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Detecting and picking the first P -wave arrivals of seismic events in seismograms is fundamental in observational seismology. Recently, several machine-learning-based algorithms have been developed to incorporate human expertise for picking P -wave arrival times automatically. One shortcoming of these models is that they pick arrival times at individual seismic stations separately, which need to be sorted and associated to identify the seismic event. Also, most of them rely on existence of P -wave arrivals in the seismograms to be picked. Here, we developed a machine-learning-based seismic event detection and P -wave arrival time picking method called MultiNet and applied it to acoustic emission (AE) waveform data recorded in laboratory experiments. The MultiNet uses 2-D waveform images from multichannel AE recordings as the input to a convolutional neural network (CNN) to detect whether there is an AE event in an image and, if so, uses a fully convolutional neural network (FCN) to pick the P -wave arrival time at each channel in the image. We tested the MultiNet using 550 known AE events recorded during syn-deformational phase transformation from olivine to spinel in Mg2GeO4 (an analogue to Mg2SiO4) in a high-pressure experiment. Waveform data of 50 events were used to train the neural networks and the rest of data were used to validate the method. At the optimal image length and detection threshold, the CNN was able to detect all 500 known events plus 48 more events missed previously. Overall, 98.7 per cent of P -wave arrival times picked by the FCN were within 0.5  |$\mu$| s from the manually picked times. The average picking errors at different channels range from 0.01 ± 0.05 to −0.06 ± 0.22  |$\mu$| s. Our method greatly reduces the amount of human labour in picking P -wave arrival times for event location and source moment tensor inversion. It can be easily adapted to process continuous waveform data of a seismic network for earthquake detection and location in real time. [ABSTRACT FROM AUTHOR]

A methodology has been developed, detailing the theory and workflow, for applying the double-difference relocation method to acoustic emission (AE) event location in high-pressure/high-temperature deformation experiments in the multi-anvil apparatus. The process is predicated on the fact that events originating from a common source region will traverse similar ray paths from the source to the receiver and display similar waveforms in seismograms. This implies their travel-time difference results only from their spatial offset and any velocity heterogeneity along the ray path is negated. To demonstrate the efficacy of this approach we applied it to a transformational faulting experiment on the isostructural olivine analogue Mg2GeO4 under controlled deformation at 2.5 GPa and 700 °C while simultaneously monitoring stress, strain, and acoustic activity. Waveforms from all 1456 AE events were cross-correlated to measure differential arrival times and construct multiplet groups of similar events. In total, 110 multiplets were identified whose size is dominated by two large groups containing 272 and 202 events. Relocation of these two multiplets using the double-difference method significantly reduces event separation and improves location uncertainty by more than an order of magnitude when compared to absolute location techniques whose uncertainty rivals that of the sample size. In particular, event locations of the two largest multiplets reveal two dense clusters whose spatial geometry closely mirrors that of macroscopic faulting displayed in computerized tomography images of the recovered sample. In this way, we are able to link specific faults with their associated AE events, which would otherwise not be possible using traditional absolute location methods. [ABSTRACT FROM AUTHOR]

Interfaces are important part of Earth's layering structure. Here, we developed a new model parametrization and iterative linearized inversion method that determines 1-D crustal velocity structure using surface wave dispersion, teleseismic P -wave receiver functions and Ps and PmP traveltimes. Unlike previous joint inversion methods, the new model parametrization includes interface depths and layer Vp / Vs ratios so that smoothness constraint can be conveniently applied to velocities of individual layers without affecting the velocity discontinuity across the interfaces. It also allows adding interface-related observation such as traveltimes of Ps and PmP in the joint inversion to eliminate the trade-off between interface depth and Vp / Vs ratio and therefore to reduce the uncertainties of results. Numerical tests show that the method is computationally efficient and the inversion results are robust and independent of the initial model. Application of the method to a dense linear array across the Wabash Valley Seismic Zone (WVSZ) produced a high-resolution crustal image in this seismically active region. The results show a 51–55-km-thick crust with a mid-crustal interface at 14–17 km. The crustal Vp / Vs ratio varies from 1.69 to 1.90. There are three pillow-like, ∼100 km apart high-velocity bodies sitting at the base of the crust and directly above each of them are a low-velocity anomaly in the middle crust and a high-velocity anomaly in the upper crust. They are interpreted to be produced by mantle magmatic intrusions and remelting during rifting events in the end of the Precambrian. The current diffuse seismicity in the WVSZ might be rooted in this ancient distributed rifting structure. [ABSTRACT FROM AUTHOR]

Seismicity of several intraplate seismic zones in the North American midcontinent is believed to be related to reactivation of ancient faults in Precambrian continental rifts by the contemporary stress field. Existence of such a rift system beneath the Wabash Valley Seismic Zone (WVSZ) is not clear. Here we obtained a crustal structural image along a 300‐km‐long profile across WVSZ using a dense linear seismic array. We first calculated teleseismic receiver functions of stations and applied the Common‐Conversion‐Point stacking method to image crustal interfaces and the Moho. We then used ambient noise cross correlation to obtain phase and group velocities of Rayleigh and Love waves. Finally, we jointly inverted the receiver function and surface wave dispersion data to determine shear wave velocity structure along the profile. The results show a thick (50‐ to 60‐km) crust with a typical Proterozoic crustal layering: a 1‐ to 2‐km thick Phanerozoic sedimentary layer, an upper crust ∼15 km thick, and a 30‐ to 40‐km‐thick lower crust. The unprecedented high‐resolution image also reveals a 50‐km‐wide high‐velocity body above an uplifted Moho and several velocity anomalies in the upper and middle crust beneath the La Salle Deformation Belt. We interpreted them as features produced by magmatic intrusions in a failed, immature continental rift during the end of Precambrian. Current seismicity in WVSZ is likely due to reactivation of ancient faults of the rift system by a combination of stress fields from the far‐field plate motion and prominent crustal and upper mantle heterogeneities in the region. Key Points: A high‐resolution crustal structure image across the Wabash Valley Seismic Zone has been obtained using a 300‐km‐long dense seismic arrayThe image reveals a high‐velocity body atop an uplifted Moho and several velocity anomalies in the crust beneath the center of Wabash ValleyThe anomalous crustal structural features suggest mantle magmatic intrusions in an immature continental rift during the end of Precambrian [ABSTRACT FROM AUTHOR]

