Xianshuihe Fault Research Articles

Numerical simulation on the influences of Wenchuan earthquake on the surrounding faults

On 12 May 2008, the devastating Wenchuan earthquake struck the Longmenshan fault zone, which comprised the eastern margin of the Tibetan Plateau, and this fault zone was predominantly a convergent boundary with a right-lateral strike-slip component. After such a large-magnitude earthquake, it was crucial to analyze the influences of the earthquake on the surrounding faults and the potential seismic activity. In this paper, a complex viscoelastic model of western Sichuan and eastern Tibet regions was constructed including the topography. Based on the findings of co-seismic static slip distribution, we calculated the stress change caused by the Wenchuan earthquake with the post-seismic relaxation into consideration. Our preliminary results indicated that: (1) The tectonic stressing rate was relatively high in Kunlun mountain pass-Jiangcuo, Ganzi-Yushu, Xianshuihe and Zemuhe faults; while in the east Kunlun and Longriba was medium; also the value was less in the Minjiang, Longmenshan, Anninghe and Huya faults. As to the Longmenshan fault, the value was 0.28×10−3 MPa/a to 0.35×10−3 MPa/a, which is coincident with the previous long recurrence interval of Wenchuan earthquake; (2) The Wenchuan earthquake not only caused the Coulomb stress decrease in the source region, but also the stress increase in the two terminals, especially the northeastern segment, which is comparatively consistent with the aftershock distribution. Meanwhile, the high concentration areas of the static slip distribution were corresponding to the Coulomb stress reductions; (3) The Coulomb stress change caused by Wenchuan earthquake showed significant increase on five major faults, which were northwestern segment of Xianshuihe fault, eastern Kunlun fault, Longriba fault, Minjiang fault and Huya fault respectively; also the Coulomb stress on the fault plane of the Yushu earthquake was faintly increased; (4) We defined the recurrence interval as the time needed to accumulate the magnitude of the stress drop, and the recurrence interval of Wenchuan earthquake was estimated about 1 714 a to 2 143 a correspondingly.

Recurrent Morphogenic Earthquakes in the Past Millennium along the Strike-Slip Yushu Fault, Central Tibetan Plateau

Abstract The magnitude ( M w ) 6.9 ( M s 7.1) Yushu earthquake of 14 April 2010 ruptured the Yushu fault of the preexisting strike-slip Ganzi–Yushu fault zone, which is an extension of the Xianshuihe fault. The earthquake resulted in widespread damage in high mountain regions of the central Tibetan Plateau. Field surveys and analyses of an excavated trench and outcrop reveal that three morphogenic earthquakes have occurred on the Yushu fault in the past millennium. Paleoseismic evidence, historical records, and radiocarbon age data show that (1) the penultimate large-magnitude earthquake (i.e., prior to the 2010 Yushu earthquake) occurred within the past 570 yr, corresponding to the 1738 M 7.1 earthquake; and (2) the third most recent event occurred between A.D. 650 and A.D. 1100, suggesting a recurrence interval of 450–680 yr for large earthquakes associated with surface rupture of the Yushu fault in the past millennium. An average slip rate of ∼2–5 mm/yr is estimated for the Yushu fault. These results are consistent with those obtained from long-term geological evidence and GPS observations. Most of the strain energy on the Ganzi–Yushu fault zone, resulting from northeastward motion of the Tibetan Plateau in response to north–south shortening of the plateau due to ongoing northward penetration of the Indian Plate into the Eurasian Plate, is released as seismic slip.

Characteristics of strong earthquake evolution around the eastern boundary faults of the Sichuan-Yunnan rhombic block

