Variation of seismic coupling with slab detachment and upper plate structure along the western Hellenic subduction zone

摘要

The western Hellenic subduction zone is characterized by a trenchward velocity of the upper plate. In the Ionian islands segment, complete seismic coupling is achieved, as is predicted by standard plate-tectonic models in which there is no slab pull force because the slab has broken off. The moderate local seismic moment rate relates to a shallow downdip limit for the seismogenic interface. This characteristic may be attributed to the ductility of the lower crust of the upper plate, which allows a decollement between the upper crust of the overriding plate and the subducting plate. Farther south, a deeper downdip limit of the seismogenic interface is indicated by thrust-faulting earthquakes, which persist much deeper in western Crete. A correspondingly larger downdip width of this seismogenic zone is consistent with the suggested larger maximum magnitude of earthquakes here. However, since the seismic moment release rate seems to be moderate in the Peloponnese and western Crete, like in in the Ionian islands, this seismically active interface cannot maintain complete seismic coupling across its larger downdip width. A cause may be the lateral addition of overweight to the part of the slab still attached in Crete, by the free fall of its part that has broken off from the surface further north, This increased slab pull reduces the compressive normal stress across the seismogenic interface and thus causes partial seismic coupling in its shallower part. However, the width of this part may provide an additional area contributing to slip in large earthquakes, which may nucleate deaper on stick-slip parts of the interface. Hints at anomalies in structure and seismicity, which need to be resolved, may relate to the present location of the edge of the tear in the slab. (C) 2004 Elsevier B.V. All rights reserved.