The Sumatra tsunami of 26 December 2004 as observed in the North Pacific and North Atlantic oceans

The M _w=9.3 megathrust earthquake of December 26, 2004 off the coast of Sumatra in the Indian Ocean generated a catastrophic tsunami that caused widespread damage in coastal areas and left more than 226,000 people dead or missing. The Sumatra tsunami was accurately recorded by a large number of tide gauges throughout the world's oceans. This paper examines the amplitudes, frequencies and wave train structure of tsunami waves recorded by tide gauges located more than 20,000 km from the source area along the Pacific and Atlantic coasts of North America.     

利用海啸数值模拟结果进行海底地震有限断层模型验证

基于地震有限断层模型进行海啸的数值模拟通常被用来估计海啸的到时、 波高等, 另一方面, 海啸数值模拟的结果也可以作为限定条件用来考察同一地震的不同断层模型之间的相对合理性。 采用国际上各地震研究机构在震后各自得出的不同的有限断层模型作为海啸源, 使用基于二维浅水波方程的海啸传播模型对2011年日本东北地震海啸的传播过程进行模拟, 以海啸模拟所得到的沿岸浪高分布、 平均波高、 最大波高等与实际观测值相比较, 进而判断由各有限断层模型所计算得到的海啸中哪个结果与实际的海啸情况更为符合, 由此推断断层模型的相对合理性。     

Tsunami Wave Analysis and Possibility of Splay Fault Rupture During the 2004 Indian Ocean Earthquake

The 2004 Indian Ocean tsunami was observed by two satellites, close in space and time, that traversed the Indian Ocean 2?h after the Sumatra–Andaman earthquake, but which observed different tsunami lead wave morphologies. The earlier satellite, Jason-1, recorded a lead wave with two peaks of similar amplitude and wavelength, while the later satellite, TOPEX/Poseidon, recorded a lead wave with only one longer wavelength uplift. To resolve this disparity, we examine the travel paths of long wavelength waves over the seafloor bathymetry. Waves traveling from the margin will traverse significantly different paths to arrive at the two satellite transects. The result is that the satellites are sensitive to different parts of the margin; Jason-1 is highly sensitive to the margin in the area of the epicenter, while TOPEX is sensitive to a more northerly section. By developing solutions of the ocean gravity wave equations, accounting for dispersion, we show that the double peak of the Jason-1 satellite observations are consistent with coseismic rupture of a splay fault of limited along-strike extent, located north of Simeulue Island. The doubly peaked morphology can be reproduced with co-activation of the subduction zone interface and the splay fault, which creates a seafloor uplift pattern with two distinct areas of uplift. The Jason-1 satellite is sensitive to a splay fault in this portion of the margin, whereas the TOPEX satellite would not be significantly affected by this uplift pattern. By back-projecting satellite observation points to the margin, we constrain the location of the proposed splay fault and find that it correlates with a bathymetric high. The aftershock locations, uplift of corals on Simeulue Island and a fault scarp on Pulau Salaut Besar are also consistent with the activation of a splay fault in the area delimited by the back-projection. Our work also shows that it is critical to fully capture gravity wave dispersion in order to represent features of the lead wave profile that may not be as well characterized by the shallow water (long-wavelength) model. It is also necessary to account for dispersion so as to precisely assess wavefront travel times; this leads us to conclude that the rupture must have reached very near to the trench and propagated with an updip rupture velocity of order 2.0?km/s or more.     

A Study of Tsunami Wave Fission in an Undistorted Experiment

To study tsunami soliton fission and split wave-breaking, an undistorted experiment was carried out which investigated tsunami shoaling on a continental shelf. Three models of the continental shelf were set up in a 205-m long 2-dimensional flume. Each shelf model was 100 m, long with slopes of either 1/100, 1/150, or 1/200. Water surface elevations were measured across the flume, including a dense cluster of wave gages installed around the point of wave-breaking. We propose new methods for calculating wave velocity and the wave-breaking criterion based on our interpretation of time series data of water surface elevation. At the point of wave-breaking, the maximum slope of water surface is between 20 to 50 deg., while the ratio of surface water particle horizontal velocity to wave velocity is from 0.5 to 1.2. The values determined by our study are larger than what has been reported by other researchers.     

