Rapid afterslip and short-term viscoelastic relaxation following the 2008 M_W7.9 Wenchuan earthquake

Significant postseismic deformation of the 2008 M W 7.9 Wenchuan earthquake has been observed from GPS data of the first 14 days after the earthquake. The possible mechanisms for the rapid postseismic deformation are assumed to be afterslip on the earthquake rupture plane and viscoelastic relaxation of coseismiclly stress change in the lower crust or upper mantle. We firstly use the constrained least squares method to find an afterslip model which can fit the GPS data best. The afterslip model can explain n...     

汶川地震发震断层破裂触发过程

2008年汶川大地震造成北川断裂、彭灌断裂、小渔洞断裂等多条断裂的破裂,呈现出复杂的破裂过程. 对此,以往地震学的研究没有对于各个发震断层的破裂先后顺序给出充分论证. 本文计算由于断层破裂在其他断层段上造成库仑应力的变化,根据其相互触发关系确定断层可能的破裂顺序. 结果表明,各断裂带可能的发震顺序为:主震在北川断裂南端(小渔洞断裂以南的北川断裂虹口段)起始,造成北川断裂的后续段落龙门山镇—清平段和彭灌断裂同时破裂,进而触发小渔洞断裂发生破裂.     

2008年5月12日汶川MW 7.9地震的震源位置与发震时刻

综合运用四川省地震台网与紫坪铺水库地震台网的观测资料,精确地测定了2008年5月12日汶川MW7.9地震的震源位置与发震时刻．指出,对汶川地震这样的大地震精准定位,必须克服或尽量减少远台观测对地震精确定位的局限性、地壳介质模型的不完善性以及识别与检测初至波震相的不一致性等因素的影响．通过分析对比、反复试验,从上述台网中精心选取了方位分布均匀、具有近震源台站约束、直达P波震相确系由初始破裂辐射出的15个地震台的直达P波到时数据,反演得出精确度比区域性地震台网常规测定的精确度高一个数量级的汶川大地震的定位结果,即:发震时刻（北京时间）:2008年5月12日14:27:57.59plusmn;0.03 s;震中位置:31.018deg;Nplusmn;0.3 km,103.365deg;Eplusmn;0.3 km;震源深度:15.5 kmplusmn;0.3 km．      

用余震确定2008年汶川M_w7.9地震的震源位置与发震时刻

利用同一地点的2个地震在相同台站记录的P波到时差是一个常数这一原理,找到了与汶川主震位置相近的余震,并利用台站距震中近,且记录波形清晰、完整的优势,对余震进行了定位,依此确定了汶川主震的位置和发震时刻.结果是震中位于31°1.26′N、103°22.50′E;震源深度7.9km;发震时刻(北京时间)为2008年5月12日14时27分58.80秒.     

基于GPS同震位移场约束反演2008年5.12汶川大地震破裂空间分布

2008年5月12日发生在四川汶川的大地震造成映秀—北川断裂和灌县—江油断裂同时破裂,分别形成了240多公里和70多公里的地表破裂带.本文以GPS观测获得的同震位移场为约束,反演地震破裂的空间分布.反演结果显示映秀—北川主破裂带倾向北西,沿破裂带的走向从南到北倾角逐渐变大,破裂断层的平均宽度在10~18 km左右.破裂断层的错动在南段以逆冲为主,在北段走滑分量逐步加大,右旋走滑成为断层破裂的主要特征.断层破裂最大段落错动量分别达到了7.8 m和7.4 m,恰好对应这次地震中地表破坏最为严重的映秀和北川地区.本次地震释放地震矩6.70×1020N·m,相应矩震级Mw=7.9.     

