中国大陆沿海地震海啸析疑

文章认为,浅源海底辑震是形成地震海啸的必要条件,但是地震海啸的形成还取决于波浪的传播机制。中国大陆沿海尽管存在着地震活动带,但不具备形成地震海啸的条件,无论在历史上还是近代,都没有发生过地震海啸。发生于琉球群岛及日本东南部近海海域的地震海啸,对中国大陆沿海或者没有影响或者影响微弱。一些中外学者曾认为中国大陆沿海历史上发生过灾害性地震海啸,这是由于误解了有关史料的结果。     

地震海啸灾害及其研究概述

１９９８年７月１７日,西北太平洋巴布亚新几内亚近海地区发生７．１级地震,并引发海啸,造成严重的生命和财产损失,约有３０００－５００人死亡或失踪,５座村庄被巨浪蚕没,本文介绍了这次地震海啸破坏情况,国内外历史上大地震海啸灾害,简述了有关专家对巴地地震海啸追踪研究的初步结果和评论。     

我国地震海啸危险性分析方法研究

美国、日本等国家已经建立了完善的海啸防灾减灾机制，对海啸危险性分析方法的研究也做了很多的工作，而我国目前在这方面尚属空白。本文首次对我国的海啸危险性分析方法进行了研究，提出了我国地震海啸危险性分析的基本原则，在此基础上，借鉴我国成熟的地震危险性分析方法，建立了我国地震海啸危险性分析方法的主要步骤。在根据我国一些历史地震海啸资料的基础上，通过统计的方法对我国珠江三角洲的海啸危险性进行了研究。     

Mwp震级在海啸地震中的应用

具有一定面波震级的海啸地震常常产生巨大的海啸，要想对海啸地震作出较早的，准确的海啸预报，尽早地确定其为可能引发海啸的事件是十分关键的。本文显示，利用P波脉冲宽度计算宽频带矩震级Mwp，可给出震源持续时间的精确估计，我们发现，结合Mwp和震源持续时间即可确定1992年的尼加拉瓜地震为一个可能的海啸地震。我们的结果显示，Mwp结合源持续时间可为海啸预报提供一种有效的工具，尤其是对于震源持续时间特别长的海地震。     

地震与海啸

海啸是严重的地震次生灾害之一,简要介绍了海啸的成因、分布、量度、传播特征及对我国的影响;并就国际及我国如何加强海啸监测,联合建设预警预报系统提出建议。     

近期海啸研究对地震研究的启发

近期海啸研究对地震研究的启发佐竹健治在一年当中，３次重大破坏性海啸给尼加拉瓜（１９９２年９月２日）、印度尼西亚弗洛勒斯岛（１９９２年１０月１２日）和日本北海道（１９９３年７月１２日）的沿海地区造成了灾害。每次海啸的浪高度均达到１０ｍ以上，共造成１５０...     

Parameters of Tsunami Source Versus Earthquake Magnitude

This study employs an empirical-analytical approach in combination with Monte-Carlo method to establish relationships between earthquake moment magnitude and upper limits of tsunami source parameters: double-amplitude of vertical bottom deformation, displaced water volume, and potential energy of initial elevation. The approach is based on the Okada solution for a finite rectangular fault and empirical scaling laws for earthquake sources. Results are compared to empirical and theoretical relationships published previously. Parameters of some recent tsunami sources, those for which USGS provides slip distribution data, are considered in light of the established relationships.     

印度洋地震海啸引发的思考

从2004年12月印度洋发生地震海啸造成的灾难中得到启示，从完善我国的应急救援、防灾救灾体系、法律法规体系，预测系统、快速反应预警系统以及启动应急预案、组建应急指挥体系、组织应急救援队伍、实施应急救援等方面，阐述了建立应急救援指挥机构和各种应急队伍的必要性，绘制了灾害处理应急流程图，分析了应急队伍所应承担的任务，提出在我国应尽快建立地震海啸预警机制的建议。     

A Revised Evaluation of Tsunami Hazards along the Chinese Coast in View of the Tohoku-Oki Earthquake

Japan’s 2011 Tohoku-Oki earthquake and the accompanying tsunami have reminded us of the potential tsunami hazards from the Manila and Ryukyu trenches to the South China and East China Seas. Statistics of historical seismic records from nearly the last 4 decades have shown that major earthquakes do not necessarily agree with the local Gutenberg-Richter relationship. The probability of a mega-earthquake may be higher than we have previously estimated. Furthermore, we noted that the percentages of tsunami-associated earthquakes are much higher in major events, and the earthquakes with magnitudes equal to or greater than 8.8 have all triggered tsunamis in the past approximately 100 years. We will emphasize the importance of a thorough study of possible tsunami scenarios for hazard mitigation. We focus on several hypothetical earthquake-induced tsunamis caused by M _w 8.8 events along the Manila and Ryukyu trenches. We carried out numerical simulations based on shallow-water equations (SWE) to predict the tsunami dynamics in the South China and East China Seas. By analyzing the computed results we found that the height of the potential surge in China’s coastal area caused by earthquake-induced tsunamis may reach a couple of meters high. Our preliminary results show that tsunamis generated in the Manila and Ryukyu trenches could pose a significant threat to Chinese coastal cities such as Shanghai, Hong Kong and Macao. However, we did not find the highest tsunami wave at Taiwan, partially because it lies right on the extension of an assumed fault line. Furthermore, we put forward a multi-scale model with higher resolution, which enabled us to investigate the edge waves diffracted around Taiwan Island with a closer view.     

