Wenchuan Ms Research Articles

Seismotectonic Mechanisms of Lushan (Ms7.0) Earthquake in the Frontal Propagation Belt of the Longmen Shan, Sichuan, China

In recent years, the apparent seismic activity around Longmen Shan and its front has included the Wenchuan (Ms8.0) Earthquake and the Lushan (Ms7.0) Earthquake, occurring in 2008 and 2013, respectively. Based on the focal mechanism solution, rupture processes, seismic intensity, surface deformation, and aftershocks of the Lushan Earthquake and the active fault on Longmen Shan, we divided the Longmen Shan and its front into two tectonic deformation belts, the Longmen Shan thrust belt and the frontal propagation belt. By comparing the differences in the tectonic deformation styles, active faults, and earthquake histories of the two belts, we propose two kinds of seismotectonic models: one is a thrusting belt characterized by napping and detachment, and the other is a frontal propagation belt characterized by thrusting and detachment folding. By analyzing the seismogenic mechanisms of thrusting and detachment folding in the frontal propagation belt during the Lushan Earthquake, we have inferred that the Lushan Earthquake was formed by thrusting and detachment folding in the frontal propagation belt. The seismogenic fault of the Lushan Earthquake was the Dayi Fault, which dips NW with a listric surface, and converges on the detachment surface. The detachment surface is the seismic source layer of the Lushan Earthquake.

Near-surface stress measurements in the Longmenshan fault belt after the 2008 Wenchuan Ms8.0 earthquake

Near-surface stress measurements in the Longmenshan fault belt after the 2008 Wenchuan Ms8.0 earthquake

Wenchuan Ms8.0 earthquake coseismic slip distribution inversion

By using GPS and gravity data before and after the Wenchuan Ms8.0 earthquake and combining data from geological surveys and geophysical inversion studies, an initial coseismic fault model is constructed. The dip angle changes of the fault slip distribution on the fault plane are inversed, and the inversion results show that the shape of the fault resembles a double-shovel. The Yingxiu–Beichuan Fault is approximately 330 km long, the surface fault dip angle is 65.1°, which gradually reduces with increasing depth to 0° at the detachment layer at a depth of 19.62 km. The Guanxian–Jiangyou Fault is approximately 90 km long, and its dip angle at the surface is 55.3°, which gradually reduces with increasing depth; the fault joins the Yingxiu–Beichuan Fault at 13.75 km. Coseismic slip mainly occurs above a depth of 19 km. There are five concentrated rupture areas, Yingxiu, Wenchuan, Hanwang, Beichuan, and Pingwu, which are consistent with geological survey results and analyses of the aftershock distribution. The rupture mainly has a thrust component with a small dextral strike–slip component. The maximum slip was more than 10 m, which occurred near Beichuan and Hanwang. The seismic moment is 7.84 × 1020 Nm (Mw7.9), which is consistent with the seismological results.

Cognitions and questions regarding crustal deformation and location forecasts of strong earthquakes

Using Global Positioning System(GPS) data to analyze the earthquake preparation characteristics of the Kunlun Ms8.1 and the Wenchuan Ms8.0 earthquakes, we review the main research developments of earthquake forecasting and the mechanisms of earthquake preparation using crustal deformation data from recent periods, and discuss the similarities and differences in the scientific approaches adopted by the Chinese and foreign scholars. We then analyze the deformation characteristics of earthquake preparation, with respect to slip and dip-slip faults. Our results show that, in order to understand the relationship between crustal deformation and earthquake preparation, research focus should be expanded from fault-scale to larger scale regions. Furthermore, the dynamic deformation characteristics associated with earthquake preparation must be considered as a multi-scale, spatial-temporal process, in order to obtain the necessary criteria for strong earthquake forecasts.

Mercury indicating inflow zones and ruptures along the Wenchuan Ms 8.0 earthquake fault

During the Wenchuan Fault Scientific Drilling Project, we determined the values of total mercury (HgT) and gaseous elemental mercury (GEM) from drilled cores and drilling mud, respectively. Geochemical analysis shows HgT values ranging from 0.24 to 6.45 ng/g for the Penguan complex and from 2.90 to 137.54 ng/g for T3 sediment. The average levels of HgT for the Penguan complex and T3 sediments are 1.81 ± 0.26 ng/g and 23.96 ± 4.80 ng/g, respectively. Major anomalous peaks of HgT appear at depth of 614,731,993 and 1,107 m, which correspond to the long-term high seismic activity during crustal deformation in response to tectonic stresses. Gaseous elemental mercury dissolved in drilling mud was also analyzed. We found fluid inflow zones with high GEM at depths of 590–750 m, suggesting that fluid-filled ruptures exist in the LMS fault zone. It indicates that mercury provides geochemical evidence for inflow zones and ruptures/fault zones in the Wenchuan Ms 8.0 earthquake fault.