Teleseismic receiver functions (RFs) are frequently used to determine depths of seismic discontinuities in the crust and upper mantle. We developed an efficient reverse-time migration (RTM) method that is applied to teleseismic receiver functions directly. Both the primary P -to- S converted phases and their crustal multiples in RFs can be used for imaging seismic discontinuities. The method uses the phase-shift-plus-interpolation algorithm to extrapolate both the source and receiver wavefields in a 3-D velocity model, which greatly reduces the computation costs compared with those using a full wave-equation numerical solver. Tests using synthetic data in various crustal models demonstrate the effectiveness of the method and its superiority over the common-conversion-point stacking method. In particular, the method handles diffraction caused by strong lateral structural variations correctly and there is no limitation on the maximum dip of the interface. We applied the method to real data of a linear array in the Wabash Valley Seismic Zone in the central USA and obtained a crustal structural image across a failed continental rift. We suggest that future passive-source seismic recording experiments for crustal scale imaging use station spacing less than 5 km, and a 2-D array with even smaller station spacing is desired for regions with strong lateral structural variations. With increasing numbers of sensors used in passive-source recording experiments nowadays, our RF-RTM method can be a useful tool for structural imaging on scales ranging from sedimentary basins, crust to lithosphere. [ABSTRACT FROM AUTHOR]

Crustal thickness (H) and Vp / Vs ratio (κ) are two important parameters in characterizing a crustal structure. Their estimates from receiver function data often have large uncertainties due to the severe trade-off between the two parameters when the receiver functions only have the Moho P -to- S converted phase and its crustal multiples are weak or missing, which are common in tectonically active regions. Here, we extended the H –κ stacking method by combining receiver functions and PmP traveltimes to determine H, κ and Vp simultaneously. We developed a deconvolution technique to measure PmP traveltimes using the post-critical Moho P -wave reflection SsPmp in the teleseismic S wave. Synthetic tests show that the new methods can reliably extract the PmP traveltimes and determine crustal thickness and velocities as long as the Moho Ps is clear in receiver functions. We applied the methods to 63 broad-band seismic stations in the western Hubei Province of China. The results confirm the rapid increase of crustal thickness revealed in a previous study from the low-elevation plains in the eastern part of the study area to the mountains in the west. The corresponding increase of crustal Poisson's ratio from low-to-intermediate values to intermediate-to-high values suggests that the crust in the eastern part is more felsic, most likely due to delamination of a portion of the mafic lower crust of the Yangtze platform in the Late Mesozoic. [ABSTRACT FROM AUTHOR]

Abstract: Earthquakes induced by reservoir impoundment pose great risk to property and lives worldwide, but studies demonstrating relationships between reservoir water levels, specific faults, and the local geology are rare. Here we show that the 2013 M 5.1 Badong Earthquake, the largest earthquake so far in the Three Gorges Reservoir (TGR) region of China, occurred at a shallow depth on a right‐lateral south‐dipping strike‐slip fault. The fault is at least 15 km long and intercepts the TGR at the NS‐running Shennongxi River. We find that the earthquake and its foreshocks and aftershocks are confined to a fractured Triassic carbonate formation that crops out in the reservoir. The precise locations of earthquakes coupled with the local geology suggest that the sequence was induced by high pore pressure due to reservoir water infiltration in a specific rock type. The newly identified fault has the potential to generate earthquakes of magnitude approaching the designed seismic intensity limit of the Three Gorges Dam. [ABSTRACT FROM AUTHOR]

Knowledge of heat flow and associated variations of temperature with depth is crucial for understanding how the Earth functions. Here, we demonstrate possible heat shielding effects that result from the occurrence of mafic intrusions/layers (granulitic rocks) within a dominantly granitic middle crust and/or ultramafic intrusions/layers (peridotitic rocks) within a dominantly granulitic lower crust; heat shielding is a familiar phenomenon in heat engineering and thermal metamaterials. Simple one-dimensional calculations suggest that heat shielding due to the intercalation of granitic, granulitic and peridotitic rocks will increase Moho temperatures substantially. This study may lead to a rethinking of numerous proposed lower crustal processes. [ABSTRACT FROM AUTHOR]

