Journal articles on the topic 'Source depth and mechanism inversion'
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Marson-Pidgeon, K. "Source Depth and Mechanism Inversion at Teleseismic Distances Using a Neighborhood Algorithm." Bulletin of the Seismological Society of America 90, no. 6 (December 1, 2000): 1369–83. http://dx.doi.org/10.1785/0120000020.
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Allamehzadeh, M., M. Dezvareh, A. M. Farahbod, D. Hatzfeld, M. Mokhtari, A. S. Moradi, M. Mostafazadeh, A. Paul, and M. Tatar. "Seismological Aspects of the 2003 Bam, Iran, Earthquake and Its Aftershock Analysis." Earthquake Spectra 21, no. 1_suppl (December 2005): 101–12. http://dx.doi.org/10.1193/1.2098167.
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The source mechanism derived from the inversion of long-period body waves revealed that the earthquake occurred on a north-south trending strike-slip fault with a thrust component. According to the source model estimated in this study, the 2003 Bam, Iran, earthquake was a multiple event formed by two subevents. The rupture following subevent one started at a depth of about 8 km. However, the depth of subevent two is about 10 km. The total seismic moment estimated from inversion processes is 8.34×1018Nm. The pulse duration of subevent one and subevent two was determined from source time function as 1.7 s and 0.8 s, respectively. Corner frequency and source radius have been calculated by using major pulse duration. The corner frequency and source radius are 0.187 Hz and 5.47 km, respectively. The aftershock events distributed along a 30 km north-south striking fault. The focal depths of aftershocks distribution show a nearly vertical alignment of aftershocks located between 6 and 20 km depth. The focal mechanism solutions of aftershocks indicate right-lateral strike-slip faulting on a north-south trending fault, parallel to the previously known Bam fault trace in the east of Bam.
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Beck, Susan L., and Howard J. Patton. "Inversion of regional surface-wave spectra for source parameters of aftershocks from the Loma Prieta earthquake." Bulletin of the Seismological Society of America 81, no. 5 (October 1, 1991): 1726–36. http://dx.doi.org/10.1785/bssa0810051726.
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Abstract Surface waves recorded at regional distances are used to study the source parameters for three of the larger aftershocks of the 18 October 1989, Loma Prieta, California, earthquake. The short-period P-wave first-motion focal mechanisms indicate a complex aftershock sequence with a wide variety of mechanisms. Many of these events are too small for teleseismic body-wave analysis; therefore, the regional surface-waves provide important long-period information on the source parameters. Intermediate-period Rayleigh- and Love-wave spectra are inverted for the seismic moment tensor elements at a fixed depth and repeated for different depths to find the source depth that gives the best fit to the observed spectra. For the aftershock on 19 October at 10:14:35 (md = 4.2), we find a strike-slip focal mechanism with right lateral motion on a NW-trending vertical fault consistent with the mapped trace of the local faults. For the aftershock on 18 October at 10:22:04 (md = 4.4), the surface waves indicate a pure reverse fault with the nodal planes striking WNW. For the aftershock on 19 October at 09:53:50 (md = 4.4), the surface waves indicate a strike-slip focal mechanism with a NW-trending vertical nodal plane consistent with the local strike of the San Andreas fault. Differences between the surface-wave focal mechanisms and the short-period P-wave first-motion mechanisms are observed for the aftershocks analyzed. This discrepancy may reflect the real variations due to differences in the band width of the two observations. However, the differences may also be due to (1) errors in the first-motion mechanism due to incorrect near-source velocity structure and (2) errors in the surface-wave mechanisms due to inadequate propagation path corrections.
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Zhu, Lupei, and Donald V. Helmberger. "Advancement in source estimation techniques using broadband regional seismograms." Bulletin of the Seismological Society of America 86, no. 5 (October 1, 1996): 1634–41. http://dx.doi.org/10.1785/bssa0860051634.
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Abstract One important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases (such as Pnl/surface wave, SV/SH). Because the misfit errors used in some waveform inversions are normalized by the data and synthetics, the amplitude information in the data has not been fully utilized. In this article, we modify the “cut and paste” source estimation technique (Zhao and Helmberger, 1994) by removing this type of normalization. It is shown that the modified method increases the stability and resolution of inversion. When multiple stations at different distance ranges are used, a distance scaling factor is introduced to compensate for the amplitude decay with distance. By applying the technique to the TERRAscope data, we have determined source mechanisms and depths of 335 southern Californian events with ML ≧ 3.5. The amplitude decays with distance are r1.13 for Pnl, r0.55 for Love waves, and r0.74 for Rayleigh waves. In contrast to generally shallow source depths reported by the southern California short-period network, the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km.
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Daniarsyad, Gatut, Aprilia Nur Vita, and Shengji Wei. "Focal Mechanism Analysis of the September 25th, 2019 Mw 6.5 Ambon Earthquake and Its Implication for Seismotectonics." IOP Conference Series: Earth and Environmental Science 873, no. 1 (October 1, 2021): 012032. http://dx.doi.org/10.1088/1755-1315/873/1/012032.
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Abstract On September 25th, 2019, an Mw 6.5 earthquake occurred in Ambon, Maluku Province, Indonesia, and caused casualties and infrastructures damages. The epicenter located in a tectonically active region with the potential strike-slip and thrust faulting earthquake sources, yet the responsible fault is still not well understood. Based on focal mechanism solutions from available seismological agencies, i.e. USGS, GFZ, GCMT, and BMKG, the earthquake has a similar strike-slip focal mechanism, although there are discrepancies on detailed source parameters. To provide a better understanding of the earthquake mechanism and seismotectonic, we apply the Cut-and-Paste (CAP) focal mechanism inversion method to broadband seismic waveforms from regional and teleseismic distances. The CAP inversion results on the regional data grouped in different distance ranges show a robust strike-slip solution. We then refine the earthquake focal depth by performing the CAPtele inversion and resulted in a depth of 12 km with similar fault plane solution as the regionals. The ruptured fault plane is resolved by a directivity analysis using azimuthal pattern of the apparent source durations, which indicates an obvious unilateral rupture propagation toward SSE direction. Our result suggests the NNW-SSE orientated fault is the ruptured fault plane, which is also consistent with the near N-S distributed aftershocks. This fault is located in a narrow sea between Seram, Ambon and Haruku island and was not reported yet in previous studies. The Coulomb failure stress (CFS) changes analysis of the mainshock shows that the Ambon earthquake has promoted the off-fault aftershocks which occurred to the west of the ruptured fault.
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Stähler, S. C., and K. Sigloch. "Fully probabilistic seismic source inversion – Part 1: Efficient parameterisation." Solid Earth 5, no. 2 (November 17, 2014): 1055–69. http://dx.doi.org/10.5194/se-5-1055-2014.
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Abstract. Seismic source inversion is a non-linear problem in seismology where not just the earthquake parameters themselves but also estimates of their uncertainties are of great practical importance. Probabilistic source inversion (Bayesian inference) is very adapted to this challenge, provided that the parameter space can be chosen small enough to make Bayesian sampling computationally feasible. We propose a framework for PRobabilistic Inference of Seismic source Mechanisms (PRISM) that parameterises and samples earthquake depth, moment tensor, and source time function efficiently by using information from previous non-Bayesian inversions. The source time function is expressed as a weighted sum of a small number of empirical orthogonal functions, which were derived from a catalogue of >1000 source time functions (STFs) by a principal component analysis. We use a likelihood model based on the cross-correlation misfit between observed and predicted waveforms. The resulting ensemble of solutions provides full uncertainty and covariance information for the source parameters, and permits propagating these source uncertainties into travel time estimates used for seismic tomography. The computational effort is such that routine, global estimation of earthquake mechanisms and source time functions from teleseismic broadband waveforms is feasible.
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Stähler, S. C., and K. Sigloch. "Fully probabilistic seismic source inversion – Part 1: Efficient parameterisation." Solid Earth Discussions 5, no. 2 (July 23, 2013): 1125–62. http://dx.doi.org/10.5194/sed-5-1125-2013.
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Abstract. Seismic source inversion is a non-linear problem in seismology where not just the earthquake parameters themselves, but also estimates of their uncertainties are of great practical importance. Probabilistic source inversion (Bayesian inference) is very adapted to this challenge, provided that the parameter space can be chosen small enough to make Bayesian sampling computationally feasible. We propose a framework for PRobabilistic Inference of Source Mechanisms (PRISM) that parameterises and samples earthquake depth, moment tensor, and source time function efficiently by using information from previous non-Bayesian inversions. The source time function is expressed as a weighted sum of a small number of empirical orthogonal functions, which were derived from a catalogue of >1000 STFs by a principal component analysis. We use a likelihood model based on the cross-correlation misfit between observed and predicted waveforms. The resulting ensemble of solutions provides full uncertainty and covariance information for the source parameters, and permits to propagate these source uncertainties into travel time estimates used for seismic tomography. The computational effort is such that routine, global estimation of earthquake mechanisms and source time functions from teleseismic broadband waveforms is feasible.
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Saetang, Kasemsak. "Focal Mechanisms of Mw 6.3 Aftershocks from Waveform Inversions, Phayao Fault Zone, Northern Thailand." International Journal of Geophysics 2017 (2017): 1–7. http://dx.doi.org/10.1155/2017/9059825.
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The focal mechanisms of Mw 6.3 aftershocks, Chiang Rai Province, Northern Thailand, were determined by using a multistation waveform inversion. Three aftershocks were selected and their waveforms were inverted for moment tensor calculation. Waveform inversions were derived from three broadband stations with three components and epicentral distances less than 250 km after all seismic stations were considered. The deviatoric moment tensor inversion was used for focal mechanism calculations. Band-pass filtering in the range of 0.03–0.15 Hz was selected for reducing low- and high-frequency noise. Source positions were created by using a single-source inversion and a grid-search method computed to optimize the waveform match. The results showed stable moment tensors and fault geometries with the southwest azimuth in the northern part of the Payao Fault Zone (PFZ) with depths shallower than 10 km. Left-lateral strike-slip with a reverse component was detected. The tectonics of the PFZ is constrained by fault-plane solutions of earthquakes. WSW directional strikes are observed in the northern part of the PFZ.
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Zhang, Jiajun, and Thorne Lay. "Duration and depth of faulting of the 22 June 1977 Tonga earthquake." Bulletin of the Seismological Society of America 79, no. 1 (February 1, 1989): 51–66. http://dx.doi.org/10.1785/bssa0790010051.
