Earthquake Rupture at Focal Depth,Part I: Structure and Rupture of the Pretorius Fault,TauTona Mine,South Africa

We analyze the structure of the Archaean Pretorius fault in TauTona mine, South Africa, as well as the rupture-zone that recently reactivated it. The analysis is part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project that utilizes the access to 3.6 km depth provided by the mining operations. The Pretorius fault is a ~10 km long, oblique-strike-slip fault with displacement of up to 200 m that crosscuts fine to very coarse grain quartzitic rocks in TauTona mine. We identify here three structural zones within the fault-zone: (1) an outer damage zone, ~100 m wide, of brittle deformation manifested by multiple, widely spaced fractures and faults with slip up to 3 m; (2) an inner damage zone, 25–30 m wide, with high density of anastomosing conjugate sets of fault segments and fractures, many of which carry cataclasite zones; and (3) a dominant segment, with a cataclasite zone up to 50 cm thick that accommodated most of the Archaean slip of the Pretorius fault, and is regarded as the ‘principal slip zone’ (PSZ). This fault-zone structure indicates that during its Archaean activity, the Pretorius fault entered the mature fault stage in which many slip events were localized along a single, PSZ. The mining operations continuously induce earthquakes, including the 2004, M2.2 event that rejuvenated the Pretorius fault in the NELSAM project area. Our analysis of the M2.2 rupture-zone shows that (1) slip occurred exclusively along four, pre-existing large, quasi-planer segments of the ancient fault-zone; (2) the slipping segments contain brittle cataclasite zones up to 0.5 m thick; (3) these segments are not parallel to each other; (4) gouge zones, 1-5 mm thick, composed of white ‘rock-flour’ formed almost exclusively along the cataclasite-host rock contacts of the slipping segments; (5) locally, new, fresh fractures branched from the slipping segments and propagated in mixed shear-tensile mode; (6) the maximum observed shear displacement is 25 mm in oblique-normal slip. The mechanical analysis of this rupture-zone is presented in Part II (Heesakkers et al., Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona mine, South Africa 2011, this volume).     

龙门山断裂带大地震孕震环境的岩石磁学证据

地震断裂带的孕震环境对于研究地震的发生至关重要.本文以汶川地震断裂带科学钻探2号（WFSD-2）钻孔岩心中的假玄武玻璃、碎裂岩及其围岩为研究对象,通过岩石磁学测试,并结合显微结构观察探讨龙门山断裂带大地震的孕震环境.WFSD-2岩心碎裂岩中的假玄武玻璃是龙门山断裂带曾经发生过大地震活动的岩石学证据,假玄武玻璃具有高磁化率特征,而碎裂岩的磁化率值与围岩相似,假玄武玻璃中的新生磁铁矿是其高磁化率值异常的重要原因之一.假玄武玻璃中较少量的新生磁铁矿暗示了假玄武玻璃的生成环境为含氧量较低的高温还原环境.大地震断裂的摩擦热是改变龙门山断裂带中假玄武玻璃岩石磁学特征的主导因素,流体作用较弱.无高磁化率异常的碎裂岩经历的温度小于300℃,推测假玄武玻璃的生成深度较深.WFSD-2岩心中20余层假玄武玻璃脉体证明映秀—北川断裂带是一条长期活跃的断裂带,龙门山断裂带上曾经发生了多次大地震断裂活动,这些大地震可能发生在深度较大、流体作用较弱的还原性孕震环境中.     

Clayey injection veins and pseudotachylyte from two boreholes penetrating the Chelungpu Fault, Taiwan: Their implications for the contrastive seismic slip behaviors during the 1999 Chi-Chi earthquake

