Geodetic Monitoring of the Crustal Movements across the Yamasaki Fault, Southwest Japan

Abstract

Data obtained by precise geodetic measurements on the network across the Yamasaki fault, Southwest Japan, were analyzed for the period of 11 years from 1975 to 1986. In order to monitor the crustal movements across the Yamasaki fault, precise geodetic measurements have been carried out since 1975 annually in November or December on a small aperture geodetic network ("Yasutomi-Usuzuku Baseline Network") across the fault. Baseline measurement, triangulation and trigonometric leveling have been applied here. The accuracy of our measurements are possibly high enough to detect relative movements among the six stations as small as 1mm. Analyzed data show non-uniform strain accumulations with reversed sense between the NE-SW directed principal strains in the northeastern and southwestern part of the network. A secular strain rate of the order of 10^-6 strain per year was estimated based on the strain data from five diagonals of the pentagonal network, and a secular WSW-down tilt rate of about 1×10^-6 rad per year was obtained from height data of all the six stations of the network. Furthermore, episodic strain and tilt changes were superposed on the secular trends. We also pointed out increases in compression rate and deviations of tilt from the secular WSW-down tilt associated with two moderate (M=3.7 and M=5.6) earthquakes having epicentral distances of about 3km from the network, and their focal depths were about 20km. These episodic movements seem to have had begun one/two years before the earthquakes and to have recovered their secular trend after the events. On the other hand, no remarkable crustal movement was detected for the moderate earthquakes having epicentral distances larger than about 15km.