A Record of Palaeo-Tsunami in the Indian Ocean

The history of catastrophic events on the Indian coast helps us to understand the frequency and magnitude of the tsunamis that occurred in the Indian Ocean. These catastrophic events have changed the coastal landscape and have left significant records for further studies. These rare events have occurred in the Indian Ocean. There have been megatsunamigenic events in the past due to volcanic eruptions and earthquakes. Those events due to earthquakes have proved more catastrophic than the volcanic activities. There has been limited official records of the causality and magnitude of palaeo-tsunamigenic events. These have been studied using the various proxies. The rate of sedimentation is a proportional tool to study the magnitude of a tsunami and this has proved to be a successful tool along with foraminiferal assemblages. Causes for a tsunami to occur are by and large, the subduction zone earthquakes of the Indian plate has been the most common source for tsunami in the Indian Ocean. More often the Andaman and Nicobar and the Indonesian islands have been vulnerable to tsunami than the mainland of India and Sri Lanka.

In summary, in the last 200 years at least three basin-wide tsunamis have occurred, with several smaller tsunami affecting one or more coastlines in the region. The December 2004 M-9 tsunami seems to have been the largest and most destructive in the last two centuries, suggesting most tsunami are likely to be smaller but still allowing the possibility that even larger tsunami could be generated in propitious circumstances.     

Contributions expected from marine geodesy to the study of tsunamis in the Mediterranean Sea

Abstract

In the Mediterranean Sea, tsunamigenic sources may be found in several areas in the belt running from Gibraltar up to the Black Sea, but they are concentrated mainly around Italy and Greece. Most of the sources are located close to the coasts and excite tsunamis reaching the coasts soon after the generation time. Tsunami research and tsunami mitigation programs are only in a very initial stage in the Mediterranean area. The present activities are focused chiefly to tsunami potential evaluation and on tsunami propagation modeling. The establishment of efficient observational networks, centers for data management and services, and systems for issuing tsunami warnings are some of the most urgent needs. In this context, the envisaged contribution of marine geodesy is twofold. First, monitoring of submarine active faults and submarine volcanic areas by means of systems capable of detecting seafloor deformation may contribute in identifying periods in which the probability of tsunami generation increases beyond a threshold value, especially in those tsunamigenic zones where geodetic observations on land are insufficient (for example, eastern Sicily in Italy and the Hellenic Arc in southern Greece). Second, since most of the active sources are close to the coastline, computations of tsunami propagation and run‐up may be significantly enhanced by a better knowledge of the bathymetry of the seabelt facing the coasts.     

A two‐parameter scheme for tsunami hazard zoning

Abstract

A necessary stage in calculations for prediction purposes is the study of the tsunami recurrence function

which yields mean recurrence of tsunami with maximum wave height not greater than a specified level h. The major problem in using these functions for prediction purposes is the fact that a well‐grounded approximation of empirical data on wave heights is difficult to obtain, because the mathematical model for prediction is an extrapolation of this function for tsunami heights whose recurrence remains uncertain. We shall show that the natural relation of observable tsunamis statistics to extremum statistics leads to the discovery of at least two and possibly three temporal scale intervals with different tsunami modes. It has also been clarified that for the 10 years < T < 10³ years range of time periods, which is the most important one for tsunami wave height prediction purposes, the tsunami recurrence is described by two parameters: frequency A of occurrence of large tsunamis and coefficient k of wave ampliflcation near the shore. As an example, a diagram of tsunami hazard zoning of the eastern Honshu coast has been plotted.     

Relevance of Bathymetry for Tsunami Run‐up Modeling

Abstract

Marine positioning is relevant for several aspects of tsunami research, observation, and prediction. These include accurate positioning of instruments on the ocean bottom for determining the deep‐water signature of the tsunami, seismic observational setups to measure the earthquake parameters, equipment to determine the tsunami characteristics during the propagation phase, and instruments to map the vertical uplift and subsidence that occurs during a dip‐slip earthquake.

In the accurate calculation of coastal tsunami run‐up through numerical models, accurate bathymetry is needed, not only near the coast (for tsunami run‐up) but also in the deep ocean (for tsunami generation and propagation). If the bathymetry is wrong in the source region, errors will accumulate and will render the numerical calculations inaccurate. Without correct and detailed run‐up values on the various coastlines, tsunami prediction for actual events will lead to false alarms and loss of public confidence.     

