Articles de revues : « Crustal scattering »

1

Sahroni, Anang, Leni Sophia Heliani, Cecep Pratama, Hidayat Panuntun et Wiwit Suryanto. « Preliminary result for crustal properties derivation related to tectonics for hazard mitigation in Eastern Indonesia using Teleseismic P Coda ». E3S Web of Conferences 325 (2021) : 01012. http://dx.doi.org/10.1051/e3sconf/202132501012.

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Eastern Indonesia is tectonically complex, formed by different plates and microplates interactions from different origins. This complexity gives geoscientists a challenge to solve the ’jigsaw’ of the complex interactions. The understanding of tectonic processes can lead to a breakthrough in both resource exploration and disaster risk reduction. We utilize teleseismic P wave coda for random coda from scattering and deterministic coda originated from the crust-mantle boundary (Moho) to derive the crustal properties, including thickness, Vp/Vs, and qualitative scattering characteristics. For the scattering properties, we apply Iterative Cross-Correlation and Stacking (ICCS) to align the waveform. At the same time, for the crust characteristic, we employ the Receiver Functions (RF) method alongside H-k stacking. The crustal thickness recovered from the RF and H-k stacking has a good correlation with the crustal origin, where the thickness in older and stable crust originated from Sundaland and Gondwana is thicker than a younger plate of the crust arc and subduction origin. The Vp/Vs is high in a region that is interpreted to be dominated by mafic lower crust originated from oceanic-oceanic subduction during Eocene, anisotropy, or by a magmatic anomaly. The P coda also correlated well with the subsurface magmatic anomaly by providing a unique pattern.

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Leary, P. C. « Quantifying crustal fracture heterogeneity by seismic scattering ». Geophysical Journal International 122, n^o 1 (juillet 1995) : 125–42. http://dx.doi.org/10.1111/j.1365-246x.1995.tb03541.x.

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Rost, S., G. A. Houseman, A. W. Frederiksen, D. G. Cornwell, M. Kahraman, S. Altuncu Poyraz, U. M. Teoman et al. « Structure of the northwestern North Anatolian Fault Zone imaged via teleseismic scattering tomography ». Geophysical Journal International 227, n^o 2 (10 juillet 2021) : 922–40. http://dx.doi.org/10.1093/gji/ggab265.

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SUMMARY Information on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and seismic hazard. We use the scattered seismic wavefield to study the subsurface structure of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 İzmit and Düzce ruptures using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7 km. Using the forward- and back-scattered energy that follows the direct P-wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve changes in lithospheric structure on a scale of 10 km or less in an area of about 130 km by 100 km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east–west, we detect strong variations in structure both north–south, across boundaries of the major blocks, and east–west, parallel to the strike of the NAFZ. The surface expression of the two strands of the NAFZ is coincident with changes on main interfaces and interface terminations throughout the crust and into the upper mantle in the tomographic sections. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and, using a tomographic approach, to resolve the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain at these depths.

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Morozov, I. B. « Crustal Scattering and Some Artifacts in Receiver Function Images ». Bulletin of the Seismological Society of America 94, n^o 4 (1 août 2004) : 1492–99. http://dx.doi.org/10.1785/012003117.

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Nielsen, L., H. Thybo, I. B. Morozov, S. B. Smithson et L. Solodilov. « TeleseismicPnarrivals : influence of mantle velocity gradient and crustal scattering ». Geophysical Journal International 152, n^o 2 (février 2003) : F1—F7. http://dx.doi.org/10.1046/j.1365-246x.2003.01873.x.

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Butler, Rhett. « Bulk, shear and scattering attenuation beneath Hawaiian Volcanos and in the oceanic crust extending to the Aloha Cabled Observatory ». Geophysical Journal International 223, n^o 1 (23 juin 2020) : 543–60. http://dx.doi.org/10.1093/gji/ggaa309.

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SUMMARY Seismic attenuation is measured from a swarm of 50 earthquakes in Kīlauea volcano in 2018, associated with caldera collapse. The traverse extends at nearly constant azimuth to the saddle between Mauna Loa and Mauna Kea, continuing to Maui beneath the distal flanks of three dormant volcanos. From Maui the traverse then extends seaward to the Aloha Cabled Observatory (ACO) on the seafloor north of O‘ahu. The effective attenuation is measured with respect to an ${\omega ^{ - 2}}$ earthquake source model. Frequency dependent ${Q_P}$ and ${Q_S}$ are derived. The initial path is shallow and uphill, the path to Maui propagates at mid-crustal depths, and the path to ACO extends through oceanic crust. The observations of ${Q_P} \le {Q_S}$ over each traverse are modelled as bulk attenuation ${Q_K}$. Several attenuation processes are observed, including ${Q_\mu }$, ${Q_K}$, $Q\sim f$, constant Q and scattering. The observation of bulk attenuation is ascribed to contrasting physical properties between basalt and water saturated vesicles. The ratio of Q values between shallow and mid-crustal propagation is used to derive an activation energy E* for the undetermined shear attenuation mechanism. A Debye relaxation peak is fit to the ${Q_S}( f )$ and ${Q_K}( f )$ observed for the mid-crustal pathway. A prior high-frequency attenuation study near Wake Island compares well with this Hawaiian Q data set, which in general shows lower values of Q than observed for Wake.

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Przybilla, Jens, Ulrich Wegler et Michael Korn. « Estimation of crustal scattering parameters with elastic radiative transfer theory ». Geophysical Journal International 178, n^o 2 (août 2009) : 1105–11. http://dx.doi.org/10.1111/j.1365-246x.2009.04204.x.

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Bertrand, E., A. Deschamps et J. Virieux. « Crustal structure deduced from receiver functions via single-scattering migration ». Geophysical Journal International 150, n^o 2 (août 2002) : 524–41. http://dx.doi.org/10.1046/j.1365-246x.2002.01723.x.

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Revenaugh, Justin. « The relation of crustal scattering to seismicity in southern California ». Journal of Geophysical Research : Solid Earth 105, B11 (10 novembre 2000) : 25403–22. http://dx.doi.org/10.1029/2000jb900304.

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Hearn, Thomas M. « Crustal attenuation from USArray ML amplitude tomography ». Geophysical Journal International 224, n^o 1 (19 septembre 2020) : 199–206. http://dx.doi.org/10.1093/gji/ggaa445.

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SUMMARY Seismic attenuation across the US is estimated using station ML magnitude data from the USArray. Station magnitudes are recalibrated back to amplitude and back projected in a 2-D tomography. Data represent the amplitudes of the horizontal components of the Lg phase. The western US shows regions of very high attenuation and contrasts with the lesser attenuation of the eastern US. Individual attenuation anomalies can be clearly tied to regional geology. Station gains show broad regional variations that match geographic regions. Most of the high-attenuation areas are regions of high geothermal activity suggesting that intrinsic attenuation dominates over scattering attenuation. An exception is the central San Andreas Fault zone because it lacks any localized heat-flow anomaly. The US east of the Rocky Mountains is bland and contains none of the high-attenuation regions of the western US. Instead, the central US has low-attenuation patches that do not obviously correspond to geologic province. Sediments of the Gulf Coast Plain, Willison Basin and Michigan Basin do show up as intermediate attenuation while the Illinois Basin, Appalachian Basin and other basins are not apparent. In Alaska, attenuation is generally less than the western US, but still much greater than the eastern US. In southeast Alaska, the Wrangell Volcanic Field causes a sizeable high-attenuation zone. The volcanic Aleutian Mountains also have high attenuation. However, moderate to high attenuation also correlates with the tertiary sedimentary basins in Alaska. The North Slope Basin does not seem to attenuate. Thicker crust and mountain roots tend to show less attenuation, if anything, but this correspondence is most likely due to differences in temperature and seismic velocity. Heat, scattering and young sedimentary basins create seismic attenuation in the continental crust.

