An example of earthquake nucleation of the strong continental earthquakes

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Introduction The study on earthquake nucleation is widely concerned by seismologists in the world. The experimental and theoretical studies indicate that earthquakes should be preceded by quasi-static slip within a nucleation zone (Oh-naka, 1992; Dodge, Beroza, 1995; Dodge, et al, 1996; Ohnaka, Kuwahara, 1990; Yamashita, Ohnaka, 1991). The earthquake nucleation process means a transition from quasi-static to quasi-dynamic rupture process, and it itself is a short-term precursor. Immediate foreshocks are local dynamic instabilities that occur during the transition from the quasi-static to the quasi-dynamic nucleation of the dynamic instability (Ohnaka, 1992). According to the recent theoretical study, immediate foreshocks can be regarded as the localized fractures accompanied by the quasi-static nucleation process of a large earthquake (Shibazaki, Matsu'ura, 1995). Therefore, foreshocks could occur during the nucleation process. The nucleation of earthquakes can be illuminated through analyzing foreshock activity in detail. Detection of the nucleation process by means of a foreshock study is a potential tool for earthquake predic-tion. The nucleation process of Izu peninsula earthquake with M=7.0 on January 14, 1978 is revealed by Ohnaka with foreshock activities. It was observed that the nucleation zone indicated by foreshocks grew at a rate of 1～40 cm/s before reaching a diameter of 10 km. The depths of foreshocks do not change much more, keep within 10 km. Recently, Hurukawa have studied the nucleation process of Off-Etorofu earthquake with MW=7.9 on December 3, 1995. The results show distinctly the nucleation process before the main shock. In the nucleation process, rupture started at the deepest point of the foreshock area, and then propagated to the shallow depth with the apparent ve-locity of 5～20 cm/s (Hurukawa, 1998). Rastogi and Mandal (1998) studied the rupture nucleation process of five Koyna medium-sized main shocks using the time-space patterns of foreshocks. They found that the nuc