Correlation length as an indicator of critical point behavior prior to a large earthquake

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. Zöller et al. [G. Zöller, 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 Zöller et al. [G. Zöller, 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.