Constraints from precariously balanced rocks on preferred rupture directions for large earthquakes on the southern San Andreas Fault

We have compared near-fault ground motions from TeraShake simulations of M_w7.7 earthquake scenarios on the southern San Andreas Fault with precariously balanced rock locations. The TeraShake scenarios with different directions of rupture generate radically different ground motions to the northwest of the Los Angeles Basin, primarily because of directivity effects, and thus provide constraints on the ground motion and rupture direction for the latest (1690) large event on that section of the San Andreas Fault. Due to the large directional near-field ground motions predicted by the simulations, we expect the precariously balanced rocks to be located primarily in the backward rupture direction or near the epicenter. Preliminary results favor persistent nucleation at or slightly northwest of the San Gorgonia Pass fault zone for large earthquakes on the southern San Andreas Fault.     

Clay gouges in the San Andreas Fault System and their possible implications

Summary In the northern and central sections of the San Andreas Fault Zone, and along Calaveras and Hayward faults, clay gouges have been found to occur on the surface and at shallow depths.It is consistent with the available geochemical data that such gouges can exist at depths down to 10 km. If extensive gouge materials exist in a fault zone then their properties will determine, to a large extent, the behavior of the fault. From known properties of clays in the presence of water we can infer that, in such cases, the tectonic stress and the stress drops for earthquakes will be low and substantial creep will take place before earthquakes.     

2014年美国加州纳帕M_W6.1地震断层参数的Sentinel-1A InSAR反演

2014年8月24日,在美国加州旧金山海湾北部的纳帕地区发生了MW6.1地震.发震断层是西纳帕断裂系统中的一部分,但是该断层之前并未被足够重视.本文利用欧洲空间局最近发射成功并刚刚投入使用的Sentinel-1A卫星获取的第一对同震干涉像对(20140807-20140831),得到了该地震的地表同震形变场,结合震后24h内区域GPS同震形变资料作为约束条件,反演了纳帕地震的断层几何参数以及滑动分布.Sentinel-1A干涉结果表明,此次地震造成了明显的地面形变,视线向最大抬升和最大沉降量均达到了10cm.联合反演结果表明,该发震断层的走向为344°,倾角为80°.主要破裂以右旋走滑为主,平均倾滑角为-146.5°,最大倾滑量达到了1.1m,位于地表下约4km,存在明显的滑动亏损现象.此次地震,累计释放地震矩达1.5×1018 N·m,约合矩震级MW6.1.该结果略小于InSAR单独约束结果,可能与Sentinel-1A像对中包含的快速震后形变分量有关.     

Crustal structure beneath the eastern part of the Coast Ranges (Diablo Range) of central California from explosion seismic and near earthquake data

Seismic refraction and near earthquake data of the U.S. Geological Survey for central California have been compiled into record sections along profiles and interpreted in terms of crustal structure. The profiles are located northeast of the San Andreas fault of central California and run parallel to the general structures. For the explosion seismic line through the centre of the Diablo Range, an uppermost layer (Franciscan formation) with P velocities of 3.6–5.0 km s^?1 decreases in thickness towards the northwest. The lower boundaries of layers with constant velocities of 5.75 and 6.8 km s^?1 are found at almost constant depths of 12 and 21 km, respectively. Between 21 and 26 km depth a well-defined low-velocity zone appears whose velocity is estimated as ～ 5.3 km s^?1 with the aid of a hedgehog inversion and the calculation of amplitudes. This zone is underlain by a layer 3–5 km thick with a velocity of 7.6 km s^?1. The upper-mantle velocity beneath the Moho at 29–30 km depth is 8.2 km s^?1. The near earthquake profiles, located ～ 20 km southwest and parallel to the explosion seismic line, follow more or less the Hayward and Calaveras fault systems. The velocity-depth distribution derived for the earthquake data is very similar to that found beneath the Diablo Range. However, the low-velocity zone at 21–26 km depth does not seem to exist everywhere along the line. The Moho is not disturbed beneath the Calaveras, Hayward and Silver Creek faults; it rises slightly from the Diablo Range towards the southwest.     