2016年全球地震海啸监测预警与数值模拟研究

回顾了国家海洋环境预报中心(国家海洋局海啸预警中心)2016年全球地震海啸监测预警的总体状况, 并基于震源生成模型和海啸传播数值模型的计算结果详细介绍了几次主要海啸事件及其影响特性。2016年全年国家海洋环境预报中心总共对全球6.5级(中国近海5.5级)以上海底地震响应了45次,发布海啸信息81期, 没有发生对我国有明显影响的海啸。结合精细化的数值模拟结果和浮标监测数据,重点介绍了苏门达腊7.8级地震海啸、厄瓜多尔7.8级地震海啸、新西兰7.1级和7.8级地震海啸, 以及所罗门7.8级地震海啸的波动特征和传播规律, 模拟结果与实测海啸波符合较好。针对厄瓜多尔7.8级地震海啸事件, 本文比较分析了均匀断层模型和多源有限断层模型对模拟结果的影响; 针对新西兰7.1级地震海啸, 探讨了色散效应对海啸波在大水深、远距离传播过程的影响规律。     

Numerical simulation of the tsunami generated by a past catastrophic landslide on the volcanic island of Ischia, Italy

The island of Ischia, Gulf of Naples, Italy, like many other volcanic islands is affected by mass failures, that are mainly related to secondary volcanic processes such as slope steepening and seismic shaking. The block resurgence of its main relief, Mount Epomeo, has been recognised to contribute cyclically to mass instability and cause landslides, that occasionally may reach the sea and start tsunamis. In this work we explore the consequences of the Ischia Debris Avalanche (IDA), a flank collapse that occurred in historical times, and involved the whole Mount Epomeo edifice including its submarine portion, and that may have caused gigantic sea waves affecting all the coasts of Ischia and of the Gulf of Naples. The IDA and the generated tsunami have been taken as the worst-case scenario for the occurrence of a new tsunami in the area. They have been simulated through numerical codes developed and maintained by the University of Bologna. The simulation shows that the IDA-induced tsunami attacks severely all the coasts of the Gulf of Naples with the highest waves obtained for the island of Ischia, the island of Capri and the peninsula of Sorrento. The propagation pattern of the IDA tsunami can be used to get hints on the impact that such an event may have had on early populations habiting Gulf of Naples, but also to get clues on the area that could be most severely hit by a tsunami generated by a smaller-scale landslide that may occur in the same source zone.     

On local probability of invasive tsunami

Abstract

Tsunami occurrence and invasive tsunami at a local area in the circum‐Pacific seismic zone were studied as a Poisson process. The tsunami height at Osaka, Japan, was related to tsunami magnitude. The exceedence frequency of invaded tsunami at Osaka showed a good fitness to the Poisson process. However, an adapted process should be introduced for exceedence frequency of tsunami occurrence in the western Pacific. The exceedence probability of invasive tsunami at Osaka was shown as a function of tsunami magnitude on a diagram with a parameter of the time period. The obtained result might be useful for the planning of coastal area, warning of invasive tsunami, and designing coastal structures as protection within the scope of tsunami economics.     

马尼拉俯冲带潜在地震海啸对华南沿海的影响

马尼拉俯冲带潜在地震海啸对我国南部沿海城市构成巨大威胁,利用情景式数值模拟技术重构灾害过程并评估危险等级有助于理解南海海啸传播规律并指导预警预报和防灾减灾工作。根据美国太平洋海洋环境研究中心(Pacific Marine Environmental Laboratory, PMEL)发布的马尼拉俯冲带断层参数设计M_w 7.5、M_w 8.1和M_w 8.5三个震级下共19个震源,应用非静压海啸数值模型(Non-hydrostatic Evolution of Ocean WAVE, NEOWAVE)模拟各震源激发海啸在南海海盆的传播过程,通过最大波辐和测点时间序列发现海啸波能量传输分布并评估代表区域危险等级。研究表明, M_w 7.5级地震海啸对我国南部沿海的影响较低,波幅一般不超过30 cm; M_w 8.1级地震海啸对华南沿海主要造成太平洋海啸预警中心定义的Ⅱ或Ⅲ级海啸危险等级,海啸影响范围和能量分布特征由震源位置决定; M_w 8.5级地震海啸主要对中国沿海构...     

