Seismic quiescence precursor to the 1983 Nihonkai-Chubu (M 7.7) earthquake, Japan

We analyzed the seismicity of Northern Honshu-Hokkaido region using the declustered earthquake catalog compiled by the Japan Meteorological Agency (JMA), for the period between January 1970 and December 1994. Making use of the ZMAP software tool, we sought to determine whether the quiescence hypothesis is applicable to 16 main shocks (M ³ 7.0) of the JMA catalog. We found a highly significant seismic quiescence prior to the May 26, 1983 Nihonkai-Chubu, MJ 7.7, earthquake. The quiescence that preceded the event lasted more than 3.5 years, and was located in the Japan Sea, off Akita and Aomori Prefectures. It was characterized by a standard deviate Z = 7.4 (Tw = 3 years), within a volume of approximately 200 by 300 by 40 km around the hypocenter. This volume contained 16 earthquakes (M ? 3.8) during the background period which lasted more than 8 years preceding the quiescence, and none during this one. A high concentration of seismic activity exceeding 15 events per year followed the main shock before the rate returned back to the previous value. No quiescence was observed before another strong event of magnitude 7.8 which occurred on July 12, 1993, 200 km north of the former, while three quiescences with the same Z-value were observed in the same region and in the same time period, not followed by any main shocks (false alarms). The probability that the Nihonkai-Chubu earthquake correlated at random with the quiescence period is estimated as approximately 1%, based on the fraction of space-time covered by alarms. The seismicity rate variations observed before and after the 1983 Nihonkai-Chubu earthquake are similar to those observed in the rupture area of the 1980 Irpinia (Italy), M 6.9 earthquake