Instability model for recurring large and great earthquakes in southern California

The locked section of the San Andreas fault in southern California has experienced a number of large and great earthquakes in the past, and thus is expected to have more in the future. To estimate the location, time, and slip of the next few earthquakes, an earthquake instability model is formulated. The model is similar to one recently developed for moderate earthquakes on the San Andreas fault near Parkfield, California. In both models, unstable faulting (the earthquake analog) is caused by failure of all or part of a patch of brittle, strain-softening fault zone. In the present model the patch extends downward from the ground surface to about 12 km depth, and extends 500 km along strike from Parkfield to the Salton Sea. The variation of patch strength along strike is adjusted by trial until the computed sequence of instabilities matches the sequence of large and great earthquakes sincea.d. 1080 reported by Sieh and others. The last earthquake was theM=8.3 Ft. Tejon event in 1857. The resulting strength variation has five contiguous sections of alternately low and high strength. From north to south, the approximate locations of the sections are: (1) Parkfield to Bitterwater Valley, (2) Bitterwater Valley to Lake Hughes, (3) Lake Hughes to San Bernardino, (4) San Bernardino to Palm Springs, and (5) Palm Springs to the Salton Sea. Sections 1, 3, and 5 have strengths between 53 and 88 bars; sections 2 and 4 have strengths between 164 and 193 bars. Patch section ends and unstable rupture ends usually coincide, although one or more adjacent patch sections may fail unstably at once. The model predicts that the next sections of the fault to slip unstably will be 1, 3, and 5; the order and dates depend on the assumed length of an earthquake rupture in about 1700.     

基于InSAR形变数据分析台湾纵谷断层南段现今运动特征

本文利用2007—2010年花东纵谷南段区域的InSAR形变数据作为约束, 采用分段断层模型和层状介质模型, 反演中国台湾东部纵谷断层南段滑动速率空间分布, 并据此分析断层运动特征。 研究结果表明, 纵谷断层南段整体以逆冲运动为主, 兼具左旋走滑运动。 纵谷断层南段的滑动速率具有空间非均匀性, 在空间上可以细分为深浅两个极值区, 浅部(0~15 km)最大滑动速率为10 cm/a, 位于深度2.5 km左右; 深部(15~30 km)最大滑动速率为21 cm/a, 位于深度25 km左右。 反演结果与用重复地震估算的深部滑动速率基本吻合。     

P-wave tomography of crust and upper mantle under Southern California: Influence of topography of Moho discontinuity

We use 146 422 P-wave arrival times from 6 347 local earthquakes recorded by the Southern California SeismicNetwork to determine a detailed three-dimensional P-wave velocity structure at 0～35 km depth. We have takeninto account the Moho depth variations, which were obtained by seismological methods. Checkerboard tests sug-gest that our inversion results are reliable. Our models provide new information on regional geological structuresof Southern California. At shallow depths P-wave velocity structure correlates with surface geological features andexpresses well variations of surface topography of the mountains and basins. The velocity structure at each layer ischaracterized by block structures bounded by large faults. Ventura Basin, Los Angeles Basin, Mojave Desert, Pen-insular Ranges, San Joaquin Valley, Sierra Nevada, and Salton Trough show respectively all-round block. SanAndreas Fault becomes an obvious boundary of the region. To its southwest, the velocity is higher, and there arestrong heterogeneity and deeper seismicity; but to its northeast, the velocity is lower and shows less variation thanto the southwest, the seismicity is shallower. To investigate the effect of the Moho geometry we conducted inver-sions for two cases: one for flat Moho geometry, another for a Moho with lateral depth variations. We found thatthe topography of the Moho greatly affects the velocity structure of the middle and lower crust. When the Mohotopography is considered, a more reasonable tomographic result can be obtained and the resulting 3-D velocitymodel fits the data better.     

