Abstract
On September 21, 1973 an earthquake of magnitude 5.1 occurred in the vicinity of the Yamasaki fault, western Kinki district. In this region, it was the largest event we had experienced since the occurrence of the earthquake swarm of 1961.
Using data obtained at three temporary observation stations and routine stations of the Tottori Microearthquake Observatory, we determined hypocenters of the main shock-aftershock sequence. Large three shocks (M>3) including the main shock have their foci concentrated in a small volume of 1-2km in diameter at a depth of 11km, where many aftershocks are also located. Another concentrated region of aftershocks is located 2km south of the first one. Consequently the size of the aftershock area is about 2km×3km, which is roughly consistent with the Utsu-Seki's formula log A=M-4.
Focal mechanism of the main shock is of a thrust type, and its maximum pressure axis lies nearly horizontal and a N64°E direction. Such type of mechanism has not been found difinitely so far for the Yamasaki fault area where events of strike-slip are large in number. Major aftershocks are estimated to have rather different types of mechanism as compared to the main shock. This implies some complexity of focal process or faulting for this earthquake.
Aftershock activity had decayed in accordance with the formula N(t)=k·t-p (p=1.5) during the period of ten days after the main shock, and had continued for more than two months. Gutenberg-Richter's b value for the aftershocks is estimated to be 0.66-0.73.
