UNUSUAL HIGH-STRESS DROP SUBEVENT DURING THE M5.5 EARTHQUAKE, THE LARGEST EVENT OF THE 1989 ITO-OKI SWARM ACTIVITY

On July 13, 1989, the Ito-oki submarie volcano was created and erupted after an earthquake swarm and volcanic tremor. At the final stage of the swarm activity, on July 9, 1989, a mainshock (M5.5) occurred. This earthquake consisted of two events separated 45 seconds from each other. The second events included high stress drop faulting (asperity). In this paper, the M5.5 earthquake was analyzed focusing on this asperity. The results are that 1) the radius and dynamic stress drop of the asperity were approximately 300m and 50MPa, respectively; 2) the epicenter of the asperity was estimated at 5km west of the erupted volcano and its depth was 6km; and 3) without this strong asperity, the waveforms of the two events were very similar, suggesting a similar source process except for the asperity.