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.     

Fortnightly variations in the flow velocity of Rutford Ice Stream, West Antarctica

Most of the ice lost from the Antarctic ice sheet passes through a few fast-flowing and highly dynamic ice streams. Quantifying temporal variations in flow in these ice streams, and understanding their causes, is a prerequisite for estimating the potential contribution of the Antarctic ice sheet to global sea-level change. Here I show that surface velocities on a major West Antarctic Ice Stream, Rutford Ice Stream, vary periodically by about 20 per cent every two weeks as a result of tidal forcing. Tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter-term variations. The existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long-term change.     

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.     

断层滑移扰动下不同辐射区岩体响应特征

为了研究不同应力波辐射区岩体稳定性,引入Haskell有限震源模型,采用修正Yoffe函数计算特定震动矩下断层滑移速度曲线,构建了等效断层滑移型震源,进而利用数值仿真手段,对P波、S波辐射区在断层滑移作用下岩体响应特征进行模拟。结果表明,断层滑移的时间效应对P波、S波辐射区的质点震动强度影响较大。在超过距离震源中心1倍处,质点在S波辐射区的质点峰值速度V_ppv(peak particle velocity)值要明显高于P波辐射区的PPV值,但位移量及获得的应变能则相对较小。     

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.     

Source rupture process of Lushan M S7.0 earthquake, Sichuan, China and its tectonic implications

The source rupture process of the M _S7.0 Lushan earthquake was here evaluated using 40 long-period P waveforms with even azimuth coverage of stations. Results reveal that the rupture process of the Lushan M _S7.0 event to be simpler than that of the Wenchuan earthquake and also showed significant differences between the two rupture processes. The whole rupture process lasted 36 s and most of the moment was released within the first 13 s. The total released moment is 1.9×10¹⁹N m with M _W=6.8. Rupture propagated upwards and bilaterally to both sides from the initial point, resulting in a large slip region of 40 km×30 km, with the maximum slip of 1.8 m, located above the initial point. No surface displacement was estimated around the epicenter, but displacement was observed about 20 km NE and SW directions of the epicenter. Both showed slips of less than 40 cm. The rupture suddenly stopped at 20 km NE of the initial point. This was consistent with the aftershock activity. This phenomenon indicates the existence of significant variation of the medium or tectonic structure, which may prevent the propagation of the rupture and aftershock activity. The earthquake risk of the left segment of Qianshan fault is worthy of attention.     

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.     

采用拉索减震支座的斜拉桥地震易损性分析

利用易损性方法对斜拉桥设置拉索减震支座与否进行了细致的研究,并得到了塔底弯矩和墩梁位移的易损性曲线.通过基于概率的分析研究表明,拉索减震支座能够有效地限制墩梁的相对位移,同时还能够适当地减小塔底的弯矩.对斜拉桥安装拉索减震支座的减震机理进行探讨表明,依靠摩擦增加阻尼以及通过拉索改变地震力的传力路径是减小斜拉桥地震响应的主要方式.     

远场长周期地震动作用下高层框剪结构地震反应分析

选取3条远场长周期地震动和2条普通地震动,在比较其能量特性的基础上,采用ETABS软件建立两栋高层框剪结构的有限元模型。对比研究了地震动类型和结构基本周期对结构弹性地震反应的影响规律。结果表明:远场长周期地震动对高层框剪结构的地震反应具有放大作用;在其作用下结构的层间剪力、楼层位移和层间位移角明显大于普通地震动作用下的结果。远场地震动作用下结构的最大层间位移角出现在中下部楼层,且明显超出现行规范1/800的限值要求。普通地震动作用下结构的最大层间位移角出现在中上部楼层,能够满足规范限值要求。随着结构基本周期的增加,远场长周期地震动作用下结构的地震反应随之增大;普通地震动作用下结构的基底剪力、顶层位移逐渐增大;最大层间位移角略有下降、出现的楼层明显上移;顶层位移对长周期地震动、结构基本周期最为敏感。建议在高层结构抗震设计时进行远场长周期地震动作用下结构的抗震性能验算;且将结构位移作为重要的控制指标。     