Field Survey and Numerical Simulations: A Review of the 1998 Papua New Guinea Tsunami

— The Papua New Guinea (PNG) tsunami of 1998 is re-examined through a detailed review of the field survey as well as numerous numerical computations. The discussion of the field survey explores a number of possible misinterpretations of the recorded data. The survey data are then employed by a numerical model as a validation tool. A Boussinesq model and a nonlinear shallow water wave (NLSW) model are compared in order to quantify the effect of frequency dispersion on the landslide-generated tsunami. The numerical comparisons indicate that the NLSW model is a poor estimator of offshore wave heights. However, due to what appears to be depth-limited breaking seaward of Sissano spit, both numerical models are in agreement in the prediction of maximum water elevations at the overtopped spit. By comparing three different hot-start initial profiles of the tsunami wave, it is shown that the initial shape and orientation of the tsunami wave is secondary to the initial displaced water mass in regard to prediction of water elevations on the spit. These numerical results indicate that agreement between numerical prediction of runup values with field recorded values at PNG cannot be used to validate either a NLSW tsunami propagation model or a specific landslide tsunami hot-start initial condition. Finally, with the use of traditional tsunami codes, a new interpretation of the PNG runup measurements is presented.     

Seismic Records of the 2004 Sumatra and Other Tsunamis: A Quantitative Study

Following the recent reports by Yuan et al. (2005) of recordings of the 2004 Sumatra tsunami on the horizontal components of coastal seismometers in the Indian Ocean basin, we build a much enhanced dataset extending into the Atlantic and Pacific Oceans, as far away as Bermuda and Hawaii, and also expanded to five additional events in the years 1995–2006. In order to interpret these records quantitatively, we propose that the instruments are responding to the combination of horizontal displacement, tilt and perturbation in gravity described by Gilbert (1980), and induced by the passage of the progressive tsunami wave over the ocean basin. In this crude approximation, we simply ignore the island or continent structure, and assume that the seismometer functions de facto as an ocean-bottom instrument. The records can then be interpreted in the framework of tsunami normal mode theory,and lead to acceptable estimates of the seismic moment of the parent earthquakes. We further demonstrate the feasibility of deconvolving the response of the ocean floor in order to reconstruct the time series of the tsunami wave height at the surface of the ocean, suggesting that island or coastal continental seismometers could complement the function of tsunameters.     

日本东北M_W9.0地震海啸在港池及邻近区域诱发的涡流危险性计算与评估分析

海啸造成的灾害与损失并非都与淹没有关,特别是港口中海啸诱导的强流会对船只及海事设施产生重要的影响及损害.由于海啸流观测数据稀缺及海啸诱导涡流机制的不确定性,过去60年海啸科学主要集中于对海啸波特征及淹没过程的研究与分析,海啸流模拟及验证工作开展较少,导致对海啸流基本特征及其造成灾害现象的曲解.开展海啸诱导的涡流研究及预警服务显得尤为重要及紧迫.考虑快速海啸预警需要,综合对比海啸诱导涡流的物理框架及模型方法,探索兼顾效率与计算精度的海啸流模拟方法是本文的核心工作及出发点.通过分析浅层湍流相干结构(TCS)产生的主要物理耗散机制,确定了考虑2D水平耗散机制的非线性浅水方程可用于海啸涡流的模拟分析.基于高精、高分辨率有限体积模型Geoclaw建立了三个精细化的港口海啸流模型,模型分辨率为5m.利用基于海啸浮标反演的海啸源模型作为初始条件,模拟分析了日本东北地震海啸在远场的海啸波流特征.海啸波流特征模拟结果与观测吻合较好,结果可信.对比发现:波驱动的自由表面流,小的位相或波幅误差就会导致大的流速误差,流的模拟和预报相对波幅来说更具挑战性.研究了海啸波流能量在港池中的分布特征,得到:港池入口及防波堤两端常被强流控制,具有极高的危险性;相对于波幅的空间变化,海啸流具有更强的空间敏感性;所建立的高分辨率海啸模型模拟再现了日本海啸在近场的涡旋结构,给出了与观测基本一致的涡流特征.最后,引入海啸流危险等级标准,分析了港口海啸流危险性等级分布、船只疏散的安全深度及回港的时间周期.针对港口、海湾同时考虑海啸波流特征的海啸预警与评估对于港口应急管理者科学决策具有重要意义.     