Study of the attenuation relationship for the Wenchuan M s 8.0 earthquake

Prediction of peak ground acceleration is an essential element in engineering seismology and it has received great attention in last few decades. In this paper, a comprehensive database of the strong-motion records of the 2008 great Wenchuan Ms 8.0 earthquake is analyzed to investigate the seismic attenuation relationship and the directivity effects. In contrast to most existing seismic attenuation models, the proposed model considers explicitly the directivity effect which has primary influence on the magnitude of ground motion. Bayesian model updating is used to obtain the model parameters and the associated uncertainty. Comparative study is performed with the well-known Boore-Joyner-Fumal empirical formula. Results show that consideration of the directivity effect is vital in modeling the seismic attenuation relationship.

Tectonic Stress State Changes Before and After the Wenchuan Ms 8.0 Earthquake in the Eastern Margin of the Tibetan Plateau

AbstractCrustal tectonic activities are essentially the consequences of the accumulation and release of in situ stress. Therefore, studying the stress state near active faults is important for understanding crustal dynamics and earthquake occurrences. In this paper, using in situ stress measurement results obtained by hydraulic fracturing in the vicinity of the Longmenshan fault zone before and after the Wenchuan Ms 8.0 earthquake and finite element modeling, the variation of stress state before and after the Wenchuan Ms 8.0 earthquake is investigated. The results show that the shear stress, which is proportional to the difference between principal stresses, increases with depth and distance from the active fault in the calm period or after the earthquakes, and tends to approach to the regional stress level outside the zone influenced by the fault. This distribution appears to gradually reverse with time and the change of fault properties such as frictional strength. With an increase in friction coefficient, low stress areas are reduced and areas with increased stress accumulation are more obvious near the fault. In sections of the fault with high frictional strengths, in situ stress clearly increases in the fault. Stress accumulates more rapidly in the fault zone relative to the surrounding areas, eventually leading to a stress field that peaks at the fault zone. Such a reversal in the stress field between the fault zone and surrounding areas in the magnitude of the stress field is a potential indicator for the occurrence of strong earthquakes.

Heavy-rainfall-induced catastrophic rockslide-debris flow at Sanxicun, Dujiangyan, after the Wenchuan Ms 8.0 earthquake

This paper uses the catastrophic rockslide at Sanxicun village in Dujianyan city as an example to investigate the formation mechanism of a rapid and long run-out rockslide-debris flow of fractured/cracked slope, under the application of a rare heavy rainfall in July 2013. The slope site could be affected by the Wenchuan Ms 8.0 Earthquake in 2008. The sliding involved the thick fractured and layered rockmass with a gentle dip plane at Sanxicun. It had the following formation process: (1) toppling due to shear failure at a high-level position, (2) shoveling the accumulative layer below, (3) forming of debris flow of the highly weathered bottom rockmass, and (4) flooding downward along valley. The debris flow destroyed 11 houses and killed 166 people. The run-out distance was about 1200 m, and the accumulative volume was 1.9 × 106 m3. The rockslide can be divided into sliding source, shear-shoveling, and flow accumulative regions. The stability of this fractured rock slope and the sliding processes are discussed at four stages of cracking, creeping, separating, and residual accumulating, under the applications of hydrostatic pressure and uplift pressure. This research also investigates the safety factors under different situations. The double rheological model (F-V model) of the DAN-W software is utilized to simulate the kinematic and dynamic processes of the shear-shoveling region and debris flow. After the shear failure occurred at a high-level position of rock, the rockslide moved for approximately 47 s downward along the valley with a maximum velocity of 35 m/s. This is a typical rapid and long run-out rockslide. Finally, this paper concludes that the identification of the potential geological hazards at the Wenchuan mountain area is crucial to prevent catastrophic rockslide triggered by heavy rainfall. The identified geological hazards should be properly considered in the town planning of the reconstruction works.

