LANDWARD LIMIT OF COSEISMIC SLIP IN GREAT SUBDUCTION ZONE EARTHQUAKES - IMPLICATIONS FOR GREAT EARTHQUAKES IN CASCADIA

The landward limit of coseismic slip is an important constraint on the shaking hazard produced by megathrust earthquakes. We have compiled coseismic slip inversions for 29 of the largest recorded Circum-Pacific megathrust earthquakes to determine how slip is related to the characteristic geologic structure of convergent margins. On average, 71% of an earthquake’s seismic moment and 79% of its asperity area occur beneath the deep-sea terrace gravity low, and 57% of the asperity area lies beneath the forearc basins within the terrace low. In SW Japan, slip in the 1923, 1944, 1946, and 1968 earthquakes was focused beneath five forearc basins. The steep gravity gradient marking the landward edge of the basins and the large asperities coincides with the 350°C isotherm on the plate boundary, but large slip (~2 m) extends landward to the 450° C isotherm. Large coseismic slip beneath the deep-sea terrace and its basins is also observed along the Aleutian, Mexico, Peru, and S. Chile subduction zones. 86% of the asperities in our sample lie on or just seaward of the gravity gradient.

For the AD 1700 Mw~9 Cascadia earthquake, early dislocation models put the megathrust locked zone (and inferred source of large slip) well offshore. Based on our observations of other great earthquakes, we suggest that the large gravity lows centered on the Eel, Coos Bay, Newport-Willapa, and Olympic basins overlie potential asperities at depth. The lows straddle the 350° C isotherm, and the steep gravity gradient marking the inboard edge of the basins coincides with the 450° C isotherm, the landward edge of the most strongly coupled portion of the transition zone, and the up-dip limit of silent slip events. The agreement is encouraging and suggests that the gradient marks the landward limit of large coseismic slip on the Cascadia megathrust. Large slip in a future great earthquake is likely to occur landward as far as the coast and possibly onshore beneath the Olympic Peninsula.