设定琉球海沟发生罕遇地震评估我国东南沿海地区的海啸风险

根据构造相似条件分析,琉球海沟与日本海沟、智利海沟、印尼巽他海沟一样具备发生9级罕遇超巨大地震的可能。在对近几年来全球发生的超巨大地震参数及构造对比分析的基础上,设定琉球海沟9.0级地震参数,并将其引发的海啸进行数值模拟研究。结果表明,该地震可引发初始波高为8m的海啸,台湾东北部半小时后遭受10m以上海啸,3~4小时左右传至浙南、闽北沿岸,近岸各处波高在1~2m;5小时左右传至浙北、粤北沿岸,浙江近岸各处波高在2m左右,广东沿海、台湾海峡由于台湾岛的正面阻挡,海啸波高低于50cm;8小时后靠近上海海岸线,最大波高约1m。海啸的上岸高度与海岸附近的海深和海岸线的形态密切相关,我国东南海域地形变化复杂、海湾众多,对海啸波有放大作用,模拟结果可能比实际海啸偏小。我国沿海地区分布着不少已建和在建的核电厂,在核电设计时未考虑海啸,一旦发生这种罕遇地震海啸则影响不可忽视,尤其是若与风暴潮、天文大潮叠加则可能出现严重后果。由于核电安全要求万无一失,故须制订有效预警和应对措施。     

冲绳海槽地区地震潜在海啸对中国东南沿海的影响研究

采用球坐标系下非线性浅水波方程, 研究日本本州M9.0大地震引发的海啸对中国东南沿海的影响, 并计算了冲绳海槽构造带上3个不同段落可能发生潜在地震引发的海啸, 分析这些海啸与日本大海啸的浪高和走时关系. 结果表明, 日本地震海啸模拟结果与日本当地报道及中国东南沿海7个验潮站的报道结果相符. 冲绳海槽构造带中段可能发生的3次不同震级（M7.0, M7.5, M8.0）潜在地震引发的海啸到达中国东南沿海的时间比日本海啸提前约4个小时, 从震源区传播3个多小时即可到达华东沿海部分验潮站. 冲绳海槽M7.5潜在地震海啸在验潮站上计算的波高与日本海啸相当, 中冲绳海槽M8.0潜在地震海啸在大陈站的波高将超过0.9 m, 在坎门站波高将超过1.8 m. 北冲绳海槽的潜在地震海啸威胁主要集中在江苏盐城、 上海一带, 南冲绳海啸主要对台湾东北部和浙江沿海产生威胁. 本文对冲绳海槽构造带上潜在地震引发海啸的模拟结果, 可为中国东南沿海地区的防震减灾、 海啸预警提供有意义的参考.      

我国地震海啸危险性初步探讨

概述了地震海啸的特征及其成因、国内外历史上发生地震海啸的情况,结合我国沿海的地震、构造、海底地貌等特点,分析了我国沿海发生地震海啸的可能性。最后简要叙述了卫星技术在地震海啸监测中的应用。     

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。     

华东沿海地震海啸模拟

利用数字地震资料及海洋地震地质基础数据, 基于Matlab平台在Windows系统下重新编译Philip liu等人编写的海啸模拟源程序, 将分层静态位移解结果与已建立的海啸传播初始场快速融合, 用交错网格蛙跳差分方法求解笛卡尔坐标系下非线性浅水波方程, 模拟华东沿海地区地震海啸传播过程, 对华东沿海地区可能遭受海啸袭击的危险做出评估, 结果显示华东沿海地区是发生地震海啸影响较弱地区。     

我国地震海啸危险性概率分析方法

日本、美国等国家在海啸危险性分析方面做了较多的研究,有相关的分析方法提出.我国地震海啸研究尚属于起步阶段,还没有形成公认的地震海啸危险性概率分析的思路和方法.简要综述了我国地震海啸的研究进展,借鉴我国已成熟的地震危险性分析方法,给出了地震海啸危险性概率分析的思路与方法,并对其关键技术问题进行了分析,为我国全面开展地震海...     

