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1
Lu, Y., L. Stehly, R. Brossier, and A. Paul. "Imaging Alpine crust using ambient noise wave-equation tomography." Geophysical Journal International 222, no. 1 (March 24, 2020): 69–85. http://dx.doi.org/10.1093/gji/ggaa145.
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SUMMARY We present an improved crustal Vs model and Moho depth map using ambient noise wave-equation tomography. The so-called ‘ambient noise wave-equation tomography’ is a method to invert seismic ambient noise phase dispersion data based on elastic waveform simulation, which accounts for 3-D and finite-frequency effects. We use cross-correlations of up to 4 yr of continuous vertical-component ambient seismic noise recordings from 304 high-quality broad-band stations in the Alpine region. We use model LSP_Eucrust1.0 obtained from traditional ambient noise tomography as initial model, and we iteratively improve the initial model by minimizing frequency-dependent phase traveltime differences between the observed and synthetic waveforms of Rayleigh waves in the period range 10–50 s. We obtain the final model after 15 iterations with ∼65 per cent total misfit reduction compared to the initial model. At crustal depth, the final model significantly enhances the amplitudes and adjusts the shapes of velocity anomalies. At Moho and upper-mantle depth, the final model corrects an obvious systematic velocity shift of the initial model. The resulting isovelocity Moho map confirms a Moho step along the external side of the external crystalline massifs of the northwestern Alps and reveals underplated gabbroic plutons in the lower most crust of the central and eastern Alps. Ambient noise wave-equation tomography turns out to be a useful tool to refine shear wave velocity models obtained by traditional ambient noise tomography based on ray theory.
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Luo, Yinhe, Yingjie Yang, Jinyun Xie, Xiaozhou Yang, Fengru Ren, Kaifeng Zhao, and Hongrui Xu. "Evaluating Uncertainties of Phase Velocity Measurements from Cross-Correlations of Ambient Seismic Noise." Seismological Research Letters 91, no. 3 (March 4, 2020): 1717–29. http://dx.doi.org/10.1785/0220190308.
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Abstract Ambient-noise tomography (ANT) has become a well-established method to image the crust and uppermost mantle structures in the past 15 yr. Having a good estimate of uncertainties of phase velocity dispersion measurements in ANT is critical as they can guide the level of data fitting in tomography. However, to date, there are still no systemic studies to evaluate these uncertainties. In this study, we obtain cross correlations with different stacking durations from 17 yr of ambient-noise data recorded at 120 stations in the United States. We analyze the variations of signal-to-noise ratio (SNR) and phase velocities of cross correlations. We find that the uncertainties of phase velocities are affected by SNRs, interstation distances, and stacking durations. However, none of those three variables can be solely used as a proxy to estimate the uncertainties of phase velocity measurements. Based on our analysis, we graphically present empirical relations of uncertainties of phase velocity measurements as a function of SNR, interstation distance, and stacking duration. These relations can be employed as a guide to estimate phase velocity uncertainties in applications of ANT, assisting in evaluating the reliability of resulting models from ANT.
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Abdetedal, M., Z. H. Shomali, and M. R. Gheitanchi. "Crust and upper mantle structures of the Makran subduction zone in south-east Iran by seismic ambient noise tomography." Solid Earth Discussions 6, no. 1 (January 2, 2014): 1–34. http://dx.doi.org/10.5194/sed-6-1-2014.
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Abstract. We applied seismic ambient noise surface wave tomography to estimate Rayleigh wave empirical Green's functions from cross-correlations to study crust and uppermost mantle structure beneath the Makran region in south-east Iran. We analysed 12 months of continuous data from January 2009 through January 2010 recorded at broadband seismic stations. We obtained group velocity of the fundamental mode Rayleigh-wave dispersion curves from empirical Green's functions between 10 and 50 s periods by multiple-filter analysis and inverted for Rayleigh wave group velocity maps. The final results demonstrate significant agreement with known geological and tectonic features. Our tomography maps display low-velocity anomaly with south-western north-eastern trend, comparable with volcanic arc settings of the Makran region, which may be attributable to the geometry of Arabian Plate subducting overriding lithosphere of the Lut block. At short periods (<20 s) there is a pattern of low to high velocity anomaly in northern Makran beneath the Sistan Suture Zone. These results are evidence that surface wave tomography based on cross correlations of long time-series of ambient noise yields higher resolution group speed maps in those area with low level of seismicity or those region with few documented large or moderate earthquake, compare to surface wave tomography based on traditional earthquake-based measurements.