Stability of genetic diversity in an intertidal goby population after exposure to tsunami disturbance

Massive tsunami disturbances have potentially detrimental effects on genetic diversity and effective population size of coastal marine species, and evaluating these effects can be useful for devising conservation strategies for coastal marine environments. Local populations of the intertidal goby Chaenogobius annularis, which are distributed on scattered rocky beaches of the Japanese Archipelago, show demographic independence without overlapping generations, making this an ideal species with which to study the effects of tsunami disturbance on genetic diversity. Some of these populations were affected by the tsunami of the 2011 off the Pacific coast of Tohoku Earthquake. Here, we investigated the change in genetic diversity of a local population of this species, which was located close to the epicenter of the earthquake, across the cohorts before and after the tsunami and evaluated the impact of the tsunami disturbance. Genetic diversity was maintained after the tsunami, and no change in the effective population size was observed. Our results suggest that the tsunami disturbance has had no marked impact on the genetic diversity of C. annularis.     

Rationale for measurement of midocean tsunami signature

Abstract

Arguments are presented to justify midocean tsunami measurements and related investigations aimed toward improvement of tsunami prediction and warning. It is postulated that midocean tsunami signatures be measured simultaneously at several locations and correlated with high‐accuracy onshore measurements.     

Managing tsunamis through early warning systems: A multidisciplinary approach

This study attempts to identify the key factors that will make a tsunami warning system most effective, to develop a framework in which results of natural science and engineering research can be effectively integrated into coastal natural hazard planning, and to develop a numerical example that illustrates how benefit-cost analysis may be used to assess early warning systems. Results of the study suggest that while the science of tsunami wave propagation and inundation is relatively advanced, our knowledge on the relationships between tsunami generation and undersea earthquakes, volcanic eruptions, and landslides remains poor, resulting in significant uncertainties in tsunami forecasting. Probabilities of damaging tsunamis for many coastal regions are still unknown, making tsunami risk assessment and management difficult. Thus it is essential to develop new techniques to identify paleo-tsunami events and to compile and develop size and frequency information on historical tsunamis for different locations. An effective tsunami early warning system must include not only the ocean technologies for accurately detecting an emerging tsunami, but also a civil communication system through which the population can be timely warned by the local government and other sources. Since minimizing the evacuation time is a key factor to make a warning system effective, adequate pre-event education and preparation of the population must be a critical component of the system. Results of a numerical example of the South Pacific region suggest that investments in a tsunami warning system in the region may lead to significant economic benefits.     

Probability distribution of the Murty‐Loomis tsunami magnitude

Abstract

In 1980, Murty and Loomis proposed a new, objective tsunami magnitude scale based on total tsunami energy. A list of 178 tsunamigenic earthquakes during the period 1815 to 1974 was given along with estimated tsunami magnitudes. In this study, we derived the probability distribution function of tsunami magnitudes based on the assumptions that (1) the occurrences of tsunamigenic earthquakes are a Poisson process, and (2) tsunami energy is a polynomial function of tsunami recurrence time. Using the data given by Murty and Loomis, the parameters of the distribution function are estimated. Comparison with the data shows that the derived distribution is a good representation of the distribution of the Murty‐Loomis tsunami magnitude.     

Positioning requirements for the tsunami warning system

Abstract

Canada has increased the number of tsunami warning stations on the Pacific Coast from two to three. The last gauge was installed at the north end of Vancouver Island, thereby filling a large gap previously existing and providing full coverage along the coast. The record of gauges at two of the three locations is accessible either by telephone or by means of meteor burst communication, alleviating the difficulties experienced during the tsunami threat of May 6, 1986, when telephone communications were disrupted by heavy use. The gauge at Langara Island will be relocated in a more accessible and also a more tsunami‐responsive location at Rennell Sound in the Queen Charlotte Islands. All tsunami gauges also serve as tide gauges, recording the water level every 15 min. In the event of a tsunami, the recording interval can be altered to every 60 s. Suggestions have been made that Canada attempt deep‐sea recording of tsunamis off its Pacific Coast. Although this would be of great scientific value, no such program is contemplated at this time.     

