Aftershock Zone Scaling

by Yan Y. Kagan

Abstract

We investigate the distribution of aftershock zones for large earthquakes (scalar seismic moment M ≥10^19.5 N m, moment magnitude, m ≥7). Mainshocks are selected from the Harvard centroid moment tensor catalog, and aftershocks are selected from the Preliminary Determination of Epicenters (NEIC) catalog. The aftershock epicenter maps are approximated by a two-dimensional Gaussian distribution; the major ellipse axis is taken as a quantitative measure of the mainshock focal zone size. The dependence of zone length, l, on earthquake size is studied for three representative focal mechanisms: thrust, normal, and strike slip. Although the numbers of mainshocks available for analysis are limited (maximum a few tens of events in each case), all earthquakes show the same scaling (M ∝ l³). No observable scaling break or saturation occurs for the largest earthquakes (M ≥10²¹ N m, m ≥8). Therefore, it seems that earthquake geometrical focal zone parameters are self-similar.

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