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Ebel, John E. « The effect of crustal scattering on observed high-frequency earthquake seismograms ». Geophysical Journal International 98, n^o 2 (août 1989) : 329–41. http://dx.doi.org/10.1111/j.1365-246x.1989.tb03356.x.

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Cao, S., B. L. N. Kennett et B. R. Goleby. « A 3D isochronal modelling technique and its applications ». Exploration Geophysics 20, n^o 2 (1989) : 205. http://dx.doi.org/10.1071/eg989205.

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Reflection seismic datasets are obtained in both the exploration of oil and mineral resources and the probing of the deep crust and the upper mantle. To interpret the datasets, considerable effort has been spent on the understanding of seismic wave propagation phenomena by simulating seismic wave propagations in some a priori physical models. A rather simple and efficient modelling technique has been developed to study elastic wave reflections with full inclusion of diffractions.This modelling technique employs an integral representation of reflections from a surface or a scatterer. High frequency asymptotic approximations are used for the propagation between the seismic source or receiver and a surface or a scatterer. At a scatterer, first order scattering is assumed. At a surface, reflection and transmission effects are estimated using the assumption of a locally plane interface and plane incident wave. With these approximations, the reflected seismograms are calculated by convolving the time derivative of a source function with a model weight function for a particular source-receiver pair. The weight function at a particular time is evaluated by a line integral along a contour of equal total travel time from source to receiver via the scattering surface (an isochron). The kernel of this integral at a reflecting point is the local reflection coefficient which which represents the effects of the amplitude of material parameter contrasts at the reflecting point, the angles between the incoming and outgoing waves and the local surface normal and the local speed of advance of the isochron on the surface, and the geometrical spreading factors from the source and receiver to the reflecting point.This modelling technique is used to investigate the validity of some of the interpretations of a deep crustal reflection profile collected in central Australia. The modelling results confirm that even with a relatively short (4 km) field spread it would be possible to pick up the reflected energy from faults with dips of about 40�. The largest fault, the Redbank Zone, has significant displacement of the crust-mantle boundary and within the fault zone, it is conceivable to have considerable variability in physical properties.The deep seismic section shows this boundary as a thick (0.5s) band of complex reflections and diffractions at the reflection time appropriate to the crust-mantle transition. Two possible structures for the crust-mantle boundary were investigated, one where the crustal faults have displaced this interface and created a 'block-faulted' geometry and the other where the crustal faults are listric near the boundary and appear to sole out on the crust-mantle interface, giving rise to an undulation of the Moho. The modelling results (Figure 1) for an undulating boundary show a band of reflections which strongly resemble the observed seismic reflection data.

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Audet, P. « Temporal Variations in Crustal Scattering Structure near Parkfield, California, Using Receiver Functions ». Bulletin of the Seismological Society of America 100, n^o 3 (14 mai 2010) : 1356–62. http://dx.doi.org/10.1785/0120090299.

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Rietbrock, A., et F. Scherbaum. « Crustal scattering at the KTB from a combined microearthquake and receiver analysis ». Geophysical Journal International 136, n^o 1 (1 janvier 1999) : 57–67. http://dx.doi.org/10.1046/j.1365-246x.1999.00708.x.

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Kennett, B. L. N., M. G. Bostock et J. K. Xie. « Guided-wave tracking in 3-D : A tool for interpreting complex regional seismograms ». Bulletin of the Seismological Society of America 80, n^o 3 (1 juin 1990) : 633–42. http://dx.doi.org/10.1785/bssa0800030633.

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Abstract The tracking of guided-wave trains by raytracing in 3-D structures can provide a means of interpreting complex seismograms at regional distances. The method relies on the interpretation of Lg as the constructive interference of multiple S reflections within the crust, and strong scattering can be simulated by the inclusion of secondary sources. This approach has been applied to Californian events observed in the southwestern United States whose records at regional distances frequently exhibit an extended and complex Lg coda with significant late energy arriving with group velocities of 2.5 km/sec or less. The general character of this energy precludes an interpretation as the result of stochastic scattering processes or dispersion in low velocity surface sediments; therefore some alternative explanation needs to be sought. The observed wavetrains of events from two sites in southwestern California (Coalinga and North Baja) at stations LAC, MNV, and ELK have been compared with the predictions from tracking guided-wave patterns. Multipathing of energy by major changes in the thickness of the crustal wave guide is consistent with the extended and often “pulse-like” nature of the Lg coda. Examples of plausible multipathing mechanisms include reflection from the ocean-continent transition, scattering associated with topographic features such as the Sierra Nevada, and resonance within the narrow corridor of Baja California.

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Shillington, Donna J., John R. Hopper et W. Steven Holbrook. « Seismic signal penetration beneath postrift sills on the Newfoundland rifted margin ». GEOPHYSICS 73, n^o 5 (septembre 2008) : B99—B107. http://dx.doi.org/10.1190/1.2972131.

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Analysis of averaged, smoothed instantaneous frequency and amplitude of seismic-reflection data collected on the Newfoundland magma-starved rifted margin along the Ocean Drilling Program (ODP) leg 210 transect demonstrate that the transparency of “transitional” crust arises from poor signal penetration. In principle, the high-frequency spectral content and amplitude of seismic reflection data should decrease with increasing traveltime as a result of absorption, geometric spreading, and scattering so long as seismic energy continues to return from deeper levels of the subsurface. As a result, if average frequency and amplitude cease to decrease with depth, background noise, rather than returning seismic energy, likely dominates the record. Average frequency increases and average amplitude remains comparatively constant below bright reflections overlying transitional crust on the Newfoundland margin. Similar patterns are not observed at the sediment-basement contact farther seaward in oceanic crust where intracrustal reflections are apparent. In thoserecords, both amplitude and frequency continue to decrease steadily for at least [Formula: see text] below the top of basement. We interpret these observations as evidence that signal penetration is comparatively poor beneath bright reflections overlying transitional basement. Consequently, the featureless appearance of transitional crust on the Newfoundland margin in seismic-reflection profiles cannot be used to make inferences about its physical properties; instead, only seismic characteristics observed in rare basement highs that rise above bright reflections in the lowermost sedimentary section can provide meaningful information. Weak signal penetration in this crustal domain is consistent with the results from site 1276 during ODP leg 210, where interlayered diabase sills and sediments were recovered above basement, which would be expected to result in high reflection coefficients and low signal penetration. The apparent lack of seismic penetration throughout the transition zone off Newfoundland also implies interlayered sills and sediments might be widespread over this crustal domain.

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Zhang, Tianrun, et Thorne Lay. « Effects of crustal structure under the Barents and Kara Seas on short-period regional wave propagation for Novaya Zemlya explosions : Empirical relations ». Bulletin of the Seismological Society of America 84, n^o 4 (1 août 1994) : 1132–47. http://dx.doi.org/10.1785/bssa0840041132.

