Abstract
Aftershocks of the 1983 Nihonkai-chubu (Japan Sea) earthquake (MJMA 7.7) were relocated using an inclined-layer velocity model, a one-layer crust in dipping contact with underlying half-space mantle. We dealt with aftershocks which occurred from June 5 through June 23, 1983, when the seismographic stations of Hirosaki University were most densely distributed in northern Tohoku, Honshu, Japan. The parameters of the velocity model were determined by applying a method of simultaneous estimation of velocity model parameters and hypocenter parameters to an ensemble of P-wave arrival times from selected aftershocks. The determined Moho interface dips nearly eastward with an angle of 6°-8°, being placed at a depth of 18 km at the western end of the aftershock area and 28 km on the Japan Sea coast of Akita and Aomori Prefectures. The relocated hypocenters concentrated intensively on an easterly dipping plane, thus clearly suggesting the geometry of the main-shock fault. The fault mapped by the aftershock distribution changes its strike halfway along the fault length, striking in N15°-20°E in the southern part and N10°-15°W in the northern part. This change in fault strike agrees with a change in the general trend of geology in the aftershock area. The dip of the fault, which had previously been uncertain, was reliably estimated to be dipping in an easterly direction with an angle of about 20° for both the northern and southern parts. The bottom of the fault is probably limited within the crust because the relocated aftershocks are confined to a depth range shallower than 20 km. When compared with hypocenters of aftershocks determined for the inclined-layer velocity model, those determined for laterally homogeneous velocity models were much more scattered. Reduction of the scatter of hypocenters has been accomplished by the introduction of the inclined-layer velocity model which adequately accounts for the lateral variation of velocity structure in and around the aftershock area.
