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The mechanism of the Iriomote earthquakes swarms

西表地震群的机制

基本信息

批准号：
05452377
负责人：
ANDO Masataka
金额：
$ 4.61万
依托单位：
KYOTO UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1993
资助国家：
日本
起止时间：
1993 至 1994
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-05452377/
关键词：
Iriomote Earthquake swarm Normal fault Extensional stress Taiwan Back arc Geothermal actin'ty Ryukyu trench 琉球列島全磁力観測震源分布火山地震

项目摘要

The Iriomote swarm activity which started suddenly in November, 1991 had been intermittently occurred in the northern part of the island. The largest event of M5.2 happened in November, 1993. Since then the activity had decreased gradually. Nevertheless events of M>3 sometimes had occurred, sugeesting the swarm holding still its activity. From this reason, the present program was initiated in the last fiscal year. In the last year installation of instruments and the examination of the data thus obtained were made.Events of the earthquake swarm is distributed like a plane, with a dimension of 20 km x 10 km, which strikes N35W and dips to the southwest with an angle of 70 deg.The focal mechanism of relatively large events M>3.5 have dominantly normal fault component that implies the earthquake swarm have been manifested under tensional stress.This year have conducted observations and analyzes of the data on the swarm activity. We conclude the process of the earthquake swarm beneath the island as follows : Under the NEE-SWW tensional stress field water (liquid phase) intruded from the upper mantle causing an increase of pore pressure leading to the earthquake swarm activity. A more detailed geochemical study is necessary on the origin of the water although a chemical analysis has implied the upper mantle as an supply source of the water. The plane-like distribution of earthquakes on which the micro-earthquakes are concentrated during the swarm activity is a basically fault plane. The dip and strike of the plane are consistent with the regional pattern of faults throughout the southern Ryukyu trench and the mechanism of several M6 class earthquakes near Taiwan.In conclusion the tensional stress field caused the Iriomote swarm activity and then the medium-size earthquakes near Taiwan. With the occurrence of the series of Taiwan events the activity completed and died out to the normal level.

1991年11月突然开始的西表岛群活动，在岛北部间歇性地发生。 M5.2最大的事件发生在1993年11月。此后活动逐渐减少。然而，有时会发生 M>3 的事件，表明虫群仍保持其活动。因此，本计划于上一财年启动。去年进行了仪器安装和数据检验。震群事件呈平面分布，尺寸为20公里×10公里，走向N35W，向西南倾角70°。较大事件M>3.5的震源机制以正断层成分为主，说明震群在拉应力作用下显现。对群体活动的数据进行了观察和分析。我们将岛下震群的过程总结如下：在NEE-SWW拉应力场下，水（液相）从上地幔侵入，导致孔隙压力增加，导致震群活动。尽管化学分析表明上地幔是水的供应源，但仍需要对水的起源进行更详细的地球化学研究。群活动时微震集中的地震平面分布基本上是断层面。该平面的倾角和走向与整个琉球海沟南部断层的区域格局以及台湾附近几次M6级地震的机制一致。综上所述，张应力场导致了西表岛群活动，进而导致了台湾附近的中型地震。随着台湾系列事件的发生，该活动逐渐结束并逐渐平息。