Internal structures and high-velocity frictional properties of Longmenshan fault zone at Shenxigou activated during the 2008 Wenchuan earthquake

This paper reports internal structures of a wide fault zone at Shenxigou,Dujiangyan,Sichuan province,China,and high-velocity frictional properties of the fault gouge collected near the coseismic slip zone during the 2008 Wenchuan earthquake.Vertical offset and horizontal displacement at the trench site were 2.8 m（NW side up）and 4.8 m（right-lateral）,respectively.The fault zone formed in Triassic sandstone,siltstone,and shale about 500 m away from the Yingxiu-Beichuan fault,a major fault in the Longmenshan fault system.A trench survey across the coseismic fault,and observations of outcrops and drill cores down to a depth of 57 m revealed that the fault zone consists of fault gouge and fault breccia of about0.5 and 250-300 m in widths,respectively,and that the fault strikes N62°E and dips 68° to NW.Quaternary conglomerates were recovered beneath the fault in the drilling,so that the fault moved at least 55 m along the coseismic slip zone,experiencing about 18 events of similar sizes.The fault core is composed of grayish gouge（GG） and blackish gouge（BG） with very complex slip-zone structures.BG contains low-crystalline graphite of about 30 %.High-velocity friction experiments were conducted at normal stresses of 0.6-2.1 MPa and slip rates of 0.1-2.1 m/s.Both GG and BG exhibit dramatic slip weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient lpto steadystate friction coefficient lssover a slip-weakening distance Dc.Deformation of GG and BG is characterized by overlapped slip-zone structures and development of sharp slickenside surfaces,respectively.Comparison of our data with those reported for other outcrops indicates that the high-velocity frictional properties of the Longmenshan fault zones are quite uniform and the high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake.

Wenchuan earthquake fault and seismic disaster

Major cases of the Ms8.0 Wenchuan earthquake are obtained through field investigations of the epicenter and high-intensity areas, and the relationships among earthquake faults, ground motion and earthquake disasters near fault zones are analyzed. Both strong deformation and ground rupture lead to significant damages of the buildings, indicating that it is necessary to keep safe distance away from active faults and to take other necessary measures. There are two reasons for that the buildings near the surface rupture zones have withstood in the strong earthquake, other than their seismic resistance capacities, with the first being the site condition, and the second the reduced effective stress and low rupture velocity. The forms of structural damages are complex in the fault areas, with shear failure and tensile and compressive damages. Those structures in urban areas that have used qualified materials and followed the building codes performed well in the earthquake. Survey results also indicate that structures of flexible materials may show better seismic performance.

Intensive Observation of Reservoirinduced Seismicity and Preliminary Analysis on the Causes of Earthquakes in the Three Gorges Reservoir Area,Yangtze River

根据第 11 五年的计划的国家关键技术 RD 节目的要求，由 26 个地震车站组成的一个使增加密度的地震网络在 2009 年 3 月 21 短时期的地震计在湖北省的节在三个峡水库区域被建立， 5 宽带数字地震计。从三月到 2009 年 12 月， 2,995 ML -0 的一个总数。8 - 2。9 地震在从 145 m 三朵峡水库水水平玫瑰扣押到 172.8m 的试用期间被检测。用双差别地震地点算法， 2,837 地震精确被重定位。小本地地震的模式聚集在三个峡水库区域的结果表演呈现了线性分布或像团的簇分发，事件的像团的簇通常在从水滨的 5 km 的距离以内，并且地震的线性分发能被扩大到 16 km 的距离离开水滨。在三座峡水库的湖北节，地震主要在 Xiangxihe 河附近在 Xiannvshan 和 Jiuwanxi 差错的北结束被集中，并且沿着在在 Badong 区域的北河岸上的 Xietan 镇区和 Shenlongxi 区域的西方的长江的河岸，与焦点的深度不到 10km ，和 4km 在平均。在水库区域的地震频率与水库水水平变化有积极关联，显示 seismicity 属于水库导致的地震。沿着在水库区域的 Shenlong 河，地震在北 Badong 县显示出三线性分布，并且根据石灰岩地区常见的地形散布了分发。在碳酸盐阶层有地下的河。当水库被扣押时，水渗入了进地下的河，因此导致地震。在 Xiannvshan 的 crossriver 片断上的区域的地震指责， Jiuwanxi 差错并且在 Xietan ， Shazhen 和 Xizhen 的区域西方，可能与弄软断绝有关 Xiannvshan 差错，或床上用品例如 Nukou 差错，连接，它将导致岩石群众的失败因此，导致地震。然而，关于被触发的机制的有说服力的结论仍然需要进一步的学习。另外， Zigui 县并且在 Badong 县的 Rangkou 镇东方的 Wenhua 和 Yanglin 向南接近区域， mininginduced 地震在水库的岸是的附近，并且上的矿发生了一些倒塌地震。

青藏高原东缘汶川地震断层陡坎类型与级联破裂模型(英文)

