Re-examination of the 1940 Shakotan-oki earthquake and the fault parameters of the earthquakes along the eastern margin of the Japan Sea

Abstract

The fault model of the 1940 Shakotan-oki, Japan, earthquake of Aug. 1, 1940 is re-examined on the basis of magnitude, aftershock distribution and tsunami data. From a careful examination of the S-P time distribution of aftershocks and comparison of the tide gauge records with numerical simulation of tsunami, the fault area is estimated to be 100 km × 35 km, the slip 1.5 m and the seismic moment 2.4 × 10 27 dyn cm. The fault parameters estimated in this study are significantly different from those of Fukao and Furumoto and do not support their conclusion that this event has a very long duration and is a tsunami earthquake. The source process time is estimated to be 30 s, which is normal for an earthquake of this size. Fault parameters of six earthquakes along the eastern margin of the Japan Sea including the Shakotan-oki event are compiled and compared with those of inter-plate earthquakes in the Pacific Ocean. Different relations from well-known scaling laws of the inter-plate shocks are found for the earthquakes in the Japan Sea. Dip angles, aspect ratios of the fault, and the average stress drop are larger in the Japan Sea than those of the Pacific events, although the seismic moment release per unit fault length is the same. The differences can be interpreted in terms of a recent plate boundary model in which the young, immature boundary between the North American and the Eurasian plates lies at the eastern margin of the Japan Sea. They also partially reflect the different excitation of the tsunami in the Japan Sea and the Pacific Ocean.