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Abstract Short-period seismic recordings at regional and upper mantle distances from underground explosions at Novaya Zemlya demonstrate that propagation across the continental shelf under the Barents and Kara Seas appears to modify the partitioning of energy between Lg and Sn phases relative to purely continental paths in the Eurasian crust. While the underwater segments of the paths are relatively short, variations in bathymetric characteristics from path to path influence the regional wave field, with systematic behavior that can be used to establish empirical amplitude corrections for regional phases. We analyze a large set of Eurasian recordings to explore the relationship between regional phase energy partitioning and bathymetric characteristics. Maximum water depth along the path is the most influential factor for the Novaya Zemlya data. It has strong linear correlations with the logarithmic rms amplitude of Lg and the ratios Sn/Lg and P/Lg. The maximum water depth probably reflects the extent of necking of the crustal wave guide under the continental margin, which may disrupt Lg modes resulting in Lg to Sn scattering, but there is surprising sensitivity to small variations in bathymetry. Empirical relations like those found here may be useful for nuclear yield estimation and discrimination for regions such as the Korean Peninsula and Persian Gulf, where many seismic phases traverse water-covered continental shelf with poorly known crustal structure.

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Tao, Jun, Zhisheng Zhang, Yunfei Wu, Leiming Zhang, Zhijun Wu, Peng Cheng, Mei Li, Laiguo Chen, Renjian Zhang et Junji Cao. « Impact of particle number and mass size distributions of major chemical components on particle mass scattering efficiency in urban Guangzhou in southern China ». Atmospheric Chemistry and Physics 19, n^o 13 (4 juillet 2019) : 8471–90. http://dx.doi.org/10.5194/acp-19-8471-2019.

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Abstract. To grasp the key factors affecting particle mass scattering efficiency (MSE), particle mass and number size distribution, PM2.5 and PM10 and their major chemical compositions, and the particle scattering coefficient (bsp) under dry conditions were measured at an urban site in Guangzhou, southern China, during 2015–2016. On an annual average, 10±2 %, 48±7 % and 42±8 % of PM10 mass were in the condensation, droplet and coarse modes, respectively, with mass mean aerodynamic diameters (MMADs) of 0.78±0.07 in the droplet mode and 4.57±0.42 µm in the coarse mode. The identified chemical species mass concentrations can explain 79±3 %, 82±6 % and 57±6 % of the total particle mass in the condensation, droplet and coarse mode, respectively. Organic matter (OM) and elemental carbon (EC) in the condensation mode, OM, (NH4)2SO4, NH4NO3, and crustal element oxides in the droplet mode, and crustal element oxides, OM, and CaSO4 in the coarse mode, were the dominant chemical species in their respective modes. The measured bsp can be reconstructed to the level of 91±10 % using Mie theory with input of the estimated chemically resolved number concentrations of NaCl, NaNO3, Na2SO4, NH4NO3, (NH4)2SO4, K2SO4, CaSO4, Ca(NO3)2, OM, EC, crustal element oxides and unidentified fraction. MSEs of particle and individual chemical species were underestimated by less than 13 % in any season based on the estimated bsp and chemical species mass concentrations. Seasonal average MSEs varied in the range of 3.5±0.1 to 3.9±0.2 m2 g−1 for fine particles (aerodynamic diameter smaller than 2.1 µm), which was mainly caused by seasonal variations in the mass fractions and MSEs of the dominant chemical species (OM, NH4NO3, (NH4)2SO4) in the droplet mode. MSEs of the dominant chemical species were determined by their lognormal size-distribution parameters, including MMADs and standard deviation (σ) in the droplet mode.

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Pham, N. D., H. Igel, J. Wassermann, M. Kaser, J. de la Puente et U. Schreiber. « Observations and Modeling of Rotational Signals in the P Coda : Constraints on Crustal Scattering ». Bulletin of the Seismological Society of America 99, n^o 2B (1 mai 2009) : 1315–32. http://dx.doi.org/10.1785/0120080101.

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Rachman, Asep Nur, et Tae Woong Chung. « Depth‐Dependent Crustal Scattering Attenuation Revealed Using Single or Few Events in South Korea ». Bulletin of the Seismological Society of America 106, n^o 4 (12 juillet 2016) : 1499–508. http://dx.doi.org/10.1785/0120150351.

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Poppe, A. R., J. S. Halekas, C. Lue et S. Fatemi. « ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies ». Journal of Geophysical Research : Planets 122, n^o 4 (avril 2017) : 771–83. http://dx.doi.org/10.1002/2017je005313.

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Gosselin, Jeremy M., Pascal Audet, Bill Fry et Emily Warren-Smith. « Seismic Constraint on Heterogeneous Deformation and Stress State in the Forearc of the Hikurangi Subduction Zone, New Zealand ». Seismic Record 1, n^o 3 (1 octobre 2021) : 145–53. http://dx.doi.org/10.1785/0320210032.

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Abstract The Hikurangi subduction zone (HSZ) is the collisional boundary between the Pacific and Australian tectonic plates along the eastern coast of the North Island of New Zealand. The region is believed to be capable of hosting large megathrust earthquakes and associated tsunamis. Recent studies observe a range of slip behavior along the plate interface, with a sharp contrast between locked and creeping parts of the megathrust along the margin. This work uses teleseismic scattering data (receiver functions [RFs]) recorded at 53 long-running seismograph stations on the North Island of New Zealand to constrain the structure and mechanical properties of the forearc in the HSZ. We observe directional variations in RF phases at P–S converted delay times (i.e., depths) associated with the overlying forearc crust and note a general correlation with spatial variations in plate coupling as well as other geophysical properties. Our results suggest differences in the nature of crustal deformation (and stress state) along the Hikurangi margin, with evidence of clockwise rotation and/or extension in the northern HSZ, where the overriding forearc crust is uncoupled from the subducting Pacific slab.

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Wu, R. S. « Seismic Wave Propagation and Scattering in Heterogeneous Crustal Waveguides Using Screen Propagators : I SH Waves ». Bulletin of the Seismological Society of America 90, n^o 2 (1 avril 2000) : 401–13. http://dx.doi.org/10.1785/0119990102.

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Martini, Francesca, Christopher J. Bean, Sean Dolan et David Marsan. « Seismic image quality beneath strongly scattering structures and implications for lower crustal imaging : numerical simulations ». Geophysical Journal International 145, n^o 2 (mai 2001) : 423–35. http://dx.doi.org/10.1046/j.1365-246x.2001.01391.x.

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Lillis, Robert J., et Xiaohua Fang. « Electron impact ionization in the Martian atmosphere : Interplay between scattering and crustal magnetic field effects ». Journal of Geophysical Research : Planets 120, n^o 7 (juillet 2015) : 1332–45. http://dx.doi.org/10.1002/2015je004841.

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Line, C. E. R., R. W. Hobbs et D. B. Snyder. « Estimates of upper-crustal heterogeneity in the Baltic Shield from seismic scattering and borehole logs ». Tectonophysics 286, n^o 1-4 (mars 1998) : 171–83. http://dx.doi.org/10.1016/s0040-1951(97)00263-1.

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Eulenfeld, Tom, et Ulrich Wegler. « Crustal intrinsic and scattering attenuation of high-frequency shear waves in the contiguous United States ». Journal of Geophysical Research : Solid Earth 122, n^o 6 (juin 2017) : 4676–90. http://dx.doi.org/10.1002/2017jb014038.

