VELOCITY STRUCTURE AND AFTERSHOCK DISTRIBUTION OF THE 1982 URAKAWA-OKI EARTHQUAKE

Aftershock distribution of the Urakawa-oki earthquake (Ms 6.8) of 1982 is investigated. A simultaneous inverse method is applied to P-wave arrival time data from the aftershocks and quarry blasts in order to estimate a three-dimensional velocity structure in the aftershock region. Strong correlations between the obtained velocity structure, geology, and gravity anomalies are observed. Hypocentral determination of the aftershocks by using the three-dimensional velocity structure shows that the epicentral distribution is a triangle and its area is 790 km2. The detailed aftershock distribution indicates that three dipping planes with high aftershock activities appeared in the aftershock region: two of the planes are almost parallel to each other with a dip angle of 50°in a depth range of 3 km to 27 km, and these planes clearly form double aftershock planes, which have been previously supposed to be an identical plane; the other plane is located perpendicularly to the double aftershock planes in a depth range of 13 km to 27 km and seems to be a conjugate plane with the double aftershock planes. The aftershocks occur in a region with P-wave velocities higher than 5 km/s, and the conjugate plane is located along the structural boundary. It is found that the rupture of the main shock took place on the conjugate plane and also might have been caused by a tectonic force derived from a collision between the Kurile and Northern Honshu Japan Arcs.

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