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.
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.
Plain Language Summary
How did the Tibetan Plateau grow to its present height and size? Models have been controversial for decades, including end‐members of continuous deformation, rigid block extrusion, and channel flow in middle‐lower crust. Here we used a recently developed joint inversion scheme to resolve several key seismic parameters simultaneously in a self‐consistent manner for a linear array in the northeast margin of the plateau, which is ideal for testing models for the plateau growth. Our joint inversion results show distinct block‐like features, which suggest a lithospheric‐scale deformation of continuous shortening as well as localized faulting at the lithospheric scale. The proposed model of Tibetan Plateau deformation and growth can reconcile the continuous deformation within the blocks and major faults at the surface.
Key Points
High‐resolution images of Vs and Vp/Vs along a profile in NE Tibet were obtained from a joint inversion with Vp constraints
The geological blocks are characterized by distinct seismic features with correlated crustal and mantle lithosphere structure
Deformation is both continuous and localized and occurs at lithospheric scale
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 30–35km to 45–50km in less than 20km 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.
•We estimated crustal thickness variation in Hubei, China using H-κ and CCP methods.•We found step-wise crustal thickness increases from 30–35km to 45–55km in the area.•The thin crust in eastern Hubei extends into the interior of mountains in western Hubei.•A double Moho was found beneath the Wudang Mts.•The results suggest eclogitization and foundering of the lower crust.
•We found a V-shape crustal thinning zone beneath southeastern China.•We suggest different crustal thinning mechanisms across the 29°N Parallel.•A “hot fingers” model was adopted to interpret our ...observation.
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 ∼30km 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.
•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 Vp/Vs ratio maps of the Dabashan orocline and adjacent areas. The crustal thickness and Vp/Vs ratio maps show prominent thick crust (50–55 km) and high Vp/Vs 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.
SUMMARY
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.
The number of broadband three‐component seismic stations in southern California has more than tripled recently. In this study we use the teleseismic receiver function technique to determine the ...crustal thicknesses and Vp/Vs ratios for these stations and map out the lateral variation of Moho depth under southern California. It is shown that a receiver function can provide a very good “point” measurement of crustal thickness under a broadband station and is not sensitive to crustal P velocity. However, the crustal thickness estimated only from the delay time of the Moho P‐to‐S converted phase trades off strongly with the crustal Vp/Vs ratio. The ambiguity can be reduced significantly by incorporating the later multiple converted phases, namely, the PpPs and PpSs+PsPs. We propose a stacking algorithm which sums the amplitudes of receiver function at the predicted arrival times of these phases by different crustal thicknesses H and Vp/Vs ratios. This transforms the time domain receiver functions directly into the H‐Vp/Vs domain without need to identify these phases and to pick their arrival times. The best estimations of crustal thickness and Vp/Vs ratio are found when the three phases are stacked coherently. By stacking receiver functions from different distances and directions, effects of lateral structural variation are suppressed, and an average crustal model is obtained. Applying this technique to 84 digital broadband stations in southern California reveals that the Moho depth is 29 km on average and varies from 21 to 37 km. Deeper Mohos are found under the eastern Transverse Range, the Peninsular Range, and the Sierra Nevada Range. The central Transverse Range, however, does not have a crustal root. Thin crusts exist in the Inner California Borderland (21–22 km) and the Salton Trough (22 km). The Moho is relatively flat at the average depth in the western and central Mojave Desert and becomes shallower to the east under the Eastern California Shear Zone (ECSZ). Southern California crust has an average Vp/Vs ratio of 1.78, with higher ratios of 1.8 to 1.85 in the mountain ranges with Mesozoic basement and lower ratios in the Mojave Block except for the ECSZ, where the ratio increases.
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.
Plain Language Summary
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. In this study, we determined focal mechanism and depth of the 2013 M 5.1 Badong Earthquake, the largest earthquake so far in the Three Gorges Reservoir (TGR) region of China. We also obtained high‐precision locations of 205 events in this earthquake sequence. The results show that these earthquakes occurred at shallow depths between 1 and 5 km on a steep south‐dipping fault. The fault is at least 15 km long and intercepts TGR at the NS‐running Shennongxi River. A field survey indicates that the earthquakes are confined to a fractured rock formation that crops out in the reservoir. The precise locations of earthquakes coupled with the local geology suggest that they were induced by high pore pressure due to reservoir water infiltration. The newly identified fault is a seismic risk to the Three Gorges Dam. The findings are important because (1) they highlight the importance of understanding the detailed local hydrologic system in risk assessment, and (2) the fact that the seismogenic fault was missed before demonstrates the limitations of preconstruction risk‐assessment surveys.
Key Points
The 2013 M 5.1 Badong Earthquake occurred at a shallow depth on a steep south‐dipping strike‐slip fault in the Three Gorges Reservoir region
The results suggest that the earthquake 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 approaching the designed seismic intensity limit of the Three Gorges Dam
A simple and unified approach is presented to solve both the elasto-dynamic and elasto-static problems of point sources in a multi-layered half-space by using the Thompson-Haskell propagator matrix ...technique. It is shown that the apparent incompatibility between the two is associated with the degeneracy of the dynamic problem when ω = 0 and both can be handled uniformly using the Jordan canonical forms of matrices. We re-derive the propagator matrices for both the dynamic and static cases. We then show that the dynamic propagator matrix and the solution converge to their static counterparts as ω → 0. Satisfactory static deformation can be obtained numerically using the dynamic solution at near-zero frequency.
The Mw 7.9 Wenchuan earthquake of 12 May 2008 was the most destructive Chinese earthquake since the 1976 Tangshan event. Tens of thousands of people were killed, hundreds of thousands were injured, ...and millions were left homeless. Here we infer the detailed rupture process of the Wenchuan earthquake by back‐projecting teleseismic P energy from several arrays of seismometers. This technique has only recently become feasible and is potentially faster than traditional finite‐fault inversion of teleseismic body waves; therefore, it may reduce the notification time to emergency response agencies. Using the IRIS DMC, we collected 255 vertical component broadband P waves at 30–95° from the epicenter. We found that at periods of 5 s and greater, nearly all of these P waves were coherent enough to be used in a global array. We applied a simple down‐sampling heuristic to define a global subarray of 70 stations that reduced the asymmetry and sidelobes of the array response function (ARF). We also considered three regional subarrays of seismometers in Alaska, Australia, and Europe that had apertures less than 30° and P waves that were coherent to periods as short as 1 s. Individual ARFs for these subarrays were skewed toward the subarrays; however, the linear sum of the regional subarray beams at 1 s produced a symmetric ARF, similar to that of the groomed global subarray at 5 s. For both configurations we obtained the same rupture direction, rupture length, and rupture time. We found that the Wenchuan earthquake had three distinct pulses of high beam power at 0, 23, and 57 s after the origin time, with the pulse at 23 s being highest, and that it ruptured unilaterally to the northeast for about 300 km and 110 s, with an average speed of 2.8 km/s. It is possible that similar results can be determined for future large dip‐slip earthquakes within 20–30 min of the origin time using relatively sparse global networks of seismometers such as those the USGS uses to locate earthquakes in near–real time.
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