印度洋地震和海啸灾害引发的若干思考

本文通过对印度洋地震和海啸灾害的反思,认真思考了灾害防御中的若干重要问题,包括尽快建构国际社会统一灾害防御体系;应当高度重视非工程措施在灾害防御中的作用以及对政府灾害管理职责的重新审视等。     

南海地震与海啸

地震海啸的形成要具备3个条件:一是有深海盆地,可以容纳巨量海水;二是海底地形隆起与拗陷反差强烈;三是存在倾滑型活断层,可发生6级以上倾滑型的地震。查南海及其周边地形地貌,北西南三面都有宽阔的大陆架,中部又是平坦的深海平原,都不具备发生地震海啸的条件,惟独东侧马尼拉海沟才具备产生地震海啸的条件。南海地壳属于大洋型地壳与大陆型地壳之间的过度类型。其断裂构造非常发育,不同地段具有明显差异。北部为拉张型,南部为挤压型,西部为剪切型,东部为俯冲型,中部是扩张型。按断裂展布方向可分为NE向、NW向、EW向、SN向4组;按断裂切割深度,可分为岩石圈断裂、地壳断裂、基底断裂和盖层断裂。这些断裂多数为活动断裂,而东缘俯冲型断裂又是发震断裂。从地震分布、震源机制解分析,南海北、西、南以及中部都不具备引发地震海啸的条件,只有台南—菲律宾地震带东西两侧的贝尼奥夫带发生的倾滑型或具倾滑分量的走滑型6级以上地震,才有可能引发海啸,并可能对南海及我国东南沿海诸省以及港澳地区产生影响。     

Near-Field Tsunami Edge Waves and Complex Earthquake Rupture

The effect of distributed coseismic slip on progressive, near-field edge waves is examined for continental shelf tsunamis. Detailed observations of edge waves are difficult to separate from the other tsunami phases that are observed on tide gauge records. In this study, analytic methods are used to compute tsunami edge waves distributed over a finite number of modes and for uniformly sloping bathymetry. Coseismic displacements from static elastic theory are introduced as initial conditions in calculating the evolution of progressive edge-waves. Both simple crack representations (constant stress drop) and stochastic slip models (heterogeneous stress drop) are tested on a fault with geometry similar to that of the M _w = 8.8 2010 Chile earthquake. Crack-like ruptures that are beneath or that span the shoreline result in similar longshore patterns of maximum edge-wave amplitude. Ruptures located farther offshore result in reduced edge-wave excitation, consistent with previous studies. Introduction of stress-drop heterogeneity by way of stochastic slip models results in significantly more variability in longshore edge-wave patterns compared to crack-like ruptures for the same offshore source position. In some cases, regions of high slip that are spatially distinct will yield sub-events, in terms of tsunami generation. Constructive interference of both non-trapped and trapped waves can yield significantly larger tsunamis than those that produced by simple earthquake characterizations.     

Tsunami Source of the 2010 Mentawai, Indonesia Earthquake Inferred from Tsunami Field Survey and Waveform Modeling

The 2010 Mentawai earthquake (magnitude 7.7) generated a destructive tsunami that caused more than 500 casualties in the Mentawai Islands, west of Sumatra, Indonesia. Seismological analyses indicate that this earthquake was an unusual “tsunami earthquake,” which produces much larger tsunamis than expected from the seismic magnitude. We carried out a field survey to measure tsunami heights and inundation distances, an inversion of tsunami waveforms to estimate the slip distribution on the fault, and inundation modeling to compare the measured and simulated tsunami heights. The measured tsunami heights at eight locations on the west coasts of North and South Pagai Island ranged from 2.5 to 9.3 m, but were mostly in the 4–7 m range. At three villages, the tsunami inundation extended more than 300 m. Interviews of local residents indicated that the earthquake ground shaking was less intense than during previous large earthquakes and did not cause any damage. Inversion of tsunami waveforms recorded at nine coastal tide gauges, a nearby GPS buoy, and a DART station indicated a large slip (maximum 6.1 m) on a shallower part of the fault near the trench axis, a distribution similar to other tsunami earthquakes. The total seismic moment estimated from tsunami waveform inversion was 1.0 × 10²¹ Nm, which corresponded to M_w 7.9. Computed coastal tsunami heights from this tsunami source model using linear equations are similar to the measured tsunami heights. The inundation heights computed by using detailed bathymetry and topography data and nonlinear equations including inundation were smaller than the measured ones. This may have been partly due to the limited resolution and accuracy of publically available bathymetry and topography data. One-dimensional run-up computations using our surveyed topography profiles showed that the computed heights were roughly similar to the measured ones.     

The 11 March 2011 East Japan Earthquake and Tsunami: Tsunami Effects on Coastal Infrastructure and Buildings

The 11 March 2011 East Japan Earthquake and Tsunami caused unprecedented damage to well-engineered buildings and coastal structures. This report presents some notable field observations of structural damage based on our surveys conducted along the Sanriku coast in April and June 2011. Engineered reinforced concrete buildings failed by rotation due to the high-velocity and deep tsunami inundation: entrapped air in the buildings and soil liquefaction by ground shaking could have contributed to the failure. The spatial distribution pattern of destroyed and survived buildings indicates that the strength of tsunami was affected significantly by the locations of well-engineered sturdy buildings: weaker buildings in the shadow zone tended to survive while jet and wake formations behind the sturdy buildings enhanced the tsunami forces. We also found that buildings with breakaway walls or breakaway windows/doors remained standing even if the surrounding buildings were washed away or destroyed. Several failure patterns of coastal structures (seawalls) were observed. Flow-induced suction pressure near the seawall crown could have caused the failure of concrete panels that covered the infill. Remarkable destruction of upright solid-concrete type seawalls was closely related with the tsunami induced scour and soil instability. The rapid decrease in inundation depth during the return-flow phase caused soil fluidization down to a substantial depth. This mechanism explains severely undermined roads and foundations observed in the area of low flow velocities.