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Abstract The 22 June 1977 (Mw = 8.2) Tonga earthquake has the longest rupture duration ever reported for a normal fault event. The 150-km depth range spanned by aftershocks of the earthquake is also unusually large. There has been substantial controversy over both the depth and duration of faulting for this great event, obscuring its tectonic significance. We study the source process of the Tonga event using long-period Rayleigh waves recorded by the Global Digital Seismograph Network (GDSN) and International Deployment of Accelerometers (IDA) networks. For a standard assumption of a Haskell source, a total duration of 84 ± 4 sec is obtained using a least-squares inversion method. We introduce the use of the spectral amplitude as a weighting factor in measuring the misfit between the data and a given source finiteness model, which reduces the scatter and improves the resolution of source duration determined from data ranging in period from 150 to 300 sec. Using a more realistic shape for the source-time function in the inversion (drawing upon results from body-wave analysis) reveals a much longer (165-sec process time) component of the source process of the Tonga earthquake. The fundamental mode Rayleigh waves do not resolve any horizontal source directivity. However, the centroid depth of the earthquake is well resolved as 96 km with 90 per cent confidence range (93, 104 km). The estimated error in the depth determination due to the uncertainties in the source finiteness and earth models is only a few kilometers. The results indicate that the rupture of the earthquake excited long-period seismic waves at depths somewhat greater than the 70 to 80 km depth range where the primary body-wave radiation occurred, favoring rupture on the steeply dipping plane of the focal mechanism. The fundamental mode Rayleigh waves with periods longer than 150 sec cannot resolve vertical extent of the faulting; however, additional information from body-wave and free oscillation analyses indicates a vertical fault extent of about 50 km with a frequency-dependent variation in seismic radiation with depth.
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Schwartz, Susan Y. "Source parameters of aftershocks of the 1991 Costa Rica and 1992 Cape Mendocino, California, earthquakes from inversion of local amplitude ratios and broadband waveforms." Bulletin of the Seismological Society of America 85, no. 6 (December 1, 1995): 1560–75. http://dx.doi.org/10.1785/bssa0850061560.
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Abstract Source parameters of aftershocks of the 22 April 1991 (MW = 7.7) Costa Rica and the 25 April 1992 (MW = 7.1) Cape Mendocino, California, earthquakes are determined using a grid search inversion of P, SH, and SV amplitude ratios recorded by sparse local networks of three-component broadband and short-period stations. The inversion procedure consists of computing synthetic seismograms for three fundamental fault orientations for all source-receiver pairs over a range of source depths; calculating the complex envelopes of the observed and synthetic seismograms to determine peak amplitudes of P, SH, and SV waves; combining the fundamental fault amplitudes for all possible values of strike, dip, and rake, at 10° increments; and determining the best fault orientation and depth as the one that yields the smallest misfit between observed and synthetic P/SH, P/SV, and SV/SH amplitude ratios. The ambiguity in the sense of motion on the nodal planes, arising due to the use of amplitude ratios, is resolved by examining P-wave polarities. The sensitivity of source parameters to uncertainties in earthquake location and crustal structure is explored. For events with good station coverage, focal mechanism determinations are stable for a wide range of assumed values of crustal structure, earthquake location, and depth. Source parameters for many of the largest events (M > 3.4) are also determined by inversion of broadband displacement waveforms using a similar grid-search technique. Comparable results were obtained using both broadband waveforms and amplitude ratios. Focal mechanism solutions for 20 aftershocks of the Costa Rica earthquake reveal a complicated faulting geometry, indicating active thrust, normal, and strike-slip faults in the back-arc of Costa Rica. The 1992 Cape Mendocino earthquake occurred at the intersection of the North American, Gorda, and Pacific plates. While the mainshock was associated with underthrusting of the Gorda plate beneath the North American plate, fault plane solutions for 70% of the 38 largest aftershocks indicate that these events result from either motion between the Gorda and Pacific plates or from internal deformation within the Gorda plate.
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O'Connell, Daniel R. H., and Lane R. Johnson. "Second-order moment tensors of microearthquakes at The Geysers geothermal field, California." Bulletin of the Seismological Society of America 78, no. 5 (October 1, 1988): 1674–92. http://dx.doi.org/10.1785/bssa0780051674.
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Abstract The Geysers geothermal field is the site of intense microseismicity which appears to be associated with steam production. It seems that focal mechanisms of earthquakes at The Geysers vary systematically with depth, but P-wave first-motion focal mechanism studies have been hampered by inadequate resolution. In this study an unconstrained frequency domain moment tensor inversion method is used to over come P-wave first-motion focal sphere distribution problems and to investigate microearthquake source properties. A goal was to investigate the feasibility of using waveforms to invert for the second-order moment tensor of microearthquakes in the complex setting of The Geysers. Derived frequency-domain moment tensors for two earthquakes were verified by mechanisms estimated from P-wave first motions and required far fewer stations. For one event, 19 P-wave first motions were insufficient to distinguish between normal-slip and strike-slip focal mechanisms, but a well-constrained strike-slip solution was obtained from the waveform principal moment inversion using data from six stations. Improved waveform focal mechanism resolution was a direct consequence of using P- and S-wave data together in a progressive velocity-hypocenter inversion to minimize Green function errors. The effects of hypocenter mislocation and velocity model Green function errors on moment tensor estimates were investigated. Synthetic tests indicate that these errors can introduce spurious isotropic and compensated linear vector dipole components as large as 26 per cent for these events, whereas principal moment orientations errors were <8°. In spite of unfavorable recording geometries and large (0.6 km) station elevation differences, the results indicate that waveform moment tensor estimates for microearthquake sources can be robust and constrain source mechanisms using data from a relatively small number of stations.
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Quintanar, Luis, J. Yamamoto, and Z. Jiménez. "Source mechanism of two 1994 intermediate-depth-focus earthquakes in Guerrero, Mexico." Bulletin of the Seismological Society of America 89, no. 4 (August 1, 1999): 1004–18. http://dx.doi.org/10.1785/bssa0890041004.
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Abstract In May and December 1994, two medium-size, intermediate-depth-focus earthquakes occurred in Guerrero, Mexico, eastward of the rupture area of the great Michoacan earthquake of September 19, 1985. Even though these are not major earthquakes (∼6.4 Mw), they were widely felt through central and southern Mexico, with minor damage at Zihuatanejo and Acapulco, located along the Pacific coast, and Mexico City. Both earthquakes, separated by ∼100 km, have similar focal depths and magnitudes, however, their focal mechanisms, based upon the polarities of first arrivals, show some differences. The May earthquake shows a clear normal faulting mechanism (φ = 307°, δ = 55°, λ = −108°), whereas the December earthquake mechanism solution suggests an initial thrust faulting (φ = 313°, δ = 62°, λ = 98°) process. Although previous analysis, including local and teleseismic stations, reported a normal faulting for the December earthquake, we find that modeling using the CMT focal mechanism solution fails to reproduce the first 5 sec of the observed P-wave signal at the nearest broadband station (Δ = 168 km) and the S-wave polarity at two strong ground-motion local stations (Δ = 32, 53 km); in fact, the best fit for these stations is obtained using the thrust focal mechanism calculated from the first-motion method. Seismic moment value and rupture duration time deduced from the teleseismic spectral analysis are: 2.0 × 1018 N-m and 6.9 sec for the May event; 2.8 × 1018 N-m and 7.1 sec for the December earthquake. From the inferred seismic moment, an average Δσ of ∼15 bars for both earthquakes is obtained. Inversion of teleseismic P-wave data indicates a better fit using the CMT focal mechanism solution (normal faulting) than the first-motion mechanism for both earthquakes, although the adjustment's differences are small for the May event; for this earthquake, the rupture consisted of two sources separated by ∼7 sec, starting at a depth of ∼40 km and then propagating downdip, reaching a depth of ∼60 km. The December earthquake however, released, all its energy at a depth of 50 km in two main sources separated by ∼10 sec. The non-double-couple components values are −0.004 and −0.01 for the May and December events, respectively, indicating that the December shock has a small contribution of non-double-couple radiation that could be the result of a changing mechanism. This result agrees with the hypothesis that a slab subducting at a shallower angle (our case) is associated with the existence of random subfaults with different fault orientations. From a tectonic point of view, the complexity of the December earthquake could be the result of the observed complexity of the stress distribution around 101°W and the existence of compressional events beneath the normal faulting earthquakes near the coastline. This feature permits the flexural stresses associated to the slab bending upward to become subhorizontal at the Guerrero region. We conclude that the May earthquake corresponds to a pure normal faulting, whereas the December shock is a complex event with a variable fault geometry.
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Colombo, Daniele, Diego Rovetta, and Ersan Turkoglu. "CSEM-regularized seismic velocity inversion: A multiscale, hierarchical workflow for subsalt imaging." GEOPHYSICS 83, no. 5 (September 1, 2018): B241—B252. http://dx.doi.org/10.1190/geo2017-0454.1.
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Seismic imaging in salt geology is complicated by highly contrasted velocity fields and irregular salt geometries, which cause complex seismic wavefield scattering. Although the imaging challenges can be addressed by advanced imaging algorithms, a fundamental problem remains in the determination of robust velocity fields in high-noise conditions. Conventional migration velocity analysis is often ineffective, and even the most advanced methods for depth-domain velocity analysis, such as full-waveform inversion, require starting from a good initial estimate of the velocity model to converge to a correct result. Nonseismic methods, such as electromagnetics, can help guide the generation of robust velocity models to be used for further processing. Using the multiphysics data acquired in the deepwater section of the Red Sea, we apply a controlled-source electromagnetic (CSEM) resistivity-regularized seismic velocity inversion for enhancing the velocity model in a complex area dominated by nappe-style salt tectonics. The integration is achieved by a rigorous approach of multiscaled inversions looping over model dimensions (1D first, followed by 3D), variable offsets and increasing frequencies, data-driven and interpretation-supported approaches, leading to a hierarchical inversion guided by a parameter sensitivity analysis. The final step of the integration consists of the inversion of seismic traveltimes subject to CSEM model constraints in which a common-structure coupling mechanism is used. Minimization is performed over the seismic data residuals and cross-gradient objective functions without inverting for the resistivity model, which is used as a reference for the seismic inversion (hierarchical approach). Results are demonstrated through depth imaging in which the velocity model derived through CSEM-regularized hierarchical inversion outperforms the results of a seismic-only derived velocity model.
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Hough, S. E., and J. G. Anderson. "High-frequency spectra observed at Anza, California: Implications for Q structure." Bulletin of the Seismological Society of America 78, no. 2 (April 1, 1988): 692–707. http://dx.doi.org/10.1785/bssa0780020692.
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Abstract Data from the Anza array in southern California have been analyzed to yield a model for the depth dependence of attenuation. The result is obtained from a formal inversion of the distance dependence of the spectral decay parameter, κ, observed from sources at a wide range of distances from single stations. The inversion procedure assumes constant Qi in plane layers and finds models which are as nearly constant with depth as possible. We find that the data cannot be explained by a model in which Qi is constant with depth and that the data generally require three-layer models. The resulting models typically give Qi for P waves between 300 and 1000 in the top 5 km, rising to 1000 to 3000 at greater depths, and decreasing to 700 to 1000 around 12 km depth. Qi for S waves is slightly higher in most cases. Because this depth dependence of Qi is generally correlated with the depths of earthquake epicenters, we suggest that Qi may be due to a pressure and temperature-controlled intrinsic attenuation mechanism.
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Miyatake, Takashi, Masahiro Iida, and Kunihiko Shimazaki. "The effect of strong-motion array configuration on source inversion." Bulletin of the Seismological Society of America 76, no. 5 (October 1, 1986): 1173–85. http://dx.doi.org/10.1785/bssa0760051173.
