Simulations of Seattle Fault Earthquakes Incorporating Realistic Near-Surface Structure

Abstract:

Earthquakes on the Seattle Fault pose a significant threat to major population centers in the Puget Sound region, Washington State. Here, we develop synthetic ground motions for a set of large crustal earthquakes (M6-7) occurring on the Seattle Fault, which vary in rupture area, slip distribution, hypocenter, and rupture velocity. Kinematic simulations are run using a finite difference code (SW4) to generate long period seismograms (> 1 s), which are then combined with stochastically generated short period waveforms (< 1 s). Our simulations use a 3-D seismic velocity model for the Pacific Northwest that we modify to include topography and more realistic shallow (< 1 km depth) seismic velocity structure. This near-surface structure includes the Holocene alluvium and artificial fill that underlie parts of Seattle (with time-averaged shear-wave velocity over the top 30 m, Vs30, ~150 m/s). We evaluate how this near-surface material interacts with the response of the deeper material in the Seattle Basin and enhances ground motion amplification. In addition, we apply generalized Pacific Northwest velocity profiles (i.e., based on a local dataset) that are a function of Vs30 throughout the model domain. We compare our results to the NGA-West2 ground motion models and interrogate the impacts of rupture directivity, basin amplification, and shallow soft soils on the resulting seismic hazard.