Energy Decay of the 2004 Sumatra Tsunami in the World Ocean

The catastrophic Indian Ocean tsunami generated off the coast of Sumatra on 26 December 2004 was recorded by a large number of tide gauges throughout the World Ocean. This study uses gauge records from 173 sites to examine the characteristics and energy decay of the tsunami waves from this event in the Indian, Atlantic and Pacific oceans. Findings reveal that the decay (e-folding) time of the tsunami wave energy within a given oceanic basin is not uniform, as previously reported, but depends on the absorption characteristics of the shelf adjacent to the coastal observation site and the time for the waves to reach the site from the source region. In general, the decay times for island and open-ocean bottom stations are found to be shorter than for coastal mainland stations. Decay times for the 2004 Sumatra tsunami ranged from about 13 h for islands in the Indian Ocean to 40–45 h for mainland stations in the North Pacific.     

Field Survey of the March 28, 2005 Nias-Simeulue Earthquake and Tsunami

On the evening of March 28, 2005 at 11:09?p.m. local time (16:09 UTC), a large earthquake occurred offshore of West Sumatra, Indonesia. With a moment magnitude (M _w) of 8.6, the event caused substantial shaking damage and land level changes between Simeulue Island in the north and the Batu Islands in the south. The earthquake also generated a tsunami, which was observed throughout the source region as well as on distant tide gauges. While the tsunami was not as extreme as the tsunami of December 26th, 2004, it did cause significant flooding and damage at some locations. The spatial and temporal proximity of the two events led to a unique set of observational data from the earthquake and tsunami as well as insights relevant to tsunami hazard planning and education efforts.     

印度尼西亚两次特大地震引发海啸和不引发海啸的比较研究

2004年12月26日苏门答腊岛安达曼海附近海域发生的9．0级地震和2005年3月28日苏门答腊岛明打威群岛北附近海域的8．7级地震,在构造环境、震级、震源深度、地震类型都相似的情况下,为何前者引发海啸,后者不引发的海啸？对此进行了对比分析,认为9．0级地震发生时,在其震源体附近的两板块相交的海沟两侧陡坡蕴育着滑坡体或和崩塌体（或者两者都有）,9．0级地震发生时,强烈的地震波,促使滑坡体的滑动或崩塌体的崩塌,推压和扰动海水,引发诲啸。而8．7级地震发生时、在其震源体附近的两板块相交的海沟两侧陡坡无滑坡体或和崩塌体,或先存滑坡体或崩塌体在9．0级地震发生时已滑坡或崩塌殆尽,当8.7地震发生时,无滑坡体滑动或崩塌体崩塌,不可能对海水有较大的扰动,故不可能引发海啸。     