Based on the existing materials of fault segmentation, characteristic earthquakes, and their empirical relationships, we calculated the parameters of the fault segments, such as length, width, magnitudes of characteristic earthquakes, etc. Constrained by GPS velocity field, the slip rates of these fault segments in depth were inversed using the 3-D half-space elastic dislocation model. As not all of the recurrence periods and co-seismic displacements of characteristic earthquakes are known, we selected the fault segments with these two parameters known and calculated the accumulation rate of average co-seismic displacement, which shows the faults’ slip rate in seismogenic layer. Then, the slip rate in depth was compared with that in seismogenic layer, the relationship between them was obtained, and this relationship was used to get the recurrence periods and co-seismic displacements of all fault segments. After the studies above, we calculated the co-seismic deformation field of all the earthquakes larger than M s6.8 from AD 1700 one by one and inversed the potential displacement in the co-seismic deformation field. Then, we divided the potential displacement by the slip rate from GPS inversion to get the influences of these fault segments, added the influences into the elapsed time of the characteristic earthquakes, and obtained the earthquake hazard degree of all the segments we studied in the form of the ratio of elapsed time to recurrence period; so, we name the ratio as the Impending Earthquake Risk (IER). Historical earthquake cases show that the fault segment is in safety when the IER is less than 1 but in danger after the IER becomes larger than 1. In 2009, the IER is larger than 1 on the following segments, 1.35 on the Tagong segment of Xianshuihe fault, 1 on the Menggu-Dongchuan segment, 1.04 on the Dongchuan-Xundian segment, and 1.09 on the Yiliang-Chengjiang segment of Xiaojiang fault.

Young, active conjugate strike–slip deformation in West Sichuan: evidence for the stress–strain pattern of the southeastern Tibetan Plateau

The active kinematics of the eastern Tibetan Plateau are characterized by the southeastward movement of a major tectonic unit, the Chuan-Dian crustal fragment, bounded by the left-lateral Xianshuihe–Xiaojiang fault in the northeast and the right-lateral Red River–Ailao Shan shear zone in the southwest. Our field structural and geomorphic observations define two sets of young, active strike–slip faults within the northern part of the fragment that lie within the SE Tibetan Plateau. One set trends NE–SW with right-lateral displacement and includes the Jiulong, Batang, and Derong faults. The second set trends NW–SE with left-lateral displacement and includes the Xianshuihe, Litang, Xiangcheng, Zhongdian, and Xuebo faults. Strike–slip displacements along these faults were established by the deflection and offset of streams and various lithologic units; these offsets yield an average magnitude of right- and left-lateral displacements of ∼15–35 km. Using 5.7–3.5 Ma as the time of onset of the late-stage evolution of the Xianshuihe fault and the regional stream incision within this part of the plateau as a proxy for the initiation age of conjugate strike–slip faulting, we have determined an average slip rate of ∼2.6–9.4 mm/year. These two sets of strike–slip faults intersect at an obtuse angle that ranges from 100° to 140° facing east and west; the fault sets define a conjugate strike–slip pattern that expresses internal E–W shortening in the northern part of the Chuan-Dian crustal fragment. These conjugate faults are interpreted to have experienced clockwise and counterclockwise rotations of up to 20°. The presence of this conjugate fault system demonstrates that this part of the Tibetan Plateau is undergoing not only southward movement, but also E–W shortening and N–S lengthening due to convergence between the Sichuan Basin and the eastern Himalayan syntaxis.

Implications from the kinematic pattern of the Longmenshan region

The boundaries of a plate are usually correlated with the locations of major already-known faults, which are well defined by local geology, regional seismicity. It facilitates for the case of the Tibetan Plateau with boundaries along the large scale faults, such as the Altyn Tagh fault. Nevertheless, for smaller scale regions, it might become much more difficult to determine the block boundaries especially when lacking of neither active faults nor historical earthquake records. The geodetic including GPS observations may become useful tools to determine the potential faults. For the purpose of obtaining the kinematic pattern across the Longmenshan fault, we performed K-means clustering analysis for 145 GPS sites in the Longmenshan region (longitude 99°–110°E, and latitude 28°–35°N) and obtained a modified ten-block system almost best-fits the GPS velocity fields. Rigid rotations of these blocks are well determined, and the velocity filed can be explained well by rigid block motion and fault slip across block boundaries. The mean residual velocity is 1.26 mm/year, less than the 2–3 mm/year uncertainty of the GPS data with 90% confidence. All the block boundaries, except for these of AH/LMS and GZ/KD, are along major faults. The AH/LMS boundary is regarded to be along Longriba fault and translates ~ 5.5 mm/year right-lateral rates and reduces the southeastward velocity of the plateau as shortening rate of < 2.0 mm/year and right-lateral slip rate of ~ 1.6 mm/year across the Longmenshan fault. The shortening rate across Minjiang fault is of ~ 2.0 mm/year. Even though there are neither already-known active faults nor active historical seismicity along GZ/KD boundary, the right-lateral slip rate is of ~ 5.6 mm/year. We inferred it as an undiscovered active fault due to the geomorphic feature and the focal mechanism of earthquakes occurred at ~ 30°N. However, it needs to be validated in the future. The left-lateral slip rates along the Xianshuihe, Anninghe and Kunlun faults are ~ 9.0 mm/year, ~ 6.9 mm/year and ~ 2.8 mm/year, respectively. The results indicate that the motion of the eastern Tibetan plateau has been accommodated significantly through the bounding faults, such as the Xianshuihe fault, Longriba fault, Kunlun fault and GZ/KD boundary. Further studies combining observations from surface surveys and depth detections are required and may provide insights into the crustal (i.e. deep into ~ 20 km or deeper) deformation pattern of the Longmenshan region and the cause of the Wenchuan earthquake.