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。     

日本南海海槽天然气水合物研究现状

本文介绍日本在其周围海域特别是南海海槽的海洋天然气水合物研究工作，首先介绍其研究简史，研究计划，研究队伍及早期研究成果，其次，介绍了西南海海槽天然气水合物的勘探、研究状况，然后,介绍了五年计划实施的东南海海槽地球物理勘探、钻探状况，最后指出，南海海槽的一些新资料深入的综合研究无疑会给世界水合物研究增添新的内容。     

冲绳海槽地震海啸的数值试验研究

在分析东海水深与地震情况基础上,我们认为冲绳海槽地震具有引发海啸的条件,并对该区海啸传播过程采用单侧破裂方式的有限移动源模型和高阶Boussinesq方程做了一维有限差分法数值模拟,给出了海啸传播过程波形和速度变化图.我们从模拟结果得到初步结论:冲绳海槽特大地震海啸传播到上海近海在5小时后,近海波高在几十厘米范围内.     

Structure and evolution of the backstop in the eastern Nankai Trough area (Japan): Implications for the soon-to-come Tokai earthquake

Abstract We present data showing that the intra-oceanic shortening now occurring south of the eastern Nankai Trough that has produced the Zenisu Ridge has also been responsible for the formation of a previous ridge now buried below the continental margin. This ridge, that we refer to as Paleo-Zenisu, is presently adjacent to the backstop and its location coincides with the outer limit of the seismogenic decollement. The subduction of the paleo-Zenisu ridge below the wedge has led to its complete reorganization and has given its identity to the Great Tokai earthquake rupture zone. The formation of paleo-Zenisu and its consequent subduction has induced the tilting of the backstop toward the northwest since ca 2 Ma. This model suggests that the backstop and possibly the wedge are dextrally sheared because they are extruded southwestward in relation to the collision of the Izu-Bonin Ridge with Japan. We use the finite motion from Zenisu to paleo-Zenisu to derive both the subduction vectors along the Nankai Trough and the shortening vectors within Zenisu-Izu. The amount of shortening absorbed within Zenisu-Izu increases toward the northeast. The corresponding subduction vectors of the Zenisu platelet below the wedge decrease accordingly to the northeast from 50 to less than 20 mm/year and the Zenisu body rotates clockwise with a pole near 36° North, 139° East. This might explain the apparent longer repetition time of great earthquakes in the Tokai area. On the other hand, the 25-35 mm/year obtained for the rate of shortening along the Zenisu thrust indicates a high seismic potential there.     

Overview of Holocene Tsunami Deposits along the Nankai,Suruga, and Sagami Troughs,Southwest Japan

Tsunami deposits provide a basis for reconstructing Holocene histories of great earthquakes and tsunamis on the Pacific Coast of southwest Japan. The deposits have been found in the past 15 years at lakes, lagoons, outcrops, and archaeological excavations. The inferred tsunami histories span 3000 years for the Nankai and Suruga Troughs and nearly 10,000 years for the Sagami Trough. The inferred histories contain recurrence intervals of variable length. The shortest of these —100–200 years for the Nankai Trough, 150–300 years for the Sagami Trough — resemble those known from written history of the past 1000–1500 years. Longer intervals inferred from the tsunami deposits probably reflect variability in rupture mode, incompleteness of geologic records, and insufficient research. The region's tsunami history could be clarified by improving the geologic distinction between tsunami and storm, dating the inferred tsunamis more accurately and precisely, and using the deposits to help quantify the source areas and sizes of the parent earthquakes.     

东海海域潜在地震海啸的数值模拟初步研究

针对东海冲绳海槽地区的地震地质背景,对东海海域潜在的地震海啸进行了预研究. 假设了冲绳海槽在发生8.5级大地震,断层错距高达15 m的极端地震情况引发的海啸对中国东部沿海地区的影响. 初步数值模拟结果表明,该地震引发的海啸的最大初始波高为4.3 m,4小时左右传至浙江沿岸,近岸各处波高为1——2 m,其中局部地区波高为2.4 m;约7——8小时靠近上海海岸线（若震源在中冲绳海槽地区,海啸传到上海最快大约7小时）,近岸波高约为1 m. 近岸区域地形变化复杂,海岛密布,局部地形条件可能会很大地影响实际各地点海啸波高,加上海啸在岸边爬高及港湾效应,估计波高还会升高. 给出了冲绳海槽南、中、北部发生潜在地震海啸的传播等时图. 笔者在东海设置了3个地震及海啸监测站,基于海啸模拟结果绘制了监测站处的海啸随时间演化曲线,分析了预研究成果对海啸预警可能发挥的作用.      

