INFLUENCE OF ASPERITIES ALONG SUBDUCTION INTERFACES ON THE STRESSING AND SEISMICITY OF ADJACENT AREAS

We have investigated the influence of large-scale fault inhomogeneities in large subduction earthquakes on the style of deformation and seismic behavior of the incoming oceanic plate and slab at intermediate depths during the earthquake cycle. The zones of the large subduction events of Rat Islands 1965, Alaska 1964 and Valparaiso 1985 have been searched for earthquakes with m(b) greater-than-or-equal-to 5.0, if available, and for time periods as long as possible. It has been found that in general the seismicity in the incoming oceanic plate clusters in front of asperities (= areas of highest seismic moment release and strongest locking) and is positioned relative to them in the direction of plate motion. It is usually lacking in areas adjacent to non-asperities, that is to zones that slip during the main event but with appreciably smaller seismic moment release, and possibly slip seismically/aseismically during the whole cycle. Similar behavior occurs in the downgoing slab at intermediate depths, where seismicity during the cycle clusters (but less strongly than in the oceanic crust) next to asperities and down-dip from them. We infer that the locking of asperities causes higher stresses associated with the earthquake cycle itself to occur in areas adjacent to asperities, both up-dip and down-dip from them along the direction of plate motion, and that such stressing is much less pronounced in the areas adjacent to non-asperities. This opens the possibility of identifying the areas of highest seismic moment release in future subduction earthquakes, and carries implications for where the highest deformation and, possibly, precursory phenomena and/or nucleation of a future event might occur.