Characteristics of Aftershock Occurrence in the Case of the Southeastern Akita Earthquake of 1970

Abstract

A study is made on the spatial and temporal changes in aftershock activity of the southeastern Akita earthquake of 1970, in relation to the characteristics of earthquake faulting in the upper crust. Aftershocks are found to have occurred almost exclusively in the so-called granitic layer of the crust. For the first 50 hours after the main shock occurrence the hypocenters of aftershocks were concentrated nearly on a plane dipping by 52° towards N64°E, which is in accordance with one of P-wave nodal planes in the focal mechanism solution of the main shock. Afterwards, the aftershock activity had a general tendency of upward migration, and spread in the northeast direction to constitute a group of shallow aftershocks which we call the eastern group. While shallow aftershocks in the western group were located approximately on the focal plane found for events in the first 50 hours, those in the eastern group are interpreted to have occurred along the other nodal plane in the mechanism solution of the main shock, indicating the existence of a conjugate fault system. Composite mechanism solutions are obtained from P-wave data for shallow aftershock groups. They are almost the same mechanism of reverse fault type for the main shock. Aftershocks deeper than 15km scattered around the horizontal boundary between the granitic and the basaltic layers. The composite mechanism solution for this group differs in its larger strike slip component from the other solutions, though it is of reverse fault type. If the nodal plane with a small dip angle is assumed to be the fault plane, the direction of slip motion agrees well with that of relative motion between the Eurasia and the Pacific plates.