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The moderate-size 2010 Yushu earthquake (Mw 6.9) caused severe seismic damage around the source region which is situated on the sparsely populated hinterland of the Tibetan Plateau. But until now the mechanisms have not been well understood. To this end, we constructed the model to simulate the fault spontaneous rupture propagation, in which the realistic fault with a curved bend is imbedded. Our modeling results show that the special fault geometry controlled the rupture behavior, which encouraged rupture propagation speed transition from subshear to supershear with the speed of 5.17 km/s larger than the local shear wave velocity. Moreover, calculation results demonstrated that strong ground motion acceleration was greatly intensified by supershear ruptures, leading to widespread destruction. This may be the main reason why serious earthquake damage happened in the Yushu earthquake. In particular, we can see from numerous numerical experiments that rupture styles will be different if geometries of fault are varied. It is confirmed that it is the special geometry of the seismogenic fault of the Yushu mainshock produced grave seismic hazard. Thus, deeply investigating fault geometry will be helpful to better understand seismic source process and seismic hazard assessment.Key Points Curved bend in the seismogenic fault for the 2010 Yushu earthquake encouraged the supershear rupture transition. Strong ground motion acceleration was greatly intensified by supershear ruptures. Special geometry of the fault of the Yushu mainshock lead to seriouos seismic damage.

The Beichuan area suffered the most serious seismic damage in the 2008 Wenchuan earthquake although the Beichuan is over 100 km away from the instrumental epicenter of the mainshock. The mechanism for this peculiar phenomenon remains unclear even though 10 years has passed since the Wenchuan event. For this purpose, we construct a spontaneous rupture model in which Gaochuan right bend (GRB) is included in the middle of Yingxiu-Beichuan fault, a major seismogenic fault for the Wenchuan event. The simulated results show that the complex geometry of the GRB played a controlling role in the rupture propagation. Once the rupture was initiated at the epicenter of the Wenchuan mainshock, it propagated spontaneously, and the rupture speed on the first segment of the fault is ∼2.79 km/s, slower than the shear wave speed of local medium. When the rupture front spread near the end of the Yingxiu-Gaochuan fault, a new rupture was re-nucleated at the curve section of the Gaochuan bend, and propagated in the northeast direction with the speed of 5.02 km/s, greater than the S wave velocity. In addition, this rupture speed transition from subshear to supershear does not need any time delay, much distinct from the case of fault stepover in which delay is needed in supershear transition. The result also demonstrates that the relation between the high values of peak ground acceleration (PGA) and the supershear rupture is strong. The large area with high values of PGA distributed in the Beichuan directly led to grave seismic catastrophe there. Therefore, this work may give some insight into why the most serious seismic damage occurred in the Beichuan area, and may have important implications for understanding earthquake dynamics and assessing seismic hazards.

We perform 2-D finite element calculations of mode II rupture along a bimaterial interface governed by regularized rate- and state-dependent friction law, with the goal of understanding how the bimaterial interface influences the strong ground motion. By comparison with properties of rupture in a homogeneous solid, we found that bimaterial mechanism is important for earthquake ruptures and influences the strong ground motion significantly. The simulated results show that mode II rupture evolves with propagation distance along a bimaterial interface to a unilateral wrinkle-like pulse in the direction of slip on the compliant side of the fault, namely in the positive direction. Strong ground motion caused by seismic waves emanated from the rupture propagation is asymmetrically distributed in space. The computed peak ground acceleration (PGA) is high in the near-fault region. Particularly, PGA is much larger in the region on the side in the positive direction. In addition, it is greater in the more compliant area of the model than that in the stiffer area with corresponding locations. Moreover, the differential PGA due to bimaterial effect increases with increasing degree of material contrast across the fault. It is hoped that the results obtained in this investigation will provide some implications for seismic hazard assessment and fault rupture mechanics.

