Remote triggering of deep earthquakes in the 2002 Tonga sequences

It is well established that an earthquake in the Earth's crust can trigger subsequent earthquakes, but such triggering has not been documented for deeper earthquakes. Models for shallow fault interactions suggest that static (permanent) stress changes can trigger nearby earthquakes, within a few fault lengths from the causative earthquake, whereas dynamic (transient) stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances. Here we present a detailed analysis of the 19 August 2002 Tonga deep earthquake sequences and show evidence for both static and dynamic triggering. Seven minutes after a magnitude 7.6 earthquake occurred at a depth of 598 km, a magnitude 7.7 earthquake (664 km depth) occurred 300 km away, in a previously aseismic region. We found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock. But the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible, thus suggesting dynamic triggering. The origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone. We propose that these events are triggered by transient effects in regions near criticality, but where earthquakes have difficulty nucleating without external influences.     

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 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.     

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...     

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.     

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.     

四川芦山Ms 7.0级地震震源机制解初步研究

 震源机制解研究是认识地震发震断层的重要手段,也是理解深部构造应力和地震发震机理的重要依据。2013年4月20日四川芦山发生M_s 7.0级地震,利用近震直达P波初动极性反演了地震机制解,同时利用全球地震台网波形记录,反演了地震机制解和矩心深度。两种方法所得发震断层走向倾角滑动角分别为208°/41°/98°和220°/46°/93°,表明这次地震为一高角度逆冲型地震,远震波形反演得到的矩心深度为12km。     

鸟取MJMA 7.3级地震震源区地震层析成像研究

采用2000年日本鸟取MJMA7.3级地震的907个余震及其地方震的24 756个P波和22 547个S波到时,确定鸟取地震震源区的P波、S波和泊松比的三维结构.在震源区地震波速变化幅度达4%,泊松比变化幅度达9%.在11 km深度发现高波速和低泊松比异常,表明这一部位为刚性的粗糙断层面,形成断裂带的锁定结构,它的破裂产生鸟取地震的主震.低波速和高泊松比异常在震源区之下的地壳中出现,这与菲律宾板块俯冲带的脱水过程和岩浆活动有关,导致Daisen弧后火山的形成.流体运动和岩浆活动,对鸟取地震的成核和破裂过程有重大的影响.     

Analysing the 1811-1812 New Madrid earthquakes with recent instrumentally recorded aftershocks

Although dynamic stress changes associated with the passage of seismic waves are thought to trigger earthquakes at great distances, more than 60 per cent of all aftershocks appear to be triggered by static stress changes within two rupture lengths of a mainshock. The observed distribution of aftershocks may thus be used to infer details of mainshock rupture geometry. Aftershocks following large mid-continental earthquakes, where background stressing rates are low, are known to persist for centuries, and models based on rate-and-state friction laws provide theoretical support for this inference. Most past studies of the New Madrid earthquake sequence have indeed assumed ongoing microseismicity to be a continuing aftershock sequence. Here we use instrumentally recorded aftershock locations and models of elastic stress change to develop a kinematically consistent rupture scenario for three of the four largest earthquakes of the 1811-1812 New Madrid sequence. Our results suggest that these three events occurred on two contiguous faults, producing lobes of increased stress near fault intersections and end points, in areas where present-day microearthquakes have been hitherto interpreted as evidence of primary mainshock rupture. We infer that the remaining New Madrid mainshock may have occurred more than 200 km north of this region in the Wabash Valley of southern Indiana and Illinois--an area that contains abundant modern microseismicity, and where substantial liquefaction was documented by historic accounts. Our results suggest that future large mid-plate earthquake sequences may extend over a much broader region than previously suspected.     

用非均匀速度模型对汶川8.0级地震余震重新定位

为了进一步探讨龙门山断裂带深部结构,根据四川省地震局提供的地震原始资料,采用Hypo 2000对汶川大地震以及震后M≥2.0级余震进行重新定位。从2008年5月12日汶川发生8.0级大地震后,截止到2011年4月15日,获得了26 278个地震记录。重新定位后对结果的总结为:(1)主震发生在龙门山推覆构造带中央断裂中段的北川-映秀断裂上,余震主要沿龙门山断裂带方向延伸,呈南北分段分布。重新定位后的到时残差为±0.35s,水平误差为±1.32km,深度误差为±5km。(2)在主震附近的映秀、理县和黑水有一条北西向的余震带与龙门山断裂带的捩断层一致。(3)在青川附近(龙门山断裂带的北端),此段成为余震密集地区,这与历史上此地很少有地震发生不吻合。     

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.     

Stress changes from the 2008 Wenchuan earthquake and increased hazard in the Sichuan basin

On 12 May 2008, the devastating magnitude 7.9 (Wenchuan) earthquake struck the eastern edge of the Tibetan plateau, collapsing buildings and killing thousands in major cities aligned along the western Sichuan basin in China. After such a large-magnitude earthquake, rearrangement of stresses in the crust commonly leads to subsequent damaging earthquakes. The mainshock of the 12 May earthquake ruptured with as much as 9 m of slip along the boundary between the Longmen Shan and Sichuan basin, and demonstrated the complex strike-slip and thrust motion that characterizes the region. The Sichuan basin and surroundings are also crossed by other active strike-slip and thrust faults. Here we present calculations of the coseismic stress changes that resulted from the 12 May event using models of those faults, and show that many indicate significant stress increases. Rapid mapping of such stress changes can help to locate fault sections with relatively higher odds of producing large aftershocks.     