Source inversion of small-magnitude events such as aftershocks or mine collapses requires use of relatively high frequency seismic waveforms which are strongly affected by small-scale heterogeneities in the crust. In this study, we developed a new inversion method called gCAP3D for determining general moment tensor of a seismic source using Green's functions of 3D models. It inherits the advantageous features of the “Cut-and-Paste” (CAP) method to break a full seismogram into the Pnl and surface-wave segments and to allow time shift between observed and predicted waveforms. It uses grid search for 5 source parameters (relative strengths of the isotropic and compensated-linear-vector-dipole components and the strike, dip, and rake of the double-couple component) that minimize the waveform misfit. The scalar moment is estimated using the ratio of L 2 norms of the data and synthetics. Focal depth can also be determined by repeating the inversion at different depths. We applied gCAP3D to the 2013 M s 7.0 Lushan earthquake and its aftershocks using a 3D crustal-upper mantle velocity model derived from ambient noise tomography in the region. We first relocated the events using the double-difference method. We then used the finite-differences method and reciprocity principle to calculate Green's functions of the 3D model for 20 permanent broadband seismic stations within 200 km from the source region. We obtained moment tensors of the mainshock and 74 aftershocks ranging from M w 5.2 to 3.4. The results show that the Lushan earthquake is a reverse faulting at a depth of 13–15 km on a plane dipping 40–47° to N46° W. Most of the aftershocks occurred off the main rupture plane and have similar focal mechanisms to the mainshock's, except in the proximity of the mainshock where the aftershocks' focal mechanisms display some variations. [ABSTRACT FROM AUTHOR]

Western Hubei Province is at the southern end of the 3000-km-long north-south-oriented Xing'anling–Taihangshan–Wulingshan topographic step in China, which separates high-rising plateaus and mountain ranges in the west from low-elevation plains in the east. We calculated teleseismic P receiver functions of 32 permanent broadband seismic stations in Hubei Province and estimated crustal thicknesses under them using the H - κ method. We also obtained detailed crustal structural images along three profiles using the CCP stacking method. The results show an east-west crustal thickness increase in the study area from 30–35 km to 45–50 km in less than 20 km of horizontal distance, most likely in a step-wise fashion. The thin crust beneath the Nanxiang and Jianghan basins in eastern Hubei extends into the interior of the Wuling Uplift and the Huangling Massif in western Hubei. The lack of mirror symmetry between the Moho and surface topography suggests that part of the mountain ranges in western Hubei is either compensated by non-Airy-type isostasy models or is not in isostatic equilibrium but supported by the strength of the lithosphere. The brittle or localized ductile deformation in the lower crust/uppermost mantle as indicated by the abrupt Moho steps seems to be decoupled with brittle deformation in the upper crust. The CCP images also reveal an apparent double Moho beneath the Wudang Mts. which is interpreted to be due to a partially eclogitized lower crust after the original cratonic mantle lithosphere was replaced by warm and hydrated mantle materials in eastern China in the Late Mesozoic. The Moho steps were formed when a segment of eclogitized lower crust became gravitationally unstable and foundered into the mantle. [ABSTRACT FROM AUTHOR]

We decompose a general seismic potency tensor into isotropic tensor, double-couple tensor and compensated linear vector dipole using the eigenvectors and eigenvalues of the full tensor. Two dimensionless parameters are used to quantify the size of the isotropic and compensated linear vector dipole components. The parameters have well-defined finite ranges and are suited for non-linear inversions of source tensors from seismic waveform data. The decomposition and parametrization for the potency tensor are used to obtain corresponding results for a general seismic moment tensor. The relations between different parameters of the potency and moment tensors in isotropic media are derived. We also discuss appropriate specification of the relative size of different source components in inversions of seismic data. [ABSTRACT FROM AUTHOR]

SUMMARY We analysed high-frequency body waves of local earthquakes to image the damage zone of the Calico fault in the eastern California shear zone. We used generalized ray theory and finite difference methods to compute synthetic seismograms for a low-velocity fault zone (FZ) to model the direct and FZ-reflected P and S traveltimes of local earthquakes recorded by a temporary array across the fault. The low velocity zone boundaries were determined by apparent traveltime delays across the fault. The velocity contrast between the fault zone and host rock was constrained by the traveltime delays of P and S waves and differential traveltimes between the direct and FZ-reflected waves. The dip and depth extent of the low velocity zone were constrained by a systematic analysis of direct P traveltimes of events on both sides of the fault. We found that the Calico fault has a ∼1.3-km-wide low velocity zone in which the P- and S-wave velocity decreased 40 and 50 per cent, respectively, with respect to the host rock. The low velocity zone dips 70° northeast and extends 3 km in depth. [ABSTRACT FROM AUTHOR]

Abstract: The Chaochou Fault, a major geological boundary in southern Taiwan is considered to be a part of the convergent plate boundary between the Eurasia Plate and the Philippine Sea Plate. We applied the Common Conversion Point stacking technique to teleseismic radial receiver functions and obtained Moho variation and crustal structure across the Chaochou Fault. In the Eurasia Plate to its west, the Moho depth is about 37km and the crust is subducting to the east beneath the Philippine Sea Plate with a dip angle of about 30° between the Backbone Belt and the Tananao Schist. In the Philippine Sea Plate, the Moho depth is about 17km. The Longitudinal Valley marks the collision boundary between the Eurasia Plate and the Philippine Sea Plate. The results suggest that the depth extent of the Chaochou Fault is about 30–35km and the fault becomes a “shallow-angle” thrust fault at depth. The Common Conversion Point image also shows several bending interfaces of velocity contrast in the crust. We proposed a simple model to explain the Philippine Sea Plate and Eurasia Plate collision process and the observed crustal deformations. [Copyright &y& Elsevier]