A note on tsunami amplitudes above submarine slides and slumps

Tsunami generated by submarine slumps and slides are investigated in the near-field, using simple source models, which consider the effects of source finiteness and directivity. Five simple two-dimensional kinematic models of submarine slumps and slides are described mathematically as combinations of spreading constant or slopping uplift functions. Tsunami waveforms for these models are computed using linearized shallow water theory for constant water depth and transform method of solution (Laplace in time and Fourier in space). Results for tsunami waveforms and tsunami peak amplitudes are presented for selected model parameters, for a time window of the order of the source duration.The results show that, at the time when the source process is completed, for slides that spread rapidly (c_R/c_T≥20, where c_R is the velocity of predominant spreading), the displacement of the free water surface above the source resembles the displacement of the ocean floor. As the velocity of spreading approaches the long wavelength tsunami velocity the tsunami waveform has progressively larger amplitude, and higher frequency content, in the direction of slide spreading. These large amplitudes are caused by wave focusing. For velocities of spreading smaller than the tsunami long wavelength velocity, the tsunami amplitudes in the direction of source propagation become small, but the high frequency (short) waves continue to be present. The large amplification for c_R/c_T1 is a near-field phenomenon, and at distances greater than several times the source dimension, the large amplitude and short wavelength pulse becomes dispersed.A comparison of peak tsunami amplitudes for five models plotted versus L/h (where L is characteristic length of the slide and h is the water depth) shows that for similar slide dimensions the peak tsunami amplitude is essentially model independent.     

On the focal mechanism of the 26.05.1975 North Atlantic event contribution from tsunami modeling

The 1975 May 26 earthquake, of magnitude Ms = 7.9, occurred in the North Atlantic close to the Azores Archipelago. Its epicentre, as given by US Geological Survey, was 17.5° W, 35.9° N, 200 km south of the Gloria Fault. Several authors determined the focal mechanism as a dextral strike-slip event with no significant dip-slip component, compatible with the relative motion between Eurasia and Nubia plates but away from the presumed plate boundary. The 1975 earthquake generated a tsunami of small amplitude, recorded at the Portuguese tide-gauge network, in Spain and Northern Africa. The peculiar location of the earthquake and tsunami source and the generation of a noticeable tsunami were already discussed by several authors, but up to now, no direct modelling of the tsunami generation and propagation was made to judge the set of source solutions obtained by seismological analysis. In this paper, we present tsunami simulations, backward ray tracing and forward non-linear shallow water simulations using data from Iberia and Azores and Northern Africa. We show that a good fit between observed data and synthetic waveforms can be obtained with a focal mechanism with no significant dip-slip component, favouring its interpretation as almost pure dextral strike-slip event located in an old fracture zone south of Gloria Fault.     

Nearshore Tsunami Inundation Model Validation: Toward Sediment Transport Applications

Model predictions from a numerical model, Delft3D, based on the nonlinear shallow water equations are compared with analytical results and laboratory observations from seven tsunami-like benchmark experiments, and with field observations from the 26 December 2004 Indian Ocean tsunami. The model accurately predicts the magnitude and timing of the measured water levels and flow velocities, as well as the magnitude of the maximum inundation distance and run-up, for both breaking and non-breaking waves. The shock-capturing numerical scheme employed describes well the total decrease in wave height due to breaking, but does not reproduce the observed shoaling near the break point. The maximum water levels observed onshore near Kuala Meurisi, Sumatra, following the 26 December 2004 tsunami are well predicted given the uncertainty in the model setup. The good agreement between the model predictions and the analytical results and observations demonstrates that the numerical solution and wetting and drying methods employed are appropriate for modeling tsunami inundation for breaking and non-breaking long waves. Extension of the model to include sediment transport may be appropriate for long, non-breaking tsunami waves. Using available sediment transport formulations, the sediment deposit thickness at Kuala Meurisi is predicted generally within a factor of 2.     

Coupling eruption and tsunami records: the Krakatau 1883 case study,Indonesia

The well-documented 1883 eruption of Krakatau volcano (Indonesia) offers an opportunity to couple the eruption’s history with the tsunami record. The aim of this paper is not to re-analyse the scenario for the 1883 eruption but to demonstrate that the study of tsunami deposits provides information for reconstructing past eruptions. Indeed, though the characteristics of volcanogenic tsunami deposits are similar to those of other tsunami deposits, they may include juvenile material (e.g. fresh pumice) or be interbedded with distal pyroclastic deposits (ash fall, surges), due to their simultaneity with the eruption. Five kinds of sedimentary and volcanic facies related to the 1883 events were identified along the coasts of Java and Sumatra: (1) bioclastic tsunami sands and (2) pumiceous tsunami sands, deposited respectively before and during the Plinian phase (26–27 August); (3) rounded pumice lapilli reworked by tsunami; (4) pumiceous ash fall deposits and (5) pyroclastic surge deposits (only in Sumatra). The stratigraphic record on the coasts of Java and Sumatra, which agrees particularly well with observations of the 1883 events, is tentatively linked to the proximal stratigraphy of the eruption.     

