The mechanism of the Iriomote earthquakes swarms

1991年11月突然开始的西表岛震群活动，在该岛北方间歇性发生。1993年11月发生了最大的M5.2事件。从那时起，活动逐渐减少。然而，M>3的事件时有发生，表明该虫群仍在活动。因此，本方案是在上一个财政年度启动的。去年，我们安装了仪器，并对所获得的资料进行了检验。震群事件呈平面分布，尺度为20公里× 10公里，走向N35 W，倾向西南，倾角70 °。3.5级地震以正断层成分为主，表明震群是在张应力作用下表现出来的，今年对震群活动的资料进行了观测分析。我们认为岛礁下震群活动的过程是：在NEE-SWW向张性应力场作用下，上地幔水（液相）侵入，引起孔隙压力增加，导致震群活动。尽管化学分析表明上地幔是水的补给源，但对水的起源还需要进行更详细的地球化学研究。震群活动时，微震集中的地震平面状分布基本上是一个断层面。该平面的倾向和走向与贯穿琉球海沟南部的区域性断裂格局和台湾附近几次6级地震的机制相一致，表明张性应力场导致了西表震群活动和台湾附近的中等地震。随着台湾系列事件的发生，这一活动逐渐完成并趋于正常水平。

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.