CANDIDATE PRECURSORS - PULSE-LIKE GEOELECTRIC SIGNALS POSSIBLY RELATED TO RECENT SEISMIC ACTIVITY IN JAPAN

Telemetric network observations of pulse-like geoelectric charge signa ls using a vertical dipole buried under the ground were undertaken at various observation sites over a wide area in Japan from April 1996. F rom continuous records of the signals during the six months following that, we attempted to select anomalous signals, possibly related to se ismic electric activity. Special attention was paid to the elimination of noise resulting from industrial and meteorological electric activi ty, comparison with other electromagnetic signals in the VLF band and the relation between the precursor period and the distance from the ev entual main shock that occurred in Japan. Four candidate precursor ele ctric signals, which were not contaminated by industrial and meteorolo gical electric activity, were then selected, of which the second appea red before the Akita-ken Nairiku-nanbu earthquake swarm, for which the maximum M = 5.9 occurred on 1996 August 11, and the third and fourth before the Chiba-ken Toho-oki earthquake, M = 6.6, on 1996 September 1 1. A tentative qualitative model explaining why the candidate precurso ry signal is related to stress building up before an earthquake is pre sented in terms of the electrification of gases released from fracturi ng rocks immediately prior to the main shock.