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3D Kinematic Models of Ground Motions of Cascadia Megathrust Earthquakes: Preliminary Results and Comparison to Paleoseismic Subsidence Data

The Cascadia Subduction Zone (CSZ) is anomalously quiet, with a dearth of moderate-to-large magnitude seismicity in the instrumented record. However, paleoseismic evidence indicates that the CSZ ruptures in large (~M8-9) megathrust earthquakes, with the last great event occurring in 1700. To quantify the seismic hazard due to great CSZ earthquakes, we employ numerical methods to simulate ground motions of many M8.7-9.2 megathrust earthquake scenarios. Here, we present preliminary results from the next generation of 3D broadband (up to 10 Hz) ground motion simulations for the CSZ, building off the work of Frankel et al. (2018) and Wirth et al. (2018). These broadband ground motions are generated using a hybrid approach, where low frequency (<1 Hz) waveforms are simulated in a 3D seismic velocity model using SPECFEM3D and are combined with high frequency (1-10 Hz) waveforms modeled stochastically. We build these scenarios using a logic tree approach, varying the event magnitude, the down-dip and up-dip limits of slip, including rupture onto secondary splay faults, and the slip distributions, which are varied both randomly and based on interseismic geodetic locking. Ultimately, these ground motions may be coupled with simulations of tsunami inundation to achieve a time-dependent understanding of seismic and tsunami hazard in coastal communities. These simulations will also be used to quantify the impacts to infrastructure and cascading hazards, such as landslides, liquefaction, and land-level change, with a focus on coastal communities in the Pacific Northwest. In addition, we highlight a subset of earthquake scenarios to evaluate how different rupture properties, as well as methods for calculating static displacements (i.e., 3-D simulations vs. a 1-D Okada model), impact estimates of coastal vertical land-level change and their comparison to paleoseismic estimates from past earthquakes. We show that the method used to calculate vertical displacements influences the distribution of coseismic uplift and subsidence, and therefore has implications for comparisons to paleoseismic data from previous Cascadia earthquakes.

Session: 3D Wavefield Simulations: From Seismic Imaging to Ground Motion Modelling - I

Type: Oral

Room: K’enakatnu 6

Date: 5/2/2024

Presentation Time: 08:15 AM (local time)

Presenting Author: Audrey Dunham

Student Presenter: No

Additional Authors