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Abstract By using the method of prediction analysis, the accuracy of source inversion of strong-motion data is investigated for various arrays, and the effects of array configuration are evaluated. In the case of a circular array in which Ns array stations are uniformly distributed within radius R and when the strike-slip fault is located at the center of the array, an empirical relationship between the array parameters and the standard deviation of the seismic moment of each subfault, normalized by the seismic moment, is obtained as σ M 0 = c ⋅ N e 2 ⋅ 10 R / 5 / N s where c is a constant, and Ne is the number of subfaults. The constant c depends upon the fault mechanism (dip slip or strike slip), aspect ratio of the fault, focal depth, the array configuration, rupture mode (unilateral or bilateral), crustal structure, etc. However, it would be useful for both the source inversion study and a future installation plan of strong-motion arrays. An application of the method to several specific cases leads us to conclude that highly dense local array is not advantagenous for source inversion, but an array covering the earthquake fault with equally spaced stations is of benefit to the source inversion. Two cases of array configuration examined are a straight-line array, which is perpendicular to the fault and another straight-line array, which is parallel to the fault. The perpendicular array is unsuitable for source inversion, but the parallel array is advantageous for source inversion. Furthermore, the inversion study for the 1979 Imperial Valley earthquake is simulated for three cases of strong-motion station distributions (case A: 20 United States stations; case B: El Centro strong-motion array; and case C: all 26 stations of the United States and Mexico). Our simulation shows that case B gives a very poor result, and that the best of three is case C.
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TIWARI, SANDIP, A. KUMAR, and J. J. WELSER. "STRADDLE-GATE TRANSISTOR: A MOSFET IN THE LIMIT OF USEFUL FIELD-EFFECT." International Journal of High Speed Electronics and Systems 10, no. 01 (March 2000): 231–45. http://dx.doi.org/10.1142/s0129156400000271.
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For transistor, the limit of usable field-effect is defined by tunneling between the source and the drain - the mechanism that competes with field-effect as device dimensions shrink to near deBroglie wavelength. This is a more fundamental constraint in the operation of a field-effect transistor than random dopants, oxide thickness, doping magnitudes and depth, gate resistivity, soft-error rates, etc. We describe here a MOSFET structure, the straddle-gate transistor, that uses inversion regions as virtual source and drain, operates within the limits placed by the other constraints, and operates at acceptable power levels with good power gain and output conductance at 10 nm channel lenth. Experimental behavior of the straddle geometry are also described to summarized the advantages accrued using electron injection from the thin inversion regions.
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Matoza, Robin S., Bernard A. Chouet, Arthur D. Jolly, Phillip B. Dawson, Rebecca H. Fitzgerald, Ben M. Kennedy, David Fee, et al. "High-rate very-long-period seismicity at Yasur volcano, Vanuatu: source mechanism and decoupling from surficial explosions and infrasound." Geophysical Journal International 230, no. 1 (January 5, 2022): 392–426. http://dx.doi.org/10.1093/gji/ggab533.
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SUMMARY Yasur volcano, Vanuatu is a continuously active open-vent basaltic-andesite stratocone with persistent and long-lived eruptive activity. We present results from a seismo-acoustic field experiment at Yasur, providing locally dense broad-band seismic and infrasonic network coverage from 2016 July 27 to August 3. We corroborate our seismo-acoustic observations with coincident video data from cameras deployed at the crater and on an unoccupied aircraft system (UAS). The waveforms contain a profusion of signals reflecting Yasur’s rapidly occurring and persistent explosive activity. The typical infrasonic signature of Yasur explosions is a classic short-duration and often asymmetric explosion waveform characterized by a sharp compressive onset and wideband frequency content. The dominant seismic signals are numerous repetitive very-long-period (VLP) signals with periods of ∼2–10 s. The VLP seismic events are ‘high-rate’, reoccurring near-continuously throughout the data set with short interevent times (∼20–60 s). We observe variability in the synchronization of seismic VLP and acoustic sources. Explosion events clearly delineated by infrasonic waveforms are underlain by seismic VLPs. However, strong seismic VLPs also occur with only a weak infrasonic expression. Multiplet analysis of the seismic VLPs reveals a systematic progression in the seismo-acoustic source decoupling. The same dominant seismic VLP multiplet occurs with and without surficial explosions and infrasound, and these transitions occur over a timescale of a few days during our field campaign. We subsequently employ template matching, stacking, and full-waveform inversion to image the source mechanism of the dominant VLP multiplet. Inversion of the dominant VLP multiplet stack points to a composite source consisting of either a dual-crack (plus forces) or pipe-crack (plus forces) mechanism. The derived mechanisms correspond to a point-source directly beneath the summit vents with centroid depths in the range ∼900–1000 m below topography. All mechanisms suggest a northeast trending crack dipping relatively shallowly to the northwest and indicate a VLP source centroid and mechanism controlled by a stable structural geologic feature beneath Yasur. We interpret the results in the framework of gas slug ascent through the conduit responsible for Yasur explosions. The VLP mechanism and timing with infrasound (when present) are explained by a shallow-buffered top-down model in which slug ascent is relatively aseismic until reaching the base of a shallow section. Slug disruption in this shallow zone triggers a pressure disturbance that propagates downward and couples at the conduit base (VLP centroid). If the shallow section is open, an explosion propagates to the surface, producing infrasound. In the case of (the same multiplet) VLPs occurring without surficial explosions and weak or no infrasound, the decoupling of the dominant VLPs at ∼900–1000 m depth from surficial explosions and infrasound strongly indicates buffering of the terminal slug ascent. This buffering could be achieved by a variety of conditions at or directly beneath the vents, such as a high-viscosity layer of crystal-rich magma, a debris cap from backfill, a foam layer, or a combination of these. The dominant VLP at Yasur captured by our experiment has a source depth and mechanism separated from surface processes and is stable over time.
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Bernard, Pascal, and Aldo Zollo. "Inversion of near-source S polarization for parameters of double-couple point sources." Bulletin of the Seismological Society of America 79, no. 6 (December 1, 1989): 1779–809. http://dx.doi.org/10.1785/bssa0790061779.
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Abstract We propose an inversion method for retrieving the focal parameters of small to moderate earthquakes by using the near-source S-wave polarizations, which are expected to be less sensitive than amplitude data to source details or propagation effects. We first studied the variability of the polarization vector for complete synthetic records generated in simple media with a shallow low-velocity layer, for various distances (0 to 50 km), source depths (1 to 15 km), and mechanisms. In the frequency band 1 to 2 Hz, a polarization fluctuation of less than 20° is found for sources deeper than 5 km. The mean angular difference between ray theory and complete field polarization is less than 10° when the complete waveform meets the two following criteria: (1) The polarization is nearly stable (less than 30° of variability) and (2) the motion is nearly horizontal (vector dip less than 30°). The inversion method uses a norm related to the angular difference in polarization between the real and synthetic waveforms generated by a point double couple. As the problem is highly nonlinear, the model space (strike, dip, slip, location) should be finely sampled and systematically explored in the whole domain of interest. We tested the inversion resolution with synthetic data for a strike-slip and a dip-slip source at 10 km in depth (no error in location) recorded at eight stations within 30 km, assuming an error of 25° on the synthetic polarization. The dip-slip inversion gives a good resolution in dip (15° of uncertainty), but a strong correlation between strike and slip, because no recording site was close to the near vertical principal axis of the stress tensor in the test. On the contrary, the near horizontal major axis of the tensor is very well constrained. The strike-slip inversion gives a good resolution in the three parameters, with uncertainties of about 10°. A 25° change in the strike, dip, or slip angle statistically results in a 25° rotation of the polarization. With numerous stations, the overdetermination of the problem reduces the model error to values lower than 25°. The absence of records in specific areas introduces additional solutions for the possible mechanisms. Finally, the sensitivity of polarization to 3 km shift in source location is globally smaller than for 25° of rotation of any fault angle. The application of this method to real data requires the evaluation of the reliability in the polarizations computed for simplified media. An uncertainty of 25° is expected to be a reasonable assumption for distances smaller than 30 km in the 1 to 2 Hz frequency band; more generally, the error is expected to increase with distance and frequency.
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PENG, LINHUI, NING WANG, and ER CHANG SHANG. "NUMERICAL SIMULATION OF EXTRACTING MODAL BACK-SCATTERING MATRIX FROM REVERBERATION IN SHALLOW-WATER WAVEGUIDE." Journal of Computational Acoustics 13, no. 02 (June 2005): 279–85. http://dx.doi.org/10.1142/s0218396x05002700.
Full textAbstract:
Modal back-scattering matrix is the key component for reverberation predicting and also important for understanding the mechanism of bottom back-scattering. To extract the modal back-scattering matrix from reverberation data is a challening topic. So far, inversion of modal back-scattering matrix from reverberation data is based on some a priori assumptions. A new method for extracting modal back-scattering matrix from reverberation data in shallow-water waveguide is proposed recently [Shang et al., 2002]. In this paper numerical simulation of extracting the modal back-scattering matrix has been performed, it is shown that the inversion by changing the source depth works very well for lower frequency. Alternatively, the inversion by using the reverberation data at different ranges is proposed and it is shown that this approach is suitable on certain conditions.
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Li, Junlun, H. Sadi Kuleli, Haijiang Zhang, and M. Nafi Toksöz. "Focal mechanism determination of induced microearthquakes in an oil field using full waveforms from shallow and deep seismic networks." GEOPHYSICS 76, no. 6 (November 2011): WC87—WC101. http://dx.doi.org/10.1190/geo2011-0030.1.
Full textAbstract:
A new, relatively high frequency, full waveform matching method was used to study the focal mechanisms of small, local earthquakes induced in an oil field, which are monitored by a sparse near-surface network and a deep borehole network. The determined source properties are helpful for understanding the local stress regime in this field. During the waveform inversion, we maximize both the phase and amplitude matching between the observed and modeled waveforms. We also use the polarities of the first P-wave arrivals and the average S/P amplitude ratios to better constrain the matching. An objective function is constructed to include all four criteria. For different hypocenters and source types, comprehensive synthetic tests showed that our method is robust enough to determine the focal mechanisms under the current array geometries, even when there is considerable velocity inaccuracy. The application to several tens of induced microseismic events showed satisfactory waveform matching between modeled and observed seismograms. Most of the events have a strike direction parallel with the major northeast-southwest faults in the region, and some events trend parallel with the northwest-southeast conjugate faults. The results are consistent with the in situ well breakout measurements and the current knowledge on the stress direction of this region. The source mechanisms of the studied events, together with the hypocenter distribution, indicate that the microearthquakes are caused by the reactivation of preexisting faults. We observed that the faulting mechanism varies with depth, from strike-slip dominance at shallower depth to normal faulting dominance at greater depth.
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Spikin, Stuart A. "Estimation of earthquake source parameters by the inversion of waveform data: Global seismicity, 1981-1983." Bulletin of the Seismological Society of America 76, no. 6 (December 1, 1986): 1515–41. http://dx.doi.org/10.1785/bssa0760061515.