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.     

探讨与汶川地震有关的焦点科学问题

基于孕震断层多锁固段脆性破裂理论和新划定的3.6版汶川地震区,再次探讨了与2008年汶川地震有关的若干科学问题,包括该震是否为主震?该震、2013年芦山MS7.0地震、2017年九寨沟M_S7.0地震之间关系?该震是否存在复发周期?该震发生前是否存在显著物理前兆?该震与紫坪铺水库之间关系?研究结果表明:汶川地震并非汶川地震区当前地震周期主震,未来将发生MS8.0~8.3标志性地震;2013年芦山地震和2017年九寨沟地震有直接联系且均与汶川地震密切相关,两者同为下一次标志性地震前的两次显著预震;汶川地震区标志性地震及其未来主震是否存在复发周期目前尚不能作出明确判断;在汶川地震前,由于自1995年12月18日后其震源体的物理状态长期保持近似不变,故不可能观测到显著的中长期与短临物理前兆;紫坪铺水库对汶川地震孕育过程的影响很小,可以忽略。     

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.     

汶川地震序列震源机制解研究

利用2008年5月12日至8月7日包括国家地震台网、 地方区域地震台网和流动地震台网在内的1023个地震台站及IRIS台站的数字地震波形记录资料, 读取P波初动极性, 并采用改进的格点尝试法对汶川地震序列震源机制解进行求解, 给出121个4.0级以上质量可靠的震源机制解。根据求解结果发现: 汶川Ms 8.0级主震P波初动解为纯逆冲型; 汶川余震序列震源机制解绝大部分是逆冲型或走滑型地震, 并具有分段分布特征; 其P轴方位有两个优势分布方向: 北西西?南东东向和北东东?南西西向。     

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.     

九寨沟Ms 7.0级地震的左旋走滑作用与动力机制

2017年8月8日四川九寨沟发生的Ms 7.0级地震是继2008年汶川Ms 8.0地震、2013年芦山Ms 7.0级地震后在青藏高原东缘发生的又一次强震。本文通过综合分析九寨沟Ms 7.0级地震及历史地震的震源机制解、余震和历史地震分布、区域应力场、活动断层等资料,来揭示九寨沟地震的发震构造与动力机制。初步研究结果表明:(1)此次地震的震中位于塔藏断裂、岷江断裂和虎牙断裂之间的交汇区,显示活动断裂的交汇区对此次地震的发生具有控制作用;(2)发震断裂为虎牙断裂,断裂走向为北西西向,倾向南西,倾角较陡,属于高倾角左旋走滑型地震;(3)震中位于虎牙断裂北段的北部地震空区,充填了1973年和1976年4次大于Mw6.0级地震空区;(4)此次地震位于2008年汶川Ms 8.0级地震的库仑应力增加区,应是汶川地震的应力传递和触发的结果;(5)此次地震位于巴颜喀拉块体的东北部顶角区,青藏高原东缘下地壳流向北东方向的挤出是驱动此次地震的动力机制。     

Decay of aftershock density with distance indicates triggering by dynamic stress

The majority of earthquakes are aftershocks, yet aftershock physics is not well understood. Many studies suggest that static stress changes trigger aftershocks, but recent work suggests that shaking (dynamic stresses) may also play a role. Here we measure the decay of aftershocks as a function of distance from magnitude 2-6 mainshocks in order to clarify the aftershock triggering process. We find that for short times after the mainshock, when low background seismicity rates allow for good aftershock detection, the decay is well fitted by a single inverse power law over distances of 0.2-50 km. The consistency of the trend indicates that the same triggering mechanism is working over the entire range. As static stress changes at the more distant aftershocks are negligible, this suggests that dynamic stresses may be triggering all of these aftershocks. We infer that the observed aftershock density is consistent with the probability of triggering aftershocks being nearly proportional to seismic wave amplitude. The data are not fitted well by models that combine static stress change with the evolution of frictionally locked faults.     

Post-earthquake ground movements correlated to pore-pressure transients

Large earthquakes alter the stress in the surrounding crust, leading to triggered earthquakes and aftershocks. A number of time-dependent processes, including afterslip, pore-fluid flow and viscous relaxation of the lower crust and upper mantle, further modify the stress and pore pressure near the fault, and hence the tendency for triggered earthquakes. It has proved difficult, however, to distinguish between these processes on the basis of direct field observations, despite considerable effort. Here we present a unique combination of measurements consisting of satellite radar interferograms and water-level changes in geothermal wells following two magnitude-6.5 earthquakes in the south Iceland seismic zone. The deformation recorded in the interferograms cannot be explained by either afterslip or visco-elastic relaxation, but is consistent with rebound of a porous elastic material in the first 1-2 months following the earthquakes. This interpretation is confirmed by direct measurements which show rapid (1-2-month) recovery of the earthquake-induced water-level changes. In contrast, the duration of the aftershock sequence is projected to be approximately 3.5 years, suggesting that pore-fluid flow does not control aftershock duration. But because the surface strains are dominated by pore-pressure changes in the shallow crust, we cannot rule out a longer pore-pressure transient at the depth of the aftershocks. The aftershock duration is consistent with models of seismicity rate variations based on rate- and state-dependent friction laws.