Simulated response of topography to ground motions from magnitude 7 earthquakes on the Seattle Fault

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Ian Stone, USGS

Wednesday, March 31, 2021 at 10:30 AM

Online-only seminar via Microsoft Teams
Erin (Wirth) Moriarty

Topography affects earthquake ground motions by amplifying shaking at topographic highs and reducing shaking at topographic lows. Amplification is typically greatest at a characteristic frequency related to the topography’s shape and varies based on the wavefield’s polarization relative to a feature’s principal axis of elongation. While topographic effects have been invoked to help explain patterns of increased damage during some major earthquakes, they are not generally considered in modeling contexts. This is largely because topographic effects are highly variable from one event to the next, and they often only manifest at higher frequencies. As a result, questions remain over how topography may contribute to the maximum predicted ground motion at a given location, and how dependent the effects are on source parameters. We simulate M7.0 earthquakes on the Seattle Fault in Washington State using a spectral element method code (SPECFEM3D) to help constrain the sensitivity of topographic effects to kinematic earthquake rupture parameters. The simulations use a 3D velocity model that includes local sedimentary basin structure and random near-surface velocity heterogeneities. A 30m mesh spacing near the surface of the model is used to specify the topography and allows for accurate ground motion prediction up to 3Hz. We isolate the effects of topography by comparing simulations with and without a topographic surface, using the ratios of peak ground velocity and spectral acceleration to identify systematic amplification. By running multiple earthquake scenarios on the same sections of fault, we evaluate the sensitivity of topographic effects to hypocenter location and slip distribution, as well as demonstrate the relative importance of topographic amplification on high frequency ground shaking.

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