Abstract
Premonitory change in the ratio of compressional to shear wave velocity, υp/υs was ascertained for two moderate size earthquakes which occurred at shallow depths in Japan. The anomalous periods preceding the earthquakes were about one year and three to four months for earthquakes of magnitudes 6.5 ahd 5.3, respectively. A similar phenomenon was also found for the Matsushiro Earthquake Swarm. Two large earthquakes located near the Japan Trench were also investigated. For those earthquakes, a post-earthquake decrease in υp/υs ratio was distinct, but forerunners were not confirmed. Appearance of premonitory change in υp/υs ratio may depend on the type of earthquake.