Partitioning of localized and diffuse deformation in the Tibetan Plateau from joint inversions of geologic and geodetic observations

The spatial complexity of continental deformation in the greater Tibetan Plateau region can be defined as the extent to which relative motion of the Indian and Asian plates is partitioned between localized slip on major faults and distributed deformation. Potency rates provide a quantitative metric for determining the magnitudes of diffuse and on-fault crustal deformation, which are proportional to strain rates within crustal micro-plates and fault slip rates, respectively. We simultaneously estimate micro-plate rotation rates, interseismic elastic strain accumulation, fault slip rates on major structures, and strain rates within 24 tectonic micro-plates inferred from active fault maps in the greater Tibetan Plateau region using quasi-static block models constrained by interseismic surface velocities at 731 GPS sites and 9 Holocene–Late Quaternary geologic fault slip rates. The joint geodetic–geologic inversion indicates that geologic slip rates are kinematically consistent with differential micro-plate motions. Estimated left-lateral slip rates on the Altyn Tagh, west-central Kunlun, and Xianshuihe faults are relatively homogeneous along strike (~ 11.5, 10.5, and 11 mm/yr, respectively) while segmentation of the eastern Kunlun fault by the intersecting Elashan and Riyueshan faults results in a decreased slip rate, consistent with geologic observations. The fraction, ϕ, of total potency rate associated with intrablock strain, uncorrected for observational noise, ranges from 0.27 in the Himalayan Range block to 0.91 in the Aksai Chin block. Monte Carlo simulations are used to quantify the likelihood that internal deformation is statistically distinguishable from the uncertainties in geodetic velocities. These simulations indicate that internal block deformation is statistically significant only within the Himalayan Range Front (where internal deformation accounts for ϕ ID = 0.11 of block potency rate budget), west-central plateau ( ϕ ID = 0.68), Ganzi-Yushu/Xianshuihe (0.58), Burma (0.09), Jiali (0.38), and Aksai Chin (0.68) blocks. In the other 18 tectonic micro-plates within the plateau region, estimated internal block potency is not currently distinguishable from the expected contribution of observational noise to residual velocities. Of the total potency budget within the Tibetan Plateau, 85% is taken up by slip on major faults, with the remaining 15% accommodated by internal processes at sub-block scale distinguishable from observational noise. The localization of the majority of plate boundary activity is also supported by the spatial distribution of modern and historical crustal earthquakes. Sixty-seven percent of the total moment released by earthquakes in the CMT catalog and 90% of historical moment since 1900 have been released within 25 km of the major faults included in the block model, representing only 12% of the characteristic half-block length scale of ~ 215 km. The localization of deformation inferred from geologic, geodetic, and seismic observations suggests that forces applied to tectonic micro-plates drive fault system activity at the India–Asia collision zone over decadal to Quaternary time scales.

Crustal thicknesses and Poisson's ratios in the eastern Tibetan Plateau and their tectonic implications