Abstract   The seismic slip that occurred during the 1999 Chi-Chi earthquake in Taiwan showed contrastive behaviors in different regions along the Chelungpu Fault: A large and smooth slip occurred in the north, while a relatively small slip associated with high-frequency seismic wave radiation occurred in the south. The core samples from shallow boreholes at northern (Fengyuan) and southern (Nantou) sites penetrating the seismic Chelungpu Fault were analyzed. The fault zones at the northern site are characterized by soft clayey material associated with clayey injection veins. This suggests that the fault zones were pressurized during ancient seismic slip events, and hence the elastohydrodynamic lubrication occurred effectively. In contrast, the fault rock from the southern site is old pseudotachylyte that has been shattered by repeated ancient seismic slip events. Statistical analysis of many pseudotachylyte fragments reveals that the degree of frictional melting tended to be low. In this case, the seismic slip is restrained by the mechanical barrier of a highly viscous melt layer. These contrastive fault rocks were produced by repeated ancient seismic slip events, but the two corresponding mechanisms of friction are likely to have also occurred during the 1999 Chi-Chi earthquake, thus causing the contrastive slip behaviors in the north and south.     

利用近场高频GPS、 强地面运动和远场地震波形数据联合反演2008年汶川MS8.0地震的震源时空破裂过程

联合近场GPS测站1-Hz运动学位移、 强震仪加速度波形和全球台站P震相波形作为约束, 以时空滑动分布约束条件和ABIC模型参数选择方法, 结合先验的滑动方向变化范围, 反演2008年汶川M_S8.0地震的震源时空破裂过程, 给出了能够综合反映震源破裂过程的统一模型。 结果表明, 汶川地震总体上存在4个主要的破裂区, 最主要的一个破裂区位于震源东北40~120 km, 断层面上的最大位错量约为10 m, 主体滑动分布在2~20 km深度范围, 破裂达到地表; 第二个主体破裂区位于断层破裂带南段, 最大滑动量达到6 m; 另外2个主体滑动区位于断层破裂带北段, 但滑动破裂量小于断层南段破裂区的滑动量, 滑动破裂值最大值为4 m, 超过1 m的区域在走向上超过70 km。 反演得到的断层滑动模型的地震矩为9.5×10²¹ Nm, 相应的矩震级为M_W7.95。 汶川地震破裂表现为单侧破裂, 起始破裂在汶川下方16 km深度, 向东北方向一致性地传播, 过程持续~120 s。 在地震发生后0~10 s内, 破裂集中在震源起始破裂区, 滑动破裂值为~1.0 m, 之后破裂向东北方向扩展, 震后20~40 s是主要的破裂时段。 在40~60 s, 破裂跨越断层南段和北段。 在80~90 s破裂最大值开始下降, 在100~110 s时, 下降为~0.5 m, 在110~120 s时, 下降为~0.1 m。 加入近场GPS测站1-Hz 波形数据与近场强震仪波形和远场长周期体波联合反演, 提高了震源破裂模型的空间分辨率, 特别是浅部滑动破裂区的分辨率, 反演的最大滑动破裂值比不用1-Hz 波形数据反演的结果增大, 表明近场1-Hz GPS波形数据对于揭示汶川地震的时空破裂过程具有重要的作用。     

Internal structures and high-velocity frictional properties of a bedding-parallel carbonate fault at Xiaojiaqiao outcrop activated by the 2008 Wenchuan earthquake

This paper reports internal structures of a bedding-parallel fault in Permian limestone at Xiaojiaqiao outcrop that was moved by about 0.5 m during the 2008 M_W7.9 Wenchuan earthquake. The fault is located about 3 km to the south from the middle part of Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system that was moved during the earthquake. The outcrop is also located at Anxian transfer zone between the northern and central segments of Yingxiu-Beichuan fault where fault system is complex. Thus the fault is an example of subsidiary faults activated by Wenchuan earthquake. The fault has a strike of 243° or N63°E and a dip of 38°NW and is nearly optimally oriented for thrust motion, in contrast to high-angle coseismic faults at most places. Surface outcrop and two shallow drilling studies reveal that the fault zone is several centimeters wide at most and that the coseismic slip zone during Wenchuan earthquake is about 1 mm thick. Fault zone contains foliated cataclasite, fault breccia, black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gouge contained in fault breccia indicate multiple slip events along this fault. But fossils on both sides of fault do not indicate clear age difference and overall displacement along this fault should not be large. We also report results from high-velocity friction experiments conducted on yellowish gouge from the fault zone using a rotary shear low to high-velocity frictional testing apparatus. Dry experiments at normal stresses of 0.4 to 1.8 MPa and at slip rates of 0.08 to 1.35 m/s reveal dramatic slip weakening from the peak friction coefficient of around 0.6 to very low steady-state friction coefficient of 0.1-0.2. Slip weakening parameters of this carbonate fault zone are similar to those of clayey fault gouge from Yingxiu-Beichuan fault at Hongkou outcrop and from Pingxi fault zone. Our experimental result will provide a condition for triggering movement of subsidiary faults or off-fault damage during a large earthquake.     