港湾中海啸流的模拟及其时空演变特征对输入条件不确定性的响应分析

一直以来,海啸波特征作为表征海啸潜在破坏性的参数指标得到了广泛应用,特别是针对近场极端海啸事件造成的灾害来说,这种表征具有较好的适用性.然而总结分析历史海啸事件造成的损失发现：在远场近岸及港湾系统中,海啸诱导的强流却是造成损失的主要原因.陆架或港湾振荡导致海啸波幅快速升降诱发强流,可能促使港工设施受到威胁及损害,进而对海啸预警服务及海事应急管理提出了新的挑战.因此,全面理解与评估海啸在港湾中诱发的灾害特征,探索港湾中海啸流的数值模拟方法,发展针对港湾尺度的海啸预警服务指导产品尤为迫切.受限于海啸流验证数据的缺乏及准确模拟海啸流技术方法的诸多不确定性,大部分海啸数值模拟研究工作主要是针对水位特征的研究及验证,可能导致对港湾中海啸灾害危险性认识的曲解与低估.本研究基于非线性浅水方程,针对夏威夷群岛三个典型港湾建立了精细化海啸数值模型（空间分辨率达到10 m）,并联合有限断层破裂模型计算分析了日本东北地震海啸在三个港湾及其邻近区域的海啸特征,波、流计算结果与实测结果吻合较好,精细化的海啸港湾模型模拟结果可信.模拟发现港湾中较小的波幅,同样可以产生强流.综合分析日本东北地震海啸波、流特征对输入条件不确定性的响应结果发现：港湾中海啸波-流能量的空间分布特征差异较大,这与港湾系统中海啸波的驻波特性相关;相比海啸波幅空间特征,海啸流特征具有更强的空间敏感性;海啸流时空分布特征对输入条件的不确定性响应比海啸波幅对这些不确定性的响应更强,海啸流的模拟与预报更有挑战性;不确定性对海啸流计算精度的影响会进一步传导放大港湾海啸流危险性的评估及对港工设施产生的应力作用的误差,合理的输入条件对海啸流的精确模拟至关重要.最后,希望通过本文的研究可以从海啸波-流特征角度更加全面认识近岸海啸灾害特征,拓展海啸预警服务的广度与深度,从而为灾害应急管理部门提供更加科学合理的辅助决策产品.     

Numerical Simulations of Tsunami Waves and Currents for Southern Vancouver Island from a Cascadia Megathrust Earthquake

The 1700 great Cascadia earthquake (M = 9) generated widespread tsunami waves that affected the entire Pacific Ocean and caused damage as distant as Japan. Similar catastrophic waves may be generated by a future Cascadia megathrust earthquake. We use three rupture scenarios for this earthquake in numerical experiments to study propagation of tsunami waves off the west coast of North America and to predict tsunami heights and currents in several bays and harbours on southern Vancouver Island, British Columbia, including Ucluelet, located on the west coast of the island, and Victoria and Esquimalt harbours inside Juan de Fuca Strait. The earthquake scenarios are: an 1100-km long rupture over the entire length of the subduction zone and separate ruptures of its northern or southern segments. As expected, the southern earthquake scenario has a limited effect over most of the Vancouver Island coast, with waves in the harbours not exceeding 1 m. The other two scenarios produce large tsunami waves, higher than 16 m at one location near Ucluelet and over 4 m inside Esquimalt and Victoria harbours, and very strong currents that reach 17 m/s in narrow channels and near headlands. Because the assumed rupture scenarios are based on a previous earthquake, direct use of the model results to estimate the effect of a future earthquake requires appropriate qualification.     