Spatio-temporal rupture process of the 2008 great Wenchuan earthquake

Focal mechanism and dynamic rupture process of the Wenchaun M _s8.0 earthquake in Sichuan province on 12 May 2008 were obtained by inverting long period seismic data from the Global Seismic Network (GSN), and characteristics of the co-seismic displacement field near the fault were quantitatively analyzed based on the inverted results to investigate the mechanism causing disaster. A finite fault model with given focal mechanism and vertical components of the long period P-waves from 21 stations with evenly azimuthal coverage were adopted in the inversion. From the inverted results as well as aftershock distribution, the causative fault of the great Wenchuan earthquake was confirmed to be a fault of strike 225°/dip 39°/rake 120°, indicating that the earthquake was mainly a thrust event with right-lateral strike-slip component. The released scalar seismic moment was estimated to be about 9.4×10²⁰-2.0×10²¹ Nm, yielding moment magnitude of M _w7.9–8.1. The great Wenchuan earthquake occurred on a fault more than 300 km long, and had a complicated rupture process of about 90 s duration time. The slip distribution was highly inhomogeneous with the average slip of about 2.4 m. Four slip-patches broke the ground surface. Two of them were underneath the regions of Wenchuan-Yingxiu and Beichuan, respectively, with the first being around the hypocenter (rupture initiation point), where the largest slip was about 7.3 m, and the second being underneath Beichuan and extending to Pingwu, where the largest slip was about 5.6 m. The other two slip-patches had smaller sizes, one having the maximum slip of 1.8 m and lying underneath the north of Kangding, and the other having the maximum slip of 0.7 m and lying underneath the northeast of Qingchuan. Average and maximum stress drops over the whole fault plane were estimated to be 18 MPa and 53 MPa, respectively. In addition, the co-seismic displacement field near the fault was analyzed. The results indicate that the features of the co-seismic displacement field were coincident with those of the intensity distribution in the meizoseismal area, implying that the large-scale, large-amplitude and surface-broken thrust dislocation should be responsible for the serious disaster in the near fault area. Supported by the National Basic Research Program of China (Grant No. 2004CB418404-4) and the National Natural Science Foundation of China (Grant Nos. 40574025 and 40874026)     

Rupture directivity effect on strong ground motion during the 12 May 2008 MW7.9 Wenchuan earthquake

We focus here on the rupture directivity effect on the spatial distribution and attenuation characteristics of near-field ground motions during the 2008 MW7.9 Wenchuan earthquake. We examine the difference between the observed ground motions in and opposite the rupture directions and compare them with Next Generation Attenuation-West2 (NGA-West2) ground motion prediction models. The isochrone directivity predictor is used to quantify the band-limited nature of the rupture directivity effect on strong ground motion. Our results show that the observed peak ground velocity (PGV) and spectral accelerations of periods of 1.0 s and longer are significantly amplified in the rupture direction, but de-amplified in the opposite direction affected by rupture directivity effect of this event. In contrast, the effect of rupture directivity on the observed peak ground acceleration (PGA) and periods of shorter than 1.0 s are relatively weak. The rupture directivity of this event shows clear period dependent and band limited characteristics with the strongest effect occurring around the period of 7.5 s.     

2008年汶川MS8.0地震多断裂破裂的近地表同震滑移及滑移分解

为深入理解汶川地震破裂的构造运动机制,本文选取典型的观测点,利用多种地质地貌标志测绘分析得到了汶川M_S8.0地震发震断裂的近地表三维同震滑移矢量.结果显示,北川—映秀断裂上的白水河—高川破裂段北西盘沿88°方位角水平滑移2.58 m、垂直滑移3.70 m;安县—灌县断裂上的白鹿—汉旺破裂北西盘沿134°方位角水平滑移1.63 m,垂直滑移2.00 m;小鱼洞破裂带南西盘沿76°~79°方位角水平滑移2.15~2.71 m,垂直滑移1.36~1.51 m.平行的白水河—高川破裂段和白鹿—汉旺破裂段合计形成1.72 m右旋走滑和3.49 m垂直断裂带的NW向水平缩短,总滑移方向（106°）与断裂带整体走向（42°）呈64°夹角,整个龙门山推覆构造带处于斜向挤压的构造环境.结合震源过程反演成果的分析显示,斜滑的白水河—高川破裂段和逆冲型白鹿—汉旺破裂段可能是在汶川地震中最大的一次子事件过程以滑移分解的形式而同时破裂形成的,滑移分解作用使两条断裂以斜滑与逆冲组合的力学性质产生破裂而非相同性质的斜滑破裂.小鱼洞破裂以低角度斜滑为主,可能是安县—灌县断裂与北川—映秀断裂以滑移分解形式同时破裂的纽带.小鱼洞断裂是龙门山断裂带长期处于斜向挤压的构造环境的产物,不只是逆冲断裂系中的捩断层.     