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Acevedo, Jorge, Gabriela Fernández-Viejo, Sergio Llana-Fúnez, Carlos López-Fernández, and Javier Olona. "Ambient noise tomography of the southern sector of the Cantabrian Mountains, NW Spain." Geophysical Journal International 219, no. 1 (July 8, 2019): 479–95. http://dx.doi.org/10.1093/gji/ggz308.
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SUMMARY This study presents the first detailed analysis of ambient noise tomography in an area of the continental upper crust in the Cantabrian Mountains (NW Spain), where a confluence of crustal scale faults occurs at depth. Ambient noise data from two different seismic networks have been analysed. In one side, a 10-short-period station network was set recording continuously for 19 months. A second set of data from 13 broad-band stations was used to extend at depth the models. The phase cross-correlation processing technique was used to compute in total more than 34 000 cross-correlations from 123 station pairs. The empirical Green's functions were obtained by applying the time–frequency, phase-weighted stacking methodology and provided the emergence of Rayleigh waves. After measuring group velocities, Rayleigh-wave group velocity tomographic maps were computed at different periods and then they were inverted in order to calculate S-wave velocities as a function of depth, reaching the first 12 km of the crust. The results show that shallow velocity patterns are dominated by geological features that can be observed at the surface, particularly bedding and/or lithology and fracturing associated with faults. In contrast, velocity patterns below 4 km depth seem to be segmented by large structures, which show a velocity reduction along fault zones. The best example is the visualization in the tomography of the frontal thrust of the Cantabrian Mountains at depth, which places higher velocity Palaeozoic rocks over Cenozoic sediments of the foreland Duero basin. One of the major findings in the tomographic images is the reduction of seismic velocities above the area in the crust where one seismicity cluster is nucleated within the otherwise quiet seismic area of the range. The noise tomography reveals itself as a valuable technique to identify shear zones associated with crustal scale fractures and hence, lower strain areas favourable to seismicity.
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Qorbani, Ehsan, Dimitri Zigone, Mark R. Handy, and Götz Bokelmann. "Crustal structures beneath the Eastern and Southern Alps from ambient noise tomography." Solid Earth 11, no. 5 (October 29, 2020): 1947–68. http://dx.doi.org/10.5194/se-11-1947-2020.
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Abstract. We study the crustal structure under the Eastern and Southern Alps using ambient noise tomography. We use cross-correlations of ambient seismic noise between pairs of 71 permanent stations and 19 stations of the Eastern Alpine Seismic Investigation (EASI) profile to derive new 3D shear velocity models for the crust. Continuous records from 2014 and 2015 are cross-correlated to estimate Green's functions of Rayleigh and Love waves propagating between the station pairs. Group velocities extracted from the cross-correlations are inverted to obtain isotropic 3D Rayleigh- and Love-wave shear-wave velocity models. Our models image several velocity anomalies and contrasts and reveal details of the crustal structure. Velocity variations at short periods correlate very closely with the lithologies of tectonic units at the surface and projected to depth. Low-velocity zones, associated with the Po and Molasse sedimentary basins, are imaged well to the south and north of the Alps, respectively. We find large high-velocity zones associated with the crystalline basement that forms the core of the Tauern Window. Small-scale velocity anomalies are also aligned with geological units of the Austroalpine nappes. Clear velocity contrasts in the Tauern Window along vertical cross sections of the velocity model show the depth extent of the tectonic units and their bounding faults. A mid-crustal velocity contrast is interpreted as a manifestation of intracrustal decoupling in the Eastern Alps that accommodated eastward escape of the Alcapa block.
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Xie, Jinyun, Yingjie Yang, and Yinhe Luo. "Improving cross-correlations of ambient noise using an rms-ratio selection stacking method." Geophysical Journal International 222, no. 2 (May 13, 2020): 989–1002. http://dx.doi.org/10.1093/gji/ggaa232.