The post-earthquake field investigations reveal that the MW7.9 Wenchuan earthquake of 12th May 2008 ruptured three NE-striking imbricate reverse faults and another NW-trending reverse fault, along the middle Longmenshan fold-and-thrust belt at the eastern margin of the Tibetan plateau. The fault-scarp features can be categorized into eight groups: simple thrust scarp, hanging-wall collapse scarp, simple pressure ridge, dextral pressure ridge, fault-related fold scarp, back-thrust pressure ridge, local normal fault scarp and crocodile-mouth-like scarp. The local normal scarp is first discovered in the reverse-faulting earthquakes as ever reported in the world. Field observation indicates that the Wenchuan earthquake surface rupture is dominated by reverse faulting with a minus right-lateral component, but the relative ratio varies from site to site. Also, the surface ruptures can be divided, for the first order, into two segments, the Yingxiu and Beichuan segments, corresponding to MW7.8 and MW7.57 events, respectively. The two segments further can be divided, for the second order, into four sub-segments in total, which are equivalent to four sub-events of MW7.46, MW7.69, MW6.99 and MW7.52, respectively. The fault segmentation, for different orders, shows a cascade-rupturing pattern and can explain why the quake time of the Wenchuan earthquake was so long as up to 100 second. Aftershock focal mechanisms are also used to constrain the fault geometry for the sub-segments, indicating that the seismogenic faults are listric at depth and in general, the fault plane becomes steeper northward, which enables the fault to accommodate larger strike-slip motion. This earthquake also confirms that the crustal shortening across the Longmenshan fold-and-thrust belt should be responsible for the growth of high topographic relief along the eastern margin of the Tibetan Plateau.

Discussion of rupture mechanisms on the seismogenic fault of the 2008 M_s8.0 Wenchuan earthquake

The May 12, 2008, Ms8.0 Wenchuan earthquake was the outcome of a recent movement of an active intra-continental thrust fault zone. The seismogenic fault of this earthquake underwent oblique-slip faulting along the central fault and pure thrust faulting along the range-front fault of the Longmenshan fault zone. The former had a steep dip and large vertical displacement, and the latter had a gentle dip and little vertical displacement. The fault zone consisted of compressive double fault ramps rup turing with right-lateral strike-slip components resulting from strain partitioning of a deep oblique slip fault in the brittle zone of the upper crust. The kinematic pattern and rupture mechanisms are complex for the seismogenic fault, as indicated by the geometric pattern of its surface ruptures, the coseismic displacement distribution and focal mechanisms of the main shock and aftershocks. As a tear fault, the NW-trending, left-lateral, strike-slip Xiaoyudong fault zone has accommodated NE-trending displacements with different shortening amounts. However, because of intense compression on the southwestern segment of the seismogenic fault, the left-lateral, strike-slip Xiaoyudong fault also carries a clear compression component. Normal faulting with a strike-slip component controls the formation of a fault-trough along the central fault, which is characterized by thrusting with a strike-slip component and strike-slip with thrusting. The fault-troughs are the product of the interaction of slip and grav ity on the seismogenic fault under specific geological and geomorphic conditions. Gravitational force exaggerated the vertical component of fault displacement, which by no means represents the actual maximum vertical displacement of the seismogenic fault.

The M_S7.1 Yushu earthquake surface rupture and large historical earthquakes on the Garzê-Yushu Fault

As revealed by field investigations,the co-seismic surface rupture zone of the 2010 MS7.1 Yushu earthquake,Qinghai is a characteristic sinistral strike-slip feature consisting of three distinct sinistral primary ruptures,with an overall strike of 310°-320° and a total length of 31 km.In addition,an approximately 2-km-long en-echelon tensile fissure zone was found east of Longbao Town;if this site is taken as the north end of the rupture zone,then the rupture had a total length of ～51 km.The surface rupture zone is composed of a series of fissures arranged in an en-echelon or alternating relationship between compressive bulges and tensile fissures,with a measured maximum horizontal displacement of 1.8 m.The surface rupture zone extends along the mapped Garzê-Yushu Fault,which implicates it as the seismogenic fault for this earthquake.Historically,a few earthquakes with a magnitude of about 7 have occurred along the fault,and additionally traces of paleoearthquakes are evident that characterize the short-period recurrence interval of large earthquakes here.Similar to the seismogenic process of the 2008 Wenchuan earthquake,the Yushu earthquake is also due to the stress accumulation and release on the block boundaries resulting from the eastward expansion of Qinghai-Tibet Plateau.However,in contrast with the Wenchuan earthquake,the Yushu earthquake had a sinistral strike-slip mechanism resulting from the uneven eastward extrusion of the Baryan Har and Sichuan-Yunnan fault blocks.

Near-field postseismic deformation along the rupture of 2008 Wenchuan earthquake and its implications

At first time,we observed the postseismic deformation,which actually is a kind of creep after slip,along the intra-plate active fault associated with the Wenchuan earthquake.To understand the near-field postseismic deformation following 2008 Wenchuan earthquake in Sichuan Province of China,we compared the fault scarp or flexure scarp profiles measured in different campaigns.Our result shows that among total 19 observation sites,on 13 sites (68% of 19) fault scarps fall back with an average about 9.7% decrease;on 5 sites (26% of 19) fault scarps present no change;and on one site (6% of 19) fault scarp continues to uplift with 12.8% increase.The variety of fault scarp we observed results mainly from near-field postseismic deformation,after slip occurred in shallow.Based on our observations,the following are demonstrated: except for the southwestern end near Yingxiu Town where coseismic slip deficit and some elastic energy residue exist there,fall back (68%) or non-changing (26%) of fault scarp shows energy balance or energy deficit due to overthrust,implying that the likelihood of occurrence of strong aftershocks of ≥ M7 becomes very small in these energy-released areas.Moreover,we suggest that a minimum of 10% error due to near-field postseismic deformation should be considered when evaluating the magnitude of historic and paleoearthquake or slip rate based on the fault scarp displacement,even though the error caused by erosion has been accounted already.