Hot Spring Gas Geochemistry in Western Sichuan Province, China After the Wenchuan Ms 8.0 Earthquake

The chemical components, He, Ne, and C isotropic ratios of gas samples collected thrice from 32 hot springs in western Sichuan Province, southwestern China in June and October 2008 and June 2009 were investigated in order to discuss the relationship between hot spring gas geochemistry and the Wenchuan Ms 8.0 earthquake. The data showed that the 3^He/4^He and δ^(13)C_(CO2) values in spring gases in the Longmenshan fault (LMSF), Xianshuihe fault (XSHF), and Minjiang fault (MJF) zones increased obviously after the great earthquake. It was estimated that up to 62% of mantle helium contributed to the spring gas in the Kangding region based on the maximum 3He/4He (7.42×10^(-6)) in June 2008. Over time the mantle derived fluid contribution to the hot springs gradually decreased, but the crustal gas components: CO_2 and CH_4 derived from organic matter and radiogenic He increased. The gas geochemical data suggested that more mantle fluids migrated into the crust in western Sichuan Province after the Wenchuan Ms 8.0 earthquake.

Crustal deformation before the 2008 Wenchuan MS8.0 earthquake studied using GPS data

We used GPS velocities from approximately 700 stations in western China to study the crustal deformation before the Wenchuan MS8.0 earthquake. The processing methods included analyses of the strain rate field, inversion of fault locking and the GPS velocity profiles. The GPS strain rate in the E-W direction in the Qinghai-Tibet block shows that extensional deformation was dominant in the western region of the block (west of 92.5°E), while compressive deformation predominated in the eastern region of the block (from 92.5°E to 100°E). On a regional scale, the hypocentral region of the Wenchuan earthquake was located at the edge of an intense compression deformation zone of about 1.9×10−8/a in an east-west direction. The characteristic deformation in the seismogenic fault was compressive with a dextral component. The compression deformation rate was greater in the fault's western region than in its eastern region, and the strain accumulation was very slow on the fault scale. The results of a fault locking inversion show that the locking fraction and slip deficit was greater in the middle-northern section of the seismogenic fault than in the southern section. The GPS velocity profile before the Wenchuan earthquake shows that the compression deformation was smaller than the dextral deformation, which is asymmetrical with respect to the distribution of co-seismic displacement. These deformation characteristics should provide some clues to the Wenchuan earthquake which occurred in the later period of the earthquake cycle.

Interseismic and coseismic displacements of the Lushan Ms 7.0 earthquake inferred from leveling measurements

By using precise leveling data observed between 1985 and 2010 across the south section of the Longmenshan fault zone, and eliminating the coseismic displacements caused by the Wenchuan Ms 8.0 earthquake, the interseismic vertical deformation field was obtained. The result shows that the Lushan region, located between the Shuangshi–Dachuan fault (front range of the Longmenshan fault) and the Xinkaidian fault (south section of the Dayi fault), is situated in the intersection zone of positive and negative vertical deformation gradient zones, indicating that this zone was locked within 25 years before the Lushan earthquake. Based on leveling data across the rupture zone surveyed between 2010 and 2013, and by eliminating the vertical deformation within 3 years before the earthquake, the coseismic vertical displacement was derived. The coseismic vertical displacement for the benchmark DD35, which is closest to the epicenter, is up to 198.4 mm (with respect to MY165A). The coseismic displacement field revealed that the northwest region (hanging wall) moved upwards in comparison with the southeastern region (foot wall), suggesting that the seismogenic fault mainly underwent thrust faulting. By comparing the coseismic and interseismic vertical deformation fields, it was found that the mechanisms of this earthquake are consistent with the elastic rebound theory; the elastic strain energy (displacement deficit) accumulated before the Lushan earthquake was released during this quake.

Study on Relationship between Gravity Field and Earthquake in Eastern Margin of Tibetan Plateau

The gravity data from 1998 to 2012 in eastern margin of Tibetan Plateau was processed to analysis the relation between gravity field and earthquake activities. The results show that long period gravity change appears to be concentrated in regions with active faults, and short term variation may be affected by change of the active tectonic in eastern margin of Tibet plateau that due to energy release and adjustment after Wenchuan Ms 7.9 and Yushu Ms 7.1 earthquake. Specifically, gravity field alternated with positive and negative from 2010 to 2011 reflects viscoelastic properties of coseismic deformation after Wuchuan earthquake; a restorational change from negative to positive during 2011-2012 provides support crust tectonics exists strain accumulate and release stage in regions.

