On July 13, 1989, the Ito-oki submarine 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 event 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 300 m and 50 MPa, respectively; 2) the epicenter of the asperity was estimated at 5 km west of the erupted volcano and its depth was 6 km; and 3) without this strong asperity, the waveforms of the two events were very similar, suggesting a similar source process except for the asperity. ¿ American Geophysical Union 1991