Crustal thicknesses and Poisson's ratios in the eastern Tibetan Plateau were estimated using the H‐k stacking method on teleseismic receiver functions. The data came mainly from the regional seismic networks in 2006–2008. Results show that the crust thickens from east to west, with a thickness difference of ∼30 km. A sharp change of crustal thickness from 41.5 to 52.5 km is observed across the Longmen Shan. Crustal thickness predictions based on the Airy‐type isostasy are consistent with the estimations from H‐k stacking, implying that the topography is approximately compensated. The study area can be divided into three parts according to the Poisson's ratios: (1) the northern Songpan‐Garze terrane and the western Qinling orogenic belt with low ratios (ν ≤ 0.26); (2) the southern Songpan‐Garze terrane, the Sanjiang fold system, and the Sichuan Basin with high‐to‐intermediate ratios (0.27 ≤ ν &lt; 0.30); and (3) the southwestern Yangtze platform with low to intermediate ratios (ν ≤ 0.27). Joint interpretation of the low average Vp values from deep seismic sounding profiles and the low Vp/Vs ratios from receiver function analysis favors the presence of felsic crust rather than significant volumes of partial melts in the northern Songpan‐Garze terrane and the western Qinling orogenic belt. The junction of the Longmen Shan fault zone, the Xianshuihe fault zone, and the Anninghe fault zone has an ultrahigh Poisson's ratio (ν ≥ 0.30), which could be considered as an evidence of local partial melt. We propose that the eastward movement of upper and middle crustal material in the southern Songpan‐Garze terrane is obstructed by the more rigid Sichuan Basin, resulting in strain accumulation along the Longmen Shan fault zone.

Pn-wave velocity and anisotropy of the western Sichuan and Longmen Mountain region, China

Pn-wave velocity and anisotropy beneath the western Sichuan and adjacent region are inverted by a tomography method using arrival data from earthquakes recorded by the Sichuan Earthquake Network. It is followed by an analysis of the uppermost mantle structure of the Longmen Mountain region and the deep tectonics of the Ms8.0 earthquake in Wenchuan. Our results reveal a prominent velocity contrast across the Longmenshan fault: low velocities in the Songpan-Ganzi mountain range and high velocities in the Sichuan basin. They indicate that the Songpan-Ganzi mountain range is underlain by a weak uppermost mantle, which is easy to create ductile flow, whereas the Sichuan basin has a mechanically strong uppermost mantle. These features reflect a difference of the lithospheric mantle beneath tectonically active mountain range and stable basin. The boundary between the high and low velocities is coincident with the Longmenshan fault. Bordered by Wenchuan, the northern part of the boundary is parallel to the fault belt and aftershock distribution, but the southern part of the boundary deviates from the fault belt and is bent toward the Sichuan basin. The Ms8.0 earthquake in Wenchuan is located at the flexural part between them, where a strong heterogeneity is shown in the velocity variation across and parallel to the boundary. This area is reasonably believed as a potential location for regional stress accumulation and release in the Longmen Mountain region, reflecting a prominent character in the deep structure around Wenchuan. Pn-wave anisotropy reveals the mantle flow that is consistent with the eastward movement of the Tibetan Plateau. This process provided deep dynamic sourse for tectonic deformation in the Longmen Mountain region and for the occurrence of the Wenchuan Ms8.0 earthquake, but it is clearly affected by the strong lithosphere of the Sichuan basin. There is no correlation between the Pn-wave anisotropy and the Longmenshan fault, hence, this fault system is only restricted within the crust. However, the mantle flow beneath the southeastern Tibetan Plateau shows a clockwise rotation along the Xianshuihe fault. We infer that these two fault systems may be controlled by different deep dynamics.

Application of ENVISAT ScanSAR interferometry to long-range fault creep: a case study of the Xianshuihe fault in the eastern Tibetan margin area

The Xianshuihe fault of Sichuan province, southwest China, is a highly active strike-slip fault approximately 350 km long. Previous studies have described left-lateral slip of up to 10–17 mm/year on the Xianshuihe fault during recent decades, as indicated by geological criteria and GPS observations. Satellite InSAR observations provide an alternative effective technique used to measure the crustal deformation on the active fault. In this article, we present ScanSAR interferometry applied to ENVISAT WSM ASAR data to produce a deformation map of about 400 × 400 km over the Xianshuihe fault zone and the Garzê-Yushu fault zone. The preliminary stacked deformation results of six WS–WS interferograms indicated a northeast–southwest deformation trend nearly perpendicular to the two fault zones and demonstrated the potential to monitor such wide-ranging deformation using ENVISAT WSS data.

