ALBERT

Description: [1]  This is a second paper in a study of statistical identification and classification of earthquake clusters using a relocated catalog of 1981–2011 seismicity in southern California and synthetic catalogs produced by the ETAS model. Here we focus on classification of event families – statistically significant clusters comprised of foreshocks , mainshocks and aftershocks – that are detected with the methodology discussed in part I of the study. The families are analyzed using their representation as time oriented tree graphs. The results (i) demonstrate that the clustering associated with the largest earthquakes, m  〉 7, is statistically different from that of small-to-medium earthquakes; (ii) establish the existence of two dominant types of small-to-medium magnitude earthquake families– burst-like and swarm-like sequences – and a variety of intermediate cluster forms obtained as a mixture of the two dominant types; (iii) suggest a simple new quantitative measure for identifying the cluster type based on its topological structure; (iv) demonstrate systematic spatial variability of the cluster characteristics on a scale of tens of kilometers in relation to heat flow and other properties governing the effective viscosity of a region; and (v) establish correlation between the family topological structure and a dozen of metric properties traditionally considered in the literature (number of aftershocks, duration, spatial properties, b -value, parameters of Omori-Utsu and Båth law, etc .). The burst-like clusters likely reflect highly-brittle failures in relatively cold regions, while the swarm-like clusters are likely associated with mixed brittle-ductile failures in regions with relatively high temperature and/or fluid content. The results of this and paper I may be used to develop improved region-specific hazard estimates and earthquake forecasts.