岩石高速摩擦实验与地震物理过程

简述了最近20年来国内外岩石高速摩擦实验研究领域的进展和动态:岩石高速摩擦实验技术的发展实现了对高滑动速率、大位移的地震过程的实验模拟;其结果揭示了岩石和断层泥在地震滑动速率下的力学性状,深化了对断层滑动弱化机制、临界滑动距离、以及地震发生过程的认识和理解;实验在假玄武玻璃成因方面取得了重要进展,并提出了断层发生地震滑动可能留下的其它地质证据,可望为研究断层滑动性状与地震物理过程提供新的思路和信息.岩石高速摩擦实验今后的发展方向主要包括:发展具有加温系统和孔隙压系统的岩石高速摩擦实验装置,研究水热作用下岩石和断层泥的高速摩擦性状;室内实验和地震资料分析相结合研究断层滑动和地震机制;室内实验和野外地质调查相结合探索断层发生地震错动的地质证据等等.     

Earthquake hazard in Marmara Region, Turkey

Earthquake hazard in the Marmara Region, Turkey has been investigated using time-independent probabilistic (simple Poissonian) and time-dependent probabilistic (renewal) models. The study culminated in hazard maps of the Marmara Region depicting peak ground acceleration (PGA) and spectral accelerations (SA)'s at 0.2 and 1 s periods corresponding to 10 and 2% probabilities of exceedance in 50 yrs. The historical seismicity, the tectonic models and the known slip rates along the faults constitute the main data used in the assignment. Based on recent findings it has been possible to provide a fault segmentation model for the Marmara Sea. For the main Marmara Fault this model essentially identifies fault segments for different structural, tectonic and geometrical features and historical earthquake occurrences. The damage distribution and pattern of the historical earthquakes have been carefully correlated with this fault segmentation model. The inter-event time period between characteristic earthquakes in these segments is consistently estimated by dividing the seismic slip estimated from the earthquake catalog by the GPS-derived slip rate of 22±3 mm/yr. The remaining segments in the eastern and southern Marmara region are also identified using recent geological, geophysical studies and historical earthquakes. The model assumes that seismic energy along the segments is released by characteristic earthquakes. For the probabilistic studies characteristic earthquake based recurrence relationships are used. Assuming normal distribution of inter-arrival times of characteristic earthquakes, the ‘mean recurrence time’, ‘covariance’ and the ‘time since last earthquake’ are developed for each segment. For the renewal model, the conditional probability for each fault segment is calculated from the mean recurrence interval of the characteristic earthquake, the elapsed time since the last major earthquake and the exposure period. The probabilities are conditional since they change as a function of the time elapsed since the last earthquake. For the background earthquake activity, a spatially smoothed seismicity is determined for each cell of a grid composed of cells of size 0.005°×0.005°. The ground motions are determined for soft rock (NEHRP B/C boundary) conditions. Western US-based attenuation relationships are utilized, since they show a good correlation with the attenuation characteristics of ground motion in the Marmara region. The possibility, that an event ruptures several fault segments (i.e. cascading), is also taken into account and investigated by two possible models of cascading. Differences between Poissonian and renewal models, and also the effect of cascading have been discussed with the help of PGA ratio maps.     

