Basin-centered asperities in great subduction zone earthquakes: A link between slip, subsidence, and subduction erosion?

Abstract

Published areas of high coseismic slip, or asperities, for 29 of thelargest Circum-Pacific megathrust earthquakes are compared to forearcstructure revealed by satellite free-air gravity, bathymetry, andseismic profiling. On average, 71% of an earthquake's seismic momentand 79% of its asperity area occur beneath the prominent gravitylow outlining the deep-sea terrace; 57% of an earthquake's asperityarea, on average, occurs beneath the forearc basins that lie withinthe deep-sea terrace. In SW Japan, slip in the 1923, 1944, 1946,and 1968 earthquakes was largely centered beneath five forearc basinswhose landward edge overlies the 350°C isotherm on the plate boundary,the inferred downdip limit of the locked zone. Basin-centered coseismicslip also occurred along the Aleutian, Mexico, Peru, and Chile subductionzones but was ambiguous for the great 1964 Alaska earthquake. Beneathintrabasin structural highs, seismic slip tends to be lower, possiblydue to higher temperatures and fluid pressures. Kilometers of lateCenozoic subsidence and crustal thinning above some of the sourcezones are indicated by seismic profiling and drilling and are thoughtto be caused by basal subduction erosion. The deep-sea terraces andbasins may evolve not just by growth of the outer arc high but alsoby interseismic subsidence not recovered during earthquakes. Basin-centeredasperities could indicate a link between subsidence, subduction erosion,and seismogenesis. Whatever the cause, forearc basins may be usefulindicators of long-term seismic moment release. The source zone forCascadia's 1700 A.D. earthquake contains five large, basin-centeredgravity lows that may indicate potential asperities at depth. Thegravity gradient marking the inferred downdip limit to large coseismicslip lies offshore, except in northwestern Washington, where thelow extends landward beneath the coast. Transverse gravity highsbetween the basins suggest that the margin is seismically segmentedand could produce a variety of large earthquakes.