Preliminary results pertaining to coseismic displacement and preseismic strain accumulation of the Lushan M S7.0 earthquake, as reflected by GPS surveying

This paper presents the coseismic displacement and preseismic deformation fields of the Lushan M _S7.0 earthquake that occurred on April 20, 2013. The results are based on GPS observations along the Longmenshan fault and within its vicinity. The coseismic displacement and preseismic GPS results indicate that in the strain release of this earthquake, the thrust rupture is dominant and the laevorotation movement is secondary. Furthermore, we infer that any possible the rupture does not reach the earth’s surface, and the seismogenic fault is most likely one fault to the east of the Guanxian-Anxian fault. Some detailed results are obtainable. (1) The southern segment of the Longmenshan fault is locked preceding the Lushan earthquake. After the Wenchuan earthquake, the strain accumulation rate in the southeast direction accelerates in the epicenter of the Lushan earthquake, and the angle between the principal compressional strain and the seismogenic fault indicates that a sinistral deformation background in the direction of the seismogenic fault precedes the Lushan earthquake. Therefore, it is evident that the Wenchuan M _S8.0 earthquake accelerated the pregnancy of the Lushan earthquake. (2) The coseismic displacements reflected by GPS data are mainly located in a region that is 230 km (NW direction) × 100 km (SW direction), and coseismic displacements larger than 10 mm lie predominantly in a 100-km region (NW direction). (3) On a large scale, the coseismic displacement shows thrust characteristics, but the associated values are remarkably small in the near field (within 70 km) of the earthquake fault. Meanwhile, the thrust movement in this 70-km region does not correspond with the attenuation characteristics of the strain release, indicating that the rupture of this earthquake does not reach the earth’s surface. (4) The laevorotation movements are remarkable in the 50-km region, which is located in the hanging wall that is close to the earthquake fault, and the corresponding values in this case correlate with the attenuation characteristics of the strain release.     

Lushan M S7.0 earthquake: A special earthquake occurs on curved fault

Relocation result shows that the aftershocks of the Lushan M _S7.0 earthquake spatially distribute in a shape like “half bowl”, indicating that the rupture structure of the mainshock is a highly curved surface. Kinematic analysis reveals that a laterally varied dislocation pattern occurs on this curved fault even though a single relative horizontal movement controls slip on this fault. Reverse slip prevails on curved fault. However, significant normal slip is predicted near the edge of north flank. Moreover, the north flank features left-lateral slip while the south flank contrarily features right-lateral slip. The relative scope of aftershock distribution implies inadequate breaking of the curved fault during the mainshock, calling for the attention to potential earthquake risk on the neighboring portions of the coseismic rupture due to significant increase of the coseismic Coulomb stress. Coseismic stress modeling also reveals that it is unnecessary for the stress on ruptured part to be unloaded following the earthquakes on the curved fault. The coseismic stress loading on ruptured elements unveils the specialty of faulting for the Lushan earthquake and we conclude that this specialty is due to the highly curved fault geometry.     

Slip model for the 2011 M_w 9.0 Sendai （Japan） earthquake and its M_w 7.9 aftershock derived from GPS data

Using GPS-measured coseismic and post-seismic displacements for the 8 h following the M w 9.0 Sendai earthquake of March 11, 2011, coseismic and post-seismic fault slip models were developed based on a layered crustal model. The geodetic moment magnitude of the main shock was measured as approximately M w 8.98. The slip exhibits clear reverse characteristics, with a maximum near the hypocenter, and a magnitude of about 23.3 m. Some strike-slip behavior may occur on the two sides of the peak rupture zone. Almost 90% of the seismic moments released by the main shock occurred at depths less than 40 km. The energy released by the fault slip in the 8 h following the main shock is approximately equal to an earthquake of M w 8.13. With a maximum of ~1.5 m, the post-seismic slip was concentrated in the southwestern part of the coseismic rupture fault, which agrees well with the location and behavior of the M w 7.9 aftershock. This implies that the post-seismic deformation in the 8 h after the main shock was mainly induced by the M w 7.9 aftershock. In addition, a post-seismic slip of 0.2-0.4 m was observed at the down-dip extension of the coseismic rupture, which may have been caused by the effect of after-slip during this period.     

