Spatial variability of seismicity parameters in aftershock zones

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Abstract
The spatial variability of thebvalue of the frequency‐magnitude relationship and the decay rate of aftershocks as described by thepvalue of the modified Omori law is investigated. By using dense spatial grids we map out the distribution ofbandpvalues within the Landers, Northridge, Morgan Hill, and Kobe aftershock sequences. Considerable spatial variability is found, withbvalues of independent subvolumes ranging from 0.6 to 1.4, andpvalues ranging from 0.6 to 1.8. These systematic and statistically highly significant differences argue that it is an oversimplification to assign one singlepandbvalue to an aftershock sequence that extends up to 100 km. The spatial distribution of these two parameters is compared with the slip distribution during the mainshock, suggesting that the areas of largest slip release correlate with highbvalue regions. We hypothesize that the frictional heat created during the event may influence thepvalue distribution within an aftershock zone, while applied shear stress, crack density and pore pressure govern the frequency‐magnitude distribution. By investigating the frequency‐magnitude distribution separately for preseismic and postseismic periods for the Morgan Hill mainshock, we find that only the volume in the vicinity of the highest slip release shows a significant increase in thebvalue, which decays to premainshock values within a year. Surrounding areas of the aftershock zone show an approximately constantbvalue with time. Because the aftershock hazard after a mainshock depends strongly on both thebandpvalue, we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters.