P波初動の偏りから見た阿蘇カルデラの構造

Abstract

The differences between the epicentral azimuths and the directions of arrivals of the initialphases are determined from the horizontal seismograms of the Volcanological Laboratory of KyotoUniversity situated inside of thr Aso caldera. The recent data in addition to the previous datagiven by Sassa (1936) exhibit a systematic deviation. We can interpret the deviation by puttingforward the following structural model under the Aso caldera. A tectonic line or zone runs acrossthe center of the caldera from east to west and the northern part inclines nearly eastwards with thedip of about 150, and the southern part inclines toward NNE with the dip of 25°. The chain of thecentral volcanocs, one of which, the Nakadake, is now active, situates on the tectonic line or zone.And moreover the sky-line of the caldera rim exhibits the similar inclination as mentioned above.