COSEISMIC GEOMAGNETIC-VARIATIONS OBSERVED AT THE 1995 HYOGOKEN-NANBU EARTHQUAKE

Impulsive co-seismic geomagnetic variations were observed at two stati ons nearly 100 km apart from the source region of the 1995 Hyogoken-Na nbu earthquake (M = 7.2). The maximum amplitude at the main shock was 0.6 similar to 1.0 nT and the duration was 20 similar to 30 seconds. T hese variations seem to have commenced at the origin time and before t he arrival of the seismic waves to the observation site of 17 seconds later. A superposed epoch analysis for the aftershocks also reveals si milar geomagnetic variations though the amplitude of them is one order of magnitude smaller than that of the main shock. A crustal dynamo me chanism is discussed as a possible cause of such co-seismic geomagneti c variations.