InSAR约束下的2008年汶川地震同震和震后形变分析

2008年5月12日,青藏高原东缘的龙门山断裂带上发生了M_w7.9级汶川地震.本文通过分析覆盖汶川地震震中区域的ALOS/PALSAR像对的方位向偏移量来选择无明显电离层扰动影响的像对进行干涉处理,获取了高精度、连续的InSAR地表形变场.在此基础上,结合高精度GPS同震形变数据,采用同震、黏弹性松弛震后形变联合反演模型同时确定了汶川地震的同震滑动分布和龙门山地区的流变结构参数.研究结果表明,汶川地震是一个断层破裂非常复杂的地震事件,其中,北川段、岳家山段、虹口段和汉旺段的滑动以逆冲为主,而青川段以右旋走滑为主.滑动主要发生在10 km深度以上的区域,最大滑动量位于虹口段的东北端,达10.7 m.地震释放的总能量为9.28×10²⁰ N·m（M_w7.91）,与地震学的结果一致.联合反演模型确定的龙门山地区中下地壳的黏性系数为2×10¹⁸ Pa·s,为青藏高原东部地区的黏性系数提供了一个可靠的下限值.如果有更长时间的震后形变观测时间序列,将为该区域提供更为可靠的流变结构.     

基于InSAR同震形变观测反演2010年新西兰南岛Mw7.1 Darfield地震同震破裂分布

2010年9月4日新西兰南岛Canterbury平原发生了M_w7.1地震,震源深度约为10 km.本次地震发生在一条震前不为人所知的断层上.我们利用覆盖整个震区的合成孔径雷达(SAR)观测资料,通过干涉处理分析获得雷达视线向(LOS)同震形变场;以此资料为约束反演了断层的几何参数以及同震破裂分布.结果显示,该地震造成四条相对独立断层的破裂.大部分的地震矩释放发生在Greendale断层(编号1-4),其错动以右旋走滑为主,最大破裂约为8.5 m.其它三条断层中,经过震源的逆冲断层最大破裂为5.1 m (编号6),位于Greendale断层以西的逆冲断层最大破裂为3.5 m (编号5),位于Greendale断层北面的走滑断层最大破裂为1.9 m(编号7).反演的Greendale断层地表滑动与地质调查得到的地表破裂在形态和数值上均吻合较好.本次地震释放的地震矩为5.0×10¹⁹N·m,矩震级为7.1.板块边界带形变场分析表明,Darfield地震的发生受边界带应变分配在该地区残留构造应力场控制,其复杂性体现了区域构造应力场的特点.地震对其周围地区的应力场影响较大,库仑应力增加区与余震分布有一定对应关系,并在2011年Christchurch 6.3级地震发震断层区域造成约0.1bar的库仑应力增加,对此地震有一定的触发作用.     

用CAP方法反演2010年6月5日山西阳曲M_S4.6地震震源机制解

利用CAP方法反演了2010年6月5日阳曲MS4.6地震震源机制解,得到震级MW为4.5,节面I走向213°、倾角47°、滑动角-161°,节面II走向109°,倾角76°,滑动角-44°,属于倾滑型;精确定位显示震中处于石岭关隆起区,CAP反演和精定位结果推断本次地震的震源深度为17～20km。震源机制解节面参数与震中附近的山根底断裂和系舟山西麓断裂产状存在差异,这两条断裂不是阳曲地震的发震断裂,由于现场野外地质考察未发现地表断裂,不排除本次地震为隐伏断层活动的结果。     

Energy partitioning during seismic slip in pseudotachylyte-bearing faults (Gole Larghe Fault, Adamello, Italy)

The determination of the earthquake energy budget remains a challenging issue for Earth scientists, as understanding the partitioning of energy is a key towards the understanding the physics of earthquakes. Here we estimate the partition of the mechanical work density into heat and surface energy (energy required to create new fracture surface) during seismic slip on a location along a fault. Earthquake energy partitioning is determined from field and microstructural analyses of a fault segment decorated by pseudotachylyte (solidified friction-induced melt produced during seismic slip) exhumed from a depth of ~ 10 km—typical for earthquake hypocenters in the continental crust. Frictional heat per unit fault area estimated from the thickness of pseudotachylytes is ~ 27 MJ m^− 2. Surface energy, estimated from microcrack density inside clast (i.e., cracked grains) entrapped in the pseudotachylyte and in the fault wall rock, ranges between 0.10 and 0.85 MJ m^− 2. Our estimates for the studied fault segment suggest that ~ 97–99% of the energy was dissipated as heat during seismic slip. We conclude that at 10 km depth, less than 3% of the total mechanical work density is adsorbed as surface energy on the fault plane during earthquake rupture.     

