The mechanism of the Iriomote earthquakes swarms

1991年11月突然开始的伊里奥莫特虫群活动在该岛北部断断续续地发生。最大的5.2级地震发生在1993年11月。从那时起，活动逐渐减少。然而，mb>3的事件有时也会发生，这表明蜂群仍然保持着它的活动。基于这个原因，本计划是在上一财政年度启动的。去年安装了仪器并检查了由此获得的数据。地震群事件呈平面分布，大小为20 km × 10 km，走向N35W，向西南倾斜70°角，相对较大的地震群震源机制以正断层成分为主，表明地震群是在张应力作用下表现出来的。今年我对蜂群活动的数据进行了观察和分析。岛下地震群的形成过程如下：在东北东-西南西向张性应力场下，水（液相）从上地幔侵入，引起孔隙压力增大，导致地震群活动。尽管化学分析表明上地幔是水的供应来源，但对水的起源还需要进行更详细的地球化学研究。地震群活动时微震集中的平面分布基本上是一个断裂面。该平面的倾角和走向与整个琉球海沟南部的区域断裂格局和台湾附近几次M6级地震的发生机制一致。综上所述，张应力场引起了伊洛莫特群的活动，进而引起了台湾附近的中等规模地震。随着台湾一系列事件的发生，活动结束并逐渐消失到正常水平。

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.