Evidence for Static and Dynamic Triggering of Seismicity Following the 24 August 2016, M W=6.0, Amatrice (Central Italy) Earthquake

The recent 24 August 2016, M (W) = 6.0, Amatrice earthquake (central Italy) and the resulting aftershock sequence represents a test case to investigate the static and dynamic stress/strain contribution to the triggering of the sequence. Here we test the hypothesis that either static or dynamic stress contribution alone may not be sufficient to explain the aftershock distribution. We first inferred the horizontal source rupture directivity together with an estimate of the surface fault projection from the analysis of the peak-ground velocity. We computed the peak-dynamic strain field, modified by source directivity, using the peak-ground velocity as a strain proxy and the Coulomb static stress change. Finally, we compared the seismicity rate map (beta-statistic) with each of the estimated fields to investigate correlations with the aftershock pattern. We found that the area of the highest values of the estimated peak-dynamic strain field better reproduces the observed asymmetry in the aftershock distribution. This suggests that, in addition to Coulomb static stress change, dynamic strain enhanced by source directivity, contributed to the triggering of the Amatrice earthquake aftershocks.