Following the devastating 2008 Wenchuan earthquake (Mw = 7.9), the 2013 Lushan strong shock (Mw = 6.6) occurred again on the Longmen Shan thrust fault. Ever since the earthquake occurred, it has been a topic of hot debate on whether it is a strong aftershock of the 2008 Wenchuan earthquake or a new independent one. To this purpose, we investigated the seismogenic fault for the Lushan earthquake, and found that it is situated on the same fault system as that for the Wenchuan earthquake, and the Lushan shock ruptured one of the asperities which were left unbroken in the 2008 Wenchuan earthquake. In addition, we analyzed the statistical properties of the Wenchuan-Lushan earthquake sequence (WLES) using three established empirical laws on aftershocks. The modelling results showed that the WLES, in which the Lushan event is regarded as one of aftershocks of the Wenchuan mainshock, well satisfied three accepted statistical laws, namely the Gutenberg-Richter relation, modified Omori-Utsu law and Bath’s law. At the same time, we observed that the weak and ductile materials between the hypocenters of the Wenchuan and Lushan shocks resulted in seismic gap in between, and the strong materials in the source region and special curved geometry of seismogenic fault for the Lushan event may be responsible for the approximately 5-year delay of the occurrence. In particular, the focal mechanism of the Lushan event, like most of the other strong aftershocks of the Wenchuan mainshock, is nearly thrust-type fault, suggesting the Lushan earthquake, together with other strong aftershocks, is controlled by the same stress environment. Most of all, we noticed that the Lushan event was triggered by the 2008 Wenhcuan main shock. Taken together, it is reasonable to infer that the 2013 Lushan event is a strong aftershock, possibly being the largest one, of the Wenchuan earthquake. Meanwhile, we propose in the paper that it is an important criterion for identification of aftershocks by means of judging whether they are triggered by the main shock or not. Identifying and forecasting strong aftershocks shall be crucial in evaluating the seismic hazard for large earthquakes.

The Lushan earthquake Ms7.0 occurred at 08:02 Beijing Time (00:02 UTC) on 20 April 2013, located on the eastern margin of the Tibetan Plateau, where the 2008 Ms8.0 Wenchuan earthquake took place. The other strong shock following the Wenchuan earthquake occurred on the Longmen Shan thrust fault. A large number of aftershocks were recorded after the mainshock. To investigate how the aftershocks were triggered, we firstly relocated the 6-month aftershock sequence by the double-difference algorithm. Next, we calculated Coulomb failure stress changes imparted by the Lushan mainshock both on the optimally oriented plane and on the aftershock nodal planes. Then we examined the correspondence between the Coulomb failure stress changes and the spatial distribution of the aftershocks. The computed results show that most of the aftershock hypocenters did not occur on the region with the positive Coulomb stress changes caused by the Lushan mainshock. Moreover, the majority of aftershock nodal planes were brought away from failure by coseismic Coulomb stresses. This implies that coseismic static Coulomb stress changes may not be the governing process for aftershock genesis. In contrast, postseismic stress transfer by processes of afterslip and pore fluid flow are assumed to play an important part in aftershock triggering, based upon the spatiotemporal distribution of the aftershocks.

The sudden and unexpected Wenchuan earthquake (Ms = 8.0) occurred on the Longmen Shan Fault, causing a large number of casualties and huge property loss. Almost no definite precursors were reported prior to this event by Chinese scientists, who made a first successful prediction of the 1975 Haicheng earthquake (M = 7.3) in China. Does the unsuccessful prediction of the Wenchuan earthquake mean earthquake prediction is inherently impossible? In order to answer this question, the paper simulated inter- and co-seismic deformation, and recurrence of strong earthquakes associated with the Longmen Shan listric thrust fault by means of viscoelastic finite element method. The modeling results show that the computed interseismic strain accumulation in the lower crust beneath the Eastern Tibet is much faster than that in the other regions. In particular, the elastic strain energy density rate accumulates very rapid in and around the Longmen Shan fault in the depth above ~25 km that may explain why the great Wenchuan earthquake occurs in the region of such a slow surface deformation rate. The modeled coseismic displacements around the fault are consistent with surface rupture, aftershock distribution, and GPS measurement. Also, the model displays the slip history on the Longmen Shan fault, implying that the average earthquake recurrence interval on the Longmen Shan fault is very long, 3,300 years, which is in good agreement with the observed by paleoseismological investigations and estimates by other methods. Moreover, the model results indicate that the future earthquake could be evaluated based on numerical computation, rather than on precursors or on statistics. Numerical earthquake prediction (NEP) seems to be a promising avenue to a successful prediction, which will play an important part in natural hazard mitigation. NEP is difficult but possible, which needs well supporting.

