On Increase of Earthquake Correlation Length Prior to Large Earthquakes in California

It is well established that earthquakes are correlated over distances greatly exceeding their source dimension. Recent studies hypothesize for important associated phenomenon: The area over which earthquake activity is correlated varies in time and might grow prior to a large earthquake. This hypothesis is supported by a wealth of observations, computer simulation, and has theoretical interpretations. Several measures of earthquake correlation lengths were recently suggested by di erent authors. Here we analyze one of these measures, (x; t), based on single-link cluster analysis of epicenters. Previous studies have shown the growth of prior to nine large earthquakes in California during 1945-2000. In this paper we study whether the reported growth of the correlation length (x; t) can be used for earthquake prediction. Our results show that reasonable retrospective prediction of large earthquakes (M 6:5) in California can be achieved by using the increase of as a signal for the approach of a large earthquake. Extensive variations of numerical parameters demonstrate the stability of this prediction method. Additionally, we compare the distributions of (x; t) close and distant in time and space to large earthquakes and nd a systematic shift re ecting the increase of the correlation length prior to large earthquakes. Premonitory increases of correlation lengths are seen most clearly in the highly fractured areas near fault junctions. Its predictive power is reduced in more homogeneous regions.