Seismicity and deformation at convergent margins due to heterogeneous coupling

Studies of the mechanics of subduction as inferred from earthquake cycle observations suggest that the distribution and style of seismicity in the seafloor, between the trench and the outer rise, and in the slab at intermediate depth, can in some cases serve to identify asperity locations along the thrust interface [ Dmowska and Lovison , 1992]. Such asperities, identified from seismic wavefield modeling, are the zones of highest seismic moment release in large underthrusting events. To the extent that asperity locations are relatively stationary from one event to the next, their locations provide the zones of highest expected moment release in future large earthquakes, and rupture often nucleates at the border of an asperity. The region of the thrust interface outside such asperities is, apparently, less well coupled and releases moment throughout the great earthquake cycle in some combination of aseismic creep and moderate seismicity. Thus it is reasonable that stress and deformation rates associated with the earthquake cycle should be most pronounced near asperities, and that this should have seismic and geodetic consequences. Three‐dimensional finite element modeling is used here to understand such stress and deformation patterns and their variation in time, in relation to heterogeneity of coupling along thrust interfaces. The stress field helps to explain the observed clustering of seafloor seismicity along the strike of the convergent margin. In cases of convergence at approximately normal incidence, like for the region of the Valparaiso, Chile, 1985 thrust event, the modeling is consistent with the observation that areas of large earthquakes in the seafloor toward the outer rise and in the slab tend to lie within corridors through thrust zone asperities, running perpendicular to the line of the trench. We seek to learn if such model stress fields are consistent with observations, for the strongly oblique subduction margin of the Rat Islands, western Aleutians, 1965 event, that active areas of the outer rise and slab at intermediate depth are offset along strike from asperity locations. Modeling results here for the stress in the seafloor raise the possibility that to explain this offset, the asperity zones along the thrust interface may have to be strung out along the direction of oblique slip, perhaps reflecting the contact path of subducting seamounts or geometric irregularities along the interface. Shear stress patterns created in the upper plate, when there is oblique subduction, suggest that favorable areas for back‐arc strike slip activity following underthrusting, as in the Adak Island, central Aleutians, region of the 1986 Andreanof Island earthquake [ Ekström and Engdahl , 1989], will also be shifted along strike from asperity locations. Our analyses show how deformation patterns on the earth's surface above asperities differ from patterns above nonasperities, and hence provide tools to identify inhomogeneous coupling from geodetic observations. We discuss possible bathymetric, topographic, and structural signals of strength of coupling, and of asperities, particularly noting that the density and extent from the trench of seafloor normal faults correlates with seismically inferred zones of strongest coupling in the central Aleutians.