QUANTITATIVE MAPPING OF A PRECURSORY SEISMIC QUIESCENCE TO THE IZU-OSHIMA 1990 (M6.5) EARTHQUAKE, JAPAN

A highly significant seismic quiescence with a standard deviate Z=10.1 , corresponding to a 99 per cent confidence level, lasted from 1987.7 up to the 1990 February 20 Izu-Oshima M6.5 earthquake. The quiescent v olume had dimensions of 30 km N-S and 10 km E-W and was centred below 14 km depth. Within the recently upgraded seismograph network of the E arthquake Research Institute (ERI), this main shock was the only one w ith a magnitude M > 5.8 in the upper 30 km of the crust for which the precursory quiescence hypothesis could be tested. Within a radius of 5 0 km, and during the observation period (1983.5-1995.9), there were no other 1.5 yr or longer periods of quiescence that were rated Z > 6.5 in the declustered earthquake catalogue, except one that was associate d with volcanic activity. The total space-time covered by alarms, incl uding the volcanic one, was less than 1 per cent at the Z = 6.5 level. The rarity of highly significant episodes of quiescence, and the corr elation in space and time suggest that a precursory seismic quiescence started 2.5 yr before the Izu-Oshima 1990 earthquake in its source vo lume and to the north of it, and that it can be recognized with an ala rm level of Z = 6.0, generating no false alarms. During the 1.5 yr qui escence window, only 10 earthquakes occurred in the quiet volume, wher eas 50 events were expected based on the rate seen at other times. In randomly selected volumes containing 50, 100 and 200 events, the anoma ly scored Z = 6.1 to 10.1. On the basis of the data from May 1983 to 1 995, there is no highly significant quiescence currently present in th e Izu-Oshima area.