Single‐link cluster analysis of earthquake aftershocks: Decay laws and regional variations

Abstract

Using single‐link cluster analysis, we investigate how various properties of aftershock sequences depend on their tectonic regime and focal depth. For International Seismological Centre earthquakes of mb ≥ 4.8, we find that earthquakes deeper than 70 km have the fewest and smallest aftershock sequences. Even after accounting for differences in detectability and maximum magnitude, we find that ridgetransform earthquakes have smaller aftershock sequences than shallow subduction zone earthquakes. Among different subduction zones, we find that zones with high moment release rates possess larger aftershock sequences. Comparing ridge‐transform zones, we find those with slower spreading rates possess larger aftershock sequences. By transposing origin times of several different aftershock sequences as if all had main shocks occurring at time zero, we are able to study the properties of aftershock sequences which individually have too few aftershocks to study by other means. Secondary events determined by single‐link cluster analysis follow a modified Omori's (power law) decay for time separations of 0.1 day to 20 days from the parent event, with p values ranging from 0.539±0.022 (intermediate‐ and deep‐focus earthquakes) to 0.928±0.024 (ridge‐transform earthquakes). We find that earthquake foreshocks and multiplets also follow a modified Omori's law. At greater times from the main shock the decay is steeper than a power law decay, more like an exponential decay. Aftershocks in the Adak catalog (mb ≥ 2.0) show a marked decrease in activity between 40 and 50 km depth. We speculate that the observed differences in number of aftershocks and p values may be caused by variations in fault heterogeneity or in fluid pressures.