Journal of Natural Disaster Science

GEOPHYSICAL STUDIES ON THE OITA EARTHQUAKE OF 1975

Syun'itiro OMOTE
Professor, Kyushu Sangyo University, Fukuoka
Akira KUBOTERA
Professor, Faculty of Science, Kyoto University, Kyoto
and
Toshio MITSUNAMI
Associate Professor, Fukuoka University of Education, Fukuoka

(Received 15 November, 1979, and in revised form 16 January, 1980)

Abstract

The results of extensive studies regarding geophysical aspects of the Oita earthquake of 1975 are compiled in this paper. The aftershocks that took place soon after the main shock were distributed in a narrow belt area of a few kilometer width and trending NW-SE, having the epicenter of the marn shock at its southeastern corner. This belt area falls exactly in an area bounded by an isoseismal line of intensity 5.5 (JMA scale) determined from questionnaire surveys. In this belt area, concentrations of earth fissures, land slides, and other earthquake damage were observed. All these events provided good reason to assume that the latent fault which generated this earthquake must lie underneath this area. This assumption is consistent with the results of fault mechanism analysis and the fault model study, derived from the initial motion distribution of seismic P waves and the crustal deformation evidence due to precise levelling survey. Of particular interest is the damage that was caused to wooden houses. Damaged houses were seen only in the belt area above mentioned, and by means of extensive field investigations of overturned gravestones, maximum ground acceleration due to this earthquake was estimated at almost 420 gals throughout the whole belt area.

Key words

earthquake, Oita earthquake