Geological Observations of Damage Asymmetry in the Structure of the San Jacinto, San Andreas and Punchbowl Faults in Southern California: A Possible Indicator for Preferred Rupture Propagation Direction

We present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the San Andreas, San Jacinto, and Punchbowl faults in southern California. The observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters, a fault zone scale of meters to tens of meters and related geomorphologic features. The observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters. In all of those scales, more damage is found on the side of the fault with faster seismic velocities at seismogenic depths. The observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks. The data are consistent with a preferred northwestward propagation direction for ruptures on all three faults. If our results are supported by additional observations, asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section. The property of a preferred rupture direction can explain anomalous behavior of historic rupture events, and may have profound implications for many aspects of earthquake physics on large faults.     

芦山MS7.0级地震余震序列重新定位及构造意义

利用四川省地震台网的震相数据和双差定位方法对芦山M_S7.0级地震及其余震序列进行了精确定位,根据余震分布确定了发震断层的位置和断层面的几何特征,并对余震活动进行了分析.结果显示,芦山M_S7.0级地震的震中位于30.28°N、102.99°E,震源深度为16.33 km.余震沿发震断层向主震两侧延伸,主要分布在长约32 km、宽约15~20 km、深度为5~24 km的范围内.地震破裂带朝西南方向扩展范围较大,东北方向略小,余震震级随时间迅速衰减.震源深度剖面清晰地显示出发震断层的逆冲破裂特征,推测发震断层为大川—双石断裂东侧约10 km的隐伏断层.该断层走向217°、倾向北西,倾角约45°,产状与大川—双石断裂相比略缓,它们同属龙门山前山断裂带的叠瓦状逆冲断层系.受发震断裂影响,部分余震沿大川—双石断裂分布,西北方向的余震延伸至宝兴杂岩体的东南缘,与汶川地震的破裂带之间存在50 km左右的地震空区,有可能成为未来发生强震的潜在危险区.     

Characterization of Damage in Sandstones along the Mojave Section of the San Andreas Fault: Implications for the Shallow Extent of Damage Generation

Following theoretical calculations that suggest shallow generation of rock damage during an earthquake rupture, we measure the degree of fracture damage in young sedimentary rocks from the Juniper Hills Formation (JHF) that were displaced 21 km along the Mojave section of San Andreas Fault (SAF) and were not exhumed significantly during their displacement. In exposures adjacent to the fault, the JHF typically displays original sedimentary fabrics and little evidence of bulk shear strain at the mesoscopic scale. The formation is, however, pervasively fractured at the microscopic scale over a zone that is about a 100 m wide on the southwest side of the SAF near Little Rock. The abundance of open fractures, the poor consolidation, and the shallow inferred burial depth imply that the damage was generated close to the surface of the Earth. The spatial correlation of this damage with a seismically active trace of the SAF suggests that it was generated by SAF slip events that by assumption were of a seismic nature throughout the displacement history of the JHF. Thus the JHF provides a very shallow upper bound for the generation of brittle damage in a seismic fault zone. The fracture fabric is characterized by preferred orientations of fractures that split grains between contact points and is consistent with overall deformation under directed compression. However, the available results cannot be used to distinguish between proposed off-fault damage mechanisms. Fracture orientations are compatible with a maximum compressive stress oriented at a high angle to the fault at about 10 m, and at a lower, more variable angle farther away from the fault. The fracture distribution and fabric are consistent with observations made of the microscale damage characteristics of the Hungry Valley Formation in the northwestern section of the SAF in the Mojave, and with previous observations of exhumed, ancestral strands of the SAF.     