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SUMMARY Stacking of ambient noise correlations is a crucial step to extract empirical Green's functions (EGFs) between station pairs. The traditional method is to linearly stack all short-duration cross-correlation functions (CCFs) over a long period of time to obtain final stacks. It requires at least several months of ambient noise data to obtain reliable phase velocities at periods of several to tens of seconds from CCFs. In this study, we develop a new stacking method named root-mean-square ratio selection stacking (RMSR_SS) to reduce the time duration required for the recovery of EGFs from ambient noise. In our RMSR_SS method, rather than stacking all short-duration CCFs, we first judge if each of the short-duration CCF constructively contributes to the recovery of EGFs or not. Then, we only stack those CCFs which constructively contribute to the convergence of EGFs. By applying our method to synthetic noise data, we demonstrate how our method works in enhancing the signal-to-noise ratio of CCFs by rejecting noise sources which do not positively contribute to the recovery of EGFs. Then, we apply our method to real noise data recorded in western USA. We show that reliable and accurate phase velocities can be measured from 15-d long ambient noise data using our RMSR_SS method. By applying our method to ambient noise tomography (ANT), we can reduce the deployment duration of seismic stations from several months or years to a few tens of days, significantly improving the efficiency of ANT in imaging crust and upper-mantle structures.
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Wang, Yadong, Fan-Chi Lin, and Kevin M. Ward. "Ambient noise tomography across the Cascadia subduction zone using dense linear seismic arrays and double beamforming." Geophysical Journal International 217, no. 3 (February 27, 2019): 1668–80. http://dx.doi.org/10.1093/gji/ggz109.
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SUMMARY In the summer of 2017, we deployed 174 three-component nodal geophones along a 130 km west–east line across the central Oregon forearc lasting about 40 d. Our goal was to evaluate the possibility of imaging the lithospheric structure in detail with a dense but short-duration sampling of passive seismic signals. In this study, we used passive recordings from the nodal array and the previous CASC93 broad-band array along the same line to calculate noise cross-correlations. Fundamental Rayleigh wave signals were observed in the cross-correlations between 3 and 15 s period. To enhance the signal and simultaneously measure the phase velocity, we employed a double beamforming method. At each period and location, a source beam and a receiver beam were selected and the cross-correlations between the two were shifted and stacked based on the presumed local velocities. A 2-D grid search was then used to find the best velocities at the source and receiver location. Multiple velocity measurements were obtained at each location by using different source and receiver pairs, and the final velocity and uncertainty at each location were determined using the mean and the standard deviation of the mean. All available phase velocities across the profile were then used to invert for a 2-D shear wave crustal velocity model. Well resolved shallow slow velocity anomalies are observed corresponding to the sediments within the Willamette Valley, and fast velocity anomalies are observed in the mid-to-lower crust likely associated with the Siletzia terrane. We demonstrate that the ambient noise double beamforming method is an effective tool to image detailed lithospheric structures across a dense and large-scale (>100 km) temporary seismic array.
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Feng, Xuping, and Xiaofei Chen. "Rayleigh-Wave Dispersion Curves from Energetic Hurricanes in the Southeastern United States." Bulletin of the Seismological Society of America 112, no. 2 (January 25, 2022): 622–33. http://dx.doi.org/10.1785/0120210192.
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ABSTRACT Seismic records during energetic storms, similar to earthquake signals, carry information about the Earth’s interior. However, the utility of these records is rare in seismic imaging, because signal onsets in storm-generated microseisms are difficult to pick. The exciting seismic sources are also unsteady. Here, we present a new method for extracting accurate Rayleigh-wave dispersion curves from broadband seismic recordings made during the passage of hurricanes by incorporating Shen et al. (2012) cross-correlation strategy with Wang et al. (2019) frequency–Bessel transform array stacking technique. We show that surface-wave dispersion curves are observed in the seismic recordings made of four hurricanes. Compared with dispersion curves from hurricanes traveling onshore or in the deepwater, dispersion curves have higher signal-to-noise ratios from hurricanes moving near the coast or along the continental shelf. The average dispersion curve obtained from stacked cross correlations of all the four hurricanes agrees closely with that obtained from yearly ambient noise. We utilize the average dispersion curve from hurricanes to obtain a reliable shear-wave velocity (VS) model. Our VS model matches that derived from annual ambient seismic noise results with an average difference less than 0.1 km/s in the crust and uppermost mantle. This study suggests that hurricane-based dispersion curves can be an effective supplement to surface-wave tomography.