Pattern of stress change and its effect on seismicity rate caused by M s8.0 Wenchuan earthquake

We use five published source models to calculate the Coulomb failure stress changes induced by the M s8.0 Wenchuan earthquake, and analyze the association between stress changes and the subsequent earthquakes. Based on the analysis of uncertainties resulting from source models, we determine the stress changes on nearby faults caused by the Wenchuan earthquake. Moreover, we focus on the seismicity rate change as a function of time on every fault under the influence of stress changes. The results indicate that the spatial distributions of aftershocks correlate well with the regions where stress is calculated to increase using the related models. The largest lobes of dropped stress lie in the west and east sides of source fault. The largest lobes of increased failure stress close to southern and northern ends of the source fault extend into the whole source failure plane. In addition, another region of increased stress lies in the Wenchuan-Yingxiu zone close to the southern segment of source fault, where a large number of aftershocks have occurred. And subsequent earthquakes seem to extend to even more remote distances; therefore, this area also has a high risk of seismic hazard. We find that the positive stress changes on nearby faults imposed by the Wenchuan earthquake produce an encouraging effect on seismicity rate. The effect is most significant on the Pengxian-Guanxian fault and Qingchuan fault, the value of seismicity rate maintains two times greater than the value before the mainshock for the next hundred years on these faults, and the time needed for the aftershock rate to recover to the pre-mainshock seismicity rate can reach up to 800–900 yr. The influence is not significant on the western Qinling fault, the Longquanshan fault, the Xianshuihe fault, the Yulongxi fault, the Anninghe fault, the Minjiang fault, and the Aba fault. Compared with the seismicity rate on these faults before the mainshock, the aftershock rate is raised by less than two times, and the time of perturbation duration is not long. The stress changes on the Fubianhe fault and Huya fault are negative, which reduce the seismicity for the next thousand years. Under the influence of stress changes caused by the Wenchuan earthquake, both Pengxian-Guanxian fault and Qingchuan fault have a high risk of earthquake occurrence.

The active fault belts in eastern Tibet margin inferred using magnetotellurics

A magnetotelluric (MT) sounding has been carried out in the eastern margin of the Tibetan plateau. The survey line is about 145 km long, trending in NEE direction and crossing the Daliangshan block in the eastern edge of the Tibetan plateau. The field measurements acquired effective data of 68 sites. Through data processing and a 2-D inversion with consideration of topography, a 2-D electrical structure model of crust and upper mantle was constructed. The structure reveals that there is a deep electrical boundary between the Daliangshan block in the west and Sichuan block in the east. West to the boundary, the crust has a relatively low resistivity with respect to the east and can be divided into three layers, the middle layer has low-resistivity with a minimum of 3-10 W·m, presumably associated with partial melt and/or salty fluids. Beneath the intersection area of the Anninghe fault, the Xianshuihe fault and the Longmenshan fault, which the MT profile crosses, the faults are separated into upper and lower sections. The upper section exhibits a nearly vertical low-resistivity zone in the upper crust, and the lower section manifests an electrical boundary in the lower crust and upper mantle. Other faults in the Daliangshan block are either nearly vertical low-resistivity zones or electrical boundaries. It is suggested that the formation of the low-resistivity layer in the middle crust is associated with the southeastward motion of the eastern margin of the Tibetan plateau, clockwise rotation of the Chuandian (Sichuan-Yunnan) block, and the westward obstruction from the Sichuan block in Huanan terrain. Seismicity, including the M 8.0 Wenchuan earthquake in the study area, is discussed. Eastern margin of Tibetan plateau. Active fault. Electrical structure. Channel flow. Wenchuan Earthquake.

Influence of fault geometry and fault interaction on strain partitioning within western Sichuan and its adjacent region

There are several major active fault zones in the western Sichuan and its vicinity. Slip rates and seismicity vary on different fault zones. For example, slip rates on the Xianshuihe fault zone are higher than 10 mm/a. Its seismicity is also intense. Slip rates on the Longmenshan fault zone are low. However, Wenchuan Ms8.0 earthquake occurred on this fault zone in 2008. Here we study the impact of fault geometry on strain partitioning in the western Sichuan region using a three-dimensional viscoelastoplastic model. We conclude that the slip partitioning on the Xianshuihe-Xiaojiang fault presents as segmented, and it is related to fault geometry and fault structure. Slip rate is high on fault segment with simple geometry and structure, and vice versa. Strain rate outside the fault is localized around the fault segment with complex geometry and fault structure. Strain partitioning on the central section of the Xianshuihe-Xiaojiang fault zone is influenced by the interaction between the Anninghe-Zemuhe fault and the Daliangshan fault zone. Striking of the Longmenshan fault zone is nearly orthogonal to the direction of eastward extrusion in the Tibetan Plateau. It leads to low slip rate on the fault zone.