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Dominguez, L. A., F. J. Sanchez-Sesma et P. M. Davis. « Scattering of Teleseismic Body Waves by the Lateral Crustal Heterogeneity at the Pacific Trench of Mexico ». Bulletin of the Seismological Society of America 101, n^o 3 (29 mai 2011) : 1281–90. http://dx.doi.org/10.1785/0120100181.

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Gaebler, Peter J., Christoph Sens-Schönfelder et Michael Korn. « The influence of crustal scattering on translational and rotational motions in regional and teleseismic coda waves ». Geophysical Journal International 201, n^o 1 (26 février 2015) : 355–71. http://dx.doi.org/10.1093/gji/ggv006.

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Olsen, Kim B., Michael Begnaud, Scott Phillips et Bo Holm Jacobsen. « Constraints of Crustal Heterogeneity and Q(f) from Regional (<4 Hz) Wave Propagation for the 2009 North Korea Nuclear Test ». Bulletin of the Seismological Society of America 108, n^o 3A (20 mars 2018) : 1369–83. http://dx.doi.org/10.1785/0120170195.

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Abstract We carried out 3D finite‐difference (FD) simulations (<4 Hz) of regional wave propagation for the 2009 North Korea nuclear explosion and compared the synthetics with instrument‐corrected records at stations INCN and TJN in South Korea. The source is an isotropic explosion with a moment magnitude of 4.1. Synthetics computed in the relatively smooth Sandia/Los Alamos National Laboratory SALSA3D (SAndia LoS Alamos 3D) velocity model significantly overpredict Rayleigh‐wave amplitudes by more than an order of magnitude while underpredicting coda amplitudes. The addition to SALSA3D of a von Karman distribution of small‐scale heterogeneities with correlation lengths of ∼1000 m, a Hurst number of 0.1, and a horizontal‐to‐vertical anisotropy of ∼5 produces synthetics in general agreement with the data. The best fits are obtained from models with a gradient in the strength of the velocity and density perturbations and strong scattering (10%) limited to the top 7.5–10 km of the crust. Deeper scattering tends to decrease the initial P‐wave amplitudes to levels much below those for the data, a critical result for methods discriminating between explosive and earthquake sources. In particular, the amplitude at the onset of Pn can be affected by as little as 2% small‐scale heterogeneity in the lower crust and upper mantle. Simulations including a constant Q of 200 (INCN) to 350 (TJN) below 1 Hz and a power‐law Q(f) formulation at higher frequencies, with an exponent of 0.3, generate synthetics in best agreement with the data. In our simulations, very limited scattering contribution from the near‐source area accumulates along the regional path.

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Finlay, Tori S., Lindsay L. Worthington, Brandon Schmandt, Nishath R. Ranasinghe, Susan L. Bilek et Richard C. Aster. « Teleseismic Scattered‐Wave Imaging Using a Large‐N Array in the Albuquerque Basin, New Mexico ». Seismological Research Letters 91, n^o 1 (30 octobre 2019) : 287–303. http://dx.doi.org/10.1785/0220190146.

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Abstract The advent of low‐cost continuously recording cable‐free autonomous seismographs, commonly referred to as nodes, enables dense spatiotemporal sampling of seismic wavefields. We create virtual source reflection profiles using P waves from five teleseismic events recorded by the Sevilleta node array experiment in the southern Albuquerque basin. The basin geology records a structurally complex history, including multiple Phanerozoic orogenies, Rio Grande rift extension, and ongoing uplift from a midcrustal magma body. The Sevilleta experiment densified the long term, regionally sparse seismograph network with 801 single channel vertical‐component 10 Hz geophone nodes deployed at ∼300 m spacing for 14 days in February 2015. Results show sediment‐basement reflections at <5 km depth and numerous sub‐basin structures. Comparisons to legacy crustal‐scale reflection images from the Consortium for Continental Reflection Profiling show agreement with structural geometries in the rift basin and upper crust. Comparisons of the teleseismic virtual reflection profiles to synthetic tests using 2D finite‐difference elastic wave propagation show strong P‐to‐Rayleigh scattering from steep basin edges. These high‐amplitude conversions dominate the record sections near the western rift margin and originate at the Loma Pelada fault, which acts as the primary contact between rift‐bounding basement‐cored fault blocks and rift basin sediments. At near offsets, these signals may interfere with interpretation of upper crustal structure, but their relatively slow moveout compared to teleseismic P‐wave multiples provides clear temporal separation from sediment‐basement reflections across most of the array. The high‐signal‐to‐noise ratio of these converted Rayleigh‐wave signals suggests that they may be useful for constraining short‐period (∼1 Hz) dispersion with strong sensitivity in the uppermost ∼1 km of the rift basin sediments.

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Frankel, Arthur, et Robert W. Clayton. « Finite difference simulations of seismic scattering : Implications for the propagation of short-period seismic waves in the crust and models of crustal heterogeneity ». Journal of Geophysical Research 91, B6 (1986) : 6465. http://dx.doi.org/10.1029/jb091ib06p06465.

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Varadan, Vijay K., Akhlesh Lakhtakia, Vasundara V. Varadan et Charles A. Langston. « Radiation characteristics of elastodynamic line sources buried in layered media with periodic interfaces. I. SH- wave analysis ». Bulletin of the Seismological Society of America 77, n^o 6 (1 décembre 1987) : 2181–91. http://dx.doi.org/10.1785/bssa0770062181.

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Abstract Using the T matrix method, or the extended boundary condition, the solution for a class of problems involving an SH line source in an elastic wave guide is determined. The boundaries of the wave guide may be periodically corrugated and the wave guide may be embedded between elastic media. Numerical results are given for a seismically interesting case of wave propagation in a one-layer crustal model over a mantle half-space with a corrugated free surface representing the Basin and Range topography in the Western United States. Analysis of the scattered fields at the surface, and of the fields radiated into the half-space, shows complicated field behavior, even with sinusoidal free surface corrugation. These results are directly applicable to regional wave propagation and scattering.

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Zhang, Bing. « Coherent Inverse Compton Scattering by Bunches in Fast Radio Bursts ». Astrophysical Journal 925, n^o 1 (1 janvier 2022) : 53. http://dx.doi.org/10.3847/1538-4357/ac3979.

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Abstract The extremely high brightness temperature of fast radio bursts (FRBs) requires that their emission mechanism must be “coherent,” either through concerted particle emission by bunches or through the exponential growth of a plasma wave mode or radiation amplitude via certain maser mechanisms. The bunching mechanism has been mostly discussed within the context of curvature radiation or cyclotron/synchrotron radiation. Here we propose a family of models invoking the coherent inverse Compton scattering (ICS) of bunched particles that may operate within or just outside of the magnetosphere of a flaring magnetar. Crustal oscillations during the flaring event may excite low-frequency electromagnetic waves near the magnetar surface. The X-mode of these waves could penetrate through the magnetosphere. Bunched relativistic particles in the charge-starved region inside the magnetosphere or in the current sheet outside the magnetosphere would upscatter these low-frequency waves to produce gigahertz emission to power FRBs. The ICS mechanism has a much larger emission power for individual electrons than curvature radiation. This greatly reduces the required degree of coherence in bunches, alleviating several criticisms of the bunching mechanism raised in the context of curvature radiation. The emission is ∼100% linearly polarized (with the possibility of developing circular polarization) with a constant or varying polarization angle across each burst. The mechanism can account for a narrowband spectrum and a frequency downdrifting pattern, as commonly observed in repeating FRBs.