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Abstract A waveform inversion algorithm, based on optimal filter theory, has been applied to the P waves from 260 of the largest earthquakes occurring during the years 1981 through 1983. Estimates of average focal depth, scalar moment, and deviatoric source mechanism have been obtained. For all except the largest events (M0 > 1027 dyne-cm), the scalar moments obtained in this study are close to, but somewhat larger than, the Harvard centroid-moment tensor (CMT) scalar moments. The CMT estimates of scalar moment are probably biased to low values due to the way unmodeled lateral heterogeneity affects the fitting procedure. For the largest events, however, source complexity can bias the scalar moments determined in this study to lower values, and the CMT scalar moments are probably more accurate. The moment tensor, CMT, and U.S. Geological Survey first-motion source mechanisms have been objectively compared by computing the locations of the vector representations of the mechanisms on the unit sphere. We find that the similarities and differences between these mechanisms can be related to the uncertainties inherent in each method for certain types of earthquakes: (1) lack of constraint on one of the nodal planes for dip-slip type mechanisms from first-motion analysis; (2) lack of resolution of the moment tensor elements defining the dip-slip component of faulting for shallow-focus earthquakes in the CMT method; and (3) lack of resolution of the moment tensor elements defining the strike-slip component of faulting in the P-wave inversion method used here. Thus, on the average, the first-motion and CMT methods yield the more reliable mechanisms for strike-slip-type earthquakes, and the method used in this study gives a more reliable result for shallow-focus earthquakes with dip-slip-type mechanisms. Finally, both of the moment tensor methods should yield reliable solutions for intermediate- and deep-focus earthquakes.
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Bezzeghoud, M., and E. Buforn. "Source parameters of the 1992 Melilla (Spain, MW = 4.8), 1994 Alhoceima (Morocco, MW = 5.8), and 1994 Mascara (Algeria, MW = 5.7) earthquakes and seismotectonic implications." Bulletin of the Seismological Society of America 89, no. 2 (April 1, 1999): 359–72. http://dx.doi.org/10.1785/bssa0890020359.
Full textAbstract:
Abstract Source mechanisms of the Melilla, Alhoceima, and Mascara earthquakes, including fault-plane solutions, waveform modeling, and spectral analysis, are presented. Strike-slip solutions have been obtained for the Melilla and Alhoceima earthquakes and reverse faulting for Mascara. Teleseismic P-wave inversion, using broadband data, show a complex mechanism for the Alhoceima earthquake formed by two subevents at very shallow depth (7 and 8 km) and scalar seismic moments of 1.1 × 10e + 17 N-m and 4.8 × 10e + 17 N-m. The Mascara earthquake was a single event, with a shallow depth (4.5 km) and a scalar seismic moment of 3.3 × 10e + 17 N-m. Results of the Alhoceima, Melilla, and Mascara earthquakes, together with 53 focal mechanisms in the region, show a regional stress regime that corresponds to horizontal compression in a NW-SE direction, associated with the convergence between Eurasia and Africa. A change of the stress regime in the plate boundary of the Ibero-Maghrebian region has been found.
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Rohadi, S., Y. H. Perdana, N. Herayndoko, B. Sunardi, T. A. Prakoso, Suliyanti, Sunardi, N. Florida, Robert Edison, and D. Karnawati. "The M 6.5 Ambon earthquake 26 September 2019: the source mechanism and the aftershock sequence characteristics." IOP Conference Series: Earth and Environmental Science 873, no. 1 (October 1, 2021): 012013. http://dx.doi.org/10.1088/1755-1315/873/1/012013.
Full textAbstract:
Abstract The area of Ambon, Maluku is located in the subduction zone in bands where the Australian plate meets the Eurasian plate, thus causing tectonic activities. The Ambon earthquake on 26th September 2019 with 6.5 Magnitude, while the Epicentral coordinates of the earthquake were determined as 3,53° S and 128,39° E and a focal depth of 10 km, according to the Agency for Meteorology Climatology and Geophysics, Indonesia. This earthquake was strongly felt at the biggest shock was felt with intensity VI-VII as unified in Ambon City, while several other areas are reported to have experienced small shaking, such as Intensity V in Masohi, and Intensity IV in Namlea and Namrole. We used a dataset of 24 waveforms of seven sensors, we determine a tabular solution, which have a large moment of 0.4573 x 1019 N-m, the depth is 6 km by minimizing the inversion residual. The method resulting strike and rake fault, with strike: 341.8°; dip; 81.5°; rake: 158.4°, and second nodal plane strike: 75.1°; dip; 68.6°; rake: 9.14°. The mechanisms were compared with those from other agency in agreement. The time decay intervals between mainshocks and significant aftershocks follow Mogi and Utsu’s Law but with a relatively faster rate of decay than that of aftershocks in general.
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Anandakrishnan, S., D. D. Blankenship, and C. R. Bentley. "Microearthquake Source Locations and Mechanisms: Ice Stream B, West Antarctica (Abstract)." Annals of Glaciology 11 (1988): 199. http://dx.doi.org/10.3189/s0260305500006558.
Full textAbstract:
An array of nine seismographic stations, each sensitive to all three components of motion, was deployed on Ice Stream B, West Antarctica, during the austral summer of 1985–86. The network was sensitive to high-frequency (=400 Hz) seismic activity within a 350 km2 area of the ice stream, and the deployment geometry allows the precise determination of depths for events beneath the 10 km2 array. Microearthquakes from both beneath and beside the ice stream were detected and recorded (Blankenship and others 1987). Inversion of P-wave and S-wave travel times and radiation patterns allows the determination of locations and fault-plane solutions for many of these events.We find that bottom events involve low-angle thrusting, in the down-stream direction, of ice or till; displacement is ∼½ cm per event over a (15 m)2 area. Such faulting is rare and releases an insignificant part of the total energy dissipated by ice flow. However, this is a possible mechanism for plucking of the ice-stream bed.Fault-plane solutions for most major surface events are consistent with the opening of tensional fractures oriented transverse to ice flow. Precise location of these events shows that they correspond to open crevasses, mapped by Vornberger and Whillans (1986), that are oriented transverse to ice flow.In addition, shear-wave splitting observed on some of the microearthquakes shows that the c-axes in the ice stream are slightly, but not strongly, anisotropic. Precise location of the sources requires the use of a detailed velocity-depth profile in the firn, which was obtained by seismic short-refraction studies (Anandakrishnan and others 1988, this volume). A density-depth profile calculated from these velocities agrees well with direct density measurements on a single core nearby (Alley and Bentley 1988, this volume).
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Anandakrishnan, S., D. D. Blankenship, and C. R. Bentley. "Microearthquake Source Locations and Mechanisms: Ice Stream B, West Antarctica (Abstract)." Annals of Glaciology 11 (1988): 199. http://dx.doi.org/10.1017/s0260305500006558.
Full textAbstract:
An array of nine seismographic stations, each sensitive to all three components of motion, was deployed on Ice Stream B, West Antarctica, during the austral summer of 1985–86. The network was sensitive to high-frequency (=400 Hz) seismic activity within a 350 km2 area of the ice stream, and the deployment geometry allows the precise determination of depths for events beneath the 10 km2 array. Microearthquakes from both beneath and beside the ice stream were detected and recorded (Blankenship and others 1987). Inversion of P-wave and S-wave travel times and radiation patterns allows the determination of locations and fault-plane solutions for many of these events. We find that bottom events involve low-angle thrusting, in the down-stream direction, of ice or till; displacement is ∼½ cm per event over a (15 m)2 area. Such faulting is rare and releases an insignificant part of the total energy dissipated by ice flow. However, this is a possible mechanism for plucking of the ice-stream bed. Fault-plane solutions for most major surface events are consistent with the opening of tensional fractures oriented transverse to ice flow. Precise location of these events shows that they correspond to open crevasses, mapped by Vornberger and Whillans (1986), that are oriented transverse to ice flow. In addition, shear-wave splitting observed on some of the microearthquakes shows that the c-axes in the ice stream are slightly, but not strongly, anisotropic. Precise location of the sources requires the use of a detailed velocity-depth profile in the firn, which was obtained by seismic short-refraction studies (Anandakrishnan and others 1988, this volume). A density-depth profile calculated from these velocities agrees well with direct density measurements on a single core nearby (Alley and Bentley 1988, this volume).
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Rebollar, Cecilio J., Luis Quintanar, Jaime Yamamoto, and Antonio Uribe. "Source process of the Chiapas, Mexico, intermediate-depth earthquake (Mw = 7.2) of 21 October 1995." Bulletin of the Seismological Society of America 89, no. 2 (April 1, 1999): 348–58. http://dx.doi.org/10.1785/bssa0890020348.
Full textAbstract:
Abstract On 21 October 1995, we recorded with a local array an earthquake that occurred at a depth of 165 km in the subduction zone of Chiapas. The Harvard focal mechanism solution indicates a normal fault responding to the down-dip tension of the subducted oceanic crust. This is the first intermediate-depth earthquake well recorded with accelerographs and seismometers in Southeastern Mexico. Peak ground accelerations (PGA) range from 21 to 436 cm/sec2 at hypocentral distances of 174 to 256 km, respectively. The recorded PGAs are larger than those of the Copala, Guerrero, earthquake of 14 September 1995, which was a shallow (16 km) thrust fault with a similar magnitude (Mw = 7.4). The large PGA generated by the Chiapas earthquake are probably due to an enhancement of the signals produced by the up-ward intraslab propagation of energy and are similar to those observed from other intermediate-depth earthquakes in the subduction zone of Japan (Molas and Yamazaki, 1995). The duration of the strongest shaking increases from about 10 sec in the southeast at the town of San Vicente (close to the Tacaná volcano) to nearly 20 sec in the northwest, in the city of Tuxtla Gutierrez located near the epicenter. Teleseismic P-wave inversion using the Harvard focal mechanism solution indicates that the seismic moment was released in three events with a total duration of about 20 sec. The results of the inversion indicate that the rupture propagated from the northwest to the southeast along a 30-km distance. From spectral analysis, we calculate a total seismic moment release of 5.2 ± 0.5 × 1019 N-m equivalent to an Mw = 7.1 magnitude event. Using three sources with an average depth of 150 km, we were able to reach a reasonable match of the first 40 sec of the displacement records recorded at the broadband seismic stations of Huatulco (HUIG) and Pinotepa Nacional (PNIG). For the station located in Tuxtla Gutierrez (TUXD), we used two sources, since only the first 5 sec were modeled. The amplitude spectrum at teleseismic distances follows a typical Brune's (1970) θ−2 model. We obtained a corner frequency of 0.045 Hz from the spectra, which is equivalent to a source radius of 15 km and a stress drop of 65 bars assuming a circular fault.
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Zhang, Jiajun, and Thorne Lay. "Effects of centroid location on determination of earthquake mechanisms using long-period surface waves." Bulletin of the Seismological Society of America 80, no. 5 (October 1, 1990): 1205–31. http://dx.doi.org/10.1785/bssa0800051205.