Static Coulomb stress changes on faults caused by the 2008 Mw 7.9 Wenchuan, China earthquake

The 12 May 2008 Mw 7.9 Wenchuan earthquake has changed the regional tectonic stress field significantly. It is important to know how such a change affects the tectonic loading processes of the faults and therefore the seismic potential of the region. Utilizing the slip distribution of the Wenchuan earthquake as the driving source we compute the changes of Coulomb failure stress ( CFS) on the neighborhood faults assuming a friction coefficient of 0.4. Our results show that the CFSs have increased on average 1.6 × 10 4 and 1.8 × 10 4 Pa at the SW and NE ends of the Longmen Shan, 2.6 × 10 4 Pa at the western segment of the Qinling Southern Frontal, 2.0 × 10 4 Pa at the northernmost part of the Longriba, 1.4 × 10 4 Pa at the southernmost part of the Xianshuihe, 1.3×10 4 Pa at the southeast end of the East Kunlun, 9.0 × 10 3 Pa at the southernmost part of the Minjiang, 2.9 × 10 3 Pa at the Baiyu, and 3.5 × 10 3 Pa at the Longxian–Baoji faults. Recurring times of these faults are advanced, about 39 and 230 years for the SW and NE segments of the Longmen Shan, ∼ 5.5 years toward the east end of the East Kunlun, and ∼ 1.6 years for the SE segments of the Xianshuihe faults. The quake also relaxed the CFS significantly on most part of the Minjiang, southwest segment of the Longriba, northwest segment of the Xianshuihe, southeast segment of the Yushu–Maqu, north part of the Xiaojinhe, Daliangshan, and Anninghe faults, with the CFS reductions ranging on average of 1.8 × 10 4, 2.5 × 10 4, 7.1 × 10 3, 2.4 × 10 3, 1.0 × 10 3, 1.0 × 10 3, and 1.6 × 10 3 Pa, respectively. Recurring times of these faults are delayed, up to 185 years for the south segment of the Minjiang, up to 18 years for the northeast of the Longriba, up to 9.8 years for the West Qinling Northern Frontal, and up to 2.7 years for the northwest segment of the Xianshuihe faults, respectively. We test sensitivity of the changes of CFS to variations of apparent fiction coefficient μ′, and find that the senses of CFS change are almost invariant with μ′ except at the Minjiang, Longriba, and Qinling Southern Frontal faults, where large normal stresses on faults induced by the Wenchuan earthquake could play an important role in manifesting CFS change when μ′ is large. The CFS changes vary slowly along seismogenic depth on most of the receiver faults.

Stress evolution and fault interactions before and after the 2008 Great Wenchuan earthquake

The 12 May 2008 Wenchuan earthquake (Mw 7.9) ruptured ∼ 300 km of the Longmen Shan fault, claiming ∼ 90,000 lives and devastating many cities in the Sichuan province, China. The coseismic stress changes due to the Wenchuan earthquake have been studied in kinematic models using the inferred coseismic fault slips, but the cause of the fault slips, the impact of other large earthquakes, and the mechanical interactions between the faults in eastern Tibet are uncertain. Here we explore these issues using a three-dimensional viscoelastoplastic dynamic model that calculates the regional stresses from tectonic and topographic loading, and simulates earthquakes and their stress perturbations. Our calculated coseismic changes of the Coulomb stress associated with the Great Wenchuan earthquake are similar to those in previous models. However, we show that the cumulative Coulomb stress changes, hence the implied earthquake risks, are significantly different when previous large earthquakes in the region are included in the model. Particularly, we show that in spite of stress increase from the Wenchuan earthquake, the southeastern segments of the Xianshuihe fault stay in a stress shadow because of the stress release by six M ≥ 6.9 events in this part of the Xianshuihe fault since 1893. We also found that interseismic locking on the Xianshuihe fault can increase the loading rate on the Longmen Shan fault by up to ∼ 50 Pa/year.