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Sambolian, S., A. Gorszczyk, S. Operto, A. Ribodetti et B. Tavakoli F. « Mitigating the ill-posedness of first-arrival traveltime tomography using slopes : application to the eastern Nankai Trough (Japan) OBS data set ». Geophysical Journal International 227, n^o 2 (7 juillet 2021) : 898–921. http://dx.doi.org/10.1093/gji/ggab262.

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SUMMARY First-arrival traveltime tomography is one of the most used velocity model building techniques especially in sparse wide-angle acquisitions for deep crustal seismic imaging cases. Relying on the inversion of a picked attribute, the absolute traveltimes, the approach is ill-posed in terms of non-uniqueness of the solution. The latter is remedied by proper regularization or the introduction of prior information. Indeed, since traveltime kernels are vulnerable to the velocity–depth ambiguity, the inversion is stabilized by the introduction of complementary data like reflections and explicit reflectors in the velocity models. Here, we propose to supplement first-arrival traveltimes by their slopes, in other words the horizontal component of the slowness vectors at the sources and/or receivers. Slopes are a crucial attribute in state of the art scattering-based or reflection-based tomographic methods like slope tomography or wavefront tomography where the differential information is needed in order to locate the scattering events position or to parametrize the wavefront. The optional but valuable injection of slopes as an objective measure in first-arrival traveltime tomography stabilizes the problem by constraining the emergence angle or in turn implicitly the turning point depth of the rays. We explain why slopes have a tremendous added value in such a tomographic problem and highlight its remedial effect in cases where the medium is unevenly illuminated. We also show that the contribution of slopes become even more significant when the acquisition is sparse as it is generally the case with ocean-bottom seismometer surveys. The inferred models from such an extended time-attributes tomography will be used as initial guesses in a full-waveform inversion workflow context. The proposed strategy is benchmarked in 2-D media against a dip section of the SEG/EAGE overthrust model and then followed by a revisit of ocean bottom seismometers data from the eastern-Nankai subduction margin as a real deep crustal case study.

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Biswas, Rahul, et Chandrani Singh. « Seismic attenuation structure across the Karakoram fault in western Tibet ». Geophysical Journal International 223, n^o 2 (18 août 2020) : 1418–31. http://dx.doi.org/10.1093/gji/ggaa376.

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SUMMARY 2-D attenuation maps are produced for the crust of western Tibet using local earthquakes which are recorded by an array of 31 broad-band stations operated from 2007 July to 2011 May. Relative contribution of scattering ($Q_{sc}^{-1}$) and intrinsic ($Q_{i}^{-1}$) attenuation have been calculated using Multiple Lapse Time Window Analysis under the assumption of uniform distribution of multiple isotropic scattering and intrinsic absorption in a medium for five different frequency bands centred at 1.5, 3, 6, 12 and 18 Hz, respectively. All the events are selected on the basis of high signal-to-noise ratio having hypocentral distance within 200 km from the respective stations. The obtained Q−1 values show a strong frequency dependent nature which can be correlated to the degree of tectonic complexity and the heterogeneities present in the medium. The intrinsic absorption is found to be the dominant mechanism at all the frequency ranges for all stations except few (WT03, WT07 and WT13) at 18 Hz, which may be correlated with the presence of partial melt, geothermal fluids, hydrothermal springs, mantle-derived fluids and radioactivity in the crust of western Tibet. We have divided the entire area into two regions across the Karakoram fault (KKF) to explore the variations of crustal attenuation properties. The first part covers the northeastern of KKF referred as Region 1 while the second part covers the southwestern of KKF referred as Region 2. The spatial variations of $Q_{i}^{-1}$ across the region exhibit significant differences between Regions 1 and 2 at all the investigated frequencies. Interestingly, Region 1 exhibits higher $Q_{i}^{-1}$ than Region 2 at lower frequencies, whereas $Q_{i}^{-1}$ shows opposite trends at higher frequencies (> 6 Hz) as it shows higher values in Region 2 than Region 1. We find that the obtained values of Q−1 are also in good agreement with the other segments of Himalaya and Tibet as well as different tectonic regions in the world.

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Leary, Peter, et Rachel Abercrombie. « Frequency dependent crustal scattering and absorption at 5-160 Hz from coda decay observed at 2.5 km Depth ». Geophysical Research Letters 21, n^o 11 (1 juin 1994) : 971–74. http://dx.doi.org/10.1029/94gl00977.

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Leng, K., J. Korenaga et T. Nissen-Meyer. « 3-D scattering of elastic waves by small-scale heterogeneities in the Earth’s mantle ». Geophysical Journal International 223, n^o 1 (8 juillet 2020) : 502–25. http://dx.doi.org/10.1093/gji/ggaa331.

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SUMMARY Small-scale heterogeneities in the Earth’s mantle, the origin of which is likely compositional anomalies, can provide critical clues on the evolution of mantle convection. Seismological investigation of such small-scale heterogeneities can be facilitated by forward modelling of elastic wave scattering at high frequencies, but doing so with conventional 3-D numerical methods has been computationally prohibitive. We develop an efficient approach for computing high-frequency synthetic wavefields originating from small-scale mantle heterogeneities. Our approach delivers the exact elastodynamic wavefield and does not restrict the geometry or physical properties of the local heterogeneity and the background medium. It combines the technique of wavefield injection and a numerical method called AxiSEM3D. Wavefield injection can decompose the total wavefield into an incident and a scattered part. Both these two parts naturally have low azimuthal complexity and can thus be solved efficiently using AxiSEM3D under two different coordinate systems. With modern high-performance computing (on an order of magnitude of 105 CPU-hr), we have achieved a 1 Hz dominant frequency for global-scale problems with strong deep Earth scattering. Compared with previous global injection approaches, ours allows for a 3-D background medium and yields the exact solution without ignoring any higher-order scattering by the background medium. Technically, we develop a traction-free scheme for realizing wavefield injection in a spectral element method, which brings in several flexibilities and simplifies the implementation by avoiding stress or traction computation on the injection boundary. For a spherical heterogeneity in the mid-lower mantle, we compare the 3-D full-wave solution with two approximate ones obtained, respectively, by the perturbation theory and in-plane (axisymmetric) modelling. As a comprehensive application, we study S-wave scattering by a 3-D ultra-low velocity zone, incorporating 3-D crustal structures on the receiver side as part of the background model.

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Rizzo, L. V., P. Artaxo, T. Müller, A. Wiedensohler, M. Paixão, G. G. Cirino, A. Arana et al. « Long term measurements of aerosol optical properties at a primary forest site in Amazonia ». Atmospheric Chemistry and Physics 13, n^o 5 (1 mars 2013) : 2391–413. http://dx.doi.org/10.5194/acp-13-2391-2013.