Rupture process of the Wenchuan <Emphasis Type="Italic">M</Emphasis>8.0 earthquake of Sichuan China: the segmentation feature

Moment tensor solution, rupture process and rupture characteristics of the great Wenchuan M8.0 earthquake are studied by using 39 long-period P and SH waveforms with evenly azimuth coverage of stations. Our results reveal that the Wenchuan M8.0 event consisted of 5 sub-events of Mw≥7.3 occurring succesively in time and space. Rupture started with a Mw7.3 introductory strike-slip faulting in the first 12 s, then within 12?40 s, two sub-events with Mw7.6 and Mw7.4 occurred within 80 km northeast from the init...     

Lushan M S7.0 earthquake: A blind reserve-fault event

In the epicenter of the Lushan M _S7.0 earthquake there are several imbricate active reverse faults lying from northwest to southeast, namely the Gengda-Longdong, Yanjing-Wulong, Shuangshi-Dachuan and Dayi faults. Emergency field investigations have indicated that no apparent earthquake surface rupture zones were located along these active faults or their adjacent areas. Only brittle compressive ruptures in the cement-covered pavements can be seen in Shuangshi, Taiping, Longxing and Longmen Townships, and these ruptures show that a local crustal shortening occurred in the region during the earthquake. Combining spatial distribution of the relocated aftershocks and focal mechanism solutions, it is inferred that the Lushan earthquake is classified as a typical blind reverse-fault earthquake, and it is advised that the relevant departments should pay great attention to other historically un-ruptured segments along the Longmenshan thrust belt and throughout its adjacent areas.     

Relocation of the mainshock and aftershock sequences of M S7.0 Sichuan Lushan earthquake

The mainshock of April 20, 2013 Sichuan Lushan M _S7.0 earthquake was relocated using a 3-D velocity model. Double difference algorithm was applied to relocate aftershock sequences of Lushan earthquake. The locations of 2405 aftershocks were determined. The location errors in E-W, N-S and U-D direction were 0.30, 0.29 and 0.59 km on average, respectively. The location of the mainshock is 102.983°E, 30.291°N and the focal depth is 17.6 km. The relocation results show that the aftershocks spread approximately 35 km in length and 16 km in width. The dominant distribution of the focal depth ranges from 10 to 20 km. A few earthquakes occurred in the shallow crust. Focal depth profiles show fault planes dip to the northwest, manifested itself as a listric thrust fault. The dip angle is steep in the shallow crust and gentle in the deep crust. Although the epicenters of aftershocks distributed mainly along both sides of the Shuangshi-Dachuan fault, the seismogenic fault may be a blind thrust fault on the eastern side of the Shuangshi-Dachuan fault. Earthquake relocation results reveal that there is a southeastward tilt aftershock belt intersecting with the seismogenic fault with y-shape. We speculate it is a back thrust fault that often appears in a thrust fault system. Lushan earthquake triggered the seismic activity of the back thrust fault.     

Deep structure beneath the southwestern section of the Longmenshan fault zone and seimogenetic context of the 4.20 Lushan M S7.0 earthquake

Magnetotelluric measurements were carried out along two profiles across the middle and southwestern sections of the Longmenshan fault zone (LMSf) from 2009 to 2011, after the 2008 Wenchuan M _W7.9 earthquake. The former profile crosses the Wenchuan event epicenter and the latter one crosses 2013 Lushan M _S7.0 event epicenter. The data were analyzed using advanced processing techniques, including phase tensor and two-dimensional inversion methods, in order to obtain reliable 2-D profiles of the electrical structure in the vicinity of the two earthquakes. A comparison of the two profiles indicates both similarities and differences in the deep crustal structure of the LMSf. West of the southwestern section, a crustal high conductivity layer (HCL) is present at about 10 km depth below the Songpan-Garzê block; this is about 10 km shallower than that under the middle section of the LMSf. A high resistivity body (HRB) is observed beneath the southwestern section, extending from the near surface to the top of upper mantle. It has a smaller size than the HRB observed below the middle section. In the middle section, there is a local area of decreased resistivity within the HRB but there is absence of this area. The 2013 Lushan earthquake occurred close to the eastern boundary of HRB and the Shuangshi-Dachuan fault, of which the seismogenic context has both common and different features in comparison with the 2008 Wenchuan event. On a large scale, the 2013 Lushan earthquake is associated with the HCL and deformation in the crust including HCL of the eastern Tibetan Plateau. In order to assess seismic risk, it is important to consider both the stress state and the detailed crustal structure in different parts of the LMSf.     