Abstract: We determine lateral variation of Moho discontinuity, crustal thickness, and Vp/Vs ratios in the vicinity of Taiwan by analyzing all the available teleseismic waveform data collected by the Broadband Array in Taiwan for Seismology network from 1998 to 2004. The crustal thickness and the average crustal Vp/Vs ratio beneath each station are obtained by stacking Ps, PpPs and PpSs+PsPs phases coherently. The best estimated crustal thickness of Taiwan from 27 broadband stations is 30km on average. The thinnest crust (11–15km) is found east of the Longitude Valley suture zone, as part of the oceanic crust of the Philippine Sea plate. In other places, the crustal thickness varies from 17–19km in northern Taiwan to 32–39km in the southwestern island. The deepest Moho is found to be 53km beneath station SSLB in central Taiwan. The average crustal Vp/Vs ratio in Taiwan is 1.74, with higher values of 1.74–1.99 in the north and lower values of 1.60–1.74 in the south. The crustal thickness variation is supported by gravity measurements in Taiwan and indicates that the collision between the Philippine Sea plate and Eurasian plate in Taiwan involves the whole crust. The thin crust and high Vp/Vs ratios in northern Taiwan are believed to be related to the volcanism in Tatun and Keelung located in the southwestern extent of the Okinawa Trough. [Copyright &y& Elsevier]

High-frequency body waves recorded by a temporary seismic array across the surface rupture trace of the 1992 Landers, California, earthquake were used to determine fault-zone structures down to the seismogenic depth. We first developed a technique to use generalized ray theory to compute synthetic seismograms for arbitrarily oriented tabular low-velocity fault-zone models. We then generated synthetic waveform record sections of a linear array across a vertical fault zone. They show that both arrival times and waveforms of P and S waves vary systematically across the fault due to transmissions and reflections from boundaries of the low-velocity fault zone. The waveform characteristics and arrival-time patterns in the record sections allow us to locate the boundaries of the fault zone and to determine its P- and S-wave velocities independently as well as its depth extent. Therefore, the trade-off between the fault-zone width and velocities can be avoided. Applying the method to the Landers waveform data reveals a low-velocity zone with a width of 270–360 m and a 35–60 per cent reduction in P and S velocities relative to the host rock. The analysis suggests that the low-velocity zone extends to a depth of ∼7 km. The western boundary of the low-velocity zone coincides with the observed main surface rupture trace. [ABSTRACT FROM AUTHOR]

Western Anatolia is one of the most seismically active continental regions in the world and much of it has been undergoing NS-directed extensional deformation since Early Miocene time. In a cooperative study, seismologists from Saint Louis University, USA and Dokuz Eylül University, Turkey, deployed five broad-band and 45 short-period seismic stations in western Anatolia between 2002 November and 2003 October. The present paper uses data collected by this network and the data from five permanent stations operated by the Kandilli Observatory and Earthquake Research Institute to map the hypocentral distribution of local earthquakes and to determine crustal structure of western Anatolia. We obtained a 1-D P-wave crustal velocity model using a generalized scheme for simultaneously obtaining earthquake locations and a crustal velocity model. Our velocity model is characterized by crustal velocities that are significantly lower than average continental values. The low velocities may be associated with high crustal temperatures, a high degree of fracture, or the presence of fluids at high pore pressure in the crust. We located 725 local earthquakes and classified them in three categories. We found that the level of seismic activity in western Anatolia is higher than previously reported. Station delays resulting from the inversion process correlate with near-surface geology and the thickness of sediments throughout the region. The hypocentral distribution of the events indicates that peak seismicity for the region occurs at depths of about 10 km. [ABSTRACT FROM AUTHOR]

Presents research which studied anomalous double-pulse teleseismic P-wave arrivals near the northern margin of the Tibetan Plateau. Azimuthal dependence of the waveform distortion; Production of the distortion; Evidence of a Moho offset; Separation between the Tibetan Plateau crust and the Qaidam Basin crust; Implications of the sharp crustal thickness change.

Slippage on a deep fault, which cannot be directly measured, can be inferred from repeating earthquakes. Here, we use a matched filter technique (MFT) to detect missing earthquakes near the Xiannvshan fault in the Three Gorges Reservoir region of Central China. We detected 13 repeating aftershock sequences (RASs) containing 107 events after a M s 4.4 local earthquake that occurred on 26 March 2014. The RASs occurred in the vicinity of the M s 4.4 main shock hypocentre and were concentrated within a depth range of 4–7 km. The short-term slip rates estimated from these RASs varied from ∼0.001 to 0.31 mm d–1. The slip rates of the RASs followed an approximately logarithmic decay with RAS duration, suggesting that the deep Xiannvshan fault behaviour tended to follow the logarithmic velocity-strengthening friction law. Relatively high-stress RAS events seemed to influence the occurrence of other RAS events whereas we found no evidence that high-stress nearby events, including a M s 4.6 local earthquake that occurred on 29 March 2014, triggered the RAS events. We suggest that the fault stress change caused by the M s 4.4 main shock played a role in triggering the RAS events. Our results demonstrate that the MFT can effectively help identify repeating aftershocks, which could help decipher fault zone rheology. [ABSTRACT FROM AUTHOR]