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Ryberg, T., W. H. Geissler, W. Jokat, X. Yuan, T. Fromm, S. Pandey, and B. Heit. "Crustal and uppermost mantle structure of the NW Namibia continental margin and the Walvis Ridge derived from ambient seismic noise." Geophysical Journal International 230, no. 1 (February 28, 2022): 377–91. http://dx.doi.org/10.1093/gji/ggac084.
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SUMMARY The Walvis Ridge (WR) is the most prominent hotspot track related to the opening in the South Atlantic Ocean. Several hypotheses have been developed to explain its origin and evolution. The presence of a massive magmatic structure at the landfall of the WR in Northwest Namibia raised speculation about the role of a hotspot during the opening of the South Atlantic ocean. To investigate its deeper velocity structure at the junction of the WR with the African continent was the focus of the amphibious seismological WALPASS experiment. In total 12 ocean-bottom seismometers and 28 broad-band land stations were installed between 2010 and 2012 to acquire seismological data. Here, we present the results of seismic ambient noise tomography to investigate to which extent the Tristan hotspot modified the crustal structure in the landward prolongation of the ridge and in the adjacent oceanic basins. For the tomography, vertical and hydrophone component cross correlations for >300 d for OBS stations and between 1 and 2 yr for land stations data were analysed. More than 49 000 velocity measurements (742 dispersion curves) were inverted for group velocity maps at 75 individual signal periods, which then had been inverted for a regional 3-D shear wave velocity model. The resulting 3-D model reveals structural features of the crust related to the continent–ocean transition and its disturbance caused by the initial formation of the WR ∼130 Ma. We found relatively thick continental crust below Northwest Namibia and below the near-shore part of the WR, a strong asymmetry offshore with typical, thin oceanic crust in the Namibe Basin (crossing over into the Angola Basin further offshore) to the North and a wide zone of transitional crust towards the Walvis Basin south of the WR.
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Zhang, Shane, Lili Feng, and Michael H. Ritzwoller. "Three-station interferometry and tomography: coda versus direct waves." Geophysical Journal International 221, no. 1 (January 28, 2020): 521–41. http://dx.doi.org/10.1093/gji/ggaa046.
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SUMMARY Traditional two-station ambient noise interferometry estimates the Green’s function between a pair of synchronously deployed seismic stations. Three-station interferometry considers records observed three stations at a time, where two of the stations are considered receiver–stations and the third is a source–station. Cross-correlations between records at the source–station with each of the receiver–stations are correlated or convolved again to estimate the Green’s function between the receiver–stations, which may be deployed asynchronously. We use data from the EarthScope USArray in the western United States to compare Rayleigh wave dispersion obtained from two-station and three-station interferometry. Three three-station interferometric methods are distinguished by the data segment utilized (coda-wave or direct-wave) and whether the source–stations are constrained to lie in stationary phase zones approximately inline with the receiver–stations. The primary finding is that the three-station direct wave methods perform considerably better than the three-station coda-wave method and two-station ambient noise interferometry for obtaining surface wave dispersion measurements in terms of signal-to-noise ratio, bandwidth, and the number of measurements obtained, but possess small biases relative to two-station interferometry. We present a ray-theoretic correction method that largely removes the bias below 40 s period and reduces it at longer periods. Three-station direct-wave interferometry provides substantial value for imaging the crust and uppermost mantle, and its ability to bridge asynchronously deployed stations may impact the design of seismic networks in the future.
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Nouibat, A., L. Stehly, A. Paul, S. Schwartz, T. Bodin, T. Dumont, Y. Rolland, and R. Brossier. "Lithospheric transdimensional ambient-noise tomography of W-Europe: implications for crustal-scale geometry of the W-Alps." Geophysical Journal International 229, no. 2 (December 24, 2021): 862–79. http://dx.doi.org/10.1093/gji/ggab520.
