Aftershock Area Expansion and Mechanical Heterogeneity of Fault Zone within Subduction Zones

We investigate global variations in the pattern of aftershock area expansion associated with large subduction zone earthquakes. The expansion of aftershock areas with time is relatively small in the subduction zones of Alaska, the Aleutians, Mexico, and parts of the Kuriles and South America. These subduction zones are interpreted to be characterized by moderate to strong interplate coupling. In contrast, the extent of aftershock expansion isgreater'for events within the subduction zones of northeast Japan and western and south Pacific where interplate coupling is inferred to be weak. These results are interpreted in terms of an asperity model where a fault zone is represented by a distribution of strong spots (asperities) and weaker zones. The immediate aftershock area is assumed to extend over an area occupied by asperities, and the subsequent expansion to occur into the surrounding weaker zones. If large asperities abut each other on a fault plane, interplate coupling is strong and little expansion of aftershock area occurs. Smaller and more sparsely distributed asperities cause weak interplate coupling and significant expansion of aftershock area. Small and densely distributed asperities result in moderate interplate coupling and little aftershock expansion.