应用动态复合震源模型模拟汶川Mw7.9地震强地面运动

2008年5月12日中国汶川地区发生Mw7.9地震,震中位置103.4°E,31.0°N.主要发震断层空间展布长达300多公里,由南西方向到北东方向呈现明显的分段性,汶川一映秀段逆冲为主兼有少量的右旋走滑分量;安县一北川段为逆冲一右旋走滑的断层错动;青川段以右旋走滑为主兼有少量逆冲分量.采用改进后的复合震源强地面运动预...     

Combination of Precise Leveling and InSAR Data to Constrain Source Parameters of the Mw = 6.5, 26 December 2003 Bam Earthquake

We used new precise leveling data acquired 40 days after the Bam earthquake in combination with radar interferometry observations from both ascending and descending orbits to investigate static deformation associated with the 2003 Bam earthquake. We invert this geodetic data set to gain insight into the fault geometry and slip distribution of the rupture. The best-fitting dislocation model is a steeply east-dipping right-lateral strike-slip fault that has a size of 11 by 8 km and strikes N2°W. We find that such smooth geometry fits available geodetic data better than previously proposed models for this earthquake. Our distributed slip model indicates a maximum strike slip of 3 m occurring about 3 to 5 km deep. The slip magnitude and depth of faulting taper to the north, where the fault approaches the Bam city. Inclusion of crustal layering increases the amount of maximum slip inferred at depth by about 4%.     

基于形变观测分析2011年日本9.0级地震与断层运动间关系

2011年3月11日日本发生9.0级地震,本文以此次地震的震间、同震和震后形变观测为约束,依据不同时段断层运动空间分布特征分析日本海沟地区强震与断层运动间关系.震间日本海沟地区,断层运动闭锁线深度约为60km,闭锁线以上从深到浅依次为断层运动强闭锁段、无震滑移段和弱闭锁段.由同震位错反演结果,2011年日本9.0级地震同震存在深浅两个滑移极值区,同震较浅的滑移极值区(同震位错量10~50m,深度小于30km)震间为断层弱闭锁段;同震较深的滑移极值区(同震位错量10~20m,深度在40km左右)震间为断层强闭锁段;而在两者之间的过渡带同震位错相对较小,震间断层运动表现为无震滑移.震后初期断层运动主要分布在在闭锁线以上的同震较深滑移极值区,而同震较浅的滑移极值区能量释放比较彻底,断层震后余滑量相对较小.依据本文同震和震间断层运动反演结果,震间强闭锁段积累10m同震位错需要100多年时间,与该区域历史上7级地震活动复发周期相当;震间弱闭锁段积累30~50m同震位错约需要300~600年时间,与相关研究给出的日本海沟9级左右地震复发周期比较一致.在实际孕震能力判定的工作中,由于不同性质的断层段在同震过程中会表现更多的组合形式,断层发震能力判定结果存在更多的不确定性,但利用区域形变观测等资料给出震间断层运动特征的研究工作对于断层强震发震能力的判定具有非常重要的实际意义.     

