Seismology: speed and size of the Sumatra earthquake

Our seismological results reveal that Indonesia's devastating Sumatra-Andaman earthquake on 26 December 2004 was 2.5 times larger than initial reports suggested--second only to the 1960 Chilean earthquake in recorded magnitude. They indicate that it slowly released its energy by slip along a 1,200-km fault, generating a long rupture that contributed to the subsequent tsunami. Now that the entire rupture zone has slipped, the strain accumulated from the subduction of the Indian plate beneath the Burma microplate has been released, and there is no immediate danger of a similar tsunami being generated on this part of the plate boundary, although large earthquakes on segments to the south still present a threat.     

Predecessors of the giant 1960 Chile earthquake

It is commonly thought that the longer the time since last earthquake, the larger the next earthquake's slip will be. But this logical predictor of earthquake size, unsuccessful for large earthquakes on a strike-slip fault, fails also with the giant 1960 Chile earthquake of magnitude 9.5 (ref. 3). Although the time since the preceding earthquake spanned 123 years (refs 4, 5), the estimated slip in 1960, which occurred on a fault between the Nazca and South American tectonic plates, equalled 250-350 years' worth of the plate motion. Thus the average interval between such giant earthquakes on this fault should span several centuries. Here we present evidence that such long intervals were indeed typical of the last two millennia. We use buried soils and sand layers as records of tectonic subsidence and tsunami inundation at an estuary midway along the 1960 rupture. In these records, the 1960 earthquake ended a recurrence interval that had begun almost four centuries before, with an earthquake documented by Spanish conquistadors in 1575. Two later earthquakes, in 1737 and 1837, produced little if any subsidence or tsunami at the estuary and they therefore probably left the fault partly loaded with accumulated plate motion that the 1960 earthquake then expended.     

The potential for giant tsunamigenic earthquakes in the northern Bay of Bengal

The great Sumatra-Andaman earthquake and Indian Ocean tsunami of 2004 came as a surprise to most of the earth science community. Although it is now widely recognized that the risk of another giant earthquake is high off central Sumatra, just east of the 2004 earthquake, there seems to be relatively little concern about the subduction zone to the north, in the northern Bay of Bengal along the coast of Myanmar. Here I show that similar indicators suggest a high potential for giant earthquakes along the coast of Myanmar. These indicators include the tectonic environment, which is similar to other subduction zones that experience giant megathrust earthquakes, stress and crustal strain observations, which indicate that the seismogenic zone is locked, and historical earthquake activity, which indicates that giant tsunamigenic earthquakes have occurred there in the past. These are all consistent with active subduction in the Myanmar subduction zone and I suggest that the seismogenic zone extends beneath the Bengal Fan. I conclude therefore that giant earthquakes probably occur off the coast of Myanmar, and that a large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard.     

Tracking the rupture of the Mw = 9.3 Sumatra earthquake over 1,150 km at teleseismic distance

On 26 December 2004, a moment magnitude Mw = 9.3 earthquake occurred along Northern Sumatra, the Nicobar and Andaman islands, resulting in a devastating tsunami in the Indian Ocean region. The rapid and accurate estimation of the rupture length and direction of such tsunami-generating earthquakes is crucial for constraining both tsunami wave-height models as well as the seismic moment of the events. Compressional seismic waves generated at the hypocentre of the Sumatra earthquake arrived after about 12 min at the broadband seismic stations of the German Regional Seismic Network (GRSN), located approximately 9,000 km from the event. Here we present a modification of a standard array-seismological approach and show that it is possible to track the propagating rupture front of the Sumatra earthquake over a total rupture length of 1,150 km. We estimate the average rupture speed to be 2.3-2.7 km s(-1) and the total duration of rupture to be at least 430 s, and probably between 480 and 500 s.     