汶川Mw7.9地震余震序列触发机制研究

余震触发机制的Dieterich解析模型被广泛应用于区域地震活动性的定量分析以及依赖时间的概率地震预测模型的建立等方面.基于滑移速率和状态相依赖的摩擦定律和弹簧-滑块模型,从Dieterich断层滑移速率方程出发,给出了静态应力扰动下触发地震的时钟提前或推后的近似解,从而明确地阐明了触发地震的产生机制与断层的演化过程密切相关,并与传统位错模型下库仑应力扰动时间提前或推后量作了比较.采用对数线性拟合方法求得了汶川Mw7.9主震后余震序列持续时间,符合Dieterich理论结果.以汶川余震序列为例,给出了两种不同的应力扰动模式在该余震序列中的应用.结果表明,经典Dieterich扰动解无法给出主震发生后即时余震数量的异常增加,而考虑主震前后剪应力速率变化的Dieterich分段解则可反映出余震发生率及个数随时间的演化特征.     

川滇地区未来强震预测与汶川Mw7.9级地震孕震过程分析

根据作者提出的孕震时空区域划分原则,确定了川滇地区地震区(带)划分的合理方案.运用孕震断层多锁固段脆性破裂理论与相关预测方法,给出了各地震区(带)未来强震的四要素预测结果.结果表明,除西昌、丽江与鲜水河地震区在较长一段时间内无破坏性地震发生外,其他地震区都将有破坏性地震发生.本文还分析了汶川大地震的孕震过程.结果表明:导致2008年5月12日汶川M_w7.9级地震的最直接原因,是1976年发生在松潘-平武的M_S7.2级双震震群事件.     

2008年汶川8.0级地震前横跨龙门山断裂带的震间形变

利用区域GPS和水准测量资料,结合地震构造背景的分析,本文研究2008年汶川8.0级地震前横跨龙门山断裂带地区的震间地壳形变,探讨引起这种形变的活动构造与动力学模式,并由此认识汶川地震的孕育与成因机制.主要结果表明：1997～2007年期间,自龙门山断裂带中段朝北西约230 km的地带内存在垂直于断裂的水平缩短变形、以及平行于断裂的水平右旋剪切变形,缩短率为1.3×10-8/a （即：0.013 mm/km/a）,角变形速率为2.6×10-8/a;同一地带在1975～1997年期间还表现出垂直上隆变形,上隆速率在龙门山前山断裂与中央断裂之间仅0.6 mm/a,而至龙门山后山断裂及其以西达2～3 mm/a.这些反映了在汶川地震之前至少10～30余年,龙门山断裂带中段的前山与中央断裂业已闭锁、并伴有应变积累.造成这种形变的主要原因是：以壳内的低速层为“解耦”带,巴颜喀拉地块上地壳朝南东的水平运动在四川盆地西缘受到华南地块的阻挡、转换成龙门山断裂带中段的逆冲运动;由于该断裂段的震间闭锁,致使西侧的巴颜喀拉地块的上地壳发生横向缩短以及平行断裂的右旋剪切变形.然而,龙门山断裂带北段在1997～2007年期间除了有大约0.9 mm/a的右旋剪切变形外,横向的缩短变形极微弱,这可能与该断裂段西侧的岷江、虎牙、龙日坝等断裂带吸收了巴颜喀拉地块朝东水平运动的大部分有关.另外,汶川地震前,横跨龙门山断裂带中段与北段的地壳形变特征的差异,与汶川地震时能量释放的空间分布吻合.     

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 about 0.5 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 μ _p to steady-state friction coefficient μ _ss over a slip-weakening distance D _c. 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.     

Variations of shear wave splitting in the 2008 Wenchuan earthquake region

Through the analysis of S-wave particle motion of local events in the shear wave window, the polariza-tion directions of the faster shear wave and the delay times between the faster and the slower shear waves were derived from seismic recordings at the stations near the fault zones. The shear wave split-ting results of seven stations in the area of Longmenshan fault zone reveal spatial variation of the po-larization directions of the fast shear wave. The directions at stations in the southeastern side of the Longmenshan fault zone (in the Sichuan Basin area) are in the NE direction, whereas the direction at station PWU (in the Plateau), which is in the northwestern side of the faults, is in the EW direction. Systematic changes of the time delays between two split shear waves were also observed. At station L5501 in the southern end of the aftershock zone, the delay times of the slower shear wave decrease systematically after the main shock. After the main shock, the delay times at station PWU were longer than those before the earthquake. Seismic shear wave splitting is caused mostly by stress-aligned microcracks in the rock below the stations. The results demonstrate changes of local stress field dur-ing the main-shock and the aftershocks. The stress in the southern part of Wenchuan seismogenic zone was released by the main-shock and the aftershocks. The crustal stresses were transferred to the northeastern part of the zone, resulting in stress increase at station PWU after the main-shock.     

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 resettling people as to avoid future disasters.It is demonstrated that the redistribution of stress induced by an earthquake should trigger su...     

