SEISMOLOGICAL EVIDENCE INDICATING RUPTURE ALONG AN EASTWARD DIPPING FAULT PLANE FOR THE 1964 NIIGATA, JAPAN EARTHQUAKE

Abstract

The Niigata earthquake of June 16, 1964. (M⁸=7.5) is studied using a body-wave inversion technique on teleseismic P waveforms of the mainshock and a joint hypocenter determination of 27 of the largest (M_JMA≥4.0) regionally recorded aftershocks. The mainshock is interpreted as being composed of two subevents with moments of 2.1×10²⁶ dyn-cm and 2.0×10²⁷ dyn-cm. The focal mechanism indicates that faulting could have occurred on either a plane dipping toward the west or a plane dipping toward the east. The initial subevent is located at a depth of 8 to 13 km, while the centroid of the second sub-event is located 27 km southwest of the first and at a depth of 3 km. The relocated aftershocks all appear to have depths of less than 15 km which is consistent with the depths obtained for the mainshock. They appear to define a trend dipping shallowly toward the east which is consistent with the relative locations of the two subevents comprising the mainshock. We conclude, therefore, that a fault plane dipping toward the east is more consistent with the relative locations of the two subevents and the relocations of the aftershocks.