LARGE-SCALE CRUSTAL MOVEMENTS BEFORE AND AFTER THE 1983 JAPAN SEA EARTHQUAKE

Abstract

Large-scale crustal activity before and after the Japan Sea earthquake with magnitude 7.7 on May 26, 1983 was investigated taking into account microearthquakes, crustal strain, crustal stress, and vertical movements and applying numerical experiments. It is found that about one month prior to the mainshock, anomalous microearthquake swarms took place at some points which were considered to be related to fracture zones. Short-term precursors were not recorded by extensometers and watertube tiltmeters deployed at distances more than 80 km from the epicenter of the mainshock. Observed strain steps explain computed steps based on an assumed fault model. Precursory uplift several years preceding the mainshock was found at the nearest Japan Sea coast to the epicenter by space-time analysis of repeated leveling surveys.