Space-time variations in the parameters of split S waves excited by local earthquakes in southern Kamchatka

An analysis of the distribution of split S-wave parameters due to small local earthquakes occurring down to a depth of 200 km beneath the PET IRIS station (in the town of Petropavlovsk-Kamchatskii) for the period 1993–2002 revealed an inhomogeneous distribution of anisotropic properties and time-dependent changes in the state of stress and strain in the sinking slab. Three time intervals have been identified from the behavior of the split-wave parameters: 1993–1995, 1996–1998, and 1999–2002. The most orderly orientation of fast-wave polarization azimuths at all depths and lower stresses in the earth was observed in 1996–1998. Regions of relatively stable and unstable parameter behavior consistent with increased and decreased P and S velocities have been identified. The central block down to a depth of 120 km and the lower part of the Benioff zone are classified as stable regions dominated by E-SE fast-wave azimuths. Unstable behavior of fast-wave azimuths was noticed for depths of 60 to 90 km and around the top of the Benioff zone. The highest values of S-wave delay time and regions where the delay times were migrating mostly occur at contacts between more rigid and weakened blocks.     

Depth and region dependence of b-value for micro-aftershocks of the May 12th, 2008 Wenchuan earthquake and its tectonic implications

Micro-aftershocks with magnitude range of 1.5?4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aftershocks of M1.5?4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks of M3.0?5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of < 5 km, the b-values are very small (~0.4), possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of ~5?11 km, where most aftershocks concentrated, the b-values become as high as ~0.9?1.0. At the depth below ~11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.

     

SPATIAL AND TEMPORAL DISTRIBUTION OF SLIP RATE DEFICIT ACROSS HAIYUAN-LIUPAN SHAN FAULT ZONE CONSTRAINED BY GPS DATA

As the northeast boundary of the Tibetan plateau, the Haiyuan-Liupan Shan fault zone has separated the intensely tectonic deformed Tibetan plateau from the stable blocks of Ordos and Alxa since Cenozoic era. It is an active fault with high seismic risk in the west of mainland China. Using geology and geodetic techniques, previous studies have obtained the long-term slip rate across the Haiyuan-Liupan Shan fault zone. However, the detailed locking result and slip rate deficit across this fault zone are scarce. After the 2008 Wenchuan M_S8.0 earthquake, the tectonic stress field of Longmen Shan Fault and its vicinity was changed, which suggests that the crustal movement and potential seismic risk of Haiyuan-Liupan Shan fault zone should be investigated necessarily. Utilizing GPS horizontal velocities observed before and after Wenchuan earthquake(1999~2007 and 2009~2014), the spatial and temporal distributions of locking and slip rate deficit across the Haiyuan-Liupan Shan fault zone are inferred. In our model, we assume that the crustal deformation is caused by block rotation, horizontal strain rate within block and locking on block-bounding faults. The inversion results suggest that the Haiyuan fault zone has a left-lateral strike-slip rate deficit, the northern section of Liupan Shan has a thrust dip-slip rate deficit, while the southern section has a normal dip-slip rate deficit. The locking depths of Maomao Shan and west section of Laohu Shan are 25km during two periods, and the maximum left-lateral slip rate deficit is 6mm/a. The locking depths of east section of Laohu Shan and Haiyuan segment are shallow, and creep slip dominates them presently, which indicates that these sections are in the postseismic relaxation process of the 1920 Haiyuan earthquake. The Liupan Shan Fault has a locking depth of 35km with a maximum dip-slip rate deficit of 2mm/a. After the Wenchuan earthquake, the high slip rate deficit across Liupan Shan Fault migrated from its middle to northern section, and the range decreased, while its southern section had a normal-slip rate deficit. Our results show that the Maomao Shan Fault and west section of Laohu Shan Fault could accumulate strain rapidly and these sections are within the Tianzhu seismic gap. Although the Liupan Shan Fault accumulates strain slowly, a long time has been passed since last large earthquake, and it has accumulated high strain energy possibly. Therefore, the potential seismic risks of these segments are significantly high compared to other segments along the Haiyuan-Liupan Shan fault zone.     