The Lushan M S7.0 earthquake and activity of the southern segment of the Longmenshan fault zone

Following the Lushan M _S7.0 earthquake on 20 April 2013, a topic of much concern is whether events of M _S7 or greater could occur again on the southern segment of the Longmenshan fault zone. In providing evidence to answer this question, this work analyzes the tectonic relationship between the Lushan event and the 2008 Wenchuan earthquake and the rupture history of the southern segment of the Longmenshan fault zone, through field investigations of active tectonics and paleoearthquake research, and our preliminary conclusions are as follows. The activity of the southern segment of the Longmenshan fault zone is much different to that of its central section, and the late Quaternary activity has propagated forward to the basin in the east. The seismogenic structure of the 2008 Wenchuan earthquake is the central-fore-range fault system, whereas that of the 2013 Lushan event is attributed to the fore-range-range-front fault system, rather than the central fault. The southern segment of the Longmenshan fault zone becomes wider towards the south with an increasing number of secondary faults, of which the individual faults exhibit much weaker surface activity. Therefore, this section is not as capable of generating a major earthquake as is the central segment. It is most likely that the 2013 earthquake fills the seismic gap around Lushan on the southern segment of the Longmenshan fault zone.     

The spatial distribution pattern of landslides triggered by the 20 April 2013 Lushan earthquake of China and its implication to identification of the seismogenic fault

After the 20 April 2013 Lushan MS6.6 earthquake occurred,investigation and identification of the seismogenic fault for this event have become a focused and debatable issue.This work prepared an initial landslide inventory map related to the Lushan earthquake based on field investigations and visual interpretation of high-resolution aerial photographs and provided evidence for solving the issue aforementioned.The analysis of three landslide-density profiles perpendicular to strike direction of the probable seismogenic fault shows that many landslides occurred on the footwall of the Shuangshi–Dachuan fault(SDF),without sudden change of landslide density near the fault.Very few landslides were detected near the Dayi fault(DF)and also no change of landslide density there.While obvious sudden change of landslide density appeared about 1–2 km from the northwest to the western Shangli fault(WSF),and the landslide density on the hanging wall of the fault is obviously higher than that of on the footwall.Therefore,we infer that the seismogenic fault for the Lushan earthquake is neither the SDF nor the DF,rather probably the WSF located between these two faults,which is an evident linear trace on the earth surface.Meanwhile,the coseismic slip did not propagate upward to the ground,implying the Lushan earthquake was spawned by a blind-thrust-fault beneath the WSF.     

Interseismic and coseismic displacements of the Lushan Ms 7.0 earthquake inferred from leveling measurements

By using precise leveling data observed between 1985 and 2010 across the south section of the Longmenshan fault zone, and eliminating the coseismic displacements caused by the Wenchuan Ms 8.0 earthquake, the interseismic vertical deformation field was obtained. The result shows that the Lushan region, located between the Shuangshi-Dachuan fault （front range of the Long- menshan fault） and the Xinkaidian fault （south section of the Dayi fault）, is situated in the intersection zone of positive and negative vertical deformation gradient zones, indicating that this zone was locked within 25 years before the Lushan earthquake. Based on leveling data across the rupture zone surveyed between 2010 and 2013, and by eliminating the vertical deformation within 3 years before the earthquake, the coseismic vertical displacement was derived. The coseismic vertical displacement for the benchmark DD35, which is closest to the epicenter, is up to 198.4 mm （with respect to MY165A）. The coseismic dis- placement field revealed that the northwest region （hanging wall） moved upwards in comparison with the southeastern region （foot wall）, suggesting that the seismogenic fault mainly underwent thrust faulting. By comparing the coseismic and interseismic vertical deformation fields, it was found that the mechanisms of this earthquake are consistent with the elastic rebound theory; the elastic strain energy （displacement deficit） accumulated before the Lu- shan earthquake was released during this quake.     

Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array

The disastrous Sumatra-Andaman earthquake of 26 December 2004 was one of the largest ever recorded. The damage potential of such earthquakes depends on the extent and magnitude of fault slip. The first reliable moment magnitude estimate of 9.0 was obtained several hours after the Sumatra-Andaman earthquake, but more recent, longer-period, normal-mode analyses have indicated that it had a moment magnitude of 9.3, about 2.5 times larger. Here we introduce a method for directly imaging earthquake rupture that uses the first-arriving compressional wave and is potentially able to produce detailed images within 30 min of rupture initiation. We used the Hi-Net seismic array in Japan as an antenna to map the progression of slip by monitoring the direction of high-frequency radiation. We find that the rupture spread over the entire 1,300-km-long aftershock zone by propagating northward at roughly 2.8 km s(-1) for approximately 8 minutes. Comparisons with the aftershock areas of other great earthquakes indicate that the Sumatra-Andaman earthquake did indeed have a moment magnitude of approximately 9.3. Its rupture, in both duration and extent, is the longest ever recorded.     