Stress, strain and earthquake activity

There are 13 papers in this special issue on stress field, crustal deformation and seismicity. The great Wenchuan earthquake is a grievous disaster, but Chinese scientists are trying to learn more from the event in order to understand better the physics of earthquakes for future hazard mitigation planning. Rapid development of GPS technology provide the possibility in the first time in history to monitor the crustal deformation in a wide range and high accuracy, however, it is an important lesson learned from the Wenchuan earthquake, that GPS deformation alone is insufficient for forecast of major earthquakes. The Longmenshan fault zone is of low strain rate and relatively low historical seismicity in comparison with a number of strike slip faults in the western Sichuan region, such as the Xianshuihe fault, Xiaojiang fault, etc. However, the Wenchuan earthquake broke out in Longmenshan fault zone as a surprise to most Chinese seismologists. Guo et al (2009) happened to make hydrofracture stress measurement at the northeastern part of the Longmenshan fault one week before the great Wenchuan earthquake, and was able to make repeated measurement after the earthquake. They found stresses were high close to the fault zone than those from boreholes farther away from the fault zone before the main quake, indicating that stress may accumulate and exceed strength of rock mass even though the strain rate is low. In contrary, high strain rate may not necessarily lead to great earthquakes if stress can be rapidly released from a number of media or strong earthquakes as well as slow or quiet earthquakes. Therefore, this issue of stress field, crustal deformation and seismicity emphasis a new point of view to investigate both strain and stress in relation to earthquake activity. Reliable stress measurement is expected to become a priority in earthquake related network construction. Chi reported deployment of 40 sets YRY-4 four -component borehole strainmeters in China as part of the National “Digital Seismic Observation Network Project”. The new generation instrument has a strain resolution of 5×10 −11 , response frequency range of DC-20Hz, and can record solid tidal strain clearly. The four-component of strainmeter provide a way to cross check reliability of the measurements, since three components are enough to determine the horizontal strain tensor components. The author was trying to show several pre-seismic strain anomalies had been observed before large earthquakes. I hope the raw data can be used to calculate eθ, eφ, eθφ and σθ, σφ, σθφ in the future to help us understand the long term stress tensor variations. There are three papers discussing different aspects

Initiation of slip along the Xianshuihe fault zone, eastern Tibet, constrained by K/Ar and fission-track ages

The Xianshuihe fault zone (XFZ) forms a boundary that accommodates crustal movement eastwards from central Tibet. The lack of well-defined time constraints has hampered the reconstruction of the geometric and kinematic evolution of the fault zone, and inhibited the development of a consistent regional tectonic model. New geochronological investigations of mica K/Ar and apatite fission-track ages on the Ganzi–Yushu segment of the XFZ indicate that fault activity started ca. 13 Ma before present, which considerably precedes the 5 Ma initiation of offset on the Xiaojiang fault (XjF) segment. Different initiation times for different fault segments clearly demonstrate that the geometric and kinematic evolution of the XFZ can be divided into two periods. The XFZ cut through the Dangjiang, Ganzi, and Gongga Shan areas, and reached the Qinghe–Yanyuan area during an early stage, from ca. 13 to 5 Ma, as a boundary fault with lateral mass transfer from the interior to southeast Tibet. At the southern edge of the XFZ, the movement took advantage of the southern segment of the Longmen Shan thrust fault – the Jinhe–Qinghe thrust fault. During the late stage, from 5 Ma to present, the fault zone passed through the Yushu, Ganzi, and Gongga Shan areas, then offset the Longmen Shan thrust fault belt and reached the Kunming area. The Jinhe–Qinghe fault was abandoned in the later period as the southern part of the XFZ, and the XjF became active because of continuous clockwise rotation of the crust around the eastern syntaxis.

Balance of seismic moment in the Songpan-Ganze region, eastern Tibet: Implications for the 2008 Great Wenchuan earthquake

The 12 May 2008 Wenchuan earthquake (Ms 8.0) occurred on the Longmen Shan fault zone, where slip rates are low and earthquakes are infrequent in comparison with other major fault zones in the Songpan-Ganze region, eastern Tibet. We have investigated the evolution of strain energy along major faults in this region by comparing the accumulation and release of seismic moment. First, we calculated the slip rates on the Longmen Shan and other major faults in the region using a three-dimensional regional-scale block model, constrained by the latest GPS data. On the Longmen Shan fault, the predicted right-lateral and dip-slip rates are respectively 1.7 ± 0.8 mm/year and 1.2 ± 1.0 mm/year along the southwestern segments, and 1.4 ± 1.1 mm/year and 3.3 ± 1.3 mm/year on the northeastern segments. These slip rates are one order of magnitude lower than those on the Xianshuihe fault and other faults in the region. Second, using the earthquake catalog, we estimated the scalar moment released on major faults in the Songpan-Ganze region between 1879 and 2007. The released seismic moment was ~ 63% of the regional scalar moment accumulated during this period. The moment deficits were found mainly on the western Xianshuihe and eastern Kunlun faults. The eastern Xianshuihe fault has been a focus of studies because of the high slip rates and frequent earthquakes, but a sequence of major quakes there since 1893 has reduced the moment deficit to 3.68 × 10 19 N m, barely enough for a Mw 7.0 event. It takes the Longmen Shan fault more than 1000 years to accumulate the seismic moment (1.15 × 10 21 N m) released during the 2008 Great Wenchuan earthquake, hence we conclude that a repeat of great earthquakes on the ruptured segment of the Longmen Shan fault is unlikely in the next few hundred years, but the unruptured southwestern segment of the Longmen Shan fault is capable of producing a Mw 7.7 earthquake in the next 50 years.

