Ray E. Wells1, Richard J. Blakely2, Yuichi Sugiyama3, and David Scholl4
We have examined the relationship between coseismic slip and forearc structure for 29 of the largest circum-Pacific megathrust earthquakes. Coseismic slip distributions were compiled from published seismic, geodetic, and tsunami waveform inversions, and we interpreted forearc structure from satellite gravity and bathymetry and marine geology. Seismogenic slip is generally focused beneath forearc deep sea terraces and basins, which are underlain by relatively high velocity arc or continental crust. Along the Nankai and Sagamai Trough of SW Japan, slip in the 1923, 1944, 1946, and 1968 earthquakes was focused beneath five forearc basins, and the presently locked Tokai source region is centered on a sixth. The steep gravity gradient marking the landward edge of the basins coincides with the landward decrease in coseismic slip and the 350¡C isotherm on the plate boundary, approximately marking the down-dip limit to stick-slip behavior. In the 1960 Chile event, the highest coseismic slip coincides with forearc basin-centered gravity lows along the entire rupture length. Similar coseismic slip beneath basins is also observed along the Aleutian, Mexico, and Peru subduction zones. Transverse forearc gravity highs which separate the basins commonly overlie areas of lower coseismic slip, as at Cape Erimo separating the 1952 and 1968 Tokachi-oki earthquakes off Hokkaido, the Shumagin gap separating the 1938 and 1946 earthquakes in S. Alaska, and the Portlock anticline separating the Kodiak and Prince William Sound asperities in 1964. If the long-term slip budget is balanced along the margin, then the intervening gravity highs may be future sources of great slip not observed historically, or more likely are regions of smaller interseismic strain accumulation, as is observed in the Shumagin gap and permitted by geodetic data at Cape Erimo and off Kodiak. The empirical relationship between high coseismic slip and forearc basins suggests that forearc basins may be useful indicators of long-term seismic moment release in some subduction zones. The inferred source zone of the 1700 AD Mw~9 Cascadia earthquake contains five very large basin-centered gravity lows, the largest of which is 350 km long off the mouth of the Columbia River. These lows, corresponding to the Eel, Coos Bay, Newport-Willapa, and Olympic basins, lie within the locked and transition zones inferred from geodetic data and may indicate potential high slip regions at depth. The steep gravity gradient marking the inboard edge of the basins and presumably the downdip limit to large coseismic slip lies beneath Grays Harbor and the westernmost Olympic Peninsula in Washington and just offshore Oregon, between the modeled 350oC and 450oC isotherms on the megathrust. Transverse gravity highs between the basins suggest the margin is seismically segmented and may produce a variety of large earthquakes.
1 U. S. Geological Survey, 345 Middlefield Road MS 975, Menlo Park, California 94025; rwells@usgs.gov
2 U. S. Geological Survey, 345 Middlefield Road MS 989, Menlo Park, California 94025
3 National Institute of Advanced Industrial Science and Technology, 1-1-3 Higashi, Tsukuba, Ibaraki, 305-8567, Japan
4 U. S. Geological Survey, 345 Middlefield Road MS 999, Menlo Park, California 94025