We introduced a P velocity model into the traditional joint inversion of P receiver function (RF) and surface wave dispersions to reduce model ambiguity. The method was implemented using a global search-based algorithm and a flexible parameterization of a sedimentary layer and spline-based parameterization that can represent sharp discontinuities. We applied the method to a dense array in SE Tibet (longitude ~97.5°E to 107°E, latitude ~25.3°N). Extensive tests using synthetic and real data suggest that the method is suitable and robust for a variety of velocity structures and Moho discontinuities and can simultaneously provide the crustal V p/ V s profile and better constrained Moho depth. The flexibility of the parameterization and the inclusion of the V p constraint are crucial in the improved model recovery. Artifacts may be created without including the sedimentary layer. Even when it is less perfect, a reasonable V p model is valuable in such a joint inversion. We showed that crustal multiples in RFs may bias the traditional H- k results when the crust structure is complex and should be avoided in a joint inversion before appropriate corrections can be made. The results from the joint inversion show two low-velocity zones (LVZs) reported previously and were identified as channels of crustal flow. A prominent isolated LVZ is observed in the mid-lower crust under the Xiaojiang fault area, which correlates with anomalously high V p/ V s ratios, indicating possible partial melting. However, the other LVZ is imaged to be in the brittle shallow upper crust without very high V p/ V s ratios, which is likely associated with crustal fault zones rather than partial melting. We observe clear low-velocity structures in the mantle beneath the two crustal LVZs, which also correlate with zones of low resistivity. The crust-mantle correlation may suggest influence of mantle processes on crustal deformation. [ABSTRACT FROM AUTHOR]

The spatial autocorrelation (SPAC) method is one of the most efficient ways to extract phase velocities of surface waves from ambient seismic noise. Most studies apply the method based on the assumption that the wavefield of ambient noise is diffuse. However, the actual distribution of sources is neither diffuse nor stationary. In this study, we examined the reliability and limitations of the SPAC method with a directional wavefield. We calculated the SPAC coefficients and phase velocities from a directional wavefield for a four-layer model and characterized the limitations of the SPAC. We then applied the SPAC method to real data in Karamay, China. Our results show that, 1) the SPAC method can accurately measure surface wave phase velocities from a square array with a directional wavefield down to a wavelength of twice the shortest interstation distance; and 2) phase velocities obtained from real data by the SPAC method are stable and reliable, which demonstrates that this method can be applied to measure phase velocities in a square array with a directional wavefield. [ABSTRACT FROM AUTHOR]

High-pressure and temperature deformation experiments interfaced with acoustic emission (AE) monitoring have been conducted to study transformational faulting in Mn 2 GeO 4 olivine, which transforms to the β phase, isostructural to wadsleyite. Metastable Mn 2 GeO 4 olivine exhibits a marked embrittlement behavior at temperatures between 800 and 1100 K, emitting numerous AEs. At each temperature, brittle deformation is characterized by a two-stage process: (1) a "preparation" stage with numerous diffusedly located low-magnitude AEs and large b values (>2), and (2) a failure stage where larger-magnitude AEs form a planar distribution with b values about 1. Microstructure analysis reveals extensive kink band development in olivine grains in the recovered samples. Kink band boundaries (KBBs), with a typical thickness of ∼100 nm, are filled with a nanometric β-Mn 2 GeO 4 "gouge". A dense array of secondary shear localizations is often present within the kink bands, suggesting significant shear deformation therein. The combined observations suggest that faulting in metastable Mn 2 GeO 4 olivine is a self-similar process, from grain-scale to the sample-scale. Both observed embrittlement behavior and the microstructure of metastable Mn 2 GeO 4 olivine are essentially identical to those in Mg 2 GeO 4 olivine we have reported previously, indicating that the physical mechanism of faulting in metastable olivine is insensitive to the specific crystallographic structure of the high-pressure phase. The low b values (about 1) observed in the faulting process in our experiments are similar to those of deep focus earthquakes in cold subduction zones. Our observed mechanism explains deep focus seismicity in cold metastable mantle wedges, provided that the self-similarity assumption holds to geological scales. • Mn 2 GeO 4 olivine rock faults when transforming to wadsleyite structure under pressure. • Transformational faulting, detected by AEs, is associated with kink band formation. • Faulting is preceded by a preparation stage with high b values and low magnitude AEs. • Main faults are characterized by b ≈ 1, with AEs distributed in a planar fashion. • Fault development is through a self-similar process. [ABSTRACT FROM AUTHOR]

We used teleseismic P -wave receiver functions and the H – κ stacking method to obtain crustal thicknesses beneath 121 permanent stations in the Middle-lower Yangtze craton and its adjacent areas, using nearly 700 teleseismic events in 2009 and 2010. We then combined them with results of previous work to map detailed Moho geometry in the region. The results show that in addition to overall thin crust of ∼30 km in thickness throughout southeastern China, there are two NS-oriented narrow zones of extensive crustal thinning in the region. The western zone passes through the Xiangzhong Basin, the Jianghan Basin, and the Nanxiang Basin, while the eastern zone follows the Tanlu fault. The two merge in the south, forming a V-shaped thin crust area in southeastern China. We suggest different geodynamic mechanisms of crustal thinning north and south of the 29°N Parallel. Crustal thinning in the northern part was caused by the westward subduction of the Pacific plate and its eastward migration of the trench. Crustal thinning in the southern part is mainly controlled by the interaction between the Philippine and the Eurasia plates. The change of subduction polarity from the northward subduction of the Philippine plate under Eurasia to the eastward high-angle subduction of the Eurasian plate under the Philippine plate in Taiwan is responsible for diverse extensional stress regime in South China. The V-shape crustal thinning zone was formed due to a mantle corner flow in the rear of subducting Eurasia plate, in a form of “hot fingers” growing from the mantle corner flow to cause the Moho uplift in the southern part of the region. [ABSTRACT FROM AUTHOR]

