Synchronous precursors in the electrical earth resistivity and the geomagnetic field in relation to an earthquake near the Yamasaki fault, Southwest Japan.

Abstract

Precursory changes both in th electrical earth resistivity and in the geomagnetic total force gradient were clearly observed in relation to the earthquake of M 5.6 which occurred near the Yamasaki fault on May 30, 1984. The observation site was at a distance of several kilometers from the epiceter. The precursors were recognized about 60-70 days before the event. First the resistivity gradually decreased during 10 days by 30% of its ordinary value. After keeping a low level for about 50 days, the resistivity began to increase, exceeding the ordinary value 2 days before the shock, and attained to a maximum of 400% after the shock. The extremely large value returned to its ordinary value within two weeks after the event. The geomagnetic total force gradient also showed about 10% increase synchronously with the resistivity change. Almost 2 days after the shock, the gradient recovered to its ordinary value. No coseismic change was recognized. Other precursor like changes of the resistivity possibly associated with several earthquakes in southwest Japan were also detected at the same observation site within the last year. We consider that the resistivity change may be caused by a small amount of change in the water content within the rocks or fissures around the fault and that the fault region is a place particularly sensitive to tectonic stress changes. A resistivity structure model to account for the observed precursors will be presented.