Full textAbstract:
Abstract Determination of shallow earthquake source mechanisms by inversion of long-period (150 to 300 sec) Rayleigh waves requires epicentral locations with greater accuracy than that provided by routine source locations of the National Earthquake Information Center (NEIC) and International Seismological Centre (ISC). The effects of epicentral mislocation on such inversions are examined using synthetic calculations as well as actual data for three large Mexican earthquakes. For Rayleigh waves of 150-sec period, an epicentral mislocation of 30 km introduces observed source spectra phase errors of 0.6 radian for stations at opposing azimuths along the source mislocation vector. This is larger than the 0.5-radian azimuthal variation of the phase spectra at the same period for a thrust fault with 15° dip and 24-km depth. The typical landward mislocation of routinely determined epicenters of shallow subduction zone earthquakes causes source moment tensor inversions of long-period Rayleigh waves to predict larger fault dip than indicated by teleseismic P-wave first-motion data. For dip-slip earthquakes, inversions of long-period Rayleigh waves that use an erroneous source location in the down-dip or along-strike directions of a nodal plane, overestimate the strike, dip, and slip of that nodal plane. Inversions of strike-slip earthquakes that utilize an erroneous location along the strike of a nodal plane overestimate the slip of that nodal plane, causing the second nodal plane to dip incorrectly in the direction opposite to the mislocation vector. The effects of epicentral mislocation for earthquakes with 45° dip-slip fault mechanisms are more severe than for events with other fault mechanisms. Existing earth model propagation corrections do not appear to be sufficiently accurate to routinely determine the optimal surface-wave source location without constraints from body-wave information, unless extensive direct path (R1) data are available or empirical path calibrations are performed. However, independent surface-wave and body-wave solutions can be remarkably consistent when the effects of epicentral mislocation are accounted for. This will allow simultaneous unconstrained body-wave and surface-wave inversions to be performed despite the well known difficulties of extracting the complete moment tensor of shallow sources from fundamental modes.
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BUCKINGHAM, MICHAEL J., and MILTON A. GARCÉS. "AIRBORNE ACOUSTICS OF EXPLOSIVE VOLCANIC ERUPTIONS." Journal of Computational Acoustics 09, no. 03 (September 2001): 1215–25. http://dx.doi.org/10.1142/s0218396x01000802.
Full textAbstract:
A recently developed theoretical model of the airborne acoustic field from an explosive volcanic eruption of the Strombolian type is described in this article. The magma column is assumed to be a circular cylinder, which is open to the atmosphere at the top, and which opens into a large magma chamber below. The magma itself is treated as a fluid, and the surrounding bedrock is taken to be rigid. An explosive source near the base of the magma column excites the natural resonances of the conduit. These resonances result in displacement of the magma surface, which acts as a piston radiating sound into the atmosphere. The source is modeled in much the same way as an underwater explosion from a high-explosive chemical such as TNT, although in the case of the volcano the detonation mechanism is the ex-solution of magmatic gases under extremely high hydrostatic pressure. The new theory shows compelling agreement with airborne acoustic signatures that were recorded in July 1994 at a distance of 150 m from the western vent of Stromboli volcano, Italy. The theoretical and observed power spectra both display the following features: (1) four energetic peaks below 20 Hz, identified as the first four longitudinal resonances of the magma column; (2) a broad minimum around 30 Hz, interpreted as a source-depth effect, occurring because the source lay close to nulls in the fifth and sixth longitudinal resonances and thus failed to excite these modes; and (3) radial resonance peaks between 35 and 65 Hz. On the basis of the theory, an inversion of the acoustic data from Stromboli yields estimates of the depth (≈100 m) and radius (≈16 m) of the magma column as well as the depth (≈83 m), spectral shape and peak shock wave pressure (≈1 GPa) of the explosive source. Most of the parameters estimated from the acoustic inversion compare favorably with the known geometry and source characteristics of Stromboli.
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Bai, Qipeng, Sidao Ni, Risheng Chu, and Zhe Jia. "gCAPjoint, A Software Package for Full Moment Tensor Inversion of Moderately Strong Earthquakes with Local and Teleseismic Waveforms." Seismological Research Letters 91, no. 6 (August 19, 2020): 3550–62. http://dx.doi.org/10.1785/0220200031.
Full textAbstract:
Abstract Earthquake moment tensors and focal depths are crucial to assessing seismic hazards and studying active tectonic and volcanic processes. Although less powerful than strong earthquakes (M 7+), moderately strong earthquakes (M 5–6.5) occur more frequently and extensively, which can cause severe damages in populated areas. The inversion of moment tensors is usually affected by insufficient local waveform data (epicentral distance <5°) in sparse seismic networks. It would be necessary to combine local and teleseismic data (epicentral distance 30°–90°) for a joint inversion. In this study, we present the generalized cut-and-paste joint (gCAPjoint) algorithm to jointly invert full moment tensor and centroid depth with local and teleseismic broadband waveforms. To demonstrate the effectiveness and explore the limitations of this algorithm, we perform case studies on three earthquakes with different tectonic settings and source properties. Comparison of our results with global centroid moment tensor and other catalog solutions illustrates that both non-double-couple compositions of the focal mechanisms and centroid depths can be reliably recovered for very shallow (<10 km) earthquakes and intermediate-depth events with this software package.
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Cirella, A., F. Romano, A. Avallone, A. Piatanesi, P. Briole, A. Ganas, N. Theodoulidis, et al. "The 2018 Mw 6.8 Zakynthos (Ionian Sea, Greece) earthquake: seismic source and local tsunami characterization." Geophysical Journal International 221, no. 2 (February 3, 2020): 1043–54. http://dx.doi.org/10.1093/gji/ggaa053.
Full textAbstract:
SUMMARY We investigated the kinematic rupture model of the 2018 Mw 6.8 Zakynthos, Ionian Sea (Greece), earthquake by using a non-linear joint inversion of strong motion data, high-rate GPS time-series and static coseismic GPS displacements. We also tested inversion results against tide-gauge recordings of the small tsunami generated in the Ionian Sea. In order to constrain the fault geometry, we performed several preliminary kinematic inversions by assuming the parameter values resulting from different published moment tensor solutions. The lowest cost function values were obtained by using the geometry derived from the United States Geological Survey (USGS) focal solution. Between the two conjugate USGS planes, the rupture model which better fits the data is the one with the N9°E-striking 39°ESE-dipping plane. The rupture history of this model is characterized by a bilateral propagation, featuring two asperities; a main slip patch extending between 14 and 28 km in depth, 9 km northeast from the nucleation and a slightly shallower small patch located 27 km southwest from the nucleation. The maximum energy release occurs between 8 and 12 s, when both patches are breaking simultaneously. The maximum slip is 1.8 m and the total seismic moment is 2.4 × 1019 Nm, corresponding to a Mw value of 6.8. The slip angle shows a dominant right-lateral strike-slip mechanism, with a minor reverse component that increases on the deeper region of the fault. This result, in addition to the observed possibility of similar mechanisms for previous earthquakes occurred in 1959 and 1997, suggests that the tectonic deformation between the Cephalonia Transform Fault Zone and the northern tip of the Hellenic Arc Subduction zone may be accommodated by prevailing right lateral low-dipping faults, occurring on re-activated structures previously experiencing (until Pliocene) compressional regime. Comparison of predicted and observed tsunami data suggests the need of a better characterization of local harbour response for this type of relatively short-wavelength events, which is important in the context of tsunami early warning. However, the suggested dominantly strike-slip character would in turn imply a reduced tsunami hazard as compared to a dominant thrust faulting regime from this source region.
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Agung Prasetyo, Retno, Supriyanto Rohadi, Nelly Florida Riama, Yusuf Hadi Perdana, Rahmat Setyo Yuliatmoko, Lewi Ristiyono, and Marzuki Sinambela. "Focal mechanism and source determination of the Padang Lawas earthquake april 30, 2020, using moment tensor inversion." E3S Web of Conferences 340 (2022): 01012. http://dx.doi.org/10.1051/e3sconf/202234001012.
Full textAbstract:
The earthquake focal mechanism is one of the essential parameters in understanding the characteristics of an earthquake source. For example, the earthquake mainshock at the Padang Lawas Regency and South Tapanuli Regency on April 30, 2020, caused some damage to houses, schools, and mosques, especially in the South Tapanuli Regency. BMKG released the first information that the earthquake was 1.17N, 99.53E, 24 kilometers northwest of Padang Lawas, North Sumatra province, at a depth of 10 kilometers, Thursday (30/4/2020) at 15.20.25 Local Time with a magnitude of 5.6. The purpose of this study is to determine the focal mechanism of the Padang Lawas earthquake mainshock on April 30, 2021, and determine the fault nodal plane that best suits local tectonic conditions. We used the moment tensor inversion method by using waveform velocity data from the BMKG mini regional station seismic network installed in 2019 and close to the epicenter, namely TTSM, PLSM, LTSM, and RRSI. We also used the velocity model from AK135. In this study, we applied the filter technique using a Butterworth bandpass filter with a lower limit of 0.05 Hz, an upper limit of 0.10 Hz, and a cut frequency of 0.13 Hz. The result of the moment tensor inversion shows the variance of the observation and calculation data is 0.3824. The earthquake source parameters show that the Nodal I plane with strike/dip/slip values is 100.1/88.3/7.5 and the Nodal II plane with strike/dip/slip values is 9.9/82.5/178.3. The value of Seismic Moment (Mo) is 0.4653E+17 Nm or Mw 5. Based on the distribution of aftershocks and the mechanism pattern of the Tapanuli earthquake source on April 30, 2020, it is consistent with the nodal plane II. The mechanism is in the form of a strike-slip fault and corresponds to the movement mechanism of the Sumatran fault system in the Toru segment. Therefore, the tectonic conditions in the southern part of Padang Lawas Regency and South Tapanuli Regency are classified as active. Thus, the government and society need to be aware and make mitigation efforts against the impact of the next earthquake in the future.
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Pak, R. M., and O. D. Hrytsai. "Modeling of earthquake source parameters on December 12, 2018 (08:49:56,16; 36,4478° N; 140,5788° E; H = 62,0 km; Mw = 4,3, Japan)." Geofizicheskiy Zhurnal 43, no. 4 (October 5, 2021): 105–18. http://dx.doi.org/10.24028/gzh.v43i4.239962.
Full textAbstract:
Modeling of earthquake source parameters, such as the orientation of the fault plane and the direction of the fault slip, is important for understanding the physics of earthquake source processes, determining the stress-strain state of the geological medium and seismic hazard estimation. For modeling source parameters of the earthquake on December 12, 2018 at 08:49:56,16 (UTC) in Japan (36,4478° N, 140,5788° E, Northern Ibaraki Pref region) at a depth of 62 km with a magnitude of Mw = 4.3, the waveforms inversion was used to determine seismic moment tensor and representation it through a focal mechanism. The earthquake source is considered as a point source of seismic waves which propagate in a medium represented by a set of horizontally homogeneous elastic layers. An algorithm for determining seismic tensor components based on the forward problem solved by the matrix method, and using the generalized inverse solution, selecting only direct waves is applied. The input data for determining seismic moment components are data of only direct P waves selected from the observed records at six seismic stations of the Japanese local network NIED F-net: TSK, YMZ, ASI, ONS, SBT, KSK. The seismic moment tensor components were determined through waveform inversion using the matrix method. The obtained results, presented through a focal mechanism, are compared to the results obtained by the National Research Institute of Earth Sciences and Resistance to Natural Disasters (NIED CMT solutions). As a result of focal mechanisms comparison, it is concluded that the proposed algorithm for determining seismic moment tensor components can be used if it is impossible to use another method, or requires some refinement for another method. This approach is especially relevant for regions with low seismicity and insufficient number of stations. In addition, this method reduces the effects of an inaccurate medium model, because direct waves are much less distorted than reflected and converted, and that increases the accuracy and reliability of the method.
