Rupture Parameters For The Pre-historic Seattle Fault Earthquake From Modeling Shoreline Uplift

Slip during large thrust earthquake often causes surface uplift and secondary rupture. Surface uplift due to an earthquake on the Seattle Fault, an east-west striking thrust fault located under downtown at 900-930 A.D., is represented by elevation changes in the shoreline of Puget Sound. We use the sharp break in slope between the landward edge of the shore terrace and the adjoining steep hill slope as a consistent feature to define the elevation of the uplifted shoreline. The data are corrected for ∼1 m change in sea level during the past 1000 years and for post-seismic uplift estimated to be up to 20% of the total based on modern analogs. We first summarize the impact of various rupture parameters, such as dip, rake, fault depth, and fault segmentation, on the changes at the surface using single fault models and multi-fault models under uniform conditions. We then model in 3-D the rupture geometry of the Seattle Fault with a realistic surface fault trace by fitting the expected uplift to the measured shoreline uplift in the area. We investigate reverse, strike-slip, normal, and mixed slips on the fault and estimate the magnitude of that earthquake. Preliminary results underscore the sensitivity of the rupture parameters to elevation changes away from the surface projection of the fault. Left-lateral strike slip cannot produce the observed uplift amplitude.