OBSERVATIONS OF THE CRUSTAL STRAINS AT THE TIME OF EARTHQUAKES AROUND KYOTO CITY

Abstract

There were about twenty sensible earthquakes, including two shocks of intensity 3 and 6 shocks of intensity 2, felt around Kyoto City for a period of one month from August 18, 1968. At Osakayama Observatory observations of the linear strains and the tilts of the crust have been made by means of some arrays and some single points equipped with extensometers or tiltmeters. According to these observations, a remarkable anomaly seemed to appear in April at latest, and to break out a swarm of earthquakes at the close of the long dry season lasting from October 1967. Furthermore, the forerunner seemed to appear half a day and half an hour before these earthquakes. Especially, the anomaly in half an hour before the earthquake on August 27 seemd to be a simple expansion of the crust. The radii of the seismic origin, the rebounded energy and the seismic magnitude were calculated by means of abrupt changes in the crustal strains during these earthquakes. Using the effects in the crustal strains caused by precipitation, the relation between the heavy rains and the outbreak of the earthquakes and that between the speed of the crustal strain and the maximum strangth of the rock-bed are discussed.