Analysis of relaxation temporal patterns in Greece through the RETAS model approach

Abstract

The temporal decay of eight aftershock sequences in the area of Greece after 1975 was examined with main shocks magnitudes of M w ≥ 6.6. The analysis was done through the restricted epidemic type aftershock sequence (RETAS) stochastic model, which enables the possibility to recognize the prevailing clustering pattern of the relaxation process in the examined areas. In four of the cases the analysis selected the epidemic type aftershock sequence (ETAS) model to offer the most appropriate depiction of the aftershock temporal distribution which presumes that all shocks to the smallest ones in the sample can cause secondary aftershocks, while for the rest of the sequences triggering potential seems to have aftershocks above a certain magnitude threshold (RETAS model) and they are expected to induce secondary activity. The models, developed on aftershock data, were also applied to forecast real seismicity after the conclusion of the aftershock sequences. For four out of eight cases, we obtained promising estimations of ensuing seismicity after the end of the sequences with models based only on aftershock data. Some features of the RETAS model simulation were also studied, like simulating magnitudes, revealing that it is reasonable to consider in the model the temporal behavior of the aftershocks’ magnitudes as well. Stochastic modeling was also applied to estimate the duration of the relaxation process, assuming that the end of each sequence is marked by the divergence of real seismicity from the modified Omori formula (MOF) model, the latter known to represent pure aftershock activity. The obtained results give an indication that possibly low stressing rate results in longer duration of the relaxation process in a region.