Records of the tsunami induced by the 2010 Chilean earthquake from Xiaoqushan seafloor observatory in the East China Sea

Sea-level variation can be induced by periodic tides, stochastic wind, air pressure, and swell. Larger sea-level variation has the potential to cause coastal disasters. In this paper, real-time continuous data obtained by the Xiaoqushan seafloor observatory in the East China Sea were analyzed employing frequency power spectral and tidal harmonic methods to extract the major components and periodicities of sea-level change. The sea-level anomaly (sla) was calculated by subtracting the tidal components from the observed sea level data. In the study period, the correlation between sla and the local north-south wind speed was high with a correlation coefficient of 0.65 at the 95% confidence level. The local wind-induced sea-level anomaly (sla wind ) was therefore computed through linear fitting. Although sla wind is one of the main components of sla, the residual sea-level anomaly (sla residual ) obtained by subtracting sla wind from sla is not zero, suggesting that there are other factors besides wind. Detailed analysis of the sea-level data at the time of the 8.8-magnitude Chilean earthquake on February 27, 2010 showed a peak sla residual value of 0.48 m at around 15:00 on February 28, which was highly coincident with the tsunami arrival time forecast by the Pacific Tsunami Warning Center. The peak sla residual event is therefore linked with the tsunami induced by the 2010 Chilean earthquake. This is the first time that a tsunami has been detected using real-time continuous data recorded by a seafloor observatory in the sea off China. Such observations are expected to improve tsunami forecast models and promote the development of a tsunami warning system and a seafloor observatory network in the East China Sea.     

大型储罐地震作用下罐壁抗失稳可靠度分析

大型储罐在地震作用下的典型破坏形式是"象足"屈曲破坏,而轴向压应力是"象足"屈曲破坏的主要因素。结合实际工程情况,运用大型通用有限元分析软件Adina对大型储罐在不同地震烈度、储液深度下的罐壁轴向压应力进行地震作用下的数值模拟,并运用可靠性分析的JC法,分析大型储罐不同储液深度、不同地震烈度下的罐壁抗失稳可靠度情况。结果表明:在同一地震烈度下,随着储罐储液深度的增加,储罐罐壁的轴向压应力将随之增大;大型储罐同一地震烈度下储液深度越大,罐壁轴向抗失稳可靠度越低;在实际工程设计中应该首先进行工程场地地震安全性评价,尽可能避开地震带。     

BKP双旋转体系与朝鲜半岛地震活动安全岛效应

朝鲜半岛位于中国东北和日本西南部的地震频发区之间,但在历史资料或仪器记录中,过去百年内没有发生过灾难性的地震。为了说明这一问题,引入了阿穆尔板块(BKP),通过对板块周缘地震进行震源机制解的分析,给定了一个科学合理的边界带,结合GPS相对位移测量,结果显示,BKP内部以郯庐断裂带为界,北部作逆时针旋转,南部朝鲜半岛作顺时针旋转的"双旋转"运动。BKP在旋转过程中,通过郯庐断裂带卸载了大部分的旋转应力。根据古登堡-里克特经验公式计算,在1900~2014年间,沿BKP边界的地震带释放总能量是朝鲜半岛地震的(M≥3.0)10~6倍,说明朝鲜半岛处于一个稳定的"安全岛"状态。通过研究,初探了阿穆尔板块边界的划定和板块边缘及内部构造运动的问题,且从多方面阐明了朝鲜半岛免于灾难性地震的原因。     

