Crustal Azimuthal Anisotropy Beneath the Southeastern Tibetan Plateau and its Geodynamic Implications

The fast orientation and magnitude of crustal azimuthal anisotropy beneath the southeastern Tibetan Plateau and adjacent areas are measured by analyzing the sinusoidal moveout of the P to S converted phase from the Moho. Beneath the tectonically active plateau, the mean magnitude is 0.48 ±Â 0.13 s, which is about twice as large as that observed in the stable Sichuan Basin (0.23 ±Â 0.10 s). The two areas are separated by the Longmenshan fault zone, a zone of devastating earthquakes including the 12 May 2008 MW 7.9 Wenchuan earthquake. Fault orthogonal fast orientations observed in the southern Longmenshan fault zone, where previous studies have revealed high crustal Vp/Vs and suggested the presence of mid-lower crustal flow, may reflect flow-induced lattice preferred orientation of anisotropic minerals. Fault parallel anisotropy in the central segment of the fault zone is most likely related to fluid filled fractures, and fault perpendicular extensional cracks are probably responsible for the observed anisotropy in the northern segment. The crustal anisotropy measurements, when combined with results from previous studies, suggest the existence of mid-lower crustal flow beneath the southeastern margin of the plateau. Comparison of crustal anisotropy obtained before and after the Wenchuan earthquake suggests that the earthquake has limited influence on whole crustal anisotropy, although temporal changes of anisotropy associated with the earthquake have been reported using splitting of shear waves from local earthquakes occurred in the upper crust

Tectonic signals documented in gravel and silt beds : A comprehensive review of the eastern Tibetan plateau

Acknowledgements This project was supported by the National Natural Science Foundation of China (42207239) and National Nonprofit Fundamental Research Grant of China, Institute of Geology, China Earthquake Administration (IGCEA1906). We gratefully acknowledge the editors of the journal and the anonymous reviewers for their useful and detailed comments and suggestions to improve the original submission.Peer reviewedPostprin

Upper Plate Response to a Sequential Elastic Rebound and Slab Acceleration During Laboratory‐Scale Subduction Megathrust Earthquakes

An earthquake‐induced stress drop on a megathrust instigates different responses on the upper plate and slab. We mimic homogenous and heterogeneous megathrust interfaces at the laboratory scale to monitor the strain relaxation on two elastically bi‐material plates by establishing analog velocity weakening and neutral materials. A sequential elastic rebound follows the coseismic shear‐stress drop in our elastoplastic‐frictional models: a fast rebound of the upper plate and the delayed and smaller rebound on the elastic belt (model slab). A combination of the rebound of the slab and the rapid relaxation (i.e., elastic restoration) of the upper plate after an elastic overshooting may accelerate the relocking of the megathrust. This acceleration triggers/antedates the failure of a nearby asperity and enhances the early slip reversal in the rupture area. Hence, the trench‐normal landward displacement in the upper plate may reach a significant amount of the entire interseismic slip reversal and speeds up the stress build‐up on the upper plate backthrust that emerges self‐consistently at the downdip end of the seismogenic zones. Moreover, the backthrust switches its kinematic mode from a normal to reverse mechanism during the coseismic and postseismic stages, reflecting the sense of shear on the interface

Methodology investigations for shear wave splitting analysis

Over the past several decades, shear wave splitting analyses have been increasingly utilized to delineate mantle structure and probe mantle dynamics. However, the reported splitting parameters (fast polarization orientations and splitting times) are frequently inconsistent among different studies, partially due to the different techniques used to estimate the splitting parameters. Here the study conduct research on methodology investigations for shear wave splitting analysis, which are composed of two sub-topics, i.e., a systematic comparison of the transverse minimization (TM) and the splitting intensity (SI) techniques and applicability of the multiple-event stacking technique (MES). Numerical experiments are conducted using both synthetic and observed data. In addition, crustal anisotropy beneath 71 broadband seismic stations situated at the eastern Tibetan Plateau and adjacent areas is investigated based on the sinusoidal moveout of P-to-S conversions from the Moho and an intra-crustal discontinuity with an average splitting time of 0.39 ± 0.19 s and dominantly fracture-parallel fast orientations. The crustal anisotropy measurements support the existences of mid/lower crustal flow in the southern Songpan-Ganzi Terrane and crustal shortening deformation beneath the Longmenshan fault zone --Abstract, page iv

Three-dimensional numerical modelling of static and transient Coulomb stress changes on intra-continental dip-slip faults

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Seismogenic structures and spatiotemporal seismicity patterns of the 2022 Ms6.0 Maerkang earthquake sequence, Sichuan, China

The 2022 Ms6.0 Maerkang earthquake sequence, Sichuan, China, occurred in an unexpected area with historically rare seismicity in the Bayan Har block. Here we relocated the earthquake sequence, inverted for the focal mechanisms of the larger events, and calculated the rupture directivity of the earthquake sequence to reveal the seismogenic structures and mechanisms of this sequence. The high-precision relocations indicate that the seismogenic structures consist of several clusters that are generally parallel to the nearby NW-trending Songgang fault, and relatively small-scale conjugate faults are also identified. The seismicity migrated from cluster one in the south to cluster two in the north during the sequence. Furthermore, the hypocenters were largely located at 5–10 km depth, thereby highlighting that the seismogenic structures are buried. The vertical fault planes of the seismogenic structures are consistent with the high-dip focal mechanism solutions from seven events. A stress field inversion based on the focal mechanisms indicates that the sequence occurred in a strike-slip environment that was controlled by a NNW–SSE-striking principal compressive stress. The different rupture directivities of the Ms5.8 (southwestward) and Ms6.0 (southeastward) events prove the existence of conjugate faults. The Ms5.8 event induced a coseismic Coulomb stress change of 1.6 MPa where the Ms6.0 event subsequently occurred, thereby highlighting that the Ms5.8 event triggered the Ms6.0 event and produced the spatiotemporal seismicity pattern of the sequence. We therefore conclude that the seismogenic structures of the 2022 Ms6.0 Maerkang earthquake sequence are previously unknown concealed conjugate structures associated with the main Songgang fault. The complex seismogenic structures and their potential to generate large earthquakes warrant the need to better understand the seismogenesis of this area and the seismic risks that may be present