Seismic hazard in Takamatsu Japan from fault trenching and paleo-liquefaction studies

Takamatsu is a harbor city of 330,000 people on the northern shore of Shikoku Island, southwest Japan. Earthquakes in the Nankai Trough, typically of magnitude M 8+, have been considered the primary source of seismic hazard for the island and Takamatsu. A major active fault system, the Median Tectonic Line, runs across the Shikoku from east to west near the north shore. There is no documented historical seismicity associated with this major fault system in Shikoku or with associated faults such as the 20 km Nagao Fault near Takamatsu. Therefore the trenches were cut across some of the more important faults to expose the record of past seismicity. Additional data on past seismicity is derived from paleo-liquefaction studies in the Takamatsu plain. This paper describes the trenching and paleo-seismic studies and discusses the implications of the findings for seismic hazard in Takamatsu.     

Field survey report on tsunami disasters caused by the 1993 Southwest Hokkaido earthquake

Detailed field work at Okushiri Island and along the southwest coast of Hokkaido has revealed quantitatively (1) the advancing direction of tsunami on land, (2) the true tsunami height (i.e., height of tsunami, excluding its splashes, as measured from the ground) and (3) the flow velocity of tsunami on land, in heavily damaged areas. When a Japanese wooden house is swept away by tsunami, bolts that tie the house to its concrete foundation resist until the last moment and become bent towards the direction of the house being carried away. The orientations of more than 850 of those bent bolts and iron pipes (all that can be measured, mostly at Okushiri Island) and fell-down direction of about 400 trees clearly display how tsunami behaved on land and caused serious damage at various places. The true tsunami height was estimated by using several indicators, such as broken tree twigs and a window pane. The flow velocity of tsunami on land was determined by estimating the hydrodynamic force exerted on a bent handrail and a bent-down guardrail by the tsunami throughin situ strength tests.Contrary to the wide-spread recognition after the tsunami hazard, our results clearly indicate that only a few residential areas (i.e., Monai, eastern Hamatsumae, and a small portion at northern Aonae, all on Okushiri Island) were hit by a huge tsunami, with true heights reaching 10 m. Southern Aonae was completely swept away by tsunami that came directly from the focal region immediately to the west. The true tsunami height over the western sea wall of southern Aonae was estimated as 3 to 4 m. Northern Aonae also suffered severe damage due to tsunami that invaded from the corner zone of the sand dune (8 m high) and tide embankment at the northern end of the Aonae Harbor. This corner apparently acted as a tsunami amplifier, and tide embankment or breakwater can be quite dangerous when tsunami advances towards the corner it makes with the coast. The nearly complete devastation of Inaho at the northern end of Okushiri Island underscored the danger of tsunami whose propagation direction is parallel to the coast, since such tsunami waves tend to be amplified and tide embankment or breakwater is constructed low towards the coast at many harbors or fishing ports. Tsunami waves mostly of 2 to 4 m in true height swept away Hamatsumae on the southeast site of Okushiri Island where there were no coastal structures. Coastal structures were effective in reducing tsunami hazard at many sites. The maximum flow velocity at northern Aonae was estimated as 10 to 18 m/s (Tsutsumi et al., 1994), and such a high on-land velocity of tsunami near shore is probably due to the rapid shallowing of the deep sea near the epicentral region towards Okushiri Island. If the advancing direction, true height, and flow velocity of tsunami can be predicted by future analyses of tsunami generation and progagation, the analyses will be a powerful tool for future assessment of tsunami disasters, including the identification of blind spots in the tsunami hazard reduction.     

