Seismic Slip Deficit in the Southwestern Forearc of the Hellenic Subduction Zone

The seismic coupling and the spatiotemporal distribution of seismicity on the presumed rupture surface of the 365 A.D. M-w 8.3 +/- 0.2 Crete event at the shallow plate contact of the Hellenic subduction zone (HSZ) southwest of Crete is investigated. The maximum cumulative average seismic slip (U-seis) on the rupture surface is estimated for different time periods using historic and instrumental seismicity catalogs and utilizing a formula that incorporates empirical relations linking rupture area and average slip of an event to its moment magnitude (M-w). Events above the completeness magnitude of the catalog are incorporated by summation while events below are taken account of by integrating over the extrapolated frequency-magnitude distribution using a truncated Gutenberg-Richter relation. Estimates of U-seis for the time intervals 0-1999, 1500-1999, and 1964-2006 using historic and instrumental catalogs suggest a considerable seismic slip deficit when compared with the total slip expected from the temporal extrapolation of geodetic measurements. This suggests a recent weakly coupled plate contact and a total slip predominantly accommodated aseismically despite the 365 A.D. event. High seismic activity up to magnitudes of about M-w 6 is found accompanying the aseismic slip at the plate contact. Investigations of the seismicity distribution hint at a spatial variability of the seismic energy release in the western forearc of the HSZ. Activity in the proximity of the 365 A. D. epicenter is observed to be an order of magnitude larger than toward the northwest between Crete and Peloponnese. This spatiotemporal variability of the seismic energy release and seismic coupling suggests that subregions of the plate contact undergo alternating periods of locking followed by intermediate magnitude events and aseismic sliding. Thus, the behavior of significant portions of the plate contact southwest of Crete may be described as conditionally stable.