3-D wave propagation simulations of Mw 6.5+ earthquakes on the Tacoma Fault, Washington state, considering the effects of topography, a geotechnical gradient, and a fault damage zone

Bulletin of the Seismological Society of America
By: , and 

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Abstract

We simulate shaking in Tacoma, Washington, and surrounding areas from Mw 6.5 and 7.0 earthquakes on the Tacoma fault. Ground motions are directly modeled up to 2.5 Hz using kinematic, finite‐fault sources; a 3D seismic velocity model considering regional geology; and a model mesh with 30 m sampling at the ground surface. In addition, we explore how adjustments to the seismic velocity model affect predicted shaking over a range of periods. These adjustments include the addition of a region‐specific geotechnical gradient, surface topography, and a fault damage zone. We find that the simulated shaking tends to be near estimates from empirical ground‐motion models (GMMs). However, long‐period (T = 5.0 s) shaking within the Tacoma basin is typically underpredicted by the GMMs. The fit between simulated and GMM‐derived short‐period (T = 0.5 s) shaking is significantly improved with the addition of the geotechnical gradient. From comparing different Mw 6.5 earthquake scenarios, we also find that the response of the Tacoma basin is sensitive to the azimuth of incoming seismic waves. In adding surface topography to the simulation, we find that average ground motion is similar to that produced from the nontopography model. However, shaking is often amplified at topographic highs and deamplified at topographic lows, and the wavefield undergoes extensive scattering. Adding a fault damage zone has the effect of amplifying short‐period shaking adjacent to the fault, while reducing far‐field shaking. Intermediate‐period shaking is amplified within the Tacoma basin, likely due to enhanced surface‐wave generation attributable to the fault damage zone waveguide. When applied in the same model, the topography and fault damage zone adjustments often enhance or reduce the effects of one another, adding further complexity to the wavefield. These results emphasize the importance of improving near‐surface velocity model resolution as waveform simulations progress toward higher frequencies.

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Publication type Article
Publication Subtype Journal Article
Title 3-D wave propagation simulations of Mw 6.5+ earthquakes on the Tacoma Fault, Washington state, considering the effects of topography, a geotechnical gradient, and a fault damage zone
Series title Bulletin of the Seismological Society of America
DOI 10.1785/0120230083
Volume 113
Issue 6
Year Published 2023
Language English
Publisher Seismological Society of America
Contributing office(s) Earthquake Science Center
Description 24 p.
First page 2519
Last page 2542
Country United States
State Washington
City Tacoma
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