Three-dimensional deformation caused by the Bam, Iran, earthquake and the origin of shallow slip deficit

Our understanding of the earthquake process requires detailed insights into how the tectonic stresses are accumulated and released on seismogenic faults. We derive the full vector displacement field due to the Bam, Iran, earthquake of moment magnitude 6.5 using radar data from the Envisat satellite of the European Space Agency. Analysis of surface deformation indicates that most of the seismic moment release along the 20-km-long strike-slip rupture occurred at a shallow depth of 4-5 km, yet the rupture did not break the surface. The Bam event may therefore represent an end-member case of the 'shallow slip deficit' model, which postulates that coseismic slip in the uppermost crust is systematically less than that at seismogenic depths (4-10 km). The InSAR-derived surface displacement data from the Bam and other large shallow earthquakes suggest that the uppermost section of the seismogenic crust around young and developing faults may undergo a distributed failure in the interseismic period, thereby accumulating little elastic strain.     

Focal mechanism analysis and parameter estimation of Zhangbei-Shangyi earthquake from differential SAR interferometry

On 10 January, 1998 an earthquake of Ms=6.2occurred in the Zhangbei-Shangyi region of North China.The surface seismic deformation was measured in the previous study using the 3 pass ERS-1/2 SAR differential interferometric technology (D-INSAR). In this note the focal mechanism of Zhangbei-Shangyi earthquake is estimated from the D-INSAR measurement of surface deformation based on a standard elastic dislocation model for seismic displacement. The inversion procedure is an iterative, linear least-squares algorithm. Through the relation between the focal parameters and displacement in the line of sight direction measured in the radar interferogram, the optimum focal parameter set is derived. The results show that the seismic fault of Zhangbei-Shangyi earthquake is a thrust fault dipping SW with a large right-lateral displacement component.The strike and dip are 95° and 30° respectively on a fault patch of 12 km long by 14 km wide. Its hypocenter is located at N40°58', E114°21', and 7.5 km in depth. The estimated slip vector is 0.728 m with a rake of 105.95°, the trend of slip is NW13.26°, and M0is 2.69×1018 N @ m.     

South American SLR stations monitoring ground displacement caused by the M8.8 Chilean earthquake of 2010

The M8.8 Chilean earthquake of February 27, 2010 caused a large surface displacement near its epicenter. Satellite Laser Ranging (SLR) station 7405, cooperatively operated by Germany and Chile about 130 km from the epicenter was affected by the earthquake. Another SLR station, No. 7406, operated jointly by the National Astronomical Observatories of the Chinese Academy of Sciences and San Juan University of Argentina, was about 600 km from the epicenter. Combining the observations of these two SLR stations with global SLR station observations, we calculate the geocentric coordinates of the two stations based on ITRF2000. The results show that the coordinates of the two stations have changed by varying degrees. SLR-7405 moved substantially towards the southwest with displacements of about 3.11, 0.52 and 0.49 m in X, Y, Z directions; the corresponding displacements for SLR-7406 are about 0.02, 0.03 and 0.02 m. This solution, as an independent result derived from SLR observations, could provide an essential external check for other positioning techniques such as GPS.     

Seismicity characteristics of the 2011 <Emphasis Type="Italic">M</Emphasis>9.0 Tohoku earthquake near the East Coast of Honshu in Japan

The spatiotemporal evolution of the M9.0 Tohoku earthquake sequence off the East Coast of Honshu in Japan on March 11,2011 and precursive seismic activity near the Japan Trench show that the earthquake sequence has foreshock-main shock-aftershock characteristics.Its foreshock sequence is characterized by a concentrated spatial distribution,low b value and the same focal mechanisms.Half an hour after the main shock,the two greatest aftershocks,with magnitudes of M7.9 and M7.7,occurred,followed by a rapid reduction in the strength of events.The aftershock activity was enhanced roughly two weeks and one month after the main event.This great earthquake ruptured bilaterally.Five hours after the main shock,the aftershock zone extended over a range that was 500 km in length and 300 km in width.A day later,the long axis of the aftershock area had expanded to about 600 km.Nine years prior to the 2011 earthquake,the seismicity in the location of the seismic source for this event enhanced significantly,with the extent of this area of enhanced seismicity being roughly equivalent to the aftershock zone.     

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 M_S7.1 Yushu earthquake, Qinghai is a char-acteristic 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.     

