A study of short-term spatiotemporal clustering features of Greek seismicity

Clustering features of crustal seismicity for the period from 2008 up to 2018 in Greece in both space and time are investigated by means of the epidemic type aftershock sequence model. With a completeness threshold of m(c) = 3.5, the data set includes 8258 events, with 19 of them of M-w >= 6.0, encouraging the detailed analysis of the short-term clustering spatiotemporal characteristics. The estimation of the model parameters is performed via the maximum likelihood estimation technique using a simulated annealing approach, after considering a normal grid superimposed on the study area with 0.1 degrees x 0.1 degrees cells. The model is tested with a residual analysis procedure along with the application of both Numbers and Log Likelihood tests. The initial estimated model with the full dataset underestimates the observed seismicity, and appears unstable, implying an explosive type model (a model with infinite decay rate). The underestimation is observed during 2008, when five M-w >= 6.0 earthquakes occurred, which is an extreme rate for Greek seismicity. Excluding this part of the catalog, during the period 2009-2018, the estimated model exhibits good agreement with the observed seismicity. The classification between background and triggered events is made by measuring the contribution of each class to the total seismicity, which presents a considerable clustering behavior.