火山体直下の地震波の速度異常 : 霧島・桜島火山の場合

概要

論文の詳細を見る
The structual anomaly beneath the Kirisima-Sakurazima volcanoes estimated from P wave velocity is concerned. The travel time residuals O-C at KVO (Kirisima Volcano Observatory, a branch station of Earthquake Research Institute) are discussed using intermediate earthquakes beneath the Kyushu-Ryukyu arc (Figs. 1 and 2). The Jeffreys-Bullen and Ichikawa-Mochizuki travel time tables are used for the calculation of arrival times. The flat layered model obtained from surface wave analyses is also used (Fig. 3). A clear discrepancy between O-C of earthquakes in south and in north at KVO is found. O-C from soutn at KVO is +1〜2 seconds and few seconds larger than from north (Fig. 4). This positive anomaly of O-C means that P wave velocity beneath the Kirisima-Sakurazima volcanoes is smaller than that beneath the north part of Kirisima. There are two reasons to suspect that the low P wave velocity occurs not in the crust but in the upper mantle; 1 (O-C from only south is positive anomaly in spite of the fact that the incident angle at KVO is larger than 70 degrees (see Fig. 4), 2) O-C of earthquake with the depth of 160 km is larger than that with 120〜130 km.
1975-02-01