In‐Situ Stress Measurements and Slip Stability of Major Faults in Beijing Region, China

AbstractIn order to estimate the upper crustal stress state and slip stability of major faults after the Wenchuan Ms 8.0 earthquake, five boreholes, i.e., Pinggu, Shisanling, Xifengsi, Miyun, and Lisiguang Memorial Hall borehole ranging from 600 to 1000 m were drilled in Beijing region. Hydraulic fracturing method was used to conduct in‐situ stress measurements in these five boreholes, and stress monitoring equipment was installed at appropriate depth and monitoring stations were constructed. Those data derived from the in‐situ stress measurements reveal that the gradient coefficients of the maximum horizontal stresses and minimum horizontal stresses versus depth are 0.0328 and 0.0221, respectively; and the magnitudes of lateral pressure coefficient such as Kav, KHv and the ratios (KHh) of the maximum horizontal stress to minimum horizontal stress are consistent with previous studies. However, the ratios (μm) of the maximum horizontal shear stress to the average horizontal stress are relatively low. It is noted that the dominant direction of the maximum horizontal stress in Beijing region is ∼EW which is the same with the tectonic stress field of North China. However, sub‐regional stress field induced by faults which shows deviation from the orientation of North China, has been also revealed. In addition, the stress regimes inferred from in‐situ stress data imply that the maximum horizontal stress (σH) is the maximum principal stress (σ1), while the intermediate (σ2) and the minimum principal stress (σ3) show some variations. It is suggested that these variations may be caused mainly by the regional tectonic evolution and the activity of faults near the boreholes. Based on these characteristic parameters, we can point out that the maximum horizontal stress in the upper crust of Beijing region can be defined as dominant stress, while the horizontal shearing stress is relatively weak. Finally, slip stability of major faults in Beijing region is estimated according to Column friction criterion and Byerlee's law, together with the frictional coefficients being assumed to be 0.2, 0.4, 0.6 and 1.0. The results reveal that when the frictional coefficient ranges from 0.6 to 1.0, the recent crustal stress state of Beijing region does not yet reach the expected limit of fault slip. The stress values of the Xifengsi borehole will reach the expected limit when the frictional coefficient is weakened to 0.4, revealing that the Babaoshan fault stands chance of slipping; the Xiadian‐Mafang fault and the Huangzhuang‐Gaoliying fault will approach to the expected limit of slip and will stand chance of slipping under this hypothesis, but only with very low possibility. However, it should be pointed out that if the frictional coefficient is weakened to as low as 0.2, the major faults in Beijing region will be unstable and slip may occur under the present‐day crustal stress state. Conclusions in this paper would be of great significance for studies on tectonic stress field and seismogeology in Beijing region, even North China.

Analysis of Strong Ground Motion Characterization in the 2013 Lushan Earthquake

Lushan earthquake on 20th April, 2013 was another thrust fault earthquake occurred at Longmen Mountain Fault Zone after 2008 Wenchuan Ms8.0 earthquake. Based on ground motion attenuation model, this paper has chosen 45 strong motion records with rupture distance less than 200km, to analyze the hanging wall effect, topographic effect and rupture directivity effect of Lushan earthquake. The results show that hanging wall effect in Lushan earthquake was not obvious as 2008 Wenchuan earthquake; ground motion in mountain areas attenuated with increasing rupture distance more quickly than that in plain areas; rupture directivity effect is obvious for two components of horizontal ground motion, which are fault-perpendicular and fault-parallel components. PGA in the forward rupture area is larger than those in the backward rupture area. With the period increased, the gap between backward and forward rupture area become small, and finally PGD in backward rupture area is greater than those in the forward rupture area.

Dynamic analysis of the stabilized Wangjiayan landslide in the Wenchuan Ms 8.0 earthquake and aftershocks

This paper discusses the effect of the May 12, 2008 Wenchuan earthquake on a landslide where stabilization work had taken place and compares the results to sites without stabilization. Contours of shear strain increment and displacement values obtained using a FLAC3D simulation show that the Wangjiayan landslide, which had stabilization work, was stable before the Wenchuan earthquake. Simulations using input strong motion accelerations from the main shock recorded at Qingping station, within the main seismic fault zone, suggest a shear strain increment about 3,000 times more than it was before the main shock. Although the stability of the stabilized landslide decreased during the earthquake, the displacement values were very small. Modeled displacements suggest that the piles played a major role in keeping the slope from sliding. The anti-slide piles were subjected to high shaking acceleration. The strong ground motion records for a time range of 10 to 30 s show that the piles generally experienced accelerations in the range from 0.0 to 3.0 g, with a few spikes of about 5.0 g. Special observation stations were set up to monitor the slope motion at Qingchuan town, 150 km far from the Wangjiayan landslide, along the main central seismic fault zone, after the Wenchuan earthquake. The variation in acceleration with time and the FOS versus time of the Wangjiayan landslide were investigated using these acceleration records of the aftershocks as input data. The aftershock accelerations did not decrease the stability of the Wangjiayan landslide, but an amplification effect on the acceleration of the slope at different elevations was illustrated.

