Mapping asperities by minima of local recurrence time: San Jacinto-Elsinore fault zones

Minima of the local recurrence time, T-L(IM), are correlated with the locations of historical main shocks in the San Jacinto (SJ)-Elsinore (E) fault zones of southern California. We define T-L(M) as the recurrence time calculated from the local b and a values of the frequency-magnitude distribution (earthquakes within 6 and 10 km from each point in the SJ and E fault zones, respectively). We propose that anomalously low T-L(IM) values map asperities. Out of six historic main shocks (1899-1968), five ruptured substantial parts of four asperities mapped by our method, which uses solely the modern earthquake catalog (M greater than or equal to 1.2, 1981.0-1998.8). We estimate the probability for this correlation to occur at random as similar to 10(-2) to 10(-3), depending on the scenario chosen. Ht appears that the minima in T-L(M) are tied to minima in b values more than to maxima in a values. Thus locations of low b values can also be used as an indication where asperities may be located and where main shocks are most likely. The Anza gap, a segment of the San Jacinto fault with low a values, is mapped as an asperity by our method. The heterogeneity in b values is strong throughout the study area. The b values range from 0.5 to 1.8 over distances of 10 km. Volumes with approximately constant b values have radii of r = 8 +/- 6 km. Our results suggest that detailed mapping of b values and local recurrence times in volumes with a few kilometers radius reveals important information about the local seismic hazard, hitherto untapped by conventional seismicity analyses.