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Singh, S. K., R. S. Dattatrayam, N. M. Shapiro, P. Mandal, J. F. Pacheco, and R. K. Midha. "Crustal and upper mantle structure of Peninsular India and source parameters of the 21 May 1997, Jabalpur earthquake (Mw = 5.8): Results from a new regional broadband network." Bulletin of the Seismological Society of America 89, no. 6 (December 1, 1999): 1631–41. http://dx.doi.org/10.1785/bssa0890061631.
Full textAbstract:
Abstract In the past two years, a permanent network of 10 broadband seismographs has become operational in the Indian Peninsular shield region. In this article, we analyze the data recorded by this network with two goals: (1) to lay ground for a quick estimation of source parameters of future earthquakes in the region and (2) to estimate the source parameters of the destructive Jabalpur earthquake of 21 May 1997 (Mw = 5.8), which occurred within the network. Toward these goals, we measure Rayleigh- and Love-wave group velocity dispersion curves in the period range of 10 to 75 sec and invert these curves to estimate the crustal and the upper mantle structure below the Indian shield region. Our best model consists of a two-layer crust; the upper layer is 14-km thick with a shear-wave velocity (Vs) of 3.55 km/sec; the corresponding values for the lower layer are 25 km and 3.85 km/sec. Vs for the upper mantle is 4.65 km/sec. Based on this structure, we perform a moment tensor (MT) inversion of the bandpassed (0.05-0.02 Hz) seismograms of the Jabalpur earthquake. The best fit is obtained for a source located at a depth of 42 km, with a seismic moment, M0, of 7.7 × 1024 dyne cm, and a focal mechanism with strike 61°, dip 64°, and rake 74°. These source parameters are similar to those previously reported in the literature, demonstrating that a routine MT inversion of moderate and large Indian Penisular earthquakes is now possible. A careful examination of the seismograms and modeling of the two closest records reveal that the Jabalpur earthquake nucleated in the lower crust at a depth of ∼36 km. It consisted of two subevents separated in time by 0.65 sec, with the second subevent located ∼2 km above the first one. The total source duration was ∼1.4 sec.
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Doser, Diane I. "Source characteristics of earthquakes along the southern San Jacinto and Imperial fault zones (1937 to 1954)." Bulletin of the Seismological Society of America 80, no. 5 (October 1, 1990): 1099–117. http://dx.doi.org/10.1785/bssa0800051099.
Full textAbstract:
Abstract Body waveform inversion techniques are used to study the source parameters of four earthquakes occurring between 1937 and 1954 along the southern San Jacinto and Imperial faults (1937 Buck Ridge, 1940 Imperial Valley, 1942 Borrego Mountain, and 1954 Salada Wash events). All earthquakes had simple rupture histories with the exception of the 1940 Imperial Valley main shock, which consisted of at least four subevents whose relative locations indicate unilateral rupture toward the southeast. Earthquakes in regions of high heat flow (>80 mW/m2) had focal depths near the base of the seismogenic zone (8 to 10 km). The 1937 Buck Ridge earthquake, located in a region of lower heat flow, however, appears to have occurred at a shallow (3 ± 2 km) depth. The location, mechanism, and aftershock distribution for the 1942 Borrego Mountain earthquake suggest it could have occurred along the Split Mountain fault, a recently identified northeast-trending cross fault located between the Elsinore and Coyote Creek faults or along an unnamed fault that parallels the trend of the Coyote Creek fault. Moment and rupture length estimates obtained from this study agree well with estimates obtained in previous studies that used different data sets.
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Kim, So Gu, and Nadeja Kraeva. "Source parameter determination of local earthquakes in Korea using moment tensor inversion of single station data." Bulletin of the Seismological Society of America 89, no. 4 (August 1, 1999): 1077–82. http://dx.doi.org/10.1785/bssa0890041077.
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Abstract The purpose of this investigation is to determine source parameters such as focal mechanism, seismic moment, moment magnitude, and source depth from recent small earthquakes in the Korcan Peninsula using broadband records of three-component single station. It is very important and worthwhile to use a three-component single station in Korea because for most Korean earthquakes it is not possible to read enough first motions of P-wave arrivals because of the poor coverage of the seismic network and the small size (ML 5.0 or less) of the events. Furthermore the recent installation of the very broadband seismic stations in Korea and use of a 3D tomography technique can enhance moment tensor inversion to determine the source parameters of small earthquakes (ML 5.0 or less) that occur at near-regional distances (Δ ≤ 500 km). The focal solution for the Youngwol earthquake of 13 December 1996 is found to be a right-lateral strike slip event with a NE strike, and the Kyongju earthquake of 25 June 1997 is found to be an oblique reverse fault with a slight component of left-lateral slip in the SE direction.
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Dreger, Douglas S., and Donald V. Helmberger. "Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake (ML = 5.2)." Bulletin of the Seismological Society of America 81, no. 4 (August 1, 1991): 1129–44. http://dx.doi.org/10.1785/bssa0810041129.
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Abstract The 1990 Upland earthquake was one of the first sizable local events to be recorded broadband at Pasadena, where the Green's functions appropriate for the path are known from a previous study. The synthetics developed in modeling the 1988 Upland sequence were available for use in rapid assessment of the activity. First-motion studies from the Caltech-USGS array data gave two solutions for the 1990 main shock based on the choice of regional velocity models. Although these focal mechanisms differ by less than 5° in strike and 20° rake, it proved possible to further constrain the solution using these derived Green's functions and a three-component waveform inversion scheme. We obtain from long-period waves a fault-plane solution of θ = 216°, δ = 77°, λ = 5.0°, M0 = 2.5 × 1024 dyne-cm, depth = 6 km, and a source duration of 1.2 sec, for which the orientation and source depth are in good agreement with the first-motion results of Hauksson and Jones (1991). Comparisons of the broadband displacement records with the high-pass Wood-Anderson simulations suggests the 1990 earthquake was a complicated event with a strong asperity at depth. Double point-source models indicate that about 30 per cent of the moment was released from a 9-km deep asperity following the initial source by 0.0 to 0.5 sec. Our best-fitting distributed fault model indicates that the timing of our point-source results is feasible assuming a reasonable rupture velocity. The rupture initiated at a depth of about 6 km and propagated downward on a 3.5 by 3.5 km (length by width) fault. Both the inversion of long-period waves and the distributed fault modeling indicate that the main shock did not rupture the entire depth extent of the fault defined by the aftershock zone. A relatively small asperity (about 1.0 km2) with a greater than 1 kbar stress drop controls the short-period Wood-Anderson waveforms. This asperity appears to be located in a region where seismicity shows a bend in the fault plane.
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Gibbons, Steven J., Esteban J. Chaves, and Mark Fisk. "The 27 February 2022 Lop Nor Earthquake: Detectability, Location, and Discrimination." Seismic Record 2, no. 2 (April 1, 2022): 137–47. http://dx.doi.org/10.1785/0320220018.
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Abstract A seismic event with mb 4.8 (Mw 4.2) was detected close to the Chinese Lop Nor nuclear test site on 27 February 2022. Waveforms recorded at regional and far regional distances in central Asia indicate greater likeness with previous earthquakes in the region than with historical nuclear tests. We investigate signal-to-noise ratio (SNR) at regional and global stations, and find the best signals in central Asia and Alaska. Lower SNR at stations in China, Europe, and Australia is likely related to the radiation pattern. A joint probabilistic location of the 2022 event and well-constrained historical nuclear tests indicates an epicenter near 41.88° N and 88.10° E, about 25 km northwest of the tunnel portion of the test site. A moment tensor inversion using high-quality regional signals indicates a nearly deviatoric source with a 72% double couple and a reverse fault mechanism. The centroid depth is 20–25 km, consistent with depth phases recorded in Alaska. The observed faulting geometry and source composition for the 2022 Lop Nor event is consistent with previous earthquakes in the region and the spatial alignment of local geomorphological features, indicating tectonic and not anthropogenic origin.
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Kumar, Rajneesh, and Savita Devi. "Thermomechanical Intereactions in Porous Generalized Thermoelastic Material Permeated with Heat Sources." Multidiscipline Modeling in Materials and Structures 4, no. 3 (March 1, 2008): 237–54. http://dx.doi.org/10.1163/157361108784890679.
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The Laplace and Fourier transforms have been employed to find the general solution to the fields equations in porous generalized thermoelastic medium subjected to thermomechanical boundary conditions permeated with various heat sources; in the transformed form. On the boundary surface, the distributed sources have been taken. To get the solution in the physical form, a numerical inversion technique has been used. The effect of continuous and moving heat sources with the thermomechanical boundary conditions; and the response of boundary sources (concentrated and continuous) with heat source varying with depth; on the normal stress component, change in volume fraction field and temperature distribution have been depicted graphically for a particular model. A particular case is also deduced from the present formulation.
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Tinker, Mark Andrew, and Susan L. Beck. "Inversion of regional surface-wave spectra for source parameters of aftershocks from the 1992 Petrolia earthquake sequence." Bulletin of the Seismological Society of America 85, no. 3 (June 1, 1995): 705–15. http://dx.doi.org/10.1785/bssa0850030705.
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Abstract Regional distance surface waves are used to study the source parameters for moderate-size aftershocks of the 25 April 1992 Petrolia earthquake sequence. The Cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock (Ms = 7.1). This underthrusting event establishes that the southern end of the North America-Gorda plate boundary is seismogenic. It was followed by two separate and distinct large aftershocks (Ms = 6.6 for both) occurring at 07:41 and 11:41 on 26 April, as well as thousands of other small aftershocks. Many of the aftershocks following the second large aftershock had magnitudes in the range of 4.0 to 5.5. Using intermediate-period surface-wave spectra, we estimate focal mechanisms and depths for one foreshock and six of the larger aftershocks (Md = 4.0 to 5.5). These seven events can be separated into two groups based on temporal, spatial, and principal stress orientation characteristics. Within two days of the mainshock, four aftershocks (Md = 4 to 5) occurred within 4 hr of each other that were located offshore and along the Mendocino fault. These four aftershocks comprise one group. They are shallow, thrust events with northeast-trending P axes. We interpret these aftershocks to represent internal compression within the North American accretionary prism as a result of Gorda plate subduction. The other three events compose the second group. The shallow, strike-slip mechanism determined for the 8 March foreshock (Md = 5.3) may reflect the right-lateral strike-slip motion associated with the interaction between the northern terminus of the San Andreas fault system and the eastern terminus of the Mendocino fault. The 10 May aftershock (Md = 4.1), located on the coast and north of the Mendocino triple junction, has a thrust fault focal mechanism. This event is shallow and probably occurred within the accretionary wedge on an imbricate thrust. A normal fault focal mechanism is obtained for the 5 June aftershock (Md = 4.8), located offshore and just north of the Mendocino fault. This event exhibits a large component of normal motion, representing internal failure within a rebounding accretionary wedge. These two aftershocks and the foreshock have dissimilar locations in space and time, but they do share a north-northwest oriented P axis.
