Minimum preshock magnitude in critical regions of accelerating seismic crustal deformation

An important factor for all models that consider the generation of a mainshock as a critical point at the end of an accelerating seismic energy release period is the minimum magnitude considered in the calculations. Using data for preshock sequences that preceded nineteen recent strong mainshocks (M ≥ 6.3) in the Aegean area, we have defined the minimum magnitude, M_min, of preshocks for which an optimum fit to the accelerated crustal deformation model has been obtained. It is shown that the average difference between the predicted mainshock magnitude, M, and this minimum preshock magnitude, M_min, is 1.8 ± 0.1 for 6.0 ≤ M ≤ 7.6, with slightly smaller values (≈ 1.6-1.8) for M≤ 7.0 and larger values (≈ 1.8-2.0) for larger events (M≥7.0). This assessment of the mainshock-preshock difference is of great practical importance for the identification of critical (preshock) regions. It is shown that similar differences are found for six regions, which, have been suggested, to be presently in a state of accelerated seismic deformation. This result can be considered as further evidence that these regions are in a critical (metastable) state, which can lead to the generation of strong mainshocks (M ≥ 6.4) in the next few years.