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Abstract. A long term experiment was conducted in a primary forest area in Amazonia, with continuous in-situ measurements of aerosol optical properties between February 2008 and April 2011, comprising, to our knowledge, the longest database ever in the Amazon Basin. Two major classes of aerosol particles, with significantly different optical properties were identified: coarse mode predominant biogenic aerosols in the wet season (January–June), naturally released by the forest metabolism, and fine mode dominated biomass burning aerosols in the dry season (July–December), transported from regional fires. Dry particle median scattering coefficients at the wavelength of 550 nm increased from 6.3 Mm−1 to 22 Mm−1, whereas absorption at 637 nm increased from 0.5 Mm−1 to 2.8 Mm−1 from wet to dry season. Most of the scattering in the dry season was attributed to the predominance of fine mode (PM2) particles (40–80% of PM10 mass), while the enhanced absorption coefficients are attributed to the presence of light absorbing aerosols from biomass burning. As both scattering and absorption increased in the dry season, the single scattering albedo (SSA) did not show a significant seasonal variability, in average 0.86 ± 0.08 at 637 nm for dry aerosols. Measured particle optical properties were used to estimate the aerosol forcing efficiency at the top of the atmosphere. Results indicate that in this primary forest site the radiative balance was dominated by the cloud cover, particularly in the wet season. Due to the high cloud fractions, the aerosol forcing efficiency absolute values were below −3.5 W m−2 in 70% of the wet season days and in 46% of the dry season days. Besides the seasonal variation, the influence of out-of-Basin aerosol sources was observed occasionally. Periods of influence of the Manaus urban plume were detected, characterized by a consistent increase on particle scattering (factor 2.5) and absorption coefficients (factor 5). Episodes of biomass burning and mineral dust particles advected from Africa were observed between January and April, characterized by enhanced concentrations of crustal elements (Al, Si, Ti, Fe) and potassium in the fine mode. During these episodes, median particle absorption coefficients increased by a factor of 2, whereas median SSA values decreased by 7%, in comparison to wet season conditions.

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Brito, J., L. V. Rizzo, P. Herckes, P. C. Vasconcellos, S. E. S. Caumo, A. Fornaro, R. Y. Ynoue, P. Artaxo et M. F. Andrade. « Physical-chemical characterization of the particulate matter inside two road tunnels in the São Paulo Metropolitan Area ». Atmospheric Chemistry and Physics Discussions 13, n^o 8 (9 août 2013) : 20839–83. http://dx.doi.org/10.5194/acpd-13-20839-2013.

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Abstract. The notable increase in biofuel usage by the road transportation sector in Brazil during recent years has significantly altered the vehicular fuel composition. Consequently, many uncertainties are currently found in particulate matter vehicular emission profiles. In an effort to better characterize the emitted particulate matter, measurements of aerosol physical and chemical properties were undertaken inside two tunnels located in the São Paulo Metropolitan Area (SPMA). The tunnels show very distinct fleet profiles: in the Jânio Quadros (JQ) tunnel, the vast majority of the circulating fleet are Light Duty Vehicles (LDVs), fuelled on average with the same amount of ethanol as gasoline. In the Rodoanel (RA) tunnel, the particulate emission is dominated by Heavy Duty Vehicles (HDVs) fuelled with diesel (5% biodiesel). In the JQ tunnel, PM2.5 concentration was on average 52 μg m−3, with the largest contribution of Organic Mass (OM, 42%), followed by Elemental Carbon (EC, 17%) and Crustal elements (13%). Sulphate accounted for 7% of PM2.5 and the sum of other trace elements was 10%. In the RA tunnel, PM2.5 was on average 233 μg m−3, mostly composed of EC (52%) and OM (39%). Sulphate, crustal and the trace elements showed a minor contribution with 5%, 1% and 1%, respectively. The average OC:EC ratio in the JQ tunnel was 1.59 ± 0.09, indicating an important contribution of EC despite the high ethanol fraction in the fuel composition. In the RA tunnel, the OC:EC ratio was 0.49 ± 0.12, consistent with previous measurements of diesel fuelled HDVs. Besides bulk carbonaceous aerosol measurement, Polycyclic Aromatic Hydrocarbons (PAHs) were quantified. The sum of the PAHs concentration was 56 ± 5 ng m−3 and 45 ± 9 ng m−3 in the RA and JQ tunnel, respectively. In the JQ tunnel, Benzo(a)pyrene (BaP) ranged from 0.9 to 6.7 ng m−3 (0.02–0.1‰ of PM2.5) in the JQ tunnel whereas in the RA tunnel BaP ranged from 0.9 to 4.9 ng m−3 (0.004–0.02‰ of PM2.5), indicating an important relative contribution of LDVs emission to atmospheric BaP. Real-time measurements performed in both tunnels provided aerosol size distributions and optical properties. The average particle count yielded 73 000 cm−3 in the JQ tunnel and 366 000 cm−3 in the RA tunnel, with an average diameter of 48 nm in the former and 39 nm in the latter. Aerosol single scattering albedo, calculated from scattering and absorption observations in the JQ tunnel, showed a minimum value of 0.4 at the peak of the morning rush hour, reached 0.6 around noon and stabilized at 0.5 in the afternoon and evening. Such single scattering albedo range is close to other tunnel studies results, despite significant biofuel usage. Given the exceedingly high Black Carbon loadings in the RA tunnel, real time light absorption measurements were possible only in the JQ tunnel. Nevertheless, using EC measured from the filters a single scattering albedo of 0.32 for the RA tunnel has been estimated. The results presented here characterize particulate matter emitted from nearly 1 million vehicles fuelled with a considerable amount of biofuel, providing an unique experimental site worldwide.

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Brito, J., L. V. Rizzo, P. Herckes, P. C. Vasconcellos, S. E. S. Caumo, A. Fornaro, R. Y. Ynoue, P. Artaxo et M. F. Andrade. « Physical–chemical characterisation of the particulate matter inside two road tunnels in the São Paulo Metropolitan Area ». Atmospheric Chemistry and Physics 13, n^o 24 (17 décembre 2013) : 12199–213. http://dx.doi.org/10.5194/acp-13-12199-2013.

