TECTONOELECTRIC SIGNAL RELATED WITH THE OCCURRENCE OF THE 1995 HYOGO-KEN NANBU EARTHQUAKE (M7.2) AND PRELIMINARY-RESULTS OF ELECTROMAGNETICOBSERVATION AROUND THE FOCAL AREA

In association with the 1995 Hyogo-ken Nanbu earthquake, we observed c hanges in the telluric field by about 10 mV/km about 10 s after the or igin time with some long-span electrical dipole network located at abo ut 70 km from the epicenter. After the occurrence of the earthquake, e lectric and magnetic measurements such as DC resistivity, VLF-MT, self -potential, and the geomagnetic total intensity across the Nojima faul t were made at several places in Awaji Island, The low resistive zone beneath the western side of the fault was found in the derived resisti vity structure of the shallower portion of the fault in Hirabayashi. O n the other hand, however, no significant anomaly associated with the fault structure was found from the measurements of total intensity and self-potential. Prior to the occurrence of the main shock, no geomagn etic or electric continuous stations existed near the focal area. Appr oximately within 1 week after the occurrence of the main shock, we set up 10 continuous observation sites for the geomagnetic total intensit y in the focal area in order to detect geomagnetic changes due to stre ss change caused by the aftershock activity. No changes in the total i ntensity in association with the individual aftershock activity have b een observed so far. However, systematic temporal trends in the geomag netic differences have been observed, especially at sites in the north ern half of the observation area.