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Bulletin of the Seismological Society of America Vol 92 No 2 pp 641 655 March 2002 Aftershock Zone Scaling by Yan Y Kagan Abstract We investigate the distribution of aftershock zones for large earthquakes scalar seismic moment M 1019 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 Introduction Earthquake rupture is characterized by three geometric quantities length of the rupture l width W and average slip u Seismic moment M is defined through these quantities as M uWl m 1 2 with d 3 However they suggested that for larger earthquakes this scaling relation breaks down with d 2 for large earthquakes or even d 1 for the largest ones The reason for postulating such a breakdown is usually formulated as follows For small earthquakes the rupture propagates entirely within the brittle crust whereas for large earthquakes rupture is confined to the upper crust layer of thickness W Thus if W W0 const 4 where M is measured in N m The magnitude calculated by 4 is used here only for illustration all pertinent computations being carried out with the moment M values Although other geometrical parameters of earthquake rupture have been correlated with seismic moment the length of the rupture is determined

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