Accelerated Preshock Deformation of Broad Regions in the Aegean Area

Abstract

—Twenty-four regions where accelerating deformation has been observed for a few decades before corresponding strong (M = 6.0–7.5) mainshocks are identified in the broader Aegean area. To a first approximation these preshock regions have elliptical shapes and the radius, R (in km), of a circle with an area equal to the corresponding ellipse is related to the moment magnitude, M, of the mainshock by the equation:¶log R = 0.42 M-0.68.¶The dimension of each preshock region is about seven to ten times larger than the rupture zone (fault length) of the corresponding mainshock. The time variation of the cumulative Benioff strain was satisfactorily fitted by a power-law relation, which is predicted by statistical physics if the mainshock to which accelerating strain rates leads is considered as a critical point. The duration, t (in years), of the accelerating Benioff strain release period is given by the relation:¶¶log t = 5.94-0.75 log s _r ¶where s _r is the mean Benioff strain rate release (per year for 10⁴ km²) in the preshock region calculated by the complete available data (M≥5.2) for the entire instrumental period (1911–1998). The importance of identifying and investigating such regions for better understanding the dynamics of the active part of the lithosphere as well as for earthquake prediction and time-dependent seismic hazard assessment is discussed.