Abstract: Trade-off between the depth of an interface and the average velocity above the interface is well known in receiver function inversion. To resolve model ambiguity, surface wave dispersion data, which are sensitive to vertical shear velocity average, are commonly combined with P receiver functions in the inversion. In this paper, we design a joint inversion scheme using the neighborhood searching algorithm to solve shear velocity structure of the crust and uppermost mantle with receiver function and surface wave dispersion data. Two sets of stretched splines are implemented in the inversion to obtain smooth velocity profiles, separated by a specifically defined Moho discontinuity. We apply this method to the dense Hi-CLIMB seismic array in the central Tibetan Plateau (TP). The Indian crust is observed to subduct and underplate the Tibetan crust between the Yarlung-Zangbo suture and the Bangong-Nujiang suture, at the latitude of about 31.5°N. The crust thickens from about 50km below the Indian foreland to over 75km south of Bangong-Nujiang suture and turns shallower at around 65km under the Qiangtang block. We identify two zones with complicated Moho structure beneath the northern Lhasa block and southern Qiangtang block, indicating highly deformed Moho morphology, and beneath the Himalaya block associated with the underthrusting of the Indian Plate, respectively. A low velocity zone is present in the mid-crust in most parts of the profile under the TP with up to more than 10% velocity reduction. The low velocity structure is not continuous, forming discontinuous patches rather than channels, calling for a revision of the channel flow model. [Copyright &y& Elsevier]

• We developed a joint inversion method by combination of surface-wave dispersions, receiver functions, Ps and SsPmp travel times. • We obtained detailed three-dimensional Vs and Vp/Vs ratio models in the area. • A northeast-dipping low Vs zone is observed in the upper crust of the Dabashan orocline. • We proposed a tectonic model on formation of Dabashan orocline. The Dabashan orocline is a large thrust belt in central China. Determination of its high-resolution three-dimensional (3D) crustal shear-wave (S -wave) velocity structure can enhance our understanding of the intracontinental evolution of the South China plate and North China plate. In this study, we estimated the Rayleigh wave phase and group velocities at periods of 5–50 s, P -wave receiver functions, and Ps and SsPmp travel times from 63 permanent stations and 29 portable stations; we also constructed a 3D S -wave velocity model and crustal thickness and V p / V s ratio maps of the Dabashan orocline and adjacent areas. The crustal thickness and V p / V s ratio maps show prominent thick crust (50–55 km) and high V p / V s ratios (1.8–1.85), suggesting a mafic lower crust in the Dabashan orocline. The 3D S -wave velocity structure shows northeast-dipping low-velocity anomalies in the upper crust of the Dabashan orocline, and high velocities extended from near the surface to the lower crust in the Hannan-Micang and Shennong-Huangling domes, suggesting deep roots of the two domes. We propose that the two domes acted as rigid indenters and rotated clockwise together with the South China plate, penetrating into the Qinling-Dabie orogenic belt. Weak sediments in the upper crust of the Dabashan orocline were compressed and blocked by the two domes and the rigid Qinling-Dabie orogenic belt, resulting in vertical accretion of the sediments and crust and subsequent formation of the northeast-dipping low-velocity anomalies and the thick crust beneath the Dabashan orocline. [ABSTRACT FROM AUTHOR]

The H‐κ method (Zhu & Kanamori, 2000, https://doi.org/10.1029/1999JB900322) has been widely used to estimate the crustal thickness (H) and the ratio of P to S velocities (VP/VS ratio, κ) with receiver functions. However, in regions where the crustal structure is complicated, the method may produce biased results, arising particularly from dipping Moho and/or crustal anisotropy. H‐κ stacking in case of azimuthal or radial anisotropy with flat Moho has been proposed but not for cases with plunging anisotropy and dipping Moho. Here we propose a generalized H‐κ method called H‐κ‐c, which corrects for these effects first before stacking. We consider rather general cases, including plunging anisotropy and dipping interfaces of multiple layers, and use harmonic functions to correct for arrival time variations of Ps and its crustal multiples with back azimuth (θ). Systematic synthetic tests show that the arrival time variations can be well fitted by cosθ and cos2θ functions even for very complex crustal structures. Correcting for the back azimuthal variations significantly enhances H‐κ stacking. We verify the feasibility of the H‐κ‐c method by applying it to 40 permanent stations in various geological setting across the Mainland China. The results show clear improvement after the harmonic corrections, with clearer multiples and stronger stacking energy, as well as more reliable H‐κ values. Large differences in H (up to 5.0 km) and κ (up to 0.09) between the new and traditional methods occur mostly in mountainous regions, where the crustal structure tends to be more complex. We caution in particular about systematic bias when the traditional method is used in the presence of dipping interfaces. The modified method is simple and applicable anywhere in the world. Plain Language Summary: Crustal thickness and crustal VP/VS ratio are important parameters for regional geology and tectonics. We propose a method that corrects for the effect of crustal anisotropy and dipping interface on receiver functions, which results in more reliable estimation of crustal thickness and VP/VS ratio. Synthetic tests and application to real data suggest that this method is powerful and widely applicable. Key Points: We propose a generalized H‐κ method with harmonic corrections on Ps and its crustal multiples in receiver functions (H‐κ‐c method)Correcting for the azimuthal variation of arrival times can significantly enhance the H‐κ stacking and obtain more robust resultsApplication of the method to real data shows clear improvement with large differences in H and κ mostly in mountainous regions [ABSTRACT FROM AUTHOR]