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Mustać, Marija, Babak Hejrani, Hrvoje Tkalčić, Seongryong Kim, Sang-Jun Lee, and Chang-Soo Cho. "Large Isotropic Component in the Source Mechanism of the 2013 Democratic People’s Republic of Korea Nuclear Test Revealed via a Hierarchical Bayesian Inversion." Bulletin of the Seismological Society of America 110, no. 1 (January 14, 2020): 166–77. http://dx.doi.org/10.1785/0120190062.
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ABSTRACT The 12 February 2013 nuclear test conducted by the Democratic People’s Republic of Korea stands out among other nuclear tests because it produced unusually large transversal motions. Previous studies found various percentages of isotropic components of the seismic moment tensor (MT), which opens up an important question about the reliability of the methods and assumptions we routinely use to recover the seismic MT in the point source approximation. Of particular interest is the data noise model that can be utilized to represent the uncertainty associated with the recorded data. If the noise is not accounted for, this may result in a range of unwanted effects such as overfitting waveform data, and, in turn, it may lead to erroneous conclusions. We thus scrutinize the analyses of the seismic MT of this explosion by performing a thorough analysis of the source depth and time utilizing newly developed Earth structure models to invert seismograms at regional distances at different frequency bands. In addition, we estimate the solution uncertainty within a hierarchical Bayesian framework that allows accounting for noise in the data. Our results show that the resulting MT of this event contains an expectedly large isotropic component (about 70%) and a dip-slip faulting.
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Guan, Jianbo, Yu Li, and Guohua Liu. "Preconditioned Conjugate Gradient Algorithm-Based 2D Waveform Inversion for Shallow-Surface Site Characterization." Shock and Vibration 2021 (December 21, 2021): 1–16. http://dx.doi.org/10.1155/2021/3164358.
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The full-waveform inversion (FWI) of a Love wave has become a powerful tool for shallow-surface site characterization. In classic conjugate gradient algorithm- (CG) based FWI, the energy distribution of the gradient calculated with the adjoint state method does not scale with increasing depth, resulting in diminished illumination capability and insufficient model updating. The inverse Hessian matrix (HM) can be used as a preprocessing operator to balance, filter, and regularize the gradient to strengthen the model illumination capabilities at depth and improve the inversion accuracy. However, the explicit calculation of the HM is unacceptable due to its large dimension in FWI. In this paper, we present a new method for obtaining the inverse HM of the Love wave FWI by referring to HM determination in inverse scattering theory to achieve a preconditioned gradient, and the preconditioned CG (PCG) is developed. This method uses the Love wave wavefield stress components to construct a pseudo-HM to avoid the huge calculation cost. It can effectively alleviate the influence of nonuniform coverage from source to receiver, including double scattering, transmission, and geometric diffusion, thus improving the inversion result. The superiority of the proposed algorithm is verified with two synthetic tests. The inversion results indicate that the PCG significantly improves the imaging accuracy of deep media, accelerates the convergence rate, and has strong antinoise ability, which can be attributed to the use of the pseudo-HM.
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Elias, Panagiotis, Ioannis Spingos, George Kaviris, Andreas Karavias, Theodoros Gatsios, Vassilis Sakkas, and Issaak Parcharidis. "Combined Geodetic and Seismological Study of the December 2020 Mw = 4.6 Thiva (Central Greece) Shallow Earthquake." Applied Sciences 11, no. 13 (June 26, 2021): 5947. http://dx.doi.org/10.3390/app11135947.
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On 2 December 2020, a moderate and shallow Mw = 4.6 earthquake occurred in Boeotia (Central Greece) near the city of Thiva. Despite its magnitude, the co-seismic ground deformation field was detectable and measurable by Sentinel-1, ascending and descending, synthetic aperture interferometry radar (InSAR) acquisitions. The closest available GNSS station to the epicenter, located 11 km west, measured no deformation, as expected. We proceeded to the inversion of the deformation source. Moreover, we reassessed seismological data to identify the activated zone, associated with the mainshock and the aftershock sequence. Additionally, we used the rupture plane information from InSAR to better determine the focal mechanism and the centroid location of the mainshock. We observed that the mainshock occurred at a shallower depth and the rupture then expanded downdip, as revealed by the aftershock distribution. Our geodetic inversion modelling indicated the activation of a normal fault with a small left-lateral component, length of 2.0 km, width of 1.7 km, average slip of 0.2 m, a low dip angle of 33°, and a SW dip-direction. The inferred fault top was buried at a depth of ~0.5 km, rooted at a depth of ~1.4 km, with its geodetic centroid buried at 1.0 km. It was aligned with the Kallithea fault. In addition, the dip-up projection of the modeled fault to the surface was located very close (~0.4 km SW) to the mapped (by existing geological observations) trace of the Kallithea fault. The ruptured area was settled in a transition zone. We suggest the installation of at least one GNSS and seismological station near Kallithea; as the activated zone (inferred by the aftershock sequence and InSAR results) could yield events with M ≥ 5.0, according to empirical laws relating to rupture zone dimensions and earthquake magnitude.
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Jost, M. L., T. Büßelberg, Ö. Jost, and H. P. Harjes. "Source parameters of injection-induced microearthquakes at 9 km depth at the KTB Deep Drilling site, Germany." Bulletin of the Seismological Society of America 88, no. 3 (June 1, 1998): 815–32. http://dx.doi.org/10.1785/bssa0880030815.
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Abstract A fluid injection-induced seismicity experiment was undertaken in the KTB (German Continental Deep Drilling Program) main borehole at 9 km depth. Several hundred microearthquakes were recorded by a three-component geophone at 4 km depth in the pilot hole of the KTB about 200 m west of the main hole. More than 100 of these events were also recorded with good signal-to-noise ratio by a 73-element temporary network at the surface. Several different clusters of microearthquakes with distinct waveforms were defined. Compound fault-plane solutions for the two most prominent clusters of seismic events were determined: a strike-slip mechanism for cluster 1 at an average depth of about 8.9 km and a strike-slip/reverse mechanism for cluster 4 (with the “main” ML = 1.2 event) at an average depth of 8.6 km. For both fault-plane solutions, the P axis is subhorizontal and oriented NNW-SSE, similar to the N160°E direction of maximum horizontal stress observed in the well bore. Both clusters were analyzed using an empirical Green's function method to derive the relative source time function (RSTF). Azimuthal variations of the RSTF were used to determine rupture directions and velocities. By combining the information about rupture directions with fault-plane solutions, it was possible to identify the active fault planes (NE striking nodal planes) for both clusters. Although injection-induced events are supposed to exhibit a dilatational component due to the tensile character of the source, the moment tensor inversion for both microearthquake clusters resulted in a double-couple contribution of about 90% and P axes similar to the direction of maximum horizontal stress observed in the borehole. The isotropic components of the moment tensors are insignificant due to the size of the location uncertainties. From records of the sensor at 4 km depth, we found seismic moments of the microearthquakes ranging from 107 to 1011 N-m. The spectra were corrected for Q [Q(f) = 420 f0.5 for P, and Q(f) = 230 f0.5 for S-waves, which were determined assuming an ω2 model]. Following Brune (1970, 1971), we found source radii between 12 and 28 m and stress drops between 0.01 and 6 MPa. The average ratio of S- to P-wave energy was determined as 14.2. Our relation between seismic moment and ML is log M0 = 1.01 ML + 9.68, and between energy and seismic moment, log E = 2.0 log M0 - 15.35. These seismic scaling relations suggest that stress drop increases with seismic moment for this data set. However, it cannot be precluded that our data, covering only somewhat more than three orders of magnitude, fall in a larger trend of constant-stress-drop scaling over many orders of magnitude due to the large scatter observed over several orders of magnitude.
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Priyobudi, Priyobudi, and Mohamad Ramdhan. "Indikasi Sesar Naik di Plampang, Pulau Sumbawa Berdasarkan Analisis Gempa Bumi 13 Juni 2020." EKSPLORIUM 42, no. 2 (November 30, 2021): 111. http://dx.doi.org/10.17146/eksplorium.2021.42.2.6273.
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ABSTRAK. Keberadaan sesar aktif dengan pola sesar naik di daerah Plampang berhasil diungkap dari sebaran hiposenter terelokasi, hasil inversi momen tensor, dan pemodelan perubahan tegangan Coulomb. Studi ini juga berhasil mengungkap sumber gempa pada sesar aktif tersebut dengan kedalaman relatif dangkal yang bisa menjadi ancaman di Pulau Sumbawa jika magnitudo maksimumnya rilis di masa yang akan datang. Hasil relokasi hiposenter menunjukkan sebaran episenter berarah barat daya–timur laut. Hal ini didukung juga oleh hasil inversi momen tensor yang menunjukkan bidang sesar berarah barat daya–timur laut (N2240E) dengan dip cukup curam (490). Penampang seismisitas vertikal pada arah dip menunjukkan adanya pola sesar naik yang semakin landai seiring bertambahnya kedalaman. Bidang sesar yang landai menunjukkan struktur decollement pada kedalaman 10–15 km dan berangsur menjadi curam sebagai struktur splay fault pada kedalaman 0–10 km. Hal tersebut konsisten dengan hasil inversi momen tensor yang menunjukkan mekanisme pergerakan sesar naik terjadi pada kedalaman 7 km. Pemodelan perubahan tegangan Coulomb menunjukkan adanya penambahan stress di luar area bidang sesar sehingga memicu terjadinya aftershocks. Sebaran gempa susulan menunjukkan adanya bidang sesar hipotetik dengan panjang 19 km dan lebar 12 km. Sesar sebesar ini berpotensi membangkitkan gempa dengan kekuatan Mw 6,4. Gempa Sumbawa 13 Juni 2020 dengan magnitudo M 5,3 disebabkan oleh sebagian kecil aktivitas dari bidang sesar tersebut.ABSTRACT. The existence of an active fault with a reverse fault mechanism in the Plampang area is successfully delineated from the distribution of the relocated hypocenter, the moment tensor inversion, and the Coulomb stress changes. This study also reveals the source of the earthquake in the active fault with a relatively shallow depth which can be a threat on Sumbawa Island if the maximum magnitude is released in the future. Seismicity from hypocenter relocation shows the distribution of the epicenter with a southwest–northeast direction. It is also supported by the moment tensor inversion result which shows the fault plane trending southwest–northeast (N2240E) with a steep dip (490). The vertical section of seismicity in the dip direction shows that the slope of the plane has a lower angle with increasing depth. The lower angle of a fault plane shows a decollement structure at a depth of 10–15 km and gradually becomes steep as a splay fault structure at a depth of 0–10 km. It is consistent with the result of moment tensor inversion which shows the mechanism of a reverse fault that occurred at a depth of 7 km. The Coulomb stress changes show the stress increasing outside the fault plane area, which triggers aftershocks. The distribution of aftershocks shows a hypothetical fault plane of 19 km long and 12 km wide. A fault of this size has the potential to generate an earthquake with a magnitude maximum of Mw 6.4. The Sumbawa earthquake on June 13, 2020, having M 5.3 was caused by a small part of the activity from the fault.