Influence of 2008 Wenchuan earthquake on earthquake occurrence trend of active faults in Sichuan-Yunnan region

The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults’ slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.

Stress changes on major faults caused by M w7.9 Wenchuan earthquake, May 12, 2008

On May 12, 2008, a magnitude 7.9 earthquake ruptured the Longmenshan fault system in Sichuan Province, China, collapsing buildings and killing tens of thousands people. As predicted, aftershocks may last for at least one year, and moreover, large aftershocks are likely to occur. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settling people as to avoid future disasters. It is demonstrated that the redistribution of stress induced by an earthquake should trigger successive seismic activity. Based on static stress triggering theory, we calculated the coseismic stress changes on major faults induced by the Wenchuan earthquake, with elastic dislocation theory and the multilayered crustal model. We also discuss the stress distribution and its significance for future seismic activity under the impact of the Wenchuan earthquake. It is shown that coulomb failure stress (CFS) increases obviously on the Daofu-Kangding segment of the Xianshuihe Fault, the Maqu and Nanping segment of the Eastern Kunlun Fault, the Qingchuan Fault, southern segment of the Minjiang Fault, Pengxian-Guanxian Fault, Jiangyou-Guangyuan Fault, and Jiangyou-Guanxian Fault. The increased stress raises the probability of earthquake occurrence on these faults. Since these areas are highly populated, earthquake monitoring and early disaster alarm system are needed. CFS increases with a magnitude of 0.03–0.06 MPa on the Qingchuan Fault, which is close to the northern end of the rapture of Wenchuan earthquake. The occurrence of some strong aftershocks, including three events with magnitude higher than 5.0, indicates that the seismic activities have been triggered by the main shock. Aftershocks seem to migrate northwards. Since the CFS change on the Lueyang-Mianxian Fault located on the NEE of the Qingchuan Fault is rather small (±0.01 MPa), the migration of aftershocks might be terminated in the area near Hanzhong City. The CFS change on the western Qinling Fault is around 10 Pa, and the impact of static triggering can be neglected. The increment of CFS on the Pengxian-Guanxian Fault and Beichuan-Yingxiu Fault southwest to the main rupture is 0.005–0.015 MPa, which would facilitate earthquake triggering in these areas. Very few aftershocks in these areas indicate that the accumulated stress has not been released sufficiently. High seismic risk is predicated in these areas due to co-seismic CFS loading. The Wenchuan earthquake released the accumulated CFS on the Fubianhe Fault, the Huya Fault, the Ha’nan-Qingshanwan Fault, and the Diebu-Bailongjiang Fault. The decrement of CFS changes on the Longquanshan Fault east to Chengdu City is about 0.002 MPa. The seismic activity will be depressed by decrement of CFS on these faults.

Interseismic slip rate of the northwestern Xianshuihe fault from InSAR data

The Xianshuihe fault is a highly active strike‐slip fault system near the eastern margin of the Tibetan plateau. We use a multi‐interferogram method to construct a map of line‐of‐sight deformation rate due to interseismic strain accumulation on the northwestern Xianshuihe fault from a decade of SAR data acquired by the ERS‐1/2 and Envisat satellites. The rate map shows a clear deformation gradient associated with the fault, but slightly offset to the northeast from the surface trace. Joint inversion of InSAR rate map and GPS data using Monte‐Carlo method, we estimate a slip rate of 9‐12 mm/yr with a locking depth of 3‐6 km at the 90% confidence level. The slip rate agrees with the Holocene fault slip rate and the historical earthquakes. Our results also suggest no significant across‐fault extension. In the future, InSAR data from both ascending and descending orbits may further constrain the 3‐D fault slip rate on this fault.