潮汐应力与云南及邻区的地震

提出了潮汐应力对发震断层的力学模式,描述了附加潮汐应力对发震断层的促滑作用方式,并将该模式应用于云南及邻区的地震,计算了173个地震震源处沿主压应力P轴和主张应力T轴方向的附加潮汐应力分量,分析了这睦量对发震断层的作用方式以及受潮汐应力促滑作用的发震断层类型,结果表明,所研究的云南及邻区发生的173个地震中,64％的地震发震断层受到潮汐应力的促滑作用,其中,受减压型促滑作用的发震断层数比例略大于受增压型促滑作用的发动层数比例;在受到潮汐应力促滑作用的111个发震断层中,走滑型发震断层占67％,倾滑斜滑型发震断层占33％,说明云南及邻区的走滑型地震较易受到潮汐应力的触发作用。     

长周期随机地震作用下超大跨斜拉桥的行波效应分析

从各个国家和地区的地震记录库中寻找出一些具有良好记录质量和基本场地资料的强震记录,并从中挑选出较为典型的长周期地震波,对比分析了不同场地条件下长周期地震波和普通地震波的平均加速度反应谱及其规范形式的分段拟合曲线.选取软土场地拟合反应谱作为目标反应谱,用迭代方法求取与目标反应谱相对应的功率谱密度函数,分析了基于长周期地震动加速度功率谱密度的特点.在此基础上,以某超大跨斜拉桥结构为背景,利用ANSYS软件建立三维有限元分析模型,分别以文中拟合的长周期地震动功率谱密度和普通地震动功率谱密度作为输入,采用直接求取位移的改进虚拟激励法,对超大跨斜拉桥结构进行了不同视波速下的行波效应分析.结果表明:长周期地震动功率谱密度的卓越频率明显低于普通地震动功率谱密度的卓越频率.长周期地震动功率谱作用下的桥梁左桥塔顶、右桥塔顶和桥面板跨中节点的位移响应功率谱值均明显大于普通地震动功率谱作用下的结果,桥梁塔顶位移响应功率谱均呈双峰分布.在本文算例中,考虑行波效应使超大跨斜拉桥结构的位移响应和弯矩响应结果有所减小.     

全球大地震破裂空间复杂度特征研究

基于有限断层反演得到的滑移数据, 用独立滑移单元的数量表征地震破裂复杂度, 据此对大地震破裂进行分组, 并研究破裂复杂度与主要震源参数之间的关系, 探讨破裂复杂度的全球及区域空间分布特征。结果表明, 矩震级很大(Mw ≥ 8.5)的事件, 地震破裂复杂度更大; 破裂复杂度较高的地震分布在浅层地壳(≤ 30 km)内的概率最大, 随着震源深度增加, 破裂复杂度对震源深度的敏感性逐渐消失; 走滑断层机制占比较大的事件, 破裂复杂度较高; 破裂复杂度与地震能矩比没有明确的关系; 破裂复杂度的空间分布特征与区域地质构造环境相关。破裂复杂度的空间分布特征可以分为3类, 第一类是板块之间简单碰撞产生的俯冲带, 板块交界处的滑动速率和方向较为一致, 这种情形下以比较简单的事件类型为主; 第二类是多板块交界处, 或者板块交界处的滑动速率和方向在整个区域内存在差异; 第三类是大陆内部的强烈挤压地带。与第一类空间分布特征相比, 第二类和第三类情形下破裂复杂度相对更高。地震破裂复杂度可以在一定程度上反映区域应力场的复杂性。     

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.     

城市地下管廊结构地震动力响应分析

地震安全问题是城市地下管廊设计中不得不考虑的部分.先使用反应位移法计算地震时管廊结构的内力,得出管廊薄弱部位.接着使用动力时程方法分析管廊的地震响应,建立土体与结构数值模型,在模型底部输入水平地震作用.采用土-结构界面接触单元,重点考虑土与结构之间的相互作用,得出结构与土之间的分离与滑移情况以及管廊在水平地震作用下的位移变形和应力分布.结果发现:地下管廊标准段结构在水平地震作用下发生明显的侧向位移,容易发生弯剪破坏,而水平方向未发生明显位移;管廊结构地震破坏时的薄弱环节在顶板、底板与侧墙的连接部位以及中隔墙的墙端,在抗震设计中需采取加固措施.     

A scaling law for slow earthquakes

Recently, a series of unusual earthquake phenomena have been discovered, including deep episodic tremor, low-frequency earthquakes, very-low-frequency earthquakes, slow slip events and silent earthquakes. Each of these has been demonstrated to arise from shear slip, just as do regular earthquakes, but with longer characteristic durations and radiating much less seismic energy. Here we show that these slow events follow a simple, unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes. We find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant, with a spectral high-frequency decay of f(-1). This scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category. The observed scale dependence of rupture velocity for these events can be explained by either a constant low-stress drop model or a diffusional constant-slip model. This new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation.     