越洋海啸的数值模拟

破坏性海啸基本上都是越洋海啸,如1960年智利海啸、2004年苏门答腊海啸。越洋海啸的传播机制与近场海啸不同,进行数值模拟所采用的数学模型也不同。本文分析比较Boussinesq方程和线性浅水方程,选用后者作为进行越洋海啸数值模拟的数学模型,基于有限差分法,运用蛙跃格式求解微分方程。以2004年苏门答腊海啸作为算例,把计算结果与NOAA和NGDC的计算结果进行对比,验证本文的数学模型和计算方法的可靠性,为以后进一步的海啸危险性分析和海啸预警等研究工作提供技术支持。     

Source mechanism of the tsunami of 2004 in the Indian Ocean: Analysis and numerical simulation

The tsunami in the Indian Ocean caused by the earthquake of December 26, 2004, near Sumatra Island had catastrophic consequences in coastal areas of many countries in this region. Notwithstanding extensive investigations of this phenomenon at various laboratories of the world, the focal mechanism of the aftershock remains unclear. The paper analyzes possible seafloor movements in the source area of the earthquake on the basis of the keyboard model of tsunamigenic earthquakes and describes numerical simulation of the generation, propagation, and runup of water surface waves in terms of this model involving vertical displacements of seafloor “keyboard-blocks.” It is shown that generated tsunami waves are essentially dependent on the combination of keyboard-block movements, which results in an irregular distribution of maximum runups along the shoreline. If the oblique nature of the subduction zone associated with the Sumatra-Andaman earthquake of December 26, 2004, is taken into account, the model results fit well the runup values observed at the Thailand shoreline. It is noted that this model of the subduction zone accounts more adequately for the tsunami wave field pattern in both areas of the Indian Ocean and other water areas such as the region of the Kurile-Kamchatka Island Arc and the Sea of Okhotsk.     

A Comparison Study of Two Numerical Tsunami Forecasting Systems

This paper presents a comparison of two tsunami forecasting systems: the NOAA/PMEL system (SIFT) and the Australian Bureau of Meteorology system (T1). Both of these systems are based on a tsunami scenario database and both use the same numerical model. However, there are some major differences in the way in which the scenarios are constructed and in the implementation of the systems. Two tsunami events are considered here: Tonga 2006 and Sumatra 2007. The results show that there are some differences in the distribution of maximum wave amplitude, particularly for the Tonga event, however both systems compare well to the available tsunameter observations. To assess differences in the forecasts for coastal amplitude predictions, the offshore forecast results from both systems were used as boundary conditions for a high-resolution model for Hilo, Hawaii. The minor differences seen between the two systems in deep water become considerably smaller at the tide gauge and both systems compare very well with the observations.     

Study on the relation of river morphology and tsunami propagation in rivers

The relations of river morphology and tsunami propagation in rivers were studied at several rivers in the Tohoku region during The Great Chilean Tsunami of 2010 and The Great East Japan Tsunami of 2011. It was found that river mouth morphological features play an important role in the intrusion of low magnitude tsunamis in which the geological and geographical conditions are an important factor. Nevertheless, the effects of these features were not found in the case of an extreme tsunami wave. As the wave enters the river, the propagation depends on other factors. It was found that the intrusion distance correlates well to the riverbed slope. The measurements of water level and riverbed slope were analyzed to propose an empirical method for estimating the damping coefficient for the tsunami propagation in rivers based on the tsunami of 2011. The proposed empirical method was used to approximate the length of the tsunami intrusion into a river by assuming that the furthest distance is given for the ratio of local tsunami wave height to the tsunami wave height at the river entrance of 0.05 (5 %). The estimated intrusion length from the proposed method in this study shows a good comparison with measurement data.