The Effect on the Nucleation and Failure of MS7.0 Lushan Earthquake Induced by the MS8.0 Wenchuan Earthquake

AbstractBased on calculations of both the post‐seismic Coulomb failure stress change and coseismic dynamic Coulomb failure stress change caused by the 2008 Wenchuan Ms8.0 earthquake, we simulate the their effects on the time of the nucleation and failure of the 2013 Lushan earthquake fault. Our calculation uses the rate‐ and state‐dependent friction law and viscoelastic relaxation in the middle‐lower crust and upper mantle. Furthermore, the triggering relationship between the Wenchuan and Lushan earthquakes is studied. The results show that the peak value of dynamic stress change on the Lushan earthquake fault induced by Wenchuan event is 0.127 MPa. With the 5‐year accumulation of the viscoelastic relaxation in the middle‐lower crust and upper mantle, the post‐seismic stress change on the Lushan earthquake fault is 0.025 MPa. The results indicate that the process of the nucleation and failure of the Lushan earthquake fault is affected by the post‐seismic Coulomb failure stress change, dynamic stress change and static stress change induced by the Wenchuan earthquake, and the effect of dynamic stress change is more obvious than others. Finally, we investigate the field of stress changes in the study area after the Lushan earthquake.

Block rotation: Tectonic response of the Sichuan basin to the southeastward growth of the Tibetan Plateau along the Xianshuihe‐Xiaojiang fault

AbstractGPS field and seismic data show that the southeastern margin of the Tibetan Plateau is tectonically and seismically active. This activity is due to the southeastward extrusion of the Chuandian fragment, a large crustal block rotating clockwise around the northeastern syntaxis of the Himalayas. The eastern boundary fault of this fragment is defined by the left‐lateral Xianshuihe‐Xiaojiang fault, which abruptly truncates the Sichuan basin of the Yangtze block. Our paper presents evidence indicating that the Sichuan basin experienced right‐lateral shear along its margin, including the Longmen Shan fault belt, as shown by the presence of a large number of interference deformation features, including S‐shaped and Z‐shaped folds and faults, aligned in an en echelon pattern. This study hypothesizes that the Sichuan basin experienced counterclockwise rotation, dragged by the left‐lateral movement along the Xianshuihe fault, and it is this rotation that was the underlying cause of the 12 May 2008 Wenchuan Ms 7.9 earthquake. During the rotation, the Sichuan basin decoupled along a subhorizontal decollement fault zone that developed along Triassic gypsum‐ and coal‐bearing rocks, at a mean depth of ~5000 m, below which the Paleozoic rocks experienced much more intense deformation than the overlying Mesozoic rocks, suggesting that the lower part of the basin experienced a larger‐scale rotation relative to the uppermost part of the basin. Based on thermal data from the western margin of the Sichuan basin and from along the Xianshuihe fault, the counterclockwise bending/rotation of the Sichuan basin initiated in late Cenozoic time (~13 Ma).

The Deep Geophysical Structure of the Middle Section of the Longmen Mountains Tectonic Belt and its Relation to the Wenchuan Earthquake