Insight into the 2004 Sumatra-Andaman earthquake from GPS measurements in southeast Asia

Data collected at approximately 60 Global Positioning System (GPS) sites in southeast Asia show the crustal deformation caused by the 26 December 2004 Sumatra-Andaman earthquake at an unprecedented large scale. Small but significant co-seismic jumps are clearly detected more than 3,000 km from the earthquake epicentre. The nearest sites, still more than 400 km away, show displacements of 10 cm or more. Here we show that the rupture plane for this earthquake must have been at least 1,000 km long and that non-homogeneous slip is required to fit the large displacement gradients revealed by the GPS measurements. Our kinematic analysis of the GPS recordings indicates that the centroid of released deformation is located at least 200 km north of the seismological epicentre. It also provides evidence that the rupture propagated northward sufficiently fast for stations in northern Thailand to have reached their final positions less than 10 min after the earthquake, hence ruling out the hypothesis of a silent slow aseismic rupture.     

Rupture of the 2004 Sumatra-Andaman earthquake inferred from direct P-wave imaging

The Sumatra-Andaman earthquake on December 26, 2004 is the first well recorded gigantic earthquake (moment magnitude MW 9.3) by modern broadband seismic and Global Positioning System networks. The rich seismic and geodetic recordings have documented unprecedented details about the earthquake rupture, coseismic and postseismic deformations. This is a report of detailed images of the rupture process using the first-arriving compressional waves recorded by the China National Digital Seismic Network (CNDSN). An improved imaging condition was employed to account for the sparse distribution of the CNDSN stations. The resulting images are consistent with the major rupture features reported by previous seismic and geodetic studies. It is found that the earthquake rupture initiated at offshore of northwestern Sumatra and propagated in the north northwest direction at a speed of 2.7 ± 0.2 km/s. The rupture continued for at least 420 s and extended about 1200-1300 km along the Andaman trough with two bursts of seismic energy.     

阿克陶MW6.6地震的发震构造及滑动特征

2016年11月25日新疆阿克陶发生了M_W6.6地震。通常震中所处断层的破裂特性与周围区域应力场的动力学特征具有紧密的联系。通过对发震构造断层的精确刻画以及区域地壳中应力释放细节的深入探究,可以加深对发震构造周围地震动力学特征的认识,同时也对判定当地未来一段时间内的地震活动趋势提供了重要参考。首先搜集整理了震源附近的26条余震震源机制,利用网格搜索法反演得到震中附近的应力场,发现该地区主压应力方向为157.36°,倾伏角为1.15°,主张应力方向为66.56°,倾伏角为34.98°,与该地所处的帕米尔高原陆内俯冲形成近东西向断裂的右旋走滑兼有逆冲的背景相一致;然后利用389条余震精定位数据,结合高斯-牛顿算法和模拟退火算法拟合得到发震断层面的走向为103.64°、倾角为65.65°,这与木吉右旋走滑断裂的几何特征基本重合;将所求应力场投影到断层上,得到滑动角为152.77°,该地震表现为右旋走滑断层;最后利用本研究获得的区域应力张量模拟得到的该状态下的各种形状断层面的相对应力分布,发现该地震发生的断层面的相对剪应力接近1,破裂方向与震源区的最优剪切力方向相同,表明...     

Ms7.0芦山地震的发生与水库有关吗？

 2013年4月20日芦山地震发生后,围绕芦山地震的发生是否与三峡水库、紫坪铺及瀑布沟等水库蓄水有关的争论较多。但定性的讨论难以给出一个确切的定论。本文拟从定量计算分析讨论三峡、紫坪铺和瀑布沟水库蓄水对芦山地震震源处弹性应力场和孔隙压力场的影响。计算结果初步表明,三峡、紫坪铺及瀑布沟水库蓄水引起芦山地震震源处的弹性应力变化仅为几十Pa;在低渗透系数(扩散系数小于1m²/s)时,3个水库蓄水造成的孔隙压力变化接近于0,对M_s^7.0芦山地震的发生几乎没有影响;当取高渗透扩散系数(10m²/s)时,只有瀑布沟水库蓄水引起震源处孔隙压力增加3.2kPa,库仑应力变化1.9kPa,然而连续延伸100km高渗透率断层基本不可能存在;在选取扩散系数1m²/s时,3个水库若均蓄水100a对M_s^7.0芦山地震震源处孔隙压力也仅仅达到kPa量级。研究结果表明,此次M_s^7.0芦山地震的发生与三峡、紫坪铺、瀑布沟等水库蓄水无关。