We make use of two-step modeling to simulate inter- and coseismic deformation within one seismic cycle by means of visco-elastic finite element method (FEM). The first step is to perform inversion to find the optimum modeling parameters by fitting the observed interseismic deformation. Then the model continues to produce a major seismic event to simulate the Wenchuan earthquake. The model results show that, in order to fit the observed interseismic deformation, it needs a “soft” lower crust and upper mantle beneath the Eastern Tibet, and a very strong lithosphere of the Sichuan basin. The computed interseismic strain accumulation in the lower crust beneath the Eastern Tibet is much faster than that in the other regions. Especially, the elastic strain energy density rate (ESED) accumulates very rapidly in and around the Longmen Shan fault in the depth above 30 km that may explain why the great Wenchuan earthquake occurs in the region of such a slow surface deformation rate. Simulations of coseismic processes reveal relationships among slip acceleration, normal stress and shear stress changes on surface of the seismogenic fault. Coseismic slip appears to initiate in the gently-dipping section of the fault, but primary slips (coseismic rupture) on both gently-dipping and steeply-dipping sections take place simultaneously. Minor fault slip accelerations decrease normal stress on fault surface to reduce frictional strength of fault, and in general increase shear stress on fault surface to enhance tectonic stress on the fault to drive the fault further slips. Both processes facilitate major slip or rupture on the listric reverse fault. Coseismic slip distribution also suggest that an important role of high-angle listric reverse faulting is to transfer overwhelming horizontal deformation of the Eastern Tibet into significant vertical displacement in the Longmen Shan during the 2008 Wenchuan earthquake.

Research on static earthquake triggering has been carried out widely in the world, and achieved remarkable results. But it is still unclear whether this model is effective to all large earthquakes. In this paper, we investigated the Coulomb stress changes of 3 megathrust earthquakes along subduction zones (the 2011 Mw9.1 Tohoku earthquake, the 2010 Mw8.8 Chile earthquake, and the 2004 Mw9.0 Sumatra-Andaman earthquake) to test the triggering effects by examining the correlation between Coulomb stress increases and spatial distribution of the following aftershocks. The calculated results suggest that there is no obvious evidence that the Coulomb stress changes caused by the 3 megathrust earthquakes promoted the occurrence of the aftershocks. There are only 47% of the encouraged aftershocks following the Tohoku earthquake. And there are 47.6% and 49.8% for the Sumatra-Andaman and the Chile earthquake, respectively. We also calculated the Coulomb stress changes with different focal models and parameters. It is still less than 60% of the promoted aftershocks in the optimal case. However, the static triggering model is good for the Wenchuan earthquake and the Chi-Chi earthquake which have enhanced more than 85% of the subsequent aftershocks. This model may be not reasonable for large subduction earthquakes. Therefore, other model should be introduced in studying earthquake triggering in subduction zone and further studies will be performed.

In this paper we first estimate the strain rate field with 1202 GPS vectors from 1999 to 2005 in the Chinese continent. Then we propose a method to make error analysis of the strain rates computed from GPS vectors based on the Monte Carlo technique. In general, the orientations of compressive principal strain rates are in agreement with those of present tectonic stresses. The strain rates in the paper confirm the tectonic features of Tibetan Plateau such as NE–W compression and shortening, E–W extension, and normal faulting along near N–S faults. At the same time, the eastward extrusion in the east part of Tibet can also be seen from the strain rate field. In the error analysis, independent computation of strain rates is repeated for a large number of times, both the absolute and relative errors as well as the mean value of strain rates are obtained through statistical theory. The error result shows that the errors of strain rates may be associated with the uncertainties of GPS measurement and the distribution of GPS stations in space. The magnitude of the strain rate error is in the range of 3–8 × 10−9/yr across China in general. However, we found that the relative errors are much larger in East China than those in the west of China. In addition, the relative errors of maximum shear strain rates are smaller than those of surface dilation rates. Also, we found that there may be some correlations between maximum shear strain rates and seismic activities. Where the maximum shear strain rates are higher, there the earthquakes occur more frequently. The computed GPS strain rates reveal the distributed nature of deformation across the whole Chinese continent, and can represent the main present-day tectonic features. The strain rate and its error in the paper may provide important constraints to geodynamical modeling.