高层结构非线性地震反应分析

从国内外地震记录库中挑选出2条具有丰富长周期信息的长周期地震波,利用正交化 HHT法对2条地震波进行时频特征分析计算,同时以2条常用地震波为参照,进行时频特征的比对分析。然后,以一实际高层结构为例,利用 ANSYS 有限元软件建立有限元数值模型,分别以前4条地震波为输入,进行该高层结构的非线性动力反应分析。结果表明：高层结构对不同类型地震波的过滤作用不同,对长周期地震波的过滤作用更加明显;长周期地震波作用下的结构响应明显大于普通地震波作用下的结构响应,其中位移响应较加速度响应明显。     

墩身高阶振型对高墩地震反应影响

采用增量动力分析的方法,全面讨论了墩身高阶振型对桥梁结构地震反应的影响.首先分析了现有规范中的桥梁抗震设计方法应用于高墩的局限性;随后采用了增量动力分析方法,在考虑墩身高阶振型效应的情况下,对高墩桥梁结构在地震作用下的性能进行了研究.结果表明:由于墩身高阶振型贡献,高墩在强震作用下可能沿墩身产生多个塑性区域;墩顶位移与墩底曲率不再同步变化,高墩墩顶极限位移不能采用规范中的方法进行计算;高墩墩身的地震剪力及弯矩需求也会由于墩身高阶振型的显著影响而变得十分复杂.最后提出了针对高墩抗震设计的一些改进措施.     

Switch of flow direction in an Antarctic ice stream

Fast-flowing ice streams transport ice from the interior of West Antarctica to the ocean, and fluctuations in their activity control the mass balance of the ice sheet. The mass balance of the Ross Sea sector of the West Antarctic ice sheet is now positive--that is, it is growing--mainly because one of the ice streams (ice stream C) slowed down about 150 years ago. Here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the Ross drainage system. We show that the flow in an area that once discharged into ice stream C has changed direction, now draining into the Whillans ice stream (formerly ice stream B). This switch in flow direction is a result of continuing thinning of the Whillans ice stream and recent thickening of ice stream C. Further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long-term climate change. We suggest that caution is needed when using observations of short-term mass changes to draw conclusions about the large-scale mass balance of the ice sheet.     

滑移隔震结构非平稳随机响应的动力特性分析

基于指数摩擦力模型，利用预估校正的数值积分方法计算滑移隔震结构的动力响应，分析了地面谐振运动下结构的加速度传递率及基底滑移率随频率比的变化规律，并发现滑移隔振结构的振动有振幅跳跃，基底爬行等非线性动力特征。将地震地面运动模拟成非平衡随机过程，基于离散小波变换得到地面运动离散小波系数的统计值，并以此作为输入，使用等价线性方法获得了滑移隔震结构在非平衡地震作用下的均方滑动位移和速度反应。最后，给出了这种结构的最大滑动位移响应随滑移面摩擦系数的变化曲线，可供设计时参考。     

Inland thinning of West Antarctic Ice Sheet steered along subglacial rifts

Current ice loss from the West Antarctic Ice Sheet (WAIS) accounts for about ten per cent of observed global sea-level rise. Losses are dominated by dynamic thinning, in which forcings by oceanic or atmospheric perturbations to the ice margin lead to an accelerated thinning of ice along the coastline. Although central to improving projections of future ice-sheet contributions to global sea-level rise, the incorporation of dynamic thinning into models has been restricted by lack of knowledge of basal topography and subglacial geology so that the rate and ultimate extent of potential WAIS retreat remains difficult to quantify. Here we report the discovery of a subglacial basin under Ferrigno Ice Stream up to 1.5?kilometres deep that connects the ice-sheet interior to the Bellingshausen Sea margin, and whose existence profoundly affects ice loss. We use a suite of ice-penetrating radar, magnetic and gravity measurements to propose a rift origin for the basin in association with the wider development of the West Antarctic rift system. The Ferrigno rift, overdeepened by glacial erosion, is a conduit which fed a major palaeo-ice stream on the adjacent continental shelf during glacial maxima. The palaeo-ice stream, in turn, eroded the 'Belgica' trough, which today routes warm open-ocean water back to the ice front to reinforce dynamic thinning. We show that dynamic thinning from both the Bellingshausen and Amundsen Sea region is being steered back to the ice-sheet interior along rift basins. We conclude that rift basins that cut across the WAIS margin can rapidly transmit coastally perturbed change inland, thereby promoting ice-sheet instability.