The 2004 Sumatra earthquake and Indian Ocean tsunami: What happened and why?

The December 26, 2004 Sumatra earthquake and the tsunami that followed killed over 300,000 people. In this paper, we analyze and discuss the geologic causes for this earthquake, the mechanisms that generated it, and follow up with a discussion on ways to prevent this type of disaster in the future.     

Determination of partial geoidal parameters over the Indian sub-continent

The paper presents values of partial geoidal parametersN, ξ and η which define the departure of the geoid from the reference spheroid, at 1° intervals over the Indian subcontinent. These values represent contributions arising from the gravity anomaly data over the entire earth's surface, except for that from a 6°×6° element around the immediate neighbourhood of the point. Complete values of these parameters at a point can be obtained simply by adding to these partial values, contributions from the 6°×6° element circumscribing that point. The objective of the study was to provide a ready basis for updating the geoidal parameters at the initial reference point as and when the density and quality of local gravity data around it improve. These computations once made, would also facilitate calculation of geoidal parameters at a fairly large number of astro-geodetic stations apart from that at the initial reference point, which should lead to considerably more precise value of the absolute datum of the Indian geodetic dasystem.     

Induction by long period geomagnetic variations in the Indian sub-continent

In the present paper storm time variations and 27-day geomagnetic periodicity have been analysed to estimate the depth of the substitute conductor, assuming an infinitely (super) conducting core model of the earth. The advantage of using data from a restricted longitude range is that the uncertainties arising from lateral contrasts in the upper mantle and contributions from Sq current systems are considerably reduced. The result of the present analysis, which has been done in the time domain, gives a value of 522 km for the depth of the substitute conductor in case of storm time variations which rises to 870 km for 27-day recurrent storms. A higher value of the depth for 27-day variations indicate that the rise in conductivity inside the earth is not like a step function rather is a gradual one. The value of 522 km for storm time variations for the Indian region is smaller than the global average. This is natural to expect because the Indian sub-continent is known to be a tectonically active region.     

Implication of source models on tsunami wave simulations for 2004 (Mw 9.2) Sumatra earthquake

Natural Hazards - Flash flood is disastrous; it losses property and life. Its effect is intensified while it occurs in semiarid region because of less preparedness. The present case conferred about...     

Anomaly map ofZ component of the Indian sub-continent from magnetic satellite data

An anomaly map of the Z component has been produced for the region of the Indian sub-continent for the first time by the Survey of India usingmagsat data. Data of thousands of kilometres of satellite tracks of varying altitude have been reduced to a common elevation of 400 km by removing the external field and linear trend. The entire data was plotted on a map of 1:6 M and mean values of 2°×2° blocks then accepted for contouring. A prominent magnetic low is reflected over the Himalayas and a prominent high over the Indian peninsula. The dividing line of positive and negative anomalies between the Himalayas and Deccan Traps falls along the Narmada lineament.     

Repeating aftershocks of the great 2004 Sumatra and 2005 Nias earthquakes

We investigate repeating aftershocks associated with the great 2004 Sumatra–Andaman (Mw 9.2) and 2005 Nias–Simeulue (Mw 8.6) earthquakes by cross-correlating waveforms recorded by the regional seismographic station PSI and teleseismic stations. We identify 10 and 18 correlated aftershock sequences associated with the great 2004 Sumatra and 2005 Nias earthquakes, respectively. The majority of the correlated aftershock sequences are located near the down-dip end of a large afterslip patch. We determine the precise relative locations of event pairs among these sequences and estimate the source rupture areas. The correlated event pairs identified are appropriately referred to as repeating aftershocks, in that the source rupture areas are comparable and significantly overlap within a sequence. We use the repeating aftershocks to estimate afterslip based on the slip-seismic moment scaling relationship and to infer the temporal decay rate of the recurrence interval. The estimated afterslip resembles that measured from the near-field geodetic data to the first order. The decay rate of repeating aftershocks as a function of lapse time t follows a power-law decay 1/t^p with the exponent p in the range 0.8–1.1. Both types of observations indicate that repeating aftershocks are governed by post-seismic afterslip.     

