COSEISMIC SUBSIDENCE IN THE 1700 GREAT CASCADIA EARTHQUAKE: CONSTRAINT ON THE RUPTURE AREA

Seismic hazard assessments for a Cascadia subduction zone earthquake are largely based on the rupture area predictions of dislocation models constrained by geodetic and geothermal data. Our study tests the consistency of the model results for the 1700 great Cascadia earthquake with compiled magnitudes of coseismic subsidence along the coast. Subsidence is estimated from marsh paleoelevation studies, via contrasts in elevation indicators (calibrated with the modern marsh) across the 1700 buried soil horizon. Coastal estimates have large uncertainties but show a consistent pattern. Greatest coseismic subsidence (~ 1-2 m) occurred in N. Oregon/ S. Washington; subsidence elsewhere was ~ 0-1 m. An independent method calculates coseismic subsidence as the subsurface depth of the buried soil corrected for eustatic sea level rise, postglacial rebound, and interseismic uplift. The resultant estimates are however uncertain due to their dependence on a dislocation model to calculate interseismic uplift. An alternative use of this approach uses paleoelevation subsidence estimates to calculate average uplift since 1700.

We use elastic dislocation models based on geodetic and thermal constraints to predict the coseismic subsidence: (A) for strain accumulation periods of (i) 800 and (ii) 550 years of plate convergence; (B) for uniform megathrust slip of 10, 20, 30 and 50 m. A better fit to the coastal data is provided by the former two models; predicted subsidence is in broad agreement with marsh estimates. Discrepancies exist however at the ends of the subduction zone. In the south, misfit may be due to break-up of the Gorda plate. A better fit would result from a narrower locked zone in southern Oregon. The discrepancy in the north may be explained if the 1700 event released only part of the accumulated strain there, consistent with net uplift in excess of eustatic sea level rise. Slip magnitude, estimated by comparing uniform slip model predictions with marsh coseismic subsidence estimates, is consistent with the magnitude 9 earthquake indicated by Japanese tsunami records. The coseismic slip was apparently greatest in N. Oregon/S. Washington (³ 30 m), declining to the north and south to < 10 m.