Precursors of the 1983 Japan Sea Earthquake

Abstract

On the basis of the space-time distribution of shallow earthquakes of M4 or above, it is pointed out that a seismic gap of the second kind (seismic quiescence) appeared in a region including the focal region of the 1983 Japan Sea earthquake, off the west coast of northern Honshu. Data are from JMA and ISC. This seismic gap appeared in about mid-1978 and continued for five years until the large earthquake of M7.7. About twelve days before the large earthquake, foreshocks including a M5.0 shock occurred near the epicenter of the main shock. Data concerning smaller earthquakes (M≥ 3.0) observed by Tohoku University also show that the seismic activity in the hypocentral region decreased from about mid-1978. Other geophysical data in this region were reexamined, and the following results were obtained. Remarkable earthquake swarms and volcanic eruptions occurred in the land areas adjacent to the seismic gap after about 1978; ground uplift at Iwasaki and the Oga Peninsula located near the eastern boundary of the seismic gap region seems to have accelerated in the past several years; tide gauge data is consistent with the recent uplift at the same places; and water-tube tiltmeter data from Tohoku University shows that a general tendency of the eastward tilt at Oga started in 1978. All these remarkable phenomena started around 1978 in the surrounding region of the seismic gap. These phenomena seem to be long-term precursory phenomena of the 1983 Japan Sea earthquake.