Modelling the pore fluid diffusion process in aftershock initiation for 2004 Sumatra earthquake: implications for marine geohazard estimation in the Andaman region

The role of fluid injection on the occurrence and migration path for the aftershocks of 2004 Sumatra earthquake (Mw 9.3) and January 2005 Andaman earthquake swarm within the aftershock sequence is investigated here from the viewpoint of pore fluid diffusion process. The Sumatra earthquake created a regionally extensive crustal rupture plane exceeding 1,200 km length below the Andaman Sea. The r–t plots (Shapiro et al. 1997) are constructed for these aftershocks in order to examine the role of poroelastic effects as rupturing progressed with time. Their main results are as follows: the r–t plot corresponding to first 3 h of aftershock activity (when only 44 events of mb ≥ 4.5 originated) reveals that 95% of the data points occurred below the modelled parabola with relatively high D value of 20 m²/s, whereas a significantly low D value of 3.5 m²/s characterises the aftershock activity for the first 24 h (when 420 events of mb ≥ 4.0 occurred). Here, the Coulomb stress was transferred from the main shock with a rapid imposition of normal stress, thus inducing the pore-pressure change that started diminishing almost immediately by fluid diffusion, at a rate, defined by the diminishing D value. The modelling results for fault seismicity at far off distances from the main epicentre are interpreted here as potential indicators for large-scale sub-seabed rupturing—consequent to stress changes induced by bending of the Indian Ocean plate. Bathymetric slopes under the Andaman subduction zone are particularly amenable to sub-marine slides where crustal E–W hinge faults inferred seismically cut across the N–S trending regional thrust and strike-slip faults. Seabed rupturing appears to allow deep-slab hydration in these areas, producing pressure gradients along the normal faults. These features are important since they can herald marine geohazards in the Andaman region.     

Transfer of stress from the 2004 Mw9.2 Sumatra subduction earthquake promoted widespread seismicity and large strike‐slip events in the Wharton Basin

The 2004 Mw9.2 Sumatra and 2012 Mw8.6 Wharton Basin (WB) earthquakes provide the unprecedented opportunity to investigate stress transfer from a megathrust earthquake to the subducting plate. Comprehensive analyses of this study revealed that the 2004 earthquake excited widespread seismicity in the WB, especially in regions of calculated stress increase greater than 0.3 bars. The 2004 earthquake stressed all three rupture planes of the 2012 Mw8.6 strike‐slip mainshock and the largest Mw8.2 aftershock with mean values of Coulomb stress between 0.3 and 2.1 bars. For the 77 Mw ≥ 4 regional events since 2012, at least one nodal plane for 95% of the events, and both nodal planes for 72% of the events experienced stress increase due to the 2004 earthquake. Results of the analyses also revealed that the regional stress directions in the WB may have controlled the sub‐fault orientations of the 2012 Mw8.6 strike‐slip earthquake.     

Geological and geomorphological characteristics of landslides triggered by the 2004 Mid Niigta prefecture earthquake in Japan

The 2004 Mid Niigta prefecture earthquake (M_JMA 6.8) triggered more than one thousand landslides in the Miocene to Quaternary sedimentary rocks in Japan. The most common landslides were shallow disrupted landslides on steep slopes, which has been common in many previous disastrous earthquakes in the world. The Mid Niigta prefecture earthquake also triggered more than one hundred deep landslides, providing valuable information on the conditions for their occurrence. A field investigation and the interpretation of aerial photographs taken before and after the earthquake suggest that reactivation of existing landslides and undercutting of slopes are the most important factors for deep landslides to be triggered by earthquakes. In addition, planar sliding surfaces seem to be essential for the generation of catastrophic landslides triggered by this earthquake. Planar bedding–parallel sliding surfaces were formed at the boundary between the overlying permeable sandstone and underlying siltstone or along the bedding planes of alternating beds of sandstone and siltstone. Sliding surfaces along the slope-parallel oxidation front were formed in the area of black mudstone. New landslides (rockslide-avalanches) occurred with the sliding surfaces in a several-cm thick tuff interbedded in siltstone. One rockslide-avalanche occurred on a slope where buckling deformation preceded the earthquake. Gentle valley bottom sediments were mobilized in many locations, probably because they were saturated and partial liquefaction had occurred due to the earthquake shaking.     