As one of the most devastating earthquakes, the 2008 Wenchuan earthquake occurred on imbricate, high-angle listric-reverse faults. The faults dip ∼70 ° above 15 km depth, become 30° to 40° below ∼15 km depth, and presumably root into a subhorizontal brittle-ductile transition zone below about 22±2 km depth. In this paper we use the viscoelastic finite-element method to simulate coseismic deformation and stress change, and recurrence of major earthquakes associated with the high-angle listric-reverse fault. Our modeling shows that changes of coseismic equivalent stress mainly occur in the vicinity of the seismogenic fault, especially the hanging wall of the fault in the depth range above 12 km. Modeled coseismic slip distribution and modeled average recurrent interval corroborate with geological, geodetic, and seismological observations. We modeled 250,000 yr slip history of the fault slip. The result shows that minor slips, which obey slip-predictable model, take place on 30°–40° dipping fault plane at depth below 15 km. When the isotropic stress accumulation on the gentle fault reaches the critical level, the entire gently dipping fault ruptures to form a large slip. The large slips on the gently dipping fault comply with a time-predictable model of recurrences. These larger slips on the gently dipping fault occur simultaneously with slips on the steeply dipping fault and might have triggered slips on the steeply dipping fault to form great events such as the 2008 Wenchuan earthquake.

Errors of the strain rates, computed from GPS data, are very important in geodynamics. In order to solve this problem, we propose a method to calculate the errors of strain rates, computed from GPS vectors with measurement errors, on the base of Monte Carlo technique. Taken the Qinghai-Tibet plateau as an example, independent computation of strain rate is repeated for large number of times, and the components of strain rates, as well as the errors, are obtained by statistical theory. The result shows that the errors of strain rates in NS, WE directions and shear components, respectively, are larger in Himalayas and both in the middle and in the east of the plateau than those in other places. In general, the strain rate in whole Qinghai-Tibet plateau is accurate and stable. Also the result suggests that the errors of the strain rates computed from GPS measurements are mainly originated from the errors of GPS data. The accuracy of GPS survey does not meet the requirements for precisely computing the strain rates in the region where there is small value of deformation, for example in east China.

In Appling studies, the strain rate in China has been computed from using different methods, resulting in quite different estimates of the strain rate the geo-statistics from GPS velocity field of Chinese continent, we obtain the velocity value at each little regularly spaced grid point, by kriging interpolation and the component of strain rate for each volume element, using a method similar to the derivation of shape functions in the finite element algorithm. Therefore the distribution of the strain rate field in whole for the Chinese continent is presented. The result shows that the orientations of principal strain rates are consistent with those of the P and T axes of focal mechanisms. The distribution of maximum shear strain rate clearly delineates some major active fault zones surrounding the Tibetan Plateau. The maximum shear strain rate is comparable with that obtained from analysis of seismic moment release. In part of the Tibetan Plateau containing normal faults and pull-apart grabens, we obtain an extensional state of strain. The absolute value of the strain rate in West China is approximately 5 times larger than that of East China, and the pattern of the strain rate field in most of the Chinese continent is controlled by the India/Eurasia collision.

The viscous properties of earth media are very important in geodynamics. Taking the physical property variation of focal media after earthquake into account, we propose a method to invert for the viscosities in the lower crust and the upper mantle from GPS temporal series measurements. Combining the genetic algorithm (GA) and the Marquardt inversion, we regard the optimum model sought by GA as the initial one to carry out inversion by the Marquardt method, in which the Jacobian matrix is produced by small perturbation. The focal region and physical properties are decided by the range of spatial distribution of the aftershocks and velocity changes of seismic wave. We apply viscoelastic finite element method to determine the viscosities in Taiwan with GPS measurements after Chi-Chi, Taiwan, earthquake. Inversion computation and digital experiments show that the inversion method is highly efficient and stable. The preliminary result suggests that the viscosities of the lower crust and the upper mantle in Taiwan is 1.2×1018Pa·s and 3.6×1019Pa·s, respectively.

