2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 231-2
Presentation Time: 9:15 AM

DOWNDIP LANDWARD LIMIT OF CASCADIA GREAT EARTHQUAKE RUPTURE


HYNDMAN, Roy D., Pacific Geoscience Centre, Geological Survey of Canada, 9860 W. Saanich, Sidney, BC V8L 4B2, Canada, rhyndman@nrcan.gc.ca

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.
Handouts
  • HyndmanGSAPosterCascadiaLandwardLimit.pdf (4.4 MB)