2008年汶川大地震孕震、同震及震后变形和应力演化全过程的数值模拟

2008年MS 8.0级汶川大地震发生在具有复杂的地质构造背景、 强烈的地表起伏、 不均匀的弹性和黏性结构的龙门山断裂带上.由于震前地震活动性不够强烈且地表构造变形较小,龙门山断裂带的地震危险性在汶川地震之前被低估.从数值模拟的角度,建立黏弹性有限元模型,考虑了初始地形、 重力、构造加载、 黏弹性松弛等因素对2008年...     

An example of landslide-inflicted damage to tunnel in the 2004 Mid-Niigata Prefecture earthquake

A large number of landslides induced by the 2004 Mid Niigata Prefecture earthquake resulted in the closure of 233 segments of national and prefectural routes in Higashiyama mountain district, and 61 localities were completely isolated. Since railway and road facilities follow closely the motion of soils, damage to these facilities has to be discussed in terms of soil deformations that they experienced. The example of Kizawa tunnel shows that even relatively small soil deformations can be large enough to cause serious cracking of tunnel lining.     

基于分裂节点法的地震同震和震后形变数值模拟及其在汶川大地震中的应用

大地震导致的同震及震后效应,对于分析不同地震之间的相互影响及区域地震危险性等有着重要的作用。文中开发了模拟地震同震及震后效应的三维黏弹性有限元程序,通过计算走滑断层震例（概念性模型）引起的同震及震后效应,并与解析/半解析解进行对比,验证了程序的可靠性。同时基于概念性模型,分析了不同介质参数对同震及震后的地表变形的影响。研究表明,地球介质的横向不均匀性对地震同震位移有显著的影响,而中下地壳上地幔的黏度对震后效应起着主要控制作用。最后将该程序应用于青藏高原东缘,计算分析了2008年MW7.9汶川大地震导致的同震及震后库仑应力变化对2013年MW6.6芦山地震及2017年MW6.5九寨沟地震的影响。结果显示,汶川地震导致的库仑应力变化在芦山地震震源附近（0.013 MPa）及九寨沟地震震源附近（0.009 MPa）都为正值,说明汶川地震可能使得两次地震提前发生。     

Investigations of earthquake probabilities in the Indian Ocean seismic belts

Gumbel's extreme-value theory is used to estimate the probability of occurrence and average return periods for earthquakes in the Indian Ocean seismic belts. The nature of seismic activity, and annual and 50 year maximum magnitudes of earthquakes are also discussed. The earthquake occurrence model of autocorrelation lends support for the periodicity of the most probable earthquake in these belts. The percentage probability of recurrence of earthquakes of magnitude 8 and above has been estimated for the region mentioned.     

从苏门答腊-安达曼到日本东北:特大地震及其引发的超级海啸的启示

2004年12月26日苏门答腊-安达曼MW9.1地震及印度洋超级海啸,以及2011年日本东北MW9.0地震及海啸与核泄漏,给人类带来了巨大的灾难。这两次灾难的接连发生充分暴露了迄今我们对于地震发生规律的认识水平还是很低的,启示我们需要继续加强对地震发生的规律性与地震预测预报的研究。在地震危险性评估中,要努力克服经验性方法的局限性,加强地应力测量以确定断层接近破裂的程度,更直接地估计地震危险性;要最大限度地运用地震、大地测量、地质、地貌等所有可资利用的资料,尽快将学术研究成果应用于防灾减灾实践。要重视不同观测资料的整合集成。要加强学科与学科之间的交叉渗透,自然科学与社会科学之间的合作交流,以及科学界与决策者和社会公众的相互沟通。要加强在海域对地震、海啸的多学科、多手段的监测工作,加强地震破裂过程复杂性的理论与应用研究,提高对地震、海啸(包括局地海啸)的监测、预测预报与预警水平。     

地震作用下西藏易贡滑坡动力响应特征分析

西藏雅鲁藏布江大峡谷地区是地震滑坡的高易发区,发生过多期地震滑坡。以西藏易贡滑坡为例,运用FLAC3D有限差分方法,对滑坡所在山体进行频响特征分析,并以此为基础对其地震波作用下的放大效应开展研究,最后对近场强震条件下山顶潜在崩滑体稳定性进行预测。研究结果发现:易贡山体整体卓越频率处于较低值,山顶卓越频率主要集中在1 Hz以下、山顶两侧卓越频率在2~6 Hz之间;在地震波作用下,易贡山体顶部及两侧出现不同程度放大,山体内部沿高度向上呈先增后减、进而再次增大的变化趋势,其计算结果与频响特征分析结果基本一致;静力条件下,潜在崩滑体基本保持稳定,其安全系数为1.27,但地震作用下的计算结果却表明其发生了失稳破坏;在考虑水平向和竖向加速度同时输入的近场强震条件下,崩滑体稳定程度将进一步下降,因此需加强近场强震条件下山体的风险分析及预测。      

