DETAILED STUDY OF THE EARTHQUAKE SEQUENCE IN 1980 OFF THE EAST COAST OF THE IZU PENINSULA, JAPAN

The earthquake sequence which occurred off the east coast of the Izu Peninsula, Japan, in 1980 is investigated in detail on the basis of a homogeneous hypocenter file (M≥2.9) carefully constructed for this purpose. Absolute and relative precisions of hypocenters are improved to be 1-2km and several hundred meters, respectively, by applying two kinds of correction to arrival times. Furthermore spectra of seismic waves recorded at OYM (Δ-50km) are examined to detect possible changes in spectral features in the course of the development of the sequence. The present sequence is the superposition of two types of activities, the swarm activity (M≤5.1, MJMA≤4.9) and the mainshock (M6.5, MJMA 6.7)-aftershock sequence. The swarm activity consisted of 28 bursts of activity, each of which lasted for about 1 to 2hr after a quiescent period of six to ten-several hours. Such swarm activity occurred within a narrow area of 6km by 2km throughout the whole sequence. The depth was also limited from 9 to 11km. Each burst of activity moved from place to place in this limited area without any significant relation to the largest shock of the sequence, occupying an area of about 1-2km square each time. It only seems to show the local fracture strength in the swarm area. The estimated fault of the largest shock (MJMA 6.7) is a vertical plane of 20km in length and 8-10km in width with a strike of N20°W. Its location agrees well with an evident cliff of the submarine topography. The hypocenter of the largest shock is 3km south of the swarm area. This place had been quiescent during the early stage of the sequence, but about 20hr before the largest shock, an M4.0 (MJMA 3.9) shock occurred there.