Two Periodic Activities of Microearthquakes near Kumamoto, Southwestern Japan : Results of a Temporary Observation in November 1978

Abstract

A tripartite microseismic network was installed near Kumamoto in Kyushu, southwestern Japan, during the period of November 15-19, 1978. Seismic activity was low underneath Kumamoto City in comparison with the surrounding areas. The distribution of epicenters was diffuse and did not appear to correlate with any of the known major faults in the area. During the period of observation, an M=1.3 earthquake and its aftershocks were observed under the Shimabara Bay, and a periodic earthquake swarm (the largest M=1.5) occurred near Ueki. The main shock and recorded aftershocks (except one out of four) of the Shimabara Bay earthquake occurred at high tide in the Bay. P wave first motions suggested a possible normal faulting. Consequently these shocks may have been triggered by an increase in earthquake-generating stress caused by tidal loading. The activity associated with the Ueki swarm occurred in cycles of about 20 hours. The activity in each cycle was characterized by the occurrence of minor shocks in the early stage, increase in magnitude in the successive stage, and finally the largest shock which was followed by a quiescent period of about 12 hours.1978年11月15-19日,熊本付近で微小地震の3点観測を行なった.熊本市街直下の地震や,周辺の顕著な活断層系に直接対応するような地震は,この期間には観測されなかった.しかし,島原湾で起きたM=1.3の地震と,植木付近で起きた群発的活動(最大のM=1.5)は,興味深い活動であった.島原湾の地震の本震と余震(4回のうち1回を除く)は,島原湾の満潮時に発生した.P波初動を調べると,正断層型のメカニズム解をもつ可能性がある.もしそうなら,海水の荷重のために満潮時に起震応力が増大し,この作用によって地震が誘発されたと考えられるかもしれない.一方,植木付近の活動は,およそ20時間前後の周期性が見られた.各サイクルの活動は特徴的で,まずごく小さな地震が起こり始め,マグニチュードがしだいに大きくなり,各サイクルの中の最大の地震が起きたあとは,約12時間ほどの静穏期にはいるという経過を3回くり返した