Several geodynamic models have been proposed for the deformation mechanism of Tibetan Plateau (TP), but it remains controversial. Here we applied a method of joint inversion of receiver functions and surface wave dispersions with P wave velocity constraint to a dense linear array in the NE Tibet. The results show that the geological blocks, separated by major faults at the surface, are characterized by distinct features in the crust, the Moho, and the uppermost mantle. The main features include crustal low-velocity zones (LVZs) with variable strengths, anomalous Vp/Vs ratios that are correlated with LVZs, a large Moho jump, and other abrupt changes near major faults, strong mantle lithosphere anomalies, and correlation of crustal and mantle velocities. The results suggest a lithospheric-scale deformation of continuous shortening as well as localized faulting, which is affected by the strength of the lithosphere blocks. The thickened mantle lithosphere can be removed, which facilitates the formation of middle-lower crustal LVZ and flow. However, such flow is likely a consequence of the deformation rather than a driving force for the outward growth of the TP. The proposed model of TP deformation and growth can reconcile the continuous deformation within the blocks and major faults at the surface. [ABSTRACT FROM AUTHOR]

Abstract: The western Junggar is situated in the southwestern Central Asian Orogenic Belt, and its origin is believed to be related to either intracontinental arc or intraoceanic arc‐related subduction system. However, the mode of subduction is still contentious. In this study, we build a high‐resolution 3‐D model of the crust of the western Junggar to constrain the subduction mode. By deploying a seismic array consisting of 31 portable broadband seismic stations from September 2013 to December 2013, we determine the Moho depth beneath every station using a H‐κ method and the Moho variations along three NW‐SE profiles using a common conversion point stacking method. We also construct a 3‐D crustal S wave velocity model using ambient noise tomography. Our results reveal significant variations of Moho depths across the study region and a pronounced Moho offset between the Zaire mountains and the western Junggar basin. Ambient noise tomography shows that a high shear velocity layer (>3.9 km/s) is observed in the middle/lower crust of the western Junggar Basin, which is overlaid by a low velocity upper crust. The high‐velocity body gradually dips northwestward with depth beneath the Zaire mountains. The speed of this high‐velocity body is consistent with the calculated S wave speed of various metamorphic facies of mid‐ocean ridge basalt under the P/T conditions of middle/lower crust in the western Junggar. Our seismic observations support a northwest paleosubduction model for the evolution of the western Junggar, in which a remnant oceanic slab is still trapped in the western Junggar. [ABSTRACT FROM AUTHOR]

We present a new 3-D lithospheric V s model for the NE Tibetan Plateau (NETP) and the western North China Craton (NCC). First, high-frequency receiver functions (RFs) were inverted using the neighborhood algorithm to estimate the sedimentary structure beneath each station. Then a 3D V s model with unprecedented resolution was constructed by jointly inverting RFs and Rayleigh wave dispersions. A low-velocity sedimentary layer with thicknesses varying from 2 to 10 km is present in the Yinchuan-Hetao graben, Ordos block, and western Alxa block. Velocities from the middle-lower crust to the uppermost mantle are generally high in the Ordos block and low in the Alxa block, indicating that the Alxa block is not part of the NCC. The thickened crust in southwestern Ordos block and western Alxa block suggests that they have been modified. Two crustal low-velocity zones (LVZs) were detected beneath the Kunlun Fault (KF) zone and western Qilian Terrane (QLT). The origin of the LVZ beneath the KF zone may be the combined effect of shear heating, localized asthenosphere upwelling, and crustal radioactivity. The LVZ in the western QLT, representing an early stage of the LVZ that has developed in the KF zone, acts as a decollement to decouple the deformation between the upper and lower crust and plays a key role in seismogenesis. We propose that the crustal deformation beneath the NETP is accommodated by a combination of shear motion, thickening of the upper-middle crust, and removal of lower crust. [ABSTRACT FROM AUTHOR]

On the eastern margin of the Himalayan orogenic belt, the rapid uplift of the Namche Barwa metamorphic terrane and significant bending of the Yarlung Zangbo suture zone occur. The formation mechanism and dynamics of the Eastern Himalaya Syntaxis (EHS) is still debated. In order to better understand the deep structures beneath the EHS, we deployed 35 broadband seismic stations around the Namche Barwa Mountain. The data were integrated with existing datasets for a 3-D teleseismic P-wave tomography. The results demonstrate complex deep structures and significantly contrasting Indian subduction styles in the eastern Himalaya. In the western region of the EHS, the Indian slab flatly subducts under southern Tibet and might extend to the Bangong-Nujiang Suture. In contrast, a (north)eastward steep subduction occurred in the eastern region of EHS. The contrasting subduction styles result in tearing and fragmentation of the Indian lithosphere between the flat and steep subducting slabs beneath the EHS. Consequently, the hot asthenospheric mantle may rise through the slab window, which might further lead to the rapid uplift of Namche Barwa and the formation of EHS. The lateral variation in subduction/collision mode and slab tearing induced asthenospheric mantle upwelling is similar to that observed in the Hellenide and Anatolide domains of the Tethyan orogen. [ABSTRACT FROM AUTHOR]

