3-D Simulations of M9 Earthquakes on the Cascadia Megathrust

Erin Wirth, Arthur Frankel, John E. Vidale, Nasser Marafi, & William J. Stephenson

Submitted August 4, 2017, SCEC Contribution #7409, 2017 SCEC Annual Meeting Poster #251

The M9 Project is an interdisciplinary, NSF-funded effort aimed at understanding and reducing the potentially catastrophic impacts of large Cascadia earthquakes on the social, built, and natural environments. This work relies on developing a suite of synthetic ground motions for various M9 earthquake rupture scenarios in Cascadia, which are then used to probabilistically evaluate the ensuing consequences (e.g., building response, landslides, liquefaction) from a megathrust event. We generated synthetic broadband seismograms (0-10 Hz) for 50 M9 earthquake scenarios on the Cascadia megathrust. Synthetic ground motions are produced by combining waveforms from 3-D finite-difference simulations at low frequencies (< 1Hz) with 1-D stochastic simulations at high frequencies (> 1 Hz). Slip consists of multiple high-stress drop subevents (M8) with short rise times on the deeper portion of the fault, superimposed on a background slip distribution with longer rise times. For non-basin sites, synthetic response spectra at 0.1-6 s periods are similar to those predicted by the BC Hydro ground motion prediction equations (GMPEs; within a factor of 1.7); at 7-10 s periods, the shallow dip of the Juan de Fuca plate results in spectral accelerations that exceed the GMPEs. For sites in the Seattle and Tacoma sedimentary basins, we find significant amplification of the synthetic response spectra (a factor of 2 to 5) relative to sites outside the basins. We also find a factor of four variation in the intensity of ground shaking depending upon several key parameters, including the down-dip limit of rupture, the slip distribution and location of strong-motion-generating subevents, and the hypocenter location. Our results demonstrate the wide range of possible ground motions from an M9 megathrust earthquake in Cascadia, and the importance of further constraining 3-D structure and key rupture parameters to refine future estimates of seismic hazard in Cascadia.

Wirth, E., Frankel, A., Vidale, J. E., Marafi, N., & Stephenson, W. J. (2017, 08). 3-D Simulations of M9 Earthquakes on the Cascadia Megathrust. Poster Presentation at 2017 SCEC Annual Meeting.

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Ground Motions