一个覆盖太平洋区域的地震海啸波幅预报系统及检验

基于线性长波方程和缓变地形近岸波幅格林公式建立了覆盖整个太平洋区域的准实时地震海啸波幅预报系统。系统利用了GPU并行加速技术,可在90 s之内完成太平洋区域32 h的海啸传播计算和中国沿海城市岸段的波幅特征值预报。筛选了自2006年以来的9次发生在太平洋区域,矩震级（Mw）超过8.0且资料丰富的历史地震海啸事件,对预报系统进行了后报检验。结果表明,线性长波模型能够很好的模拟海啸在大洋中的传播过程;格林公式能够较为准确的估算缓变水深和开阔地形条件下的近岸海啸最大波幅,波幅预警准确率可达80%,基本满足海啸预警需求。以2011年日本Mw9.0地震海啸为例,评估了该系统对中国城市岸段的波幅预警能力,结论基本合理。需要注意的是,利用该系统计算对海啸源特别敏感的近场海啸波幅可能产生较大偏差。提出了若要进一步提高定量海啸波幅预警的准确率,可从以下两个方面加强研究和业务实践:一是采用多数据联合反演方法提升海啸源的精度;二是提高格林公式的适用性,或者构建高效的近岸精细化海啸数值预报系统。     

精细化地震海啸波流实时预警系统研究与应用

基于Okada模型和非线性浅水波模型,结合高精度多层嵌套网格针对我国浙江沿海的温州和台州地区建立了越洋–近海–局部的精细化地震海啸波流实时预警系统,近岸的分辨率为900 m。该预警系统包括了并行化的数值计算模块,基于Python 2D绘图库的计算结果可视化处理模块,以及通过Python语言将所有经过数值计算的图形与动画产品集成在一个网页上的产品集成模块。一旦地震发生,该系统可根据地震的震源参数信息在10 min内完成数值计算、可视化处理,以及产品集成。选取2011年日本东北9.0级地震海啸结合实测数值对该系统进行模拟验证,进一步应用该系统模拟计算了日本南海海槽和琉球海沟潜在极端海啸的影响规律。结果表明,该预警系统可有效地提高地震海啸实时预警的时效性和准确度,为海啸的预警、减灾,以及辅助决策提供科学依据。     

Microbial status in seawater and coastal sediments during pre- and post-tsunami periods in the Bay of Bengal, India

Tsunami, the natural disaster, which occurred on December 26, 2004 in the Indian Ocean, caused severe damage to mankind in the coastal areas. Total loss of life and economic loss because of this disaster have been estimated by various agencies but its effect on microbial density has not been probed. With our previous results on microbial populations in four locations of the Chennai coast of the Bay of Bengal, India in the pre‐tsunami period, the change in microbial populations was studied after the tsunami at regular intervals in the same locations. Coastal sediment and seawater samples were collected from four different locations after 5–10 h and thereafter at intervals of every 7 days up to 28 days post‐tsunami. Bacteria, fungi and actinomycetes were isolated from the marine samples by serial dilution on respective media. Before the tsunami, the bacterial population was higher in seawater samples than the sediments, whereas fungi and actinomycetes were recorded only in the sediments. The microbial population remarkably increased 5–10 h post‐tsunami in all the marine samples irrespective of the location. However, it slowly declined in the subsequent days and became similar to that of the population recorded before the tsunami. The population of gram‐positive bacteria increased whereas the gram‐negative bacterial population decreased after the tsunami. Further, populations of pathogenic bacteria such as coliform and vibrios did not increase after the tsunami. It has been observed that the increase in populations of bacteria and actinomycetes even after 28 days of tsunami may be due to the introduction of foreign microorganisms that developed the ability to survive in the extreme environment by exhibiting special characteristics such as pigmentation and production of exopolysaccharides.     

有限断层模型在2015年9月16日智利Mw8.3级地震海啸数值模拟中的应用与评估

2015年9月16日22时54分（当地时间）智利中部近岸发生M_w8.3级地震,震源深度25 km。同时,强震的破裂区长200 km,宽100 km,随之产生了中等强度的越洋海啸。海啸影响了智利沿岸近700 km的区域,局部地区监测到近5 m的海啸波幅和超过13 m的海啸爬坡高度。太平洋区域的40多个海啸浮标及200多个近岸潮位观测站详细记录了此次海啸的越洋传播过程,为详细研究此次海啸近场及远场传播及演化规律提供了珍贵的数据。本文选择有限断层模型和自适应网格海啸数值模型建立了既可以兼顾越洋海啸的计算效率又可以实现近场海啸精细化模拟的高分辨率海啸模型。模拟对比分析了海啸的越洋传播特征,结果表明采用所建立的模型可以较好地再现远场及近场海啸特征,特别是对近场海啸的模拟结果非常理想。表明有限断层可以较好地约束近场、特别是局部区域的破裂特征,可为海啸预警提供更加精确的震源信息,结合高分辨率的海啸数值预报模式实现海啸传播特征的精细化预报。本文结合观测数据与数值模拟结果初步分析了海啸波的频散特征及其对模型结果的影响。同时对观测中典型的海啸波特征进行的简要的总结。谱分析结果表明海啸波的能量主要分布在10~50 min周期域内。这些波特征提取是现行海啸预警信息中未涉及,但又十分重要的预警参数。进一步对这些波动特征的详细研究将为海啸预警信息及预警产品的完善提供技术支撑。     