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SUMMARY A full understanding of the dynamics of mountain ranges such as the Alps requires the integration of available geological and geophysical knowledge into a lithospheric-scale 3-D geological model. As a first stage in the construction of this geo-model, we derive a new 3-D shear wave velocity model of the Alpine region, with a spatial resolution of a few tens of kilometres, making it possible to compare with geological maps. We use four years of continuous vertical-component seismic noise records to compute noise correlations between more than 950 permanent broad-band stations complemented by ∼600 temporary stations from the AlpArray sea-land seismic network and the Cifalps and EASI linear arrays. A specific pre-processing is applied to records of ocean–bottom seismometers in the Liguro-Provençal basin to clean them from instrumental and oceanic noises. We first perform a 2-D transdimensional inversion of the traveltimes of Rayleigh waves to compute group-velocity maps from 4 to $150\, \mathrm{ s}$. The data noise level treated as an unknown parameter is determined with a Hierarchical Bayes method. A Fast Marching Eikonal solver is used to update ray path geometries during the inversion. We use next the group-velocity maps and their uncertainties to derive a 3-D probabilistic Vs model. The probability distributions of Vs at depth and the probability of presence of an interface are estimated at each location by exploring a set of 130 million synthetic four-layer 1-D Vs models. The obtained probabilistic model is refined using a linearized inversion. Throughout the inversion for Vs, we include the water column where necessary. Our Vs model highlights strong along-strike changes of the lithospheric structure, particularly in the subduction complex between the European and Adriatic plates. In the South-Western Alps, our model confirms the existence of a low-velocity structure at $50-80\, \mathrm{ km}$ depth in the continuation of the European continental crust beneath the subduction wedge. This deep low-velocity anomaly progressively disappears towards the North-Western and Central Alps. The European crust includes lower crustal low-velocity zones and a Moho jump of $\sim \, 8-12$ km beneath the western boundary of the External Crystalline Massifs of the North-Western Alps. The striking fit between our Vs model and the receiver function migrated depth section along the Cifalps profile documents the reliability of the Vs model. In light of this reliability and with the aim to building a 3-D geological model, we re-examine the geological structures highlighted along the Cifalps profile.
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Wiesenberg, L., C. Weidle, A. El-Sharkawy, M. Timkó, S. Lebedev, and T. Meier. "Measuring the phase of ambient noise cross correlations: anisotropic Rayleigh and Love wave tomography across the Oman Mountains." Geophysical Journal International, June 22, 2022. http://dx.doi.org/10.1093/gji/ggac232.
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Summary Ambient seismic noise tomography has, over the last two decades, developed into a well established tool for imaging seismic properties of the Earth’s crust. Fundamental mode Rayleigh and Love wave phase velocity dispersion curves can be measured from ambient noise cross correlation functions (CCF) either using a high-frequency approximation theory, or by fitting the spectrum of the CCF to a Bessel function. Here, we advance the latter approach and present an automated algorithm that fits the phase of the Hankel function to the phase of the causal symmetric part of the CCF in order to determine phase velocity curves as continuous functions of frequency. Synthetic tests verify the reliability of the proposed method in the presence of low signal-to-noise ratio (SNR). Moreover, usage of the phase allows for robust phase velocity measurements at longer periods than when using the zero crossings of the Bessel function only and is, therefore, particularly useful at short inter-station distances. In the frequency domain, acceptable bandwidths of smooth phase velocity curves are obtained in an automated procedure using a set of fine-tuned quality criteria. We apply the method to 2.5 years of continuous waveform data recorded by 58 temporary and permanent broad band seismic stations in northern Oman. We obtain 1072 and 670 phase velocity curves for Rayleigh and Love waves, respectively, in the period range of 2 - 40 s. The data are inverted for isotropic and azimuthally anisotropic period-dependent phase velocity maps. Synthetic reconstruction tests show that the phase velocity maps have a lateral resolution of ∼30 km. The results suggest distinctly different middle to lower crustal architecture between the northern and eastern Oman Mountains. Azimuthal anisotropy shows contrasting fast propagation orientations in the shallow and deep crust, which we attribute to stress-induced and structural anisotropy in the upper crust and to lattice-preferred orientation in the lower crust.
