火山性地震BL・BHの初動の押し引き分布

Abstract

Volcanic microearthquakes called "B-type" are closely related with explosive activity.Previous researchers devided B-type earthquakes into subclasses by the predominant frequen-cy. However, hypocenters and focal mechanism of the individual subclasses in B-typeearthquakes have not been determined because of the unclear onsets.In order to detect the initial motions clearly, seismometers were installed in the boreholesaround the active crater of Sakurajima Volcano. In this paper, B-type earthquakes whosedominant frequecies are 1-3 Hz and 5-8 Hz, are named as BL-type and BH-type earthquakes, respectively. The hypocenters and distribution of the initial motions of BL-type and BH-type earthquakes were examined. The results were summarised as follows.(1) Focal depths of BL-type earthquakes were determined at the depth of less than 2 kmbelow the sea level. BH-type earthquakes had deeper hypocenters than BL-type. Themaximum depth of BH-type was 2.5 km.(2) Most of BL-type earthquakes which occurred isolatedly and BH-type earthquakeshad all compressional arrivals. BL-type earthquakes which occurred in swarms had allcompressional or all dilatational arrivals, with the ratio of 61% to 35%. The initial motionsof the remaiders were mixed with compressions and dilatations. The distributions of initialmotions did not show that focal mechanisms of BL-type and BH-type earthquakes werequadrant types.