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Feng, Zhiren, Bo Jin, Hongfu Lei, and Di Liu. "Spatiotemporal Characteristics of Reservoir-Induced Earthquakes Using P-Wave Velocity Structures." Shock and Vibration 2021 (June 29, 2021): 1–11. http://dx.doi.org/10.1155/2021/6250086.
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In this study, through the seismic phase observation report obtained by a river reservoir seismic network from September 2012 to July 2015, the joint inversion method of source and velocity structure is employed, combined with the regional seismic geological environment data. Such method gives the P-wave velocity structure of different sections in the reservoir area and tries to find out the impact range of the reservoir water depth. And the impact of reservoir water infiltration on crustal medium is discussed. The obtained location results show that the earthquake activity is mainly concentrated in the first reservoir tail and the second reservoir head. As such, the spatial distribution characteristics of the recorded earthquakes are given, and the resolution results of detection board at different depths are speculated and discussed.
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Nigam, R., A. G. Kosovichev, P. H. Scherrer, and J. Schou. "LINE ASYMMETRY AND EXCITATION MECHANISM OF SOLAR OSCILLATIONS." Symposium - International Astronomical Union 185 (1998): 195–98. http://dx.doi.org/10.1017/s0074180900238618.
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In his opening address at the conference Dr. Tim Brown posed the line asymmetry problem between velocity and intensity as a puzzle in helioseismology that has been resisting theoretical explanation for many years. It was the observations of Duvall et al. (1993) that for the first time indicated that the power spectrum of solar acoustic modes show varying amounts of asymmetry. In particular, the velocity and intensity power spectra revealed an opposite sense of asymmetry. Many doubted the correctness of the experiment and thought it to be a puzzling result (Abrams & Kumar, 1996). Many authors have investigated this problem theoretically and have found that there is an inherent asymmetry whenever there is a localized source exciting the solar oscillations (Gabriel, 1995; Roxburgh & Vorontsov, 1995; Abrams & Kumar, 1996; Nigam et al. 1997). This problem has important implications in helioseismology where the eigenfrequencies are generally determined by assuming that the power spectrum was symmetric and can be fitted by a Lorentzian. This leads to systematic errors in the determination of frequencies and, thus, affects the results of inversions (Rhodes et al. 1997). In this paper we offer an explanation for the difference in parity of the two asymmetries and estimate the depth and type of the sources that are responsible for exciting the solar p-modes.
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Lin, Jiun-Ting, Wu-Lung Chang, Diego Melgar, Amanda Thomas, and Chi-Yu Chiu. "Quick determination of earthquake source parameters from GPS measurements: a study of suitability for Taiwan." Geophysical Journal International 219, no. 2 (August 5, 2019): 1148–62. http://dx.doi.org/10.1093/gji/ggz359.
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SUMMARY We test the feasibility of GPS-based rapid centroid moment tensor (GPS CMT) methods for Taiwan, one of the most earthquake prone areas in the world. In recent years, Taiwan has become a leading developer of seismometer-based earthquake early warning systems, which have successfully been applied to several large events. The rapid determination of earthquake magnitude and focal mechanism, important for a number of rapid response applications, including tsunami warning, is still challenging because of the limitations of near-field inertial recordings. This instrumental issue can be solved by an entirely different observation system: a GPS network. Taiwan is well posed to take advantage of GPS because in the last decade it has developed a very dense network. Thus, in this research, we explore the suitability of the GPS CMT inversion for Taiwan. We retrospectively investigate six moderate to large (Mw6.0 ∼ 7.0) earthquakes and propose a resolution test for our model, we find that the minimum resolvable earthquake magnitude of this system is ∼Mw5.5 (at 5 km depth). Our tests also suggest that the finite fault complexity, often challenging for the near-field methodology, can be ignored under such good station coverage and thus, can provide a fast and robust solution for large earthquake directly from the near field. Our findings help to understand and quantify how the proposed methodology could be implemented in real time and what its contributions could be to the overall earthquake monitoring system.
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Kumar, Sushil, Hiroaki Negishi, Jim Mori, and Tamao Sato. "Role of crustal fluid in triggering moderate to major earthquakes: evidence from aftershock data of two recent large tremors." Journal of Nepal Geological Society 38 (September 25, 2008): 29–38. http://dx.doi.org/10.3126/jngs.v38i0.31479.
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A number of models have been proposed for the role of fluids and high pore pressures in the mechanics of fault slip and the nucleation of earthquakes, e.g., dilatancy-diffusion, mineral dehydration, frictional heating, fluid pressure-activated fault valves and hydrofracturing, partially sealed fault zones, a spatially varying stress tensor without hydrofracturing, and fluid involved weak and strong patch failures. In this study, the availability of fluid in the upper and lower crust was analysed carefully, as the fluid may be responsible for triggering large earthquakes. The anomalies observed in the three-dimensional tomographic images from the source regions of the 2001 Gujarat and 1995 Kobe earthquakes, obtained after inversion of aftershock data, can be attributed to the presence of the fluid. A tomographic inversion was also applied to the aftershock data from the 26 January 2001 Bhuj earthquake (Mw 7.7) in the state of Gujarat in western India. We used arrival times from 8,374 P and 7,994 S waves of 1,404 aftershocks recorded on 25 temporary seismic stations. It seems that the aftershock distribution corresponds to the high-velocity anomalies. Low P- to S-wave velocity ratio (Vp/Vs) anomalies are generally found at depths of 10 to 35 km, i.e. the depth range of the aftershock distribution. However, relatively high Vp/Vs and low Vs characterise the deeper region below the hypocentre of the mainshock, at depths of 35 to 45km.This anomaly may be due to a weak fractured and fluid-filled rock matrix, which might have contributed to triggering this earthquake. This anomaly exists in the depth range of 35 to 45 km, and extends 10 to 12 km laterally. This earthquake occurred on a relatively deep and steeply dipping reverse fault with a large stress drop.
Similarly, the 17 January 1995 Kobe earthquake (M 7.2) in southwest Japan had a strike-slip focal mechanism and it caused a rupture at a 17 km depth. The Kobe main shock hypocentre is located in a distinctive zone characterised by low P- and S-wave velocities and a high Poisson's ratio. This anomaly exists in a depth range of 16to 21 km, and extends 15to 20km laterally. This anomaly can be attributed to a fluid-filled, fractured rock matrix that contributed to the initiation of the Kobe earthquake. The existence of fluids in and below the seismogenic layer may affect the long-term structural and compositional evolution of the fault zone, change the fault zone strength, and alter the local stress regime. These influences can be exerted through the physical role of fluid pressure and a variety of chemical effects, such as stress corrosion and pressure solution. These influences would have enhanced stress concentration in the seismogenic layer leading to mechanical failure of a strong asperity, and thus may have contributed to the nucleation of the Kobe earthquake.
The area of low Vs and high Vp/Vs values can be seen in a depth range of35 to45km beneath the main shock hypocentre. These features are very similar to the velocity anomaly also observed, in a depth range of 16 to 21 km, in the hypocentre region of the1995 Kobe earthquake. Such an anomaly possibly indicates the existence of a fluid-filled, fractured rock matrix, which may have contributed to the initiation of large earthquakes. The fluid in a depth range of35to45km might have also triggered the 200I Gujarat earthquake.
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Gong, Zhang, Li, Wen, Zhao, Hou, and Shan. "Multi-Sensor Geodetic Observations and Modeling of the 2017 Mw 6.3 Jinghe Earthquake." Remote Sensing 11, no. 18 (September 16, 2019): 2157. http://dx.doi.org/10.3390/rs11182157.
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The Mw 6.3 Jinghe earthquake struck Xingjiang Province, China, on 8 August 2017 (05:15:04 UTC); the epicenter was near the Kusongmuxieke Piedmont Fault (KPF) of the northern Tian Shan Mountains. We used multi-source and multi-track satellite Synthetic Aperture Radar (SAR) imagery and Interferometric SAR (InSAR) techniques to reconstruct the coseismic displacement field from different line-of-sight geometries. To reduce the phase artifacts, we employed multi-temporal scenes acquired by Sentinel-1, and reconstructed the coseismic deformation through a temporal averaging strategy. Together with a single interferometric pair obtained using the Phased Array type L-band Synthetic Aperture Radar 2 (PALSAR2) sensor aboard the Advanced Land Observing Satellite 2 (ALOS2), we obtained five displacement maps with slightly different viewing geometries; all of which were used to constrain a geodetic inversion to retrieve the fault geometry parameters and slip distribution. Based on the focal mechanism and regional geology, we constructed multiple fault models that differ in dip direction (south and north dipping), and various striking angles. Both models fit the InSAR displacement maps, but have slip distributions of different depths. The slip depth of the south dipping model, with a dip of ~42°, is the most consistent with the relocated earthquake sequence and regional geological structure. Through the geodetic inversion, the maximum slip (0.25 m) occurred at 14.05 km and the associated rake was 89.56°. The result implies that the seismogenic fault is a blind thrust fault north of KPF (towards the foreland). Considering the relative locations of the suggested blind fault, the KPF, and the continuing north to south (N–S) shortening of the Tian Shan Mountains, this fault could be formed by the northward propagation of the regional fold-thrust belt.
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Valerio, Emanuela, Mariarosaria Manzo, Francesco Casu, Vincenzo Convertito, Claudio De Luca, Michele Manunta, Fernando Monterroso, Riccardo Lanari, and Vincenzo De Novellis. "Seismogenic Source Model of the 2019, Mw 5.9, East-Azerbaijan Earthquake (NW Iran) through the Inversion of Sentinel-1 DInSAR Measurements." Remote Sensing 12, no. 8 (April 24, 2020): 1346. http://dx.doi.org/10.3390/rs12081346.
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In this work, we investigate the Mw 5.9 earthquake occurred on 7 November 2019 in the East-Azerbaijan region, in northwestern Iran, which is inserted in the tectonic framework of the East-Azerbaijan Plateau, a complex mountain belt that contains internal major fold-and-thrust belts. We first analyze the Differential Synthetic Aperture Radar Interferometry (DInSAR) measurements obtained by processing the data collected by the Sentinel-1 constellation along ascending and descending orbits; then, we invert the achieved results through analytical modelling, in order to better constrain the geometry and characteristics of the seismogenic source. The retrieved fault model shows a rather shallow seismic structure, with a center depth at about 3 km, approximately NE–SW-striking and southeast-dipping, characterized by a left-lateral strike-slip fault mechanism (strike = 29.17°, dip = 79.29°, rake = −4.94°) and by a maximum slip of 0.80 m. By comparing the inferred fault with the already published geological structures, the retrieved solution reveals a minor fault not reported in the geological maps available in the open literature, whose kinematics is compatible with that of the surrounding structures, with the local and regional stress states and with the performed field observations. Moreover, by taking into account the surrounding geological structures reported in literature, we also use the retrieved fault model to calculate the Coulomb Failure Function at the nearby receiver faults. We show that this event may have encouraged, with a positive loading, the activation of the considered receiver faults. This is also confirmed by the distribution of the aftershocks that occurred near the considered surrounding structures. The analysis of the seismic events nucleated along the left-lateral strike-slip minor faults of the East-Azerbaijan Plateau, such as the one analyzed in this work, is essential to improve our knowledge on the seismic hazard estimation in northwestern Iran.