Geophysical Center, RAS, Molodezhnaya 3, 117296 Moscow, Russian Federation

A large earthquake preparation is often manifested in correlation of seismicity in an area whose characteristic dimension greatly exceeds a dimension of source of main shock. Zfller et al. [G. Zfller, S. Hainzl, J. Kurths, Observation of growing correlation length as an indicator for critical point behavior prior to large earthquakes, J. Geophys. Res. 106 (2001) 2167– 2176] show the growth of correlation length of earthquakes prior to nine large earthquakes in California according to a power low. We argue that the algorithm of correlation length estimation proposed by Zfller et al. [G. Zfller, S. Hainzl, J. Kurths, Observation of growing correlation length as an indicator for critical point behavior prior to large earthquakes, J. Geophys. Res. 106 (2001) 2167–2176] can result in a decrease of correlation length preceding its precursory growth before large earthquakes if the area in which earthquake activity is correlated grows with time during a main shock preparation. The correlation length analysis of acoustic emission events recorded in laboratory experiments on destruction of rocks and correlation length analysis of intermediate magnitude earthquakes in the area of large earthquakes preparation on Kamchatka and in Italy confirms the theoretical argument. This effect can be considered as an additional premonitory pattern of large earthquake preparation.

Seismic quiescence and accelerating seismic energy release are considered as possible spatio-temporal patterns of the preparation process of the 6 September 2002 Palermo, Italy, earthquake (M 5.8). The detailed properties of the quiescence are analyzed applying the RTL algorithm. The RTL algorithm is based on the analysis of the RTL prognostic parameter, which is designed in such a way that it has a negative value if, in comparison with long-term background, there is a deficiency of events in the time–space vicinity of the tested point. The RTL parameter increases if activation of seismicity takes place. The RTL algorithm identified that a seismic quiescence started from the beginning of November 2001 and reached its minimum at the end of May 2002. The Palermo 2002 earthquake occurred 2 months after the RTL parameter restored its long-term background level. The application of a log-periodic time-to-failure model gives a ‘‘predicted’’ (in retrospect) magnitude M= 6.2 main shock on 5 May 2002.