A slip tendency analysis to test mechanical and structural control on aftershock rupture planes

Large portions of intraplate regions are characterised by relatively uniform stress fields with moderate to large main shock fault-ruptures nucleating on planes successfully predicted by 2D frictional fault reactivation theory. Here we use a slip tendency analysis, based on the notion that slip on a fault is controlled by the ratio of shear stress to normal stress acting on the plane of weakness, to test whether aftershock sequences are also governed by fault reactivation theory within the regional stress field. We observe that aftershocks for two well-documented seismic sequences occurring in extensional and compressional environments, the 1997 Mw = 6.0 Colfiorito sequence (Central Italy) and the 1999 Mw = 7.5 Chi-Chi sequence (Taiwan), respectively, nucleate on planes favourably oriented for frictional fault reactivation. In particular, 89% of 329 and 81% of 121 events for the Colfiorito and Chi-Chi sequences respectively, are the result of fault reactivation processes on geological structures that represent well oriented planes within the regional stress field. This suggests that stress rotations induced by the main shock for these two intracontinental sequences are unlikely.In addition, the percentage of well oriented aftershock rupture planes reaches 100% for Colfiorito and 86% for Chi-Chi if we consider a magnitude threshold above Mw = 3.7 and Mw = 5.0, respectively. We interpret this as the fact that stress heterogeneities if present are generally localised and can influence only small structures capable of generating small magnitude aftershocks.