Time variations in teleseismic reisduals prior to the magnitude 5.1 Bear Valley earthquake of February 24, 1972

Summary Teleseismic arrivals for large earthquakes occurring in the Circum-Pacific seismic source regions have been analysed forP-velocity variations prior to the February 24, 1972, Bear Valley earthquake at several stations in the vicinity of Bear Valley, California. The teleseismic arrivals have been analysed by the method of two-station residuals and corrected for observed azimuthal variations. The data covers the time period of July, 1971, through April, 1972, and suggests that during part of January, 1972, aP-velocity anomaly occurred beneath station BVL 2 km from the epicenter of the magnitude 5.1 Bear Valley event. A maximum vertical travel time delay of 0.15 seconds is observed. No other anomalous behavior associated with this event is suggested by the data for the other stations ranging from 7 to 19 km from the event's epicenter. These results support an anomalous zone of limited size with a maximum horizontal extent of less than 5 km perpendicular and about 10 km parallel to the San Andreas fault relative to the epicenter and confined within a 5 to 10 km portion of the uppermost crust. Also aP-velocity delay for waves travelling essentially along the intermediate stress axis would imply in this case that theP-velocity anomaly is caused by a bulk modulus mechanism such as that proposed by the dilatancy-fluid flow theory.     

S-wave Velocity Models in the Scotia Sea Region, Antarctica, from Nonlinear Inversion of Rayleigh Waves Dispersion

—More than 60 events recorded by four recently deployed seismic broadband stations around Scotia Sea, Antarctica, have been collected and processed to obtain a general overview of the crust and upper mantle seismic velocities.¶Group velocity of the fundamental mode of Rayleigh waves in the period between 10 s to 30–40 s is used to obtain the S-wave velocity versus depth along ten different paths crossing the Scotia Sea region. Data recorded by two IRIS (Incorporated Research Institutions for Seismology) stations (PMSA, EFI) and the two stations of the OGS-IAA (Osservatorio Geofisico Sperimentale—Instituto Antarctico Argentino) network (ESPZ, USHU) are used.¶The Frequency-Time Analysis (FTAN) technique is applied to the data set to measure the dispersion properties. A nonlinear inversion procedure, "Hedgehog," is performed to retrieve the S-wave velocity models consistent with the dispersion data.¶The average Moho depth variation on a section North to South is consistent with the topography, geological observations and Scotia Sea tectonic models.¶North Scotia Ridge and South Scotia Ridge models are characterised by similar S-wave velocities ranging between 2.0 km/s at the surface to 3.2 km/s to depths of 8 km/s. In the lower crust the S-wave velocity increases slowly to reach a value of 3.8 km/s. The average Moho depth is estimated between 17 km to 20 km and 16 km to 19 km, respectively, for the North Scotia Ridge and South Scotia Ridge, while the Scotia Sea, bounded by the two ridges, has a faster and thinner crust, with an average Moho depth between 9 km and 12 km.¶On other paths crossing from east to west the southern part of the Scotia plate and the Antarctic plate south of South Scotia Ridge, we observe an average Moho depth between 14 km and 18 km and a very fast upper crust, compared to that of the ridge. The S-wave velocity ranges between 3.0 and 3.6 km/s in the thin (9–13 km) and fast crust of the Drake Passage channel. In contrast the models for the tip of the Antarctic Peninsula consist of two layers with a large velocity gradient (2.3–3.0 km/s) in the upper crust (6-km thick) and a small velocity gradient (3.0–4.0) in the lower crust (14-km thick).     

Spatial-temporal characterization of the San Andreas Fault by fault-zone trapped waves at seismic experiment site,Parkfield, California

In this article, we review our previous research for spatial and temporal characterizations of the San Andreas Fault (SAF) at Parkfield, using the fault-zone trapped wave (FZTW) since the middle 1980s. Parkfield, California has been taken as a scientific seismic experimental site in the USA since the 1970s, and the SAF is the target fault to investigate earthquake physics and forecasting. More than ten types of field experiments (including seismic, geophysical, geochemical, geodetic and so on) have been carried out at this experimental site since then. In the fall of 2003, a pair of scientific wells were drilled at the San Andreas Fault Observatory at Depth (SAFOD) site; the main-hole (MH) passed a ～200-m-wide low-velocity zone (LVZ) with highly fractured rocks of the SAF at a depth of ～3.2 km below the wellhead on the ground level (Hickman et al., 2005; Zoback, 2007; Lockner et al., 2011). Borehole seismographs were installed in the SAFOD MH in 2004, which were located within the LVZ of the fault at ～3-km depth to probe the internal structure and physical properties of the SAF. On September 282004, a M6 earthquake occurred ～15 km southeast of the town of Parkfield. The data recorded in the field experiments before and after the 2004 M6 earthquake provided a unique opportunity to monitor the co-mainshock damage and post-seismic heal of the SAF associated with this strong earthquake. This retrospective review of the results from a sequence of our previous experiments at the Parkfield SAF, California, will be valuable for other researchers who are carrying out seismic experiments at the active faults to develop the community seismic wave velocity models, the fault models and the earthquake forecasting models in global seismogenic regions.     

中、缅、印交界地区的地震分布特征及震源机制解的研究

利用1965——1981年mb4.0的580个地震,研究了中、缅、印交界地区的地震空间分布特征,得到地震主要在阿萨姆块体周围及凹向块体的断裂带上成带或成群分布;在缅甸北部大约由20N到26N存在倾斜地震带,其倾向由南到北逐渐由东转向南东东,其倾角由30变为50;地震带厚度为20——30km;作了38个地震的机制解,机制解表明,在缅甸北部、阿萨姆块体及其相邻地区压力轴为北东方向且近于水平,反映了印度板块以北东方向挤压欧亚板块.      

ESTIMATING THE WENCHUAN EARTHQUAKE RUPTURE PROCESS USING NEAR FIELD STRONG MOTION RECORDS AND COSEISMIC DISPLACEMENTS

Based on the extensive near field broadband strong-motion records with uniform azimuthal coverage and coseismic displacements, the rupture process of 2008 Wenchuan earthquake is inversed by the non-negative least square method and multiple-time window technique. The possible rupture sequence among southern Beichuan Fault, Pengguan Fault and Xiaoyudong Fault and the initial rupture time of high dip angle part of southern Beichuan Fault are analyzed from kinetic aspects, which have been seldom focused on. The results indicate that:(1) The near field waveform fitting residuals and the coseismic displacements show that only a bilateral rupture occurs on the intersection between the southern Beichuan Fault and Xiaoyudong Fault can the synthetic records of the stations located near the southwestern end of the Beichuan Fault conform to the observed ones, and meanwhile, the Pengguan fault cannot generate large slips on its southwestern part. The possible rupture sequence is that the earthquake started at the low dip angle part of Beichuan Fault and propagates to the Pengguan Fault in the shallow area, the Xiaoyudong Fault is triggered by the Pengguan Fault, and then producing bilateral rupture on the high dip angle part of Beichuan Fault at the intersection with the Xiaoyudong Fault. (2) Through analysis of the synthetic second packet records of stations at the southwest area of the fault, we obtain the initial rupture time on the high dip angle part of Beichuan Fault may have a 8s stagnation. In terms of timing, there may be rupture sequence between the southern Beichuan Fault and Pengguan Fault which are parallel to each other. The rupture of the southern shallow part of Beichuan Fault with high dip angle may lag behind the Pengguan Fault. At the same time, there may be a multipoint rupture in the southern section of the Beichuan Fault. (3) There is a good correspondence between the area on the fault with larger slip rate and the surrounding stations with larger PGV. In areas where slip rate on the fault plane is large, the stations tend to have larger peak ground velocities.     

An upper mantleP-wave velocity model for eastern and southern Africa

AP-wave velocity model for the upper mantle beneath eastern and southern Africa is proposed. The top 250 km of the model is characterized by relatively low velocities similar to those deduced for the upper mantle beneath the western United States of America. At greater depths, the velocities gradually change to normal mantle values.