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Sadeghisorkhani, Hamzeh, Ólafur Gudmundsson, Ka Lok Li, Ari Tryggvason, Björn Lund, and Karin Högdahl. "Shear-wave structure of southern Sweden from precise phase-velocity measurements of ambient-noise data." Geophysical Journal International, December 18, 2020. http://dx.doi.org/10.1093/gji/ggaa598.
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Summary Rayleigh-wave phase-velocity tomography of southern Sweden is presented using ambient seismic noise at 36 stations (630 station pairs) of the Swedish National Seismic Network (SNSN). We analyze one year (2012) of continuous recordings to get the first crustal image based on the ambient-noise method in the area. Time-domain cross-correlations of the vertical component between the stations are computed. Phase-velocity dispersion curves are measured in the frequency domain by matching zero crossings of the real spectra of cross-correlations to the zero crossings of the zeroth-order Bessel function of the first kind. We analyze the effect of uneven source distributions on the phase-velocity dispersion curves and correct for the estimated velocity bias before tomography. To estimate the azimuthal source distribution to determine the bias, we perform inversions of amplitudes of cross-correlation envelopes in a number of period ranges. Then, we invert the measured and bias-corrected dispersion curves for phase-velocity maps at periods between 3 and 30 s. In addition, we investigate the effects of phase-velocity bias corrections on the inverted tomographic maps. The difference between bias corrected and uncorrected phase-velocity maps is small ($< 1.2 \%$), but the correction significantly reduces the residual data variance at long periods where the bias is biggest. To obtain a shear velocity model, we invert for a one-dimensional velocity profile at each geographical node. The results show some correlation with surface geology, regional seismicity and gravity anomalies in the upper crust. Below the upper crust, the results agree well with results from other seismological methods.
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Qorbani, Ehsan, Petr Kolínský, Irene Bianchi, Dimitri Zigone, and Götz Bokelmann. "Upper crustal structure at the KTB drilling site from ambient noise tomography." Geophysical Journal International, June 28, 2022. http://dx.doi.org/10.1093/gji/ggac233.
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Summary In this study, we show results from ambient noise tomography around the KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), a continental deep drilling site located at the western edge of the Bohemian Massif, within the Variscan belt of Europe. At the KTB site, crustal rocks have been drilled down to 9 km depth. Before the drilling activity started, several active seismic surveys had been performed to explore its surroundings during the ’80s and early ’90s, in the frame of an extensive exploration of the area aimed at unraveling the characteristics of the continental lower crust that is exposed at surface in this location. Despite the exploration campaigns held at and around the KTB drilling site, there are important targets that are worth further investigation; these are related in particular to the obduction of lower crustal units to the surface, and to the mechanism of orogenic processes in general. Here we present a new 3D shear-wave velocity model of the area from cross-correlations of ambient seismic noise. The model is obtained by a unique data-set composed of two years of continuous data recorded at nine 3-component temporary stations (installed from July 2012 to July 2014) located on top and around the drilling site, and together with the data from 19 permanent stations throughout the region. This paper is focusing on the upper crustal layers, and we show velocity variations at short scales that correlate well with known geological structures in the region of the KTB site, at the surface and at depth. These are used to discuss features that are less well-resolved at present.
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Tan, Jiayan, Charles A. Langston, and Sidao Ni. "Shallow Shear-Wave Velocity Structure in Oklahoma Based on the Joint Inversion of Ambient Noise Dispersion and Teleseismic P-Wave Receiver Functions." Bulletin of the Seismological Society of America, December 8, 2020. http://dx.doi.org/10.1785/0120200246.
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ABSTRACT Ambient noise cross-correlations, used to obtain fundamental-mode Rayleigh-wave group velocity estimates, and teleseismic P-wave receiver functions are jointly modeled to obtain a 3D shear-wave velocity model for the crust and upper mantle of Oklahoma. Broadband data from 82 stations of EarthScope Transportable Array, the U.S. National Seismic Network, and the Oklahoma Geological Survey are used. The period range for surface-wave ambient noise Green’s functions is from 4.5 to 30.5 s constraining shear-wave velocity to a depth of 50 km. We also compute high-frequency receiver functions at these stations from 214 teleseismic earthquakes to constrain individual 1D velocity models inferred from the surface-wave tomography. Receiver functions reveal Ps conversions from the Moho, intracrustal interfaces, and shallow sedimentary basins. Shallow low-velocity zones in the model correlate with the large sedimentary basins of Oklahoma. The velocity model significantly improves the agreement of synthetic and observed seismograms from the 6 November 2011 Mw 5.7 Prague, Oklahoma earthquake suggesting that it can be used to improve earthquake location and moment tensor inversion of local and regional earthquakes.