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Abstract. The notable increase in biofuel usage by the road transportation sector in Brazil during recent years has significantly altered the vehicular fuel composition. Consequently, many uncertainties are currently found in particulate matter vehicular emission profiles. In an effort to better characterise the emitted particulate matter, measurements of aerosol physical and chemical properties were undertaken inside two tunnels located in the São Paulo Metropolitan Area (SPMA). The tunnels show very distinct fleet profiles: in the Jânio Quadros (JQ) tunnel, the vast majority of the circulating fleet are light duty vehicles (LDVs), fuelled on average with the same amount of ethanol as gasoline. In the Rodoanel (RA) tunnel, the particulate emission is dominated by heavy duty vehicles (HDVs) fuelled with diesel (5% biodiesel). In the JQ tunnel, PM2.5 concentration was on average 52 μg m−3, with the largest contribution of organic mass (OM, 42%), followed by elemental carbon (EC, 17%) and crustal elements (13%). Sulphate accounted for 7% of PM2.5 and the sum of other trace elements was 10%. In the RA tunnel, PM2.5 was on average 233 μg m−3, mostly composed of EC (52%) and OM (39%). Sulphate, crustal and the trace elements showed a minor contribution with 5%, 1%, and 1%, respectively. The average OC : EC ratio in the JQ tunnel was 1.59 ± 0.09, indicating an important contribution of EC despite the high ethanol fraction in the fuel composition. In the RA tunnel, the OC : EC ratio was 0.49 ± 0.12, consistent with previous measurements of diesel-fuelled HDVs. Besides bulk carbonaceous aerosol measurement, polycyclic aromatic hydrocarbons (PAHs) were quantified. The sum of the PAHs concentration was 56 ± 5 ng m−3 and 45 ± 9 ng m−3 in the RA and JQ tunnel, respectively. In the JQ tunnel, benzo(a)pyrene (BaP) ranged from 0.9 to 6.7 ng m−3 (0.02–0.1‰ of PM2.5) whereas in the RA tunnel BaP ranged from 0.9 to 4.9 ng m−3 (0.004–0. 02‰ of PM2.5), indicating an important relative contribution of LDVs emission to atmospheric BaP. Real-time measurements performed in both tunnels provided aerosol size distributions and optical properties. The average particle count yielded 73 000 cm−3 in the JQ tunnel and 366 000 cm−3 in the RA tunnel, with an average diameter of 48 nm in the former and 39 nm in the latter. Aerosol single scattering albedo, calculated from scattering and absorption observations in the JQ tunnel, indicates a value of 0.5 associated with LDVs. Such single scattering albedo is 20–50% higher than observed in previous tunnel studies, possibly as a result of the large biofuel usage. Given the exceedingly high equivalent black carbon loadings in the RA tunnel, real time light absorption measurements were possible only in the JQ tunnel. Nevertheless, using EC measured from the filters, a single scattering albedo of 0.31 for the RA tunnel has been estimated. The results presented here characterise particulate matter emitted from nearly 1 million vehicles fuelled with a considerable amount of biofuel, providing a unique experimental site worldwide.

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Xu, Song, Xiaoming Tang, Yuanda Su et Chunxi Zhuang. « Seismic shear wave anisotropy of an anisotropic rock containing aligned cracks : theory and applications to experiment and field data ». Geophysical Journal International 220, n^o 1 (12 octobre 2019) : 404–14. http://dx.doi.org/10.1093/gji/ggz456.

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SUMMARY Cracks universally exist in Earth's crustal rocks. Many rocks are intrinsically anisotropic, which, when coupled with crack-induced anisotropy, significantly affect seismic wave propagation through the rocks. Using the method of sphere equivalency of effective scattering, we have developed a technique to model the effective moduli of transversely isotropic (TI) media containing cracks. The modelling results show that the wave characteristics are significantly affected by the interaction of the two anisotropy mechanisms. To validate the validity and accuracy, the theory was applied to a recent experiment made with a vertical transverse isotropy (VTI) medium containing cracks and shows significantly better agreement with the data. For a more realistic situation, the new modelling was applied to interpret the borehole acoustic anisotropy measurement results from a fractured VTI formation, showing that the theory can adequately explain the anisotropic characteristics of the field data. With the validation and testing, the theoretical results advocated in this study can be used with confidence.

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Lee, En‐Jui, Po Chen, Thomas H. Jordan, Phillip B. Maechling, Marine A. M. Denolle et Gregory C. Beroza. « Full‐3‐D tomography for crustal structure in Southern California based on the scattering‐integral and the adjoint‐wavefield methods ». Journal of Geophysical Research : Solid Earth 119, n^o 8 (août 2014) : 6421–51. http://dx.doi.org/10.1002/2014jb011346.

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Hartse, Hans E., W. Scott Phillips, Michael C. Fehler et Leigh S. House. « Single-station spectral discrimination using coda waves ». Bulletin of the Seismological Society of America 85, n^o 5 (1 octobre 1995) : 1464–74. http://dx.doi.org/10.1785/bssa0850051464.

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Abstract Spectral discrimination methods applied at regional distances are based on the observation that there is a difference in the frequency content of the seismic waves radiated from the source region of earthquakes and explosions. We have investigated a spectral discrimination method that exploits the scattered energy contained in the event codas of seismograms to measure the relative source excitation of earthquakes and explosions as a function of frequency. Key strengths of the coda method include the following: (1) it is not affected by source radiation pattern because the scattering process averages the radiation over the focal sphere, (2) path corrections are not required with the coda method because coda waves are scattered throughout a common crustal volume, (3) it can be successfully applied using data from only a single station, and (4) it can be successfully applied using data with clipped direct arrivals. To implement the coda method, we first estimate a “type curve,” which represents the average shape of all the individual coda decay curves for a particular frequency band. The shape of the type curve (as well as the shape of all individual coda decay curves) is controlled by the scattering and attenuation properties of the crustal volume sampled by the coda waves (a common path effect). The amplitude of the type curve is not defined, and we arbitrarily adjust it to have a zero mean. The amplitude difference between an individual event coda decay curve and the type curve provides a relative measure of the event's source size within the particular frequency band. We define this amplitude difference as the “relative source factor.” Our approach exploits all of an event's usable coda for each spectral measurement, thus providing a more stable measure of relative source size than could be obtained from using a predefined, fixed-length time window. Analysis of 27 Nevada Test Site explosions and 15 southern Great Basin earthquakes (3.1 ≦ M ≦ 4.7), shows that the source factor difference between the 0.5- to 1- and 2- to 4-Hz bands plotted against the 1- to 2-Hz source factor is a stable event discriminant. The method reliably distinguishes small explosions from small earthquakes (3.0 < M < 3.5), even if the explosions are overburied. Because the coda method requires only one seismic station, we believe that it would be especially useful in identifying small, clandestine underground nuclear tests that occur far from established networks.

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González Álvarez, Itahisa N., Sebastian Rost, Andy Nowacki et Neil D. Selby. « Small-scale lithospheric heterogeneity characterization using Bayesian inference and energy flux models ». Geophysical Journal International 227, n^o 3 (29 juillet 2021) : 1682–99. http://dx.doi.org/10.1093/gji/ggab291.

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SUMMARY Observations from different disciplines have shown that our planet is highly heterogeneous at multiple scale lengths. Still, many seismological Earth models tend not to include any small-scale heterogeneity or lateral velocity variations, which can affect measurements and predictions based on these homogeneous models. In this study, we describe the lithospheric small-scale isotropic heterogeneity structure in terms of the intrinsic, diffusion and scattering quality factors, as well as an autocorrelation function, associated with a characteristic scale length (a) and RMS fractional velocity fluctuations (ε). To obtain this characterization, we combined a single-layer and a multilayer energy flux models with a new Bayesian inference algorithm. Our synthetic tests show that this technique can successfully retrieve the input parameter values for 1- or 2-layer models and that our Bayesian algorithm can resolve whether the data can be fitted by a single set of parameters or a range of models is required instead, even for very complex posterior probability distributions. We applied this technique to three seismic arrays in Australia: Alice Springs array (ASAR), Warramunga Array (WRA) and Pilbara Seismic Array (PSAR). Our single-layer model results suggest intrinsic and diffusion attenuation are strongest for ASAR, while scattering and total attenuation are similarly strong for ASAR and WRA. All quality factors take higher values for PSAR than for the other two arrays, implying that the structure beneath this array is less attenuating and heterogeneous than for ASAR or WRA. The multilayer model results show the crust is more heterogeneous than the lithospheric mantle for all arrays. Crustal correlation lengths and RMS velocity fluctuations for these arrays range from ∼0.2 to 1.5 km and ∼2.3 to 3.9 per cent, respectively. Parameter values for the upper mantle are not unique, with combinations of low values of the parameters (a < 2 km and ε < ∼2.5 per cent) being as likely as those with high correlation length and velocity variations (a > 5 km and ε > ∼2.5 per cent, respectively). We attribute the similarities in the attenuation and heterogeneity structure beneath ASAR and WRA to their location on the proterozoic North Australian Craton, as opposed to PSAR, which lies on the archaean West Australian Craton. Differences in the small-scale structure beneath ASAR and WRA can be ascribed to the different tectonic histories of these two regions of the same craton. Overall, our results highlight the suitability of the combination of an energy flux model and a Bayesian inference algorithm for future scattering and small-scale heterogeneity studies, since our approach allows us to obtain and compare the different quality factors, while also giving us detailed information about the trade-offs and uncertainties in the determination of the scattering parameters.