2015年尼泊尔Mw7.8地震震源机制InSAR反演及强地面运动模拟

2015年4月25日,在尼泊尔中部发生了M_w7.8地震.本文利用ALOS-2和SENTINEL-1A宽幅数据获取了该地震大范围的同震形变场,并反演了该地震断层破裂的几何特征及运动机制,继而以此为约束资料反演地震强地面运动.InSAR结果显示本次地震造成了巨大的地表形变,LOS向最大抬升量达到1.3 m,最大下沉量达到0.7 m.震源机制反演得到的最优的滑动分布模型表明,断层的走向为291°,倾角为7.6°,倾滑主要分布在深度为12~18 km范围,主倾滑分布范围在长度上达到了140 km,该范围内的平均倾滑角为95°.本次地震最大倾滑量达到5.3 m,位于深度15 km处.累计释放地震矩达 6.5×10²⁰N·m,约合矩震级M_w7.8.该地震发生在印度与欧亚板块俯冲逆冲界面之间,发震构造推断为主喜马拉雅逆冲断裂,属于典型的喜马拉雅型——低角度逆断层型强震.以该滑动分布模型参数为基础利用随机振动的有限断层模型进行尼泊尔地震的强地面运动模拟,结果显示最大地震烈度为Ⅸ度,烈度分布的范围及烈度等级与USGS模型结果对比具有很高的符合度.     

1966年邢台强震群震前、 同震和震后断层运动特征分析

在1966年3月邢台强震群水准观测基础上, 本文首先对同震位移和数值模拟结果进行了对比, 认为邢台强震群发震断层浅层是牛家桥阳断层和永福庄断层, 深部发震断层是倾向北西的北东向断层; 然后以区域水准观测为约束, 通过反演给出了邢台强震群震前断层无震滑移、 同震断层位错和震后断层余滑空间分布。 结果表明, 震前显著无震滑移主要发生在3月8日6.8级前震的发震断层上, 而3月20日宁晋7.2级和6.7级地震发震断层深部同震位错相对弱的断层段, 震前也存在较为明显的无震滑移; 震后断层余滑主要分布在束鹿凹陷西侧的牛家桥阳断层。 从邢台强震群过程中断层运动时空演化特征可以看出, 强震破裂成核过程中震前断层无震滑移既可能发生于前震发震断层, 也可能发生于主震发震断层, 震后断层位移空间分布与同震位错具有互补性; 而震前断层无震滑移是否为有关震前前兆的真正原因, 是值得进一步深入研究的问题。     

Coupling of tectonic loading and earthquake fault slips at subduction zones

Because of the viscoelastic behaviour of the earth, accumulation of elastic strain energy by tectonic loading and release of such energy by earthquake fault slips at subduction zones may take place on different spatial scales. If the lithospheric plate is acted upon by distant tectonic forces, strain accumulation must occur in a broad region. However, an earthquake releases strain only in a region comparable to the size of the rupture area. A two-dimensional finite-element model of a subduction zone with viscoelastic rheology has been used to investigate the coupling of tectonic loading and earthquake fault slips. A fault lock-and-unlock technique is employed so that the amount of fault slip in an earthquake is not prescribed, but determined by the accumulated stress. The amount of earthquake fault slip as a fraction of the total relative plate motion depends on the relative sizes of the earthquake rupture area and the region of tectonic strain accumulation, as well as the rheology of the rock material. The larger the region of strain accumulation is compared to the earthquake rupture, the smaller is the earthquake fault slip. The reason for the limited earthquake fault slip is that the elastic shear stress in the asthenosphere induced by the earthquake resists the elastic rebound of the overlying plate. Since rapid permanent plate shortening is not observed at subduction zones, there must be either strain release over a large region or strain accumulation over a small region over earthquake cycles. The former can be achieved only by significant aseismic fault slip between large subduction earthquakes. The most likely mechanism for the latter is the accumulation of elastic strain around isolated locked asperities of the fault, which requires significant aseismic fault slip between asperities.     