Understanding earthquake triggering and fault slip behavior based on improved earthquake catalogs

Large earthquakes can alter the subsurface stress field on active faults across broad spatial and temporal windows, which would promote or inhibit slip on these faults. Besides dynamic ruptures during regular earthquakes, faults can also slip at a steady rate without any seismic radiation. In between these two extremes, slow slip events have been documented within the conditionally stable transition zone. Recent advancements in seismic instrumentation and computer programs provide an unprecedented opportunity to capture weak seismic events, and the resulting complete catalogs can be used to understand physical mechanisms of earthquake interactions from nearby to long-range distances, as well as diverse faulting processes inside the Earth. Earthquakes are routinely picked and located by analysts at seismic network centers. However, a significant fraction of events are missed, especially during intensive aftershock or swarm sequences. These missing events can be detected by a semi-automatic template matching method, which uses waveforms of existing events as templates to scan through continuous data for new events with high similarities. This dissertation focuses on improved understanding of fault slip behaviors and earthquake interactions based on improved catalogs from the template matching method. I first present studies on earthquake interactions in both continental-continental (Tibet) and oceanic-continental (North Island of New Zealand and Nicoya Peninsula) convergent environments following large mainshocks at nearby and far-field distances. The obtained results suggest that transient stress carried by passing seismic waves can trigger fault slip at long-range distances, and the aftershock sequence can be driven by continuing fault slip following the mainshock rupture. The second group of studies focuses on seismic activities prior to the 2008 Mw 7.9 Wenchuan earthquake, as well as the 2010 Mw 7.2 El Mayor-Cucapah earthquake. The primary target is to decipher diverse fault slip behaviors and understand their roles in mainshock nucleation.Ph.D

Roughness of fault surfaces over nine decades of length scales

International audienc

Integrated Geophysical Studies of The Northeastern Caribbean Plate, Eastern Tibetan Plateau in China and Panhandle Field (Texas)

There are three different areas and research projects that are the targets of this proposed dissertation. These projects have the common theme of integration of a variety of data but are at different scales. The first one focuses on plate scale interactions in northeastern Caribbean at a lithospheric scale. The second one focuses on crustal scale deformation in the eastern Tibetan Plateau. The last project is in the Panhandle oil and gas field in Texas and targets the structure of basement and seismic attributes of reservoirs. The first project targets slab dip variations of the Caribbean Plate along the Muertos Trough, which bounds the Puerto Rico-Virgin Islands tectonic block on the south. Results of this research are intended to contribute to understanding the kinematic evolution of this region and the associated natural hazards. Two-dimensional gravity modeling is being employed to address this question for the first time. Integration of the potential field data and seismic data is providing details on the geometry of the subducting slab of the northeastern Caribbean plate. To further reveal the 3D deep structural geometry, P wave tomography is also being used in this research. The second project aims to explore the lithospheric structure across the Songpan-Ganzi terrane, LMS and western Sichuan basin by undertaking an integrated analysis of deep seismic profiling, gravity, magnetic, and geologic data. Based on our new results and previous research, a 2D numerical simulation was conducted using three initial models with different viscosity structures. These modeling results show that the uplift and crustal thickening observed can be due simply to ductile deformation in the middle crust west of the LMSFZ. xiv The third project focuses on the igneous basement structures of Panhandle Field in northern Texas by integrating 3D seismic reflection data, aeromagnetic data and gravity data. The gravity and magnetic data are the useful tool to identify these igneous basement structures. The high-resolution 3D seismic data were collected in Gray County, Texas. I employ volumetric seismic attributes such as instantaneous frequency, instantaneous phase and cosine instantaneous phase derived from 3D seismic data to better characterize subtle features such as collapse features, faulting and fracturing within the deposits that are difficult to detect on conventional 3D seismic data displays. Our research show the igneous basement has been broken into a series of grabens and horsts bounding with normal and reverse faults due to rifting, uplifting and left-lateral deformation

Seismic tomography and anisotropy : studies of intraplate seismic zones

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 24, 2010).The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file.Dissertation advisor: Dr. Eric Sandvol.Vita.Ph. D. University of Missouri--Columbia 2009.Both body and surface wave tomography has been applied to some significant intraplate seismic zones to explore the cause of intraplate earthquakes. Pn tomography in the Central and Eastern United States reveals that the major intraplate seismic zones are all near the edges of high-velocity anomalies in the upper mantle. It indicates that the origin of intraplate earthquakes could be related to the rheological boundaries around rigid lithospheric roots where stress may accumulate. A joint teleseismic and local P tomographic study shows that the New Madrid Seismic Zone is associated with a local, NE-SW trending low-velocity anomaly in the lower crust and upper mantle, suggestive of a weak zone caused by local deformation due to rheological contrast. Rayleigh wave tomography in the northeastern Tibetan Plateau discovers a lithospheric velocity boundary that separates the low-velocity northeastern Tibet and the high-velocity Ordos and Sichuan blocks. Again, the velocity contrast zone is also linked to significant intraplate earthquakes.Includes bibliographical reference