Some thoughts on seismotectonics of major earthquake occurrence zones in China

A major earthquake occurrence zone means a place where M≥6 events have occurred since the Holocene and similar shocks may happen again in the future. The dynamic context of the major earthquake occurrence zones in China is primarily associated with the NNE-directed push of the India plate,next with the westward subduction of the Pacific plate. The Chinese mainland is a grand mosaic structure of many crust blocks bounded by faults and sutures. When it is suffered from boundary stresses,deformation takes place along these faults or sutures while the block interiors remain relatively stable or intact. Since the Quaternary,for example,left slip on the Xianshuihe-Xiaojiang fault zone in southwestern China has produced a number of fault-depression basins in extensional areas during periods Q1 and Q2. In the Q3,the change of stress orientation and enhancement of tectonic movement made faults of varied trends link each other,and continued to be active till present day,producing active fault zones in this region. Usually major earthquakes occur at some special locations on these active fault zones. During these events,in the epicenter areas experience intensive deformation characterized by large-amplitude rise and fall of neighboring sections,generation of horst-graben systems and dammed rivers. The studies on palaeoearthquakes suggest that major shocks of close magnitudes often repeated for several times at a same place. By comparison of the Chi-Chi,Taiwan event in 1999 and Yuza,Yunnan event in 1955,including contours of accelerations and intensities,destruction of buildings,and in contrast to the Xigeda formation in southwestern China,a sandwich model is established to account for the mechanism of deformation caused by major earthquakes. This model consists of three layers,i. e. the two walls of a fault and the ruptured zone intercalated between them. This ruptured zone is just the loci where stress is built up and released,and serves as a channel for seismic waves.     

Plate-boundary deformation associated with the great Sumatra-Andaman earthquake

The Sumatra-Andaman earthquake of 26 December 2004 is the first giant earthquake (moment magnitude M(w) > 9.0) to have occurred since the advent of modern space-based geodesy and broadband seismology. It therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events. Here we report estimates of the ground displacement associated with this event, using near-field Global Positioning System (GPS) surveys in northwestern Sumatra combined with in situ and remote observations of the vertical motion of coral reefs. These data show that the earthquake was generated by rupture of the Sunda subduction megathrust over a distance of >1,500 kilometres and a width of <150 kilometres. Megathrust slip exceeded 20 metres offshore northern Sumatra, mostly at depths shallower than 30 kilometres. Comparison of the geodetically and seismically inferred slip distribution indicates that approximately 30 per cent additional fault slip accrued in the 1.5 months following the 500-second-long seismic rupture. Both seismic and aseismic slip before our re-occupation of GPS sites occurred on the shallow portion of the megathrust, where the large Aceh tsunami originated. Slip tapers off abruptly along strike beneath Simeulue Island at the southeastern edge of the rupture, where the earthquake nucleated and where an M(w) = 7.2 earthquake occurred in late 2002. This edge also abuts the northern limit of slip in the 28 March 2005 M(w) = 8.7 Nias-Simeulue earthquake.     

Triggering of earthquake aftershocks by dynamic stresses

It is thought that small 'static' stress changes due to permanent fault displacement can alter the likelihood of, or trigger, earthquakes on nearby faults. Many studies of triggering in the near-field, particularly of aftershocks, rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault. Here we report a comparison of the aftershock pattern of the moment magnitude Mw = 7.3 Landers earthquake, not only with static stress changes but also with transient, oscillatory stress changes transmitted as seismic waves (that is, 'dynamic' stresses). Dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone. These dynamically weakened faults may fail after the seismic waves have passed by, and might even cause earthquakes that would not otherwise have occurred. We find similar asymmetries in the aftershock and dynamic stress patterns, the latter being due to rupture propagation, whereas the static stress changes lack this asymmetry. Previous studies have shown that dynamic stresses can promote failure at remote distances, but here we show that they can also do so nearby.     

Assessment of damage to buildings and farms during the 2011 <Emphasis Type="Italic">M</Emphasis> 9.0 earthquake and tsunami in Japan from remote sensing data

At 13:46 on March 11,2011(Beijing time),a great earthquake of magnitude of 9.0 occurred off the Pacific coast of northeastern Japan(Tohoku Region).After the earthquake,the Center for Earth Observation and Digital Earth(CEODE),Chinese Academy of Sciences,quickly launched a major disaster emergency response to assess the disaster.In this paper,we have extracted buildings and farms in the heavily devastated area that includes Iwate,Miyagi,and Fukushima Prefectures,based on multi-spectral remote sensing data.We have assessed building damage by overlaying seismic intensity and building distributions.In addition,we have assessed the extent of building and farm damage caused by the tsunami through the establishment of a tsunami impact assessment model that is based on terrain and distance from the coastline.The results were partially verified by high-resolution images obtained after the disaster.The results show that about 76% of buildings in Miyagi Prefecture were affected by seismic intensity 6 and above,with 24% of the buildings and 12% of the farms destroyed by the tsunami.These huge losses were echoes at lesser amounts in Fukushima and Iwate.The results provide the support of basic data,which can be further developed by economic and social aspects of the disaster assessment.     

