A Search for Temporal Variations in V_p/V_s around Suruga Bay

Abstract

The suggestion was tested that anomalously low velocity of compressional waves around Suruga Bay at the northeast end of Nankai trough might be a precursor for an impending great Tokai earthquake. Temporal variations in the ratio of compressional to shear velocities in the region were analyzed during the period from January 1951 to June 1976 by using the events which have frequently occurred in the eastern part of the Kanto district.
Long term premonitory variations in V_p/V_s ratios relating with a forthcoming large earthquake with magnitude about 8.0 could not be detected. On the other hand, a significant decrease of about 15% in V_p/V_s was observed over a 5-year period preceding the 1965 Shizuoka earthquake (M6.1, H=20km). This anomaly, time seems to be unusually longer for its magnitude.
The circumstances might be well explained a new model which takes the thermal process into consideration for earthquake forerunners.