Great earthquake at 7.3 ka inferred from tsunami deposits in the Sukumo Bay area,Southwestern Japan

Tsunami deposits in Kyushu Island, Southwestern Japan, have been attributed to the 7.3 ka Kikai caldera eruption, but their origin has not been confirmed. We analyzed an 83-cm-thick Holocene event deposit in the SKM core, obtained from incised valley fill in the coastal lowlands near Sukumo Bay, Southwestern Shikoku Island. We confirmed that the event deposit contains K-Ah volcanic ash from the 7.3 ka eruption. The base of the event deposit erodes the underlying inner-bay mud, and the deposit contains material from outside the local terrestrial and marine environment, including angular quartz porphyry from a small inland exposure, oyster shell debris, and a coral fragment. Benthic foraminifers and ostracods in the deposit indicate various habitats, some of which are outside Sukumo Bay. The sand matrix contains low-silica volcanic glass from the late stage of the Kikai caldera eruption. We also documented the same glass in an event deposit in the MIK1 core, from the incised Oyodo River valley in the Miyazaki Plain on Southeastern Kyushu. These two 7.3 ka tsunami deposits join other documented examples that are widely distributed in Southwestern Japan including the Bungo Channel and Beppu Bay in Eastern Kyushu, Tachibana Bay in Western Kyushu, and Zasa Pond on the Kii Peninsula as well as around the caldera itself. The tsunami deposits near the caldera have been divided into older and younger 7.3 ka tsunami deposits, the younger ones matching the set of widespread deposits. We attribute the younger 7.3 ka tsunami deposits to a large tsunami generated by a great interplate earthquake in the Northern part of the Ryukyu Trench and (or) the Western Nankai Trough just after the late stage of the Kikai caldera eruption and the older 7.3 ka tsunami deposits to a small tsunami generated by an interplate earthquake or Kikai caldera eruption.     

Field survey of the East Java earthquake and tsunami of June 3, 1994

A field survey of the June 3, 1994 East Java earthquake tsunami was conducted within three weeks, and the distributions of the seismic intensities, tsunami heights, and human and house damages were surveyed. The seismic intensities on the south coasts of Java and Bali Islands were small for an earthquake with magnitudeM 7.6. The earthquake caused no land damage. About 40 minutes after the main shock, a huge tsunami attacked the coasts, several villages in East Java Province were damaged severely, and 223 persons perished. At Pancer Village about 70 percent of the houses were swept away and 121 persons were killed by the tsunami. The relationship between tsunami heights and distances from the source shows that the Hatori's tsunami magnitude wasm=3, which seems to be larger for the earthquake magnitude. But we should not consider this an extraordinary event because it was pointed out byHatori (1994) that the magnitudes of tsunamis in the Indonesia-Philippine region generally exceed 1–2 grade larger than those of other regions.     

Millennium‐scale recurrent uplift inferred from beach deposits bordering the eastern Nankai Trough,Omaezaki area,central Japan

A flight of Holocene marine terraces on the southwestern coast of Cape Omaezaki of central Japan provides evidence of recurrent millennium‐scale uplift events. We reconstructed the uplift history of these terraces by using facies analysis of drill core and geoslicer samples, environmental analysis of trace fossils, and ¹⁴C age determinations. Coastal uplift can be identified by the displacement of beach deposits such as foreshore deposits, which represent the intertidal swash zone of a wave‐dominated sandy coast. Three levels of former beach deposits facing the Nankai Trough were identified near the coast in the Omaezaki area. The highest of these, dated at about 3020–2880 BC, records a maximum of 2.2–2.7 m of emergence. The middle beach surface, of minimum age 370–190 BC, shows 1.6–2.8 m of emergence. The lowest beach surface, which is older than 1300–1370 AD, records 0.4–1.6 m of emergence. Our analysis of vertical crustal deformation data during the Holocene in this region suggests that rapid and strong uplift was restricted to the southwestern coast of the Omaezaki area and was probably caused by high‐angle thrusting on subsidiary faults branching from the underlying plate boundary megathrust.