2017年9月8日墨西哥沿岸Mw8.2级地震海啸观测数据分析与模拟

2017年9月8日4时49分（UTC）,墨西哥瓦哈卡州沿岸海域（15.21°N,93.64°W）发生M_w8.2级地震,震源深度30 km。强震在该海域引发海啸,海啸对震源附近数百千米范围内造成了严重影响。位于太平洋上的多个海啸监测网络捕捉到了海啸信号并详细记录了此次海啸的传播过程。本文选用了近场2个DART浮标和6个验潮站的水位数据,通过潮汐调和分析和滤波分离出海啸信号,对近场海啸特征值进行了统计分析,并采用小波变换分析方法进一步分析了海啸的波频特征。基于Okada弹性位错理论断层模型计算得到了强震引发的海底形变分布,并采用MOST海啸模式对本次海啸事件近场传播特征进行了模拟,模拟结果与观测吻合较好。最后,基于实测和模拟结果,详细分析了此次地震海啸的近场分布特征,发现除受海啸源的强度和几何分布特征影响外,近岸海啸波还主要受地形特征控制,在与特定地形相互作用后波幅产生放大效应,会进一步加剧海啸造成的灾害。     

2015年9月16日智利Mw 8.3级地震:海啸波从近场到远场的特征分析

On September 16, 2015, an earthquake with magnitude of M_w 8.3 occurred 46 km offshore from Illapel, Chile,generating a 4.4-m local tsunami measured at Coquimbo. In this study, the characteristics of tsunami are presented by a combination of analysis of observations and numerical simulation based on sources of USGS and NOAA. The records of 16 DART buoys in deep water, ten tidal gauges along coasts of near-field, and ten coastal gauges in the far-field are studied by applying Fourier analyses. The numerical simulation based on nonlinear shallow water equations and nested grids is carried out to provide overall tsunami propagation scenarios, and the results match well with the observations in deep water and but not well in coasts closed to the epicenter. Due to the short distance to the epicenter and the shelf resonance of southern Peru and Chile, the maximum amplitude ranged from 0.1 m to 2 m, except for Coquimbo. In deep water, the maximum amplitude of buoys decayed from9.8 cm to 0.8 cm, suggesting a centimeter-scale Pacific-wide tsunami, while the governing period was 13–17 min and 32 min. Whereas in the far-field coastal region, the tsunami wave amplified to be around 0.2 m to 0.8 m,mostly as a result of run-up effect and resonance from coast reflection. Although the tsunami was relatively moderate in deep water, it still produced non-negligible tsunami hazards in local region and the coasts of farfield.     

Submersible observations of manganese nodule and crust deposits on the Tenpo Seamount,Northwestern Pacific

Pavements of manganese nodules and crusts and outcrops of Miocene limestones were observed on the flanks and flat top of the Tenpo Seamount during three Shinkai 2000 dives. The pre‐Miocene volcano supplied nuclei of volcanic rocks and hydrothermal manganese deposits, and subsequent slow or no sedimentation promoted deposition of abundant hydrogenetic nodules and crusts, mainly on the upper flank of the seamount. Nodule pavements generally cover calcareous sand surface sediments, while crusts cover hard outcrops composed probably of volcanic rocks. The fields of crusts and nodules are sparsely distributed with each other on scales of meters to tens of meters. The on‐site observation suggests the deposits have encountered tectonic and/or mass movements that resulted in unusual occurrences of densely stacked nodules and occasionally the nodules resting directly on crusts or hard substrates. Mineralogical and chemical compositions reveal that for nodules and crusts the encrusting manganese layers of around 1 cm thickness are composed of hydrogenetic vernadite, and diagenetic influence is negligible.