Seismicity Changes During Earthquake Preparation

Seismicity changes play an important role not only in intermediate-term earthquake prediction, but also in studies on earthquake preparation and hazard estimation. Thus, a close investigation on the seismicity changes associated with a strong earthquake would be useful for understanding the seismo-tectonics in the epicentral region and the preparation process of this strong earthquake. Recently, a statistical method has been developed to reveal the seismicity changes associated with strong earthquakes [Sobolev and Tyupkin, 1997]. This method is called the RTL algorithm, where the RTL is a parameter reflecting the combination of three functions of distance, time and rupture length, respectively. Previous studies indicated that similar variations of seismicity patterns were detected during the preparation of some strong earthquakes in Russia and Japan [Sobolev and Tyupkin, 1997; Huang et al., 2001]. As a case study, we investigated the seismicity changes associated with the M = 7.3 Tottori earthquake by applying the RTL algorithm to the earthquake catalog of the Japan Meteorological Agency [Huang and Nagao, 2002]. We also present an idea of improving the reliability of a precursor (e.g., seismicity anomaly in this study). The preliminary results indicated that a seismic quiescence started in 1999 and reached its minimum in May 2000. Further analyses indicated that this quiescence is unlikely an artifact due to the changes of model parameters. The close investigation showed that the quiescence, which appeared around the epicenter during 1999-2000, is the only alarm group in study space and time. The random test indicated that the revealed seismicity anomaly is a significant one. Therefore, the detected anomaly is most probably a precursor of the Tottori earthquake. The seismicity changes revealed in this study may give better understanding of the seismo-tectonics in the region and the preparation process of the Tottori earthquake. Sobolev, G. A., and Tyupkin, Y. S., Low-seismicity precursors of large earthquakes in Kamchatka, Volc. Seis., 18, 433-446, 1997. Huang, Q., Sobolev, G. A., and Nagao, T., Characteristics of the seismic quiescence and activation patterns before the M=7.2 Kobe earthquake, Tectonophysics, 337, 99-116, 2001. Huang, Q.,Nagao, T., Seismic quiescence before the 2000 M=7.3 Tottori earthquake. Geophys. Res. Lett., Vol. 29, No. 12, 10.1029/2001GL013835, 2002.