Lithospheric deformation under pre- and post-seismic stress fields along the Nicobar–Sumatra subduction margin during 2004 Sumatra mega-event and its tectonic implications

A high-resolution study was carried out under pre-seismic (i.e., static) and post-seismic stress fields (i.e., dynamic) in a space–time frame along the Nicobar–Sumatra margin. The study reveals that the descending lithosphere records minimum stress obliquity, predominant thrust movement, and down-dip least compressive stress axis under static stress field in northwest Sumatra (sector III). The imbalance between down-dip component of slab pull force and viscous resistive force possibly caused cyclic stress loading in compressive field around the flexing zone ( 25 km), and that undergone brittle failure through generation of mega-thrust event on 26th December' 2004. A sharp decrease in stress obliquity towards north (sector II), redressing of least compressive stress axes from horizontal to down-dip direction, and increasing thrust movements under dynamic stress field account for continued upward shortening of the lithosphere. The weak thinner zone (i.e., between  159 and  217 km depth), an age-discontinuity portion, possibly was collapsed through rapid enhancement of stress-induced weakening and strain localization following the 2004 Sumatra mega-shock. It is also well appreciated in the literature that such shallow disruption of the lithosphere is inevitable in the upper mantle, if the slab is weakened or broken there, and this phenomenon is not uncommon below the Sumatra.     

汶川地震中锚固岩质边坡的动力响应特征

岩质边坡的地震响应是当前岩土工程界所关注的重要问题之一。以紫坪铺工程进水口边坡为研究对象,以现场监测数据为基础,从时间和空间的角度综合分析了汶川地震前、后支护荷载和边坡位移的变化特征,并讨论了坡体形态、岩体结构以及支护荷载对地震力作用的协同影响。研究结果表明：受地震力作用影响,坡体变形和支护荷载发生突变,主要受主震影响;地震引起的支护荷载变化量在100 kN以内,未超出设计荷载的5%;边坡水平向产生的永久位移在12 mm范围内,岩体变形深度受结构面分布控制;边坡中部变形最大,沿水平方向具有明显的临空面放大效应,而沿高程方向,没有明显垂直放大效应。研究成果可加深对锚固岩体动力响应机制的认识,同时为以位移控制的边坡工程抗震设计提供参考。     

Modeling the 2004 Sumatra tsunami at Seychelles Islands: site-effect analysis and comparison with observations

The M _w = 9.1 mega-thrust Sumatra–Andaman earthquake that occurred on December 26, 2004, was followed by a devastating tsunami that killed hundreds of thousands of people and caused catastrophic effects on human settlements and environments along many coasts of the Indian Ocean, where even countries very far from the source were affected. One of these cases is represented by the Republic of Seychelles, where the tsunami reached the region about 7 h after the earthquake and produced relevant damages, despite the country was more than 4,500 km far from the seismic source. In the present work, we present and discuss a study of the 2004 Sumatra tsunami by means of numerical simulations with the attention focused on the effects observed at the Seychelles Archipelago, a region never previously investigated with this approach. The case is interesting since these islands lay on a very shallow oceanic platform with steep slopes so as the ocean depth changes from thousands to few tens of meters over short distances, with significant effects on the tsunami propagation features: the waves are strongly refracted by the oceanic platform and the tsunami signal is modified by the introduction of additional frequencies. The study is used also to validate the UBO-TSUFD numerical code on a real tsunami event in the far field, and the results are compared with the available observations, i.e., the sea level time series recorded at the Pointe La Rue station, Mahé Island, and run-up measurements and inundation lines surveyed few weeks after the tsunami at Praslin Island, where the tsunami hit during low tide. Synthetic results are found in good agreement with observations, even though some of the observations remain not fully solved. Moreover, simulations have been run in high-tide condition since the 2004 Sumatra tsunami hitting at high tide can be taken as the worst-case scenario for the Seychelles islands and used for tsunami hazard and risk assessments.