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Waveguide or Not? Expected Ground Motions in the Greater Los Angeles Area From the ShakeOut scenario

Description: 

Since its introduction in 2008, the ShakeOut scenario of a M7.8 northwestward rupture on the southern San Andreas fault (SSAF, Jones et al., 2008) has been widely used as a basis for a world-