Abstract: Here we report estimates of crustal thickness and Poisson's ratios for northern Vietnam, based on teleseismic receiver function analysis of observations from a dense broadband seismic array. The seismic array comprised of 24 stations distributed evenly over northern Vietnam, from whose records we examined 190 teleseismic events of Mw>5.5 for the period 2006–2008. Using the radial receiver functions calculated from teleseismic records at individual stations, the optimum crustal thickness and Vp/Vs (where Vp and Vs are the velocities of P- and S-waves, respectively) ratio beneath each station were determined using the H–κ (where H is crustal thickness and κ is defined as the Vp/Vs ratio) stacking algorithm. Determined values of crustal thickness range from 26.5km to 36.4km, with an average of 31.0±2.1km. The simple pattern of variation of crustal thickness in the northeastern region of the study area, with a mean of ~31km, suggests that the sector belongs to the craton of the South China block. A highly variable crustal thickness is found over the northwestern region of northern Vietnam, ranging from ~29.5km to ~36.4km, implying that complex tectonic processes have taken place in this region. The thinnest crust is found in the Red River Delta, where it ranges from 26.5km to 30.4km, which is suggestive of a recent rifting process. The determinations of crustal thickness show a good linear correlation with Bouguer gravity anomalies. Lower values of Poisson's ratio in the northeastern and Red River Delta sectors suggest a more felsic crust, and larger values in the northwestern sector suggest lithospheric extension in the Song Da depression. The findings enhance our understanding of the geotectonic architecture of the northern Vietnam region. [Copyright &y& Elsevier]

Highlights: [•] A database of 312 moment tensor solutions for the 2009 L’Aquila seismic sequence (central Italy) is presented. [•] Stress tensors for the study region and selected sub-volumes are computed. [•] A relation between M w and M l properly calibrated for the study area is furnished. [Copyright &y& Elsevier]

Abstract: We determined the location, size, mechanism, and the frequency content of the first 4.0s of the 2011 Tohoku–Oki earthquake. Since the beginning of this earthquake is very small, we develop a comparative approach against a near-by reference earthquake, the master event. We first determined the water depth near the master event using the differential timing between the water phase pwP reflected from the air–water interface and the depth phase pP reflected from the water–crust interface. Then we located the master event using the well-known ocean bathymetry in the area. After calibrating teleseismic arrays (Δ=30° to 90°) at short periods for timing and amplitude with respect to the master event, we were able to determine the initiation of the main event. It began as a small (Mw =4.9) thrust event located at 38.19°N, 142.68°E at a depth of 21km, and, a few seconds later, evolved into a slower extremely large slip event up-dip. [Copyright &y& Elsevier]

Abstract: We report the first application of the Cut and Paste (CAP) method to compute earthquake focal mechanisms in the Messina Straits area by waveform inversion of Pnl and surface wave segments. This application of CAP has furnished new knowledge about low-magnitude earthquake mechanics that will be useful for improved understanding of the local geodynamics. This is possible because the CAP inversion technique can be applied to small earthquakes, for which traditional moment tensor inversion methods are not appropriate and P-onset focal mechanisms in the study area fail because of a lack of sufficient observations. We estimate the focal mechanisms of 23 earthquakes with local magnitudes in the range of 3–4 occurring in the 2004–2008 time period, and recorded by the broadband stations of the Italian National Seismic Network and the Mediterranean Very Broadband Seismographic Network (MedNet) run by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The solutions show that normal faulting is the prevailing style of seismic deformation in the northern part of the study area while co-existence of normal faulting and strike-slip has been detected in the southern part. In the whole area of investigation the T-axes of focal mechanisms display a preferential northwest-southeast direction of extension. Combined with the findings of previous investigations, this improved database of focal mechanisms allows us to better detail the transitional area between the extensional domain related to subduction trench retreat (southern Calabria) and the compressional one associated with continental collision (western-central Sicily). The observed spatial change of seismic deformation regime offers new data to current seismotectonic and seismic hazard investigations in the area of Messina Straits where a magnitude 7.2 earthquake caused more than 60,000 casualties on 28 December 1908. [Copyright &y& Elsevier]

Abstract: We determined crustal structure along the latitude 30°N through the eastern Tibetan Plateau using a teleseismic receiver function analysis. The data came mostly from seismic stations deployed in eastern Tibet and western Sichuan region from 2004 to 2006. Crustal thickness and Vp/Vs ratio at each station were estimated by the H–k stacking method. On the profile, the mean crustal thickness and Vp/Vs ratio were found to be 62.3km and 1.74 in the Lhasa block, 71.2km and 1.79 near the Bangong–Nujiang suture, 66.3km and 1.80 in the Qiangtang block, 59.8km and 1.81 in the Songpan–Garze block, and 42.9km and 1.76 in the Yangtze block, respectively. The estimated crustal thicknesses are consistent with predictions based on the topography and the Airy isostasy, except near the Bangong–Nujiang suture and in the Qiangtang block where the crust is 5–10km thicker than predicted, indicating that the crust may be denser, possibly due to mafic underplating. We also inverted receiver functions for crustal velocity structure along the profile, which reveals a low S-wave velocity zone in the lower crust beneath the eastern Tibetan Plateau, although the extent of the low-velocity zone varies considerably. The low-velocity zone, together with previous results, suggests limited partial melting and localized crustal flow in the lower crust of the eastern Tibetan Plateau. [Copyright &y& Elsevier]

We analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 Landers earthquake. A grid-search method with station delay corrections is used to locate events that do not have catalogue locations. The quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct S waves at stations close to and off the fault zone. Approximately 70 per cent of the events with S–P times of less than 2 s, including many clearly off the fault, produce considerable trapped wave energy. This distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the Landers rupture zone. The time difference between the S arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth. The dispersion measured from the trapped waves is weak. These results imply that the seismic trapping structure at the Landers rupture zone is shallow and does not extend continuously along-strike by more than a few kilometres. Synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2–4 km, a width of approximately 200 m, an S-wave velocity reduction relative to the host rock of approximately 30–40 per cent and an S-wave attenuation coefficient of approximately 20–30. The fault zone waveguide north of the array appears to be shallower and weaker than that south of the array. The waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break. [ABSTRACT FROM AUTHOR]