Torsional phenomena in 2008 great Wenchuan earthquake

It is well known that there are some torsional damages in earthquakes. In Taibai park, Jiangyou city, Sichuan province, most of the stone statues, which were placed upon the banisters of one zigzag bridge, exhibited different torsional phenomena in 2008 Wenchuan earthquake. This paper introduces the torsional phenomena of all the statues on the zigzag bridge firstly. Then one eccentricity model is established and the equivalent rotational accelerations are calculated in order to analyze the causes of the to...     

汶川大地震前非台风扰动现象的研究

为了区分汶川大地震的震前扰动现象中的台风因素和非台风因素,本文研究了中国大陆宽带地震仪在汶川大地震前记录到的异常扰动信号的时频特征.研究结果表明汶川大地震的震前扰动主要由两种扰动构成,二者动态特征完全不同.其中优势频率为0.2～0.25 Hz的扰动主要与台风Rammasun有关.这种台风扰动在沿海地区较强,在内陆地区较弱,其震动源在靠近台风运动路径的海底.另一种优势频率为0.1～0.18 Hz的扰动与台风无关,这种非台风扰动在地震发生前约10 h突然急剧增强,其最大值出现在地震爆发时刻.非台风扰动在靠近震中的地区较强,在沿海和西部地区较弱.震源扰动扫描算法计算初步定位的结果显示其震动源不在海底,而是分散在震中附近的内陆地区.汶川大地震前的非台风扰动是否与汶川大地震有关,值得进行更深入的研究.     

汶川地震前龙门山断裂带闭锁程度和滑动亏损分布研究

利用1999—2007期GPS水平速度场数据,采用Defnode负位错反演程序估算了龙门山断裂在汶川地震前的闭锁程度和滑动亏损分布,结合龙门山断裂带附近地表水平应变率场结果,综合分析了震前地壳变形特征.反演结果表明,震前龙门山断裂中北段处于完全闭锁状态,闭锁深度达到21 km(闭锁比例0.99)左右,垂直断层方向的挤压滑动亏损速率约为2.2 mm/a,平行断层方向的右旋滑动亏损速率约为4.6 mm/a.龙门山断裂南段只有地表以下12 km闭锁程度较高(闭锁比例0.99),垂直断层方向滑动亏损速率约为1.4 mm/a,平行断层方向滑动亏损速率约为4.6 mm/a;在12~16 km处闭锁比例约为0.83,垂直断层方向滑动亏损速率约为1.2 mm/a,平行断层方向滑动亏损速率约为3.8 mm/a;在16~21 km处闭锁比例约为0.75,垂直断层方向滑动亏损速率约为1.1 mm/a,平行断层方向滑动亏损速率约为3.5 mm/a.在21~24 km处整条断裂均逐步转变为蠕滑.上述反演结果与区域应变计算获得的龙门山断裂带中北段整体应变积累速率较低、南段应变积累速率较高相一致,均表明中北段闭锁程度高、南段闭锁程度稍低,该特征可以较好地解释汶川地震时从震中向北东向单向破裂现象.     

2008年汶川M_S8.0地震前地震活动异常特征

分析了汶川地震前地震活动时空演化特征．结果表明:①汶川地震前38a龙门山断裂带及其附近形成5级地震背景空区,震前6.5a形成ML4.0地震孕震空区,震前1a孕震空区内部及其两端相继发生多次ML4.0—5.0地震,空区缩小;②中国大陆西部及邻区2001年以来处于大震活跃时段,而中国大陆内部地震活动水平非常低,出现非常显著的7级、6级和5级地震平静;③南北地震带7级以上地震在时间上具有准周期特征,空间上存在由南向北迁移的特点,汶川地震的发生符合这一规律;④1998年以来南北地震带中段为7级地震空段,汶川地震就发生该空段内;⑤2003年云南大姚地震后,南北地震带地震活动显著增强,且在中、南段形成4.6级以上地震环形分布,四川及其附近表现为异常平静,同时震群活动显著,且在4.6级地震平静区内形成震群空区,汶川地震就发生震群空区的边缘,震前8个月,震群频度出现高值异常;⑥汶川地震前7个月,青藏块体大范围ML≥4.0地震平静103d,2008年1月13日以后平静区逐渐解体,至汶川地震前４级地震平静区缩小到巴颜喀拉地块,汶川地震就发生在巴颜喀拉地块的东边界带上,汶川地震前3 个多月,孕震空区内部出现NW走向的3级地震条带,与龙门山断裂带斜交．