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Rizzo, L. V., P. Artaxo, T. Müller, A. Wiedensohler, M. Paixão, G. G. Cirino, A. Arana et al. « Long term measurements of aerosol optical properties at a pristine forest site in Amazonia ». Atmospheric Chemistry and Physics Discussions 12, n^o 9 (10 septembre 2012) : 23333–401. http://dx.doi.org/10.5194/acpd-12-23333-2012.

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Abstract. A long term experiment was conducted in a pristine area in the Amazon forest, with continuous in situ measurements of aerosol optical properties between February 2008 and April 2011, comprising, to our knowledge, the longest database ever in Amazonia. Two types of aerosol particles, with significantly different optical properties were identified: coarse mode predominant biogenic aerosols in the wet season (January–June), naturally released by the forest metabolism, and fine mode dominated biomass burning aerosols in the dry season (July–December), transported from regional fires. Dry particle median scattering coefficients at the wavelength of 550 nm increased from 6.3 Mm−1 to 22 Mm−1, whereas absorption at 637 nm increased from 0.5 Mm−1 to 2.8 Mm−1 from wet to dry season. Most of the scattering in the dry season was attributed to the predominance of fine mode particles (40–80% of PM10 mass), while the enhanced absorption coefficients are attributed to the presence of light absorbing aerosols from biomass burning. As both scattering and absorption increased in the dry season, the single scattering albedo (SSA) did not show a significant seasonal variability, in average 0.86 ± 0.08 at 637 nm for dry particles. Measured particle optical properties were used to estimate the aerosol forcing efficiency at the top of the atmosphere. Results indicate that in this pristine forest site the radiative balance was dominated by the cloud cover, or, in other words, the aerosol indirect effect predominated over the direct effect, particularly in the wet season. Due to the high cloud fractions, the aerosol forcing efficiency was below −3.5 W m−2 in 70% of the wet season days and in 46% of the dry season days. These values are lower than the ones reported in the literature, which are based on remote sensing data. Besides the seasonal variation, the influence of external aerosol sources was observed occasionally. Periods of influence of the Manaus urban plume were detected, characterized by a consistent increase on particle scattering (factor 2.5) and absorption coefficients (factor 5). Episodes of biomass burning and mineral dust particles advection from Africa were observed between January and April, characterized by enhanced concentrations of fine mode (PM2.0), crustal elements (Al, Si, Ti, Fe) and potassium. During these episodes, median particle absorption coefficients increased by a factor of 2, whereas median SSA values decreased by 7%, in comparison to wet season conditions.

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Filippucci, Marilena, Salvatore Lucente, Edoardo Del Pezzo, Salvatore de Lorenzo, Giacomo Prosser et Andrea Tallarico. « 3D-Kernel Based Imaging of an Improved Estimation of (Qc) in the Northern Apulia (Southern Italy) ». Applied Sciences 11, n^o 16 (16 août 2021) : 7512. http://dx.doi.org/10.3390/app11167512.

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We investigate crustal seismic attenuation by the coda quality parameter (Qc) in the Gargano area (Southern Italy), using a recently released dataset composed of 191 small earthquakes (1.0 ≤ ML ≤ 2.8) recorded by the local OTRIONS and the Italian INGV seismic networks, over three years of seismic monitoring. Following the single back-scattering theoretical assumption, Qc was computed using different frequencies (in the range of 2–16 Hz) and different lapse times (from 10 to 40 s). The trend of Qc vs. frequency is the same as that observed in the adjacent Umbria-Marche region. Qc at 1 Hz varies between 11 and 63, indicating that the area is characterized by active tectonics, despite the absence of high-magnitude earthquakes in recent decades. The 3D mapping procedure, based on sensitivity kernels, revealed that the Gargano Promontory is characterized by very low and homogeneous Qc at low frequencies, and by high and heterogeneous Qc at high frequencies. The lateral variations of Qc at 12 Hz follow the trend of the Moho in this region and are in good agreement with other geophysical observations.

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Deshwal, P. S., et S. Mudgal. « Scattering of Love waves due to the presence of a rigid barrier of finite depth in the crustal layer of the earth ». Proceedings Mathematical Sciences 108, n^o 1 (février 1998) : 81–94. http://dx.doi.org/10.1007/bf03161315.

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Kent, Graham M., Robert S. Detrick, Stephen A. Swift, John A. Collins et Isaac I. Kim. « Evidence from Hole 504B for the origin of dipping events in oceanic crustal reflection profiles as out-of-plane scattering from basement topography ». Geology 25, n^o 2 (1997) : 131. http://dx.doi.org/10.1130/0091-7613(1997)025<0131:efhfto>2.3.co;2.

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Gregersen, Søren, Lene Vandur Nielsen et Peter Voss. « Evidence of stretching of the lithosphere under Denmark ». Geological Survey of Denmark and Greenland (GEUS) Bulletin 15 (10 juillet 2008) : 53–56. http://dx.doi.org/10.34194/geusb.v15.5043.

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The structure of the lithosphere under Denmark has been investigated in relation to adjacent regions of Sweden and Germany. The most interesting result of the study is that the 120 km thick lithosphere under Denmark appears to be a stretched version of the Swedish lithosphere, which is more than twice as thick. During the international project Tele seismic Tomography across the Tornquist Zone (Tor), field work and international interpretation were carried out between 1996 and 2002. Following the field work period, modelvelocity computations were undertaken based on ob ser vations of distant earthquakes (e.g. Arlitt 1999; Shomali et al. 2002; Voss et al. 2006), and recently an evaluation of the Tor results was completed (Nielsen 2007). The Tor project investigates deeper parts of the Earth than previous projects, and in particular the depth interval 50–300 km, which is below the crystalline crust. The investigations have included many geophysical features such as teleseismic P-wave tomography, Rayleigh wave velocities, shear wave splitting and wave scattering. We have distinguished between relatively high- and low-velocity zones, which also show variations in anisotropy and scatter characteristics. Generalised high-velocity zones correspond to the lithosphere, while generalised relatively low-velocity zones are equivalent to the asthenosphere. The main outcome of the combined studies is that the deep lithosphere can be divided into three blocks separated approximately along the national boundaries between Sweden and Denmark and between Denmark and Germany. The boundaries between the blocks are steep, almost vertical. The Denmark block has lithosphere properties between those to the north and south. Based on previous crustal studies and the Tor results, we suggest that the Denmark block has evolved by stretching. The details in the new evaluation are derived from teleseismic tomography. Here we present a synthesis of the many derived models in the light of the new evaluation.

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Articles de revues sur le sujet « Crustal scattering »

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