Sedimentary Basins: A Deeper Look at Seattle and Portland’s Earthquake Hazards

Abstract:

Seattle, Washington and Portland, Oregon, two major metropolitan areas in the Pacific Northwest, are vulnerable to earthquakes on active local faults, deep intraslab earthquakes, and megathrust earthquakes on the Cascadia Subduction Zone (CSZ). Both cities are located within sedimentary basins that could increase this hazard. The Seattle basin is ~8 km in depth and is located beneath downtown Seattle. The 6-km-deep Tualatin basin (McPhee et al., 2014) sits below and west of downtown Portland with the shallow Portland basin to the northeast. Unlike other West Coast sedimentary basins, the Tualatin contains a higher-velocity Columbia River basalt layer between sediment layers. The velocity contrast between stiff bedrock surrounding the basins and soft sediment within can cause seismic waves to amplify greatly, increasing shaking intensity and duration at the surface. For example, our observations show amplification of seismic waves by factors of 2 - 4 within the Seattle basin. Basin geometry can also increase local shaking by converting incident S-waves to surface waves, and focusing S-waves at basin edges.

We characterize effects of the Seattle, Tualatin and Portland basins by modeling with 3-D numerical methods. To evaluate these effects, we use data from the 2001 M6.8 Nisqually, the 2009 M4.5 Kingston, and the 2006 M3.8 Vancouver earthquakes recorded by stations operated by the US Geological Survey (10 - 25 stations) and the Pacific Northwest Seismic Network (7 - 81 stations). Time differences between S-waves and S-converted-to-P-waves at basin/bedrock interfaces as well as reverberations from teleseisms (global earthquakes) are used to constrain the basin depth and structure of the three basins. Basin effects are modeled using a 3D finite difference program to generate synthetic seismograms. Results will be used to improve the Seattle and Portland 3D velocity models and to better understand and predict amplification of strong motion. We also plan similar analyses to assess the shaking hazards for Portland due to local earthquakes and great earthquakes on the CSZ.