Characteristic Crustal Movements Observed in the Rokko Active Faults Area Relating to the 1984 Yamasaki Earthquake

Abstract

Characteristic crustal movements related to the earthquake of magnitude 5.6 near the Yamasaki fault, southwestern part of Hyogo Prefecture were observed at three observation stations in the Rokko active faults area. The observation stations were located about 60km southeast of the epicenter. Although these stations appeared a little too far to detect the strain changes caused by the earthquake of magnitude 5.6, the coseismic and post-seismic changes were observed very clearly. The changes corresponding to the aftershock activities were also found on the strain change curves obtained by subtracting the earth tide variations from the original records. Two anomalous changes which started about a week and about two days before the main shock were seen on most of the records of extensometers. However, it is not clear whether these events were the precursory phenomena or the meteorological disturbances. The anomalous tilt changes were not so remarkable as the strain changes in the case of the present earthquake.
It dose not seem that the observed strain changes were immediately caused by the effects of earthquake sources in the uniform and homogeneous elastic medium. It is supposed that they were caused by the amplified effects of tectonic stress changes or of relative movements of the crustal blocks which were triggered by the occurrence of earthquakes. Therefore, in order to detect the slight changes of crustal movements in relation to occurrence of earthquakes, it might be of great advantage to observe them in the vicinity of active faults.