Some considerations on the faulting mechanism of the southeastern Akita earthquake of October 16, 1970

Abstract

The faulting process of the southeastern Akita earthquake of October 16, 1970 (M=6.2) that took place on inland of the Tohoku region has been investigated from available seismic and geodetic data. A synthetic study on the radiation pattern of P-wave first motions, spatial distribution of aftershocks, dynamic ground motions in the nearfield, vertical tectonic movements along a nearby leveling route, and on teleseismic observations of S waves, suggests two possible faulting mechanisms. One is a thrust faulting of a western block over an eastern block along a westerly dipping fault plane with N-S strike. This mechanism seems consistent with tectonic features of this region, a belt-like distribution of earthquake damages along this trend, and with the location of the epicenter near the presumed extension of a fault caused by the 1896 earthquake. Another possible mechanism is also thrust faulting of a northeastern block over a southwestern block along a northeasterly dipping fault plane. Spatial distribution of aftershocks inclining northeastwards appears to support the second mechanism. Comparisons of the various observations with the corresponding theoretical calculations for a number of fault models slightly favor the first mechanism, but cannot yield a definite conclusion. The fault parameters such as dimensions, seismic moment, average dislocation, rise time, fracture velocity and stress drop are estimated for four probable models. Pre-earthquake data are also examined in relation to the faulting mechanism.