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.     

The use of earthquake rate changes as a stress meter at Kilauea volcano

Stress changes in the Earth's crust are generally estimated from model calculations that use near-surface deformation as an observational constraint. But the widespread correlation of changes of earthquake activity with stress has led to suggestions that stress changes might be calculated from earthquake occurrence rates obtained from seismicity catalogues. Although this possibility has considerable appeal, because seismicity data are routinely collected and have good spatial and temporal resolution, the method has not yet proven successful, owing to the non-linearity of earthquake rate changes with respect to both stress and time. Here, however, we present two methods for inverting earthquake rate data to infer stress changes, using a formulation for the stress- and time-dependence of earthquake rates. Application of these methods at Kilauea volcano, in Hawaii, yields good agreement with independent estimates, indicating that earthquake rates can provide a practical remote-sensing stress meter.     

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.     

SeisComP3地震监测软件系统及其在海啸预警系统建设中的应用

 SeisComP3 软件系统是近些年发展起来的一款免费的、部分开源的地震实时监测与自动处理系统。根据地震监测、大地震海啸预警等工作需求,本研究组应用SeisComP3 系统提供的实用工具进行辅助功能开发,实现地震数据的实时汇集、共享与自动处理,为大地震海啸预警提供及时、可靠的地震基本参数。同时,基于SeisComP3 系统地震自动定位结果和实时波形数据,应用W 震相方法快速反演海啸强震震源机制解,为海啸数据模拟程序提供实际发震断层面参数解,提高后续海啸数值预报的准确性。实际应用表明：SeisComP3 软件系统为海啸业务提供了重要的数据保障和科技支撑,促进了海啸预警预报工作的发展。     

宝鸡市地震灾害区划

根据宝鸡市地质构造特征与历史地震的时空变化及李玉龙的地震带的划分标准,将宝鸡市划分为两个危害性地震地区;（１）陇县－千阳－宝鸡市区;（２）岐山,扶风,眉县交界地区与一个相对稳定区。     

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.     

2010年智利强震引发的海啸对台湾周边海域的影响

2010年2月27日南美洲智利中部近岸发生强震并引发海啸.利用海啸期间台湾周边海域验潮站的潮位资料,分析了此次海啸对台湾周边海域的影响,海啸波通过太平洋于震后25.5h到达台湾周边海域,最大波高达44 cm.进而从理论上讨论了海啸传播时间和波高变化的简单计算方法,并引入了波高衰减因子.结果表明,该简单计算方法能快速且较准确地计算出海啸波的传播时间,引入的波高衰减因子,可在一定程度上为台湾周边海域海啸的方便快捷的预警提供参考.     

Numerical simulation of March 11, 2011 Honshu, Japan tsunami

In order to predict tsunami hazards through numerical simulation,by using the focal mechanisms as well as fault parameters of Japan’s 2011 Tohoku Earthquake provided by National Geological Survey(referred to as USGS),we proposed a numerical model to simulate the Honshu,Japan tsunami.Numerical computing is conducted to investigate the security along the coast.We also analyzed the simulation results and distribution of tsunami disaster,trying to achieve a more reasonable tsunami warning program.Our numerical model is composed of simulation of surface deformation after the earthquake and the tsunami propagation process which is based on two dimensional shallow water equations.The simulation results show the characteristics of the tsunami propagation,and arrival times on recorder points are consistent with tsunami observation.This model can be applied to evaluate the security of the coastal area and obtain more accurate tsunami warning.