Remarkable Crustal Strain Change Observed at the Yasutomi Station of AIST near the Yamasaki Fault

Abstract

At the Yasutomi station of AIST located near the Yamasaki fault, a crustal strain field has been measured by a borehole strainmeter. Although the crustal strain rates were almost constant for the period before around April 2002, a conspicuous compressive strain change was observed for the period from June to around September 2002. The corresponding changes were too small to be detected from groundwater pressures and GPS measurement at the Yasutomi station of AIST, and extensometers at the Yasutomi and Osawa stations of Kyoto University. Moreover, the phenomenon was not detected at GPS stations of GEONET around the Yasutomi station, either. These observations indicate that the phenomenon was limited to be local and caused by a small source. Since it was not accompanied by earthquakes, it was possibly caused by unstable and aseismic slip occurring on the Yasutomi fault, which is a part of the Yamasaki fault system. Supposing that a left-lateral slip with 1mm on the shallow part of the Yasutomi fault, all observed results can be explained.