1303年山西洪洞8级地震震源断层研究

1303年在山西洪洞附近发生的8级巨大地震, 是中国根据现存较为详细的文献记载史料所确定的最早的一次8级地震。 这次地震距今已有700多年的历史, 而地震所在区域至今仍有持续不断的小地震活动。 本文根据地震破裂区1981年至2013年的中小地震精定位地震目录, 采用震源断层面拟合方法, 反演得到了1303年山西洪洞地震的震源断层面参数: 走向19.3°、 倾角88.5°、 滑动角-170.0°。 断层面长75.5 km, 宽26.2 km, 深度为地下11.12 ～37.35 km。 将地震破裂区的地震精确定位资料以近东西向的洪洞断裂为界划分为地震北段和地震南段, 分段进行地震震源断层拟合, 反演得到洪洞地震北段震源断层面参数: 走向13.7°、 倾角76.6°、 滑动角-157.6°。 断层面长32.7 km, 宽21.7 km, 深度为地下11.97～32.86 km; 南段震源断层面参数: 走向20.3°、 倾角87.1°、 滑动角-154.6°。 断层面长45.9 km, 宽16.6 km, 深度为地下9.32 km～25.50 km。 无论是分段还是不分段, 反演得到的洪洞地震震源断层均是右倾的近直立断层, 属于右旋走向滑动性质。 分段计算得到的地震北段震源断层深度比南段更深, 将反演得到的震源断层与临汾盆地深部构造最新研究成果进行了分析对比, 北段震源断层深度及倾角大小与深地震剖面推测得到的深大断裂几乎相同。 震源断层在地表的投影与洪洞地震的高烈度区能够较好地对应。     

2008年于田MW7.2地震滑动分布反演及三维同震形变场解算

基于ENVISAT ASAR升降轨数据, 利用InSAR获取2008年于田M_W7.2地震同震形变场; 采用SDM反演本次地震断层滑动分布; 使用PSCMP正演获取于田M_W7.2地震南北向模拟形变量, 并结合升降轨同震形变场, 解算三维同震形变场。 同震形变场分析表明, 2008年于田M_W7.2地震以正断为主, 且带有走滑运动特征, 破裂带走向为NNE向。 同震滑动分布反演结果显示, 断层沿走向被分为4段F1、 F2、 F3、 F4, 其滑动分布集中在0~14 km区间, 以F2、 F3段为主, 最大滑动量约5.31 m, 位于F2段深部2.76 km处; 沿破裂带走向, 左旋走滑位移与垂直位移比值有增大的趋势; 反演获得的地震矩M₀=5.58×10¹⁹N·m, 相当于矩震级M_W7.1。 三维同震形变场解算结果显示, 断层上盘整体表现为沉降, 断层下盘整体表现为隆升, 且沉降量明显大于隆升量, 表明地震以正断破裂为主; 除靠近断裂带中上部表现为向东南运动外, 上盘整体上表现为向西南运动; 断层下盘则整体表现为向东北运动, 证明破裂兼有左旋走滑运动。 滑动分布反演、 正演与三维同震形变场解算结果皆表明, 于田M_W7.2地震破裂以正断为主, 且带有一定的左旋走滑。     

Interplate Earthquake Fault Slip During Periodic Earthquake Cycles in a Viscoelastic Medium at a Subduction Zone

--A 2-D finite-element-method (FEM) numerical experiment of earthquake cycles at a subduction zone is performed to investigate the effect of viscoelasticity of the earth on great interplate earthquake fault slip. We construct a 2-D viscoelastic FEM model of northeast Japan, which consists of an elastic upper crust and a viscoelastic mantle wedge under gravitation overlying the subducting elastic Pacific plate. Instead of the dislocation model prescribing an amount of slip on a plate interface, we define an earthquake cycle, in which the plate interface down to a depth is locked during an interseismic period and unlocked during coseismic and postseismic periods by changing the friction on the boundary with the master-slave method. This earthquake cycle with steady plate subduction is periodically repeated to calculate the resultant earthquake fault slip.¶As simulated in a previous study (Wang, 1995), the amount of fault slip at the first earthquake cycle is smaller than the total relative plate motion. This small amount of fault slip in the viscoelastic medium was considered to be one factor explaining the small seismic coupling observed at several subduction zones. Our simulation, however, shows that the fault slip grows with an increasing number of repeated earthquake cycles and reaches an amount comparable to the total relative plate motion after more than ten earthquake cycles. This new finding indicates that the viscoelasticity of the earth is not the main factor in explaining the observed small seismic coupling. In comparison with a simple one-degree-of-freedom experiment, we demonstrate that the increase of the fault slip occurs in the transient state from the relaxed initial state to the stressed equilibrium state due to the intermittent plate loading in a viscoelastic medium.