GREAT SLIP IN GREAT SUBDUCTION EARTHQUAKES OCCURS UNDER FOREARC BASINS

Asperities, or areas of greater seismic slip and moment release, have been recognized for many great subduction zone earthquakes. The origin of these areas of high slip and their role in earthquake recurrence are much debated. Asperities may have geologic significance, and they are commonly correlated with strong upper plate blocks. However, along the Nankai Trough of SW Japan, well constrained regions of maximum co-seismic slip in the 1923, 1944, 1946 and 1968 thrust events correlate with the bathymetric and gravity lows of forearc basins. This suggests a link between basin formation and the slip process. Asperities for other subduction zone earthquakes, usually determined by waveform inversion techiniques, were compared to forearc structure revealed by global bathymetic and gravity data. Asperities for the 1938 Alaskan Peninsula (Mw 8.2), 1957 central Aleutians (Mw 8.6), 1965 Rat Island (Mw 8.7), and 1986 Andreanof (Mw 8.0) earthquakes coincide with the location and size of basin-centered forearc gravity lows along the Aleutian terrace. Off southern Chile, offshore basins have little bathymetric expression, but are visible in the free-air gravity and seismic profiles. Slip maxima in the 1960 earthquake (Mw 9.5) coincides well with the size and distribution of basin-centered forearc gravity lows. Along the Cascadia subduction zone, large offshore gravity lows coincide with the geodetically-defined coupled zone and suggest large-scale seismic segmentation similar to SW Japan. Basin-centered asperities suggest that basins grow by interseismic subsidence driven by tectonic erosion at depth. Subduction erosion has been well documented along sediment-starved subduction zones and is inferred to be an important process globally. Our data suggests that erosion is coincident with seismic slip. Once forearc lows are created, lower fault-normal stresses beneath them may at times help to localize slip. Along sediment-dominated subduction zones, the location of forearc basins may be a useful indicator of long-term seismic moment release.