Many researchers calculated strain rate of significant differences from the same GPS measurement data. In this paper, we use the Kriging method in geostatistics to GPS velocity field. Interpolating the scattered GPS velocity data of Qinghai-Tibetan plateau and its adjacent areas to grid point values by Kriging, we calculate the strain rates from these nodal values in each grid cell similar to derivative of shape functions (essentially Lagrange interpolation function) in finite element algorithm, and obtain the stable distribution of strain rate field in Qinghai-Tibetan plateau. The results show that the main part of Qinghai-Tibetan plateau is in the state of compression in north-south direction, and extension in west-east direction. On the contrary, in the eastern part of Tibet, the strain rate is compressive in west-east and extensional in north-south direction. The orientations of principal strain rates are consistent with those of the P axis and T axis in focal mechanism. The high values of maximum compressive principal strain rates are located in the Himalayan main boundary thrust zone (MBT) and the surrounded regions. The maximum extensive principal strain rates are higher than those of the compressive ones in the main part of the interior of Qinghai-Tibetan plateau. Also, the surface dilation strain rate shows that it is in the state of surface compression in Himalayan and its surrounded areas, and in the state of surface extension in the interior of Qinghai-Tibetan plateau. The distribution of maximum shear strain rate clearly displays the outlines of some main active fault zones. The result of the strain rate in this study suggests that the contemporary tectonic strain of Tibet inherits the long term geological deformation.

Based on data of focal mechanism and geological survey, the boundary forces and shear force applied beneath the upper crust in the Sichuan-Yunnan area (96° ~ 104°E, 22° ~ 30°N) of southwest China are inversed using the pseudo-3D genetic algorithm finite element method (GA-FEM). The inversion method is discussed, and reliable result is obtained through calculation. The result shows that the direction of shear force on the bottom of the upper crust in the Sichuan-Yunnan rhombus block applied by the lower crust is south-southeast, which is roughly consistent with the orientation of present crust movement revealed by GPS measurements, but significant shear forces are not found beneath the upper crust in other places of the study area. Combined with other research data, we conclude that the lower crust drags the upper crust in the Sichuan-Yunnan rhombus block since the lower crust is easy to flow than the upper crust in the process of mass movement toward east and southeast when the Qinghai-Xizang Plateau pushes it. The model displacements inversed from stress field agree with present displacements measured by GPS. This shows that our result is reasonable.

Rheology of continental lithosphere is an important basic issue in geodynamics, but not well resolved yet. There are observational and experimental evidences to support a model of strong mantle - ductile weak lower crust near the Moho; there are also evidences to support a contrast model of weak ductile mantle - strong crust. In this study, we use CMT data of Harvard with corrected focal depth to analyze the depth variation of plunge angle of principal stresses in continental China. It is found that the plunge angles are either nearly horizontal or close to vertical for very shallow earthquakes, while there are great varieties in plunge angle of earthquakes in the middle and lower crust and earthquakes in the mantle. This feature is distinctively different from results of regions with weak ductile lower crust, where at the base of earthquake generation layer, plunge angles of principal stresses are found nearly horizontal or vertical similar to the case of very shallow earthquakes. It may indicate that both kinds of crust-mantle rheology models exist in China. Therefore, integrated detailed study on regional lithospheric rheology is necessary.

Stress release model used to be applied to seismicity study of large historical earthquakes in a space of large scale. In this paper, we improve the stress release model, and discuss whether the stress release model is still applicable or not in the case of smaller spatio-temporal scale and weaker earthquakes. As an example of testing the model, we have analyzed the M≥6 earthquakes in recent about 100 years. The result shows that the stress release model is still applicable. The earthquake conditional probability intensity in Taiwan area is calculated with the improved stress release model. We see that accuracy of earthquake occurrence time predicted by the improved stress release model is higher than that by Poisson model in the test of retrospect earthquake prediction.