Abstract:Investigation of the deep geophysical structure of the Longmen Mountains tectonic belt and its relation to the Wenchuan Earthquake is important for the study of earthquakes. By using magnetotelluric sounding profiles of the Luqu–Zhongjiang and Anxian–Suining; seismic sounding profiles of the Sichuan Maowen–Chongqing Gongtan, the Qinghai Huashi Gorge–Sichuan Jianyang, and the Batang–Zizhong; and magnetogravimetric data of the Longmen Mountains region, the deep geophysical structure of the Songpan–Ganzi block, the western Sichuan foreland basin, and the Longmen Mountains tectonic belt and their relation was discussed. The eastward extrusion of the Qinghai–Tibet Plateau thrusts the Songpan–Ganzi block upon the Yangtze block, which obstructs the eastward movement of the Qinghai–Tibet Plateau. The Maoxian–Wenchuan, Beichuan–Yingxiu, and Anxian–Guanxian faults of the Longmen Mountains fault belt dip to northwest with different dip angles and gradually converge in the deeper parts. Geophysical structure suggests that an intracrustal low‐velocity, low‐resistivity, and high‐conductivity layer is common between the middle and upper crust west of the Longmen Mountains tectonic belt but not in the upper Yangtze block. The Sichuan Basin has a thick low‐resistance sedimentary layer on a stable high‐resistance basement; moreover, there are secondary paleohighs and depression structures at the lower part of the western Sichuan foreland basin with characteristic of high magnetic anomalies, whereas the Songpan–Ganzi block has a high resisitivity cover of upper crust and continues to a low‐resistance layer. Considering the Longmen Mountains tectonic belt as the boundary, there are Bouguer gravity anomalies of “one belt between two zones.” Thus, we infer that there is a corresponding relation between the inferred crystalline basement of the Songpan block and the underlying basin basement of the Longmen Mountains fault belt. Furthermore, there may be an extensive ancient Yangtze block, which is west of the Ruoergai block. In addition, the crust–mantle ductile shear zone under the Longmen Mountains tectonic belt is the main fault, whereas the Beichuan–Yingxiu and Anxian–Guanxian faults at the surface are earthquake faults. The Wenchuan Ms 8.0 earthquake might be attributed to the collision of the Yangtze block and the Qinghai–Tibet Plateau. The eastward obduction of the eastern edge of the Qinghai–Tibet Plateau and eastward subduction of its deeper part under the influence of the collision of the Indian, Pacific, and Philippine Plates with the Eurasia Plate might have caused the Longmen Mountains tectonic belt to cut the Moho and extend to the middle and upper crust; thus, creating high stress concentration and rapid energy release zone.

Automatic recognition of seismic intensity based on RS and GIS: a case study in Wenchuan Ms8.0 earthquake of China.

In recent years, earthquakes have frequently occurred all over the world, which caused huge casualties and economic losses. It is very necessary and urgent to obtain the seismic intensity map timely so as to master the distribution of the disaster and provide supports for quick earthquake relief. Compared with traditional methods of drawing seismic intensity map, which require many investigations in the field of earthquake area or are too dependent on the empirical formulas, spatial information technologies such as Remote Sensing (RS) and Geographical Information System (GIS) can provide fast and economical way to automatically recognize the seismic intensity. With the integrated application of RS and GIS, this paper proposes a RS/GIS-based approach for automatic recognition of seismic intensity, in which RS is used to retrieve and extract the information on damages caused by earthquake, and GIS is applied to manage and display the data of seismic intensity. The case study in Wenchuan Ms8.0 earthquake in China shows that the information on seismic intensity can be automatically extracted from remotely sensed images as quickly as possible after earthquake occurrence, and the Digital Intensity Model (DIM) can be used to visually query and display the distribution of seismic intensity.

Insights gained from the seismicity around the Zipingpu reservoir before the Wenchuan MS8.0 earthquake

The 2008 Wenchuan MS8.0 earthquake occurred on Longmenshan fault zone (LMSF), which is at the eastern margin of the Tibetan plateau. The epicenter is near the Shuimogou earthquake swarm, which was thought to be triggered by the Zipingpu reservoir after its impounding in 2004. People have speculated that the large earthquake was triggered by the water filling of the reservoir. To figure out the role of the Zipingpu reservoir on the earthquake, the local seismicity recorded by the Zipingpu local seismic network during the period from 31 July 2004 to 11 May 2008 were analyzed in detail. The distribution of hypocenters showed that most earthquakes occurred on Yingxiu-Beichuan fault (YBF) in the reservoir area with hypocenters depth less than 10 km, which is a major source fault of the Wenchuan earthquake. Useful information on fault geometry in the depth was also obtained. The spatial-temporal distribution of hypocenters demonstrated clear migration pattern that indicated pore-pressure diffusion, it also showed a hydraulic diffusivity (D) of 0.7 m 2 /s. Previous experiments show the existence of the synergism process of the fault under a meta-instability state before fault sliding. It enhances the stress on the stronger portion of the fault and the synergism degree by reducing strength of the weak portions and by increasing the total length of weak portions. According to this view, the pore pressure diffusion by water filling of Zipingpu reservoir increased the total length of weak portions and enhanced the stress at the focal.