DOWNDIP LANDWARD LIMIT OF CASCADIA GREAT EARTHQUAKE RUPTURE

In this study we have summarized and assessed the various constraints to the downdip landward limit of rupture for the Cascadia great earthquakes off western North America. This limit is a primary control for ground motion hazard at near-coastal cities. The studies also provide information on the physical controls of subduction thrust rupture globally. The principal constraints are: (1) “Locked/transition” zones from geodetic deformation (GPS, repeated leveling, tide gauges); (2) Rupture zone from paleoseismic coastal marsh subsidence, 'paleo-geodesy'; (3) Temperature on the thrust for the seismic-aseismic transition; (4) Change in thrust seismic reflection character downdip from thin seismic to thick ductile; (5) Forearc mantle corner aseismic serpentinite and talc overlying the thrust; (6) Updip limit of ETS slow slip; (7) Rupture area associations with shelf-slope basins; (8) Depth limit for small events on the thrust; (9) Landward limit of earthquakes on the Nootka transform fault. The most reliable constraints for the limit of large rupture displacement, >10 m, are generally just offshore in agreement with thermal control for this hot subduction zone, but well offshore central Oregon and near the coast of northern Washington. The limit for 1-2 m rupture that can still provide strong shaking is less well estimated 25-50 km further landward. The forearc mantle corner and the updip extent of ETS slow slip are significantly landward from the other constraints. Surprisingly, there is a downdip gap between the best other estimates for the great earthquake rupture zone and the ETS slow slip. In this gap, plate convergence may occur as continuous slow creep.