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Miller, M. S., P. Zhang, M. P. Dahlquist, A. J. West, T. W. Becker, and C. W. Harris. "Inherited lithospheric structures control arc-continent collisional heterogeneity." Geology, February 5, 2021. http://dx.doi.org/10.1130/g48246.1.
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From west to east along the Sunda-Banda arc, convergence of the Indo-Australian plate transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and it can be viewed as the temporal transition of this process along strike. We collected a range of complementary geological and geophysical data to place constraints on the geometry and history of arc-continent collision. Utilizing ~4 yr of new broadband seismic data, we imaged the structure of the crust through the uppermost mantle. Ambient noise tomography shows velocity anomalies along strike and across the arc that are attributed to the inherited structure of the incoming and colliding Australian plate. The pattern of anomalies at depth resembles the system of salients and embayments that is present offshore western Australia, which formed during rifting of east Gondwana. Previously identified changes in geochemistry of volcanics from Pb isotope anomalies from the inner arc islands correlate with newly identified velocity structures representing the underthrusted and subducted Indo-Australian plate. Reconstruction of uplift from river profiles from the outer arc islands suggests rapid uplift at the ends of the islands of Timor and western Sumba, which coincide with the edges of the volcanic-margin protrusions as inferred from the tomography. These findings suggest that the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Therefore, the initial template of plate structure controls orogenesis.
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Sadeghi-Bagherabadi, Amir. "High-Resolution Crustal S-wave Velocity Model and Moho Geometry Beneath the Southeastern Alps: New Insights From the SWATH-D Experiment." Frontiers in Earth Science 9 (March 31, 2021). http://dx.doi.org/10.3389/feart.2021.641113.
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We compiled a dataset of continuous recordings from the temporary and permanent seismic networks to compute the high-resolution 3D S-wave velocity model of the Southeastern Alps, the western part of the external Dinarides, and the Friuli and Venetian plains through ambient noise tomography. Part of the dataset is recorded by the SWATH-D temporary network and permanent networks in Italy, Austria, Slovenia and Croatia between October 2017 and July 2018. We computed 4050 vertical component cross-correlations to obtain the empirical Rayleigh wave Green’s functions. The dataset is complemented by adopting 1804 high-quality correlograms from other studies. The fast-marching method for 2D surface wave tomography is applied to the phase velocity dispersion curves in the 2–30 s period band. The resulting local dispersion curves are inverted for 1D S-wave velocity profiles using the non-perturbational and perturbational inversion methods. We assembled the 1D S-wave velocity profiles into a pseudo-3D S-wave velocity model from the surface down to 60 km depth. A range of iso-velocities, representing the crystalline basement depth and the crustal thickness, are determined. We found the average depth over the 2.8–3.0 and 4.1–4.3 km/s iso-velocity ranges to be reasonable representations of the crystalline basement and Moho depths, respectively. The basement depth map shows that the shallower crystalline basement beneath the Schio-Vicenza fault highlights the boundary between the deeper Venetian and Friuli plains to the east and the Po-plain to the west. The estimated Moho depth map displays a thickened crust along the boundary between the Friuli plain and the external Dinarides. It also reveals a N-S narrow corridor of crustal thinning to the east of the junction of Giudicarie and Periadriatic lines, which was not reported by other seismic imaging studies. This corridor of shallower Moho is located beneath the surface outcrop of the Permian magmatic rocks and seems to be connected to the continuation of the Permian magmatism to the deep-seated crust. We compared the shallow crustal velocities and the hypocentral location of the earthquakes in the Southern foothills of the Alps. It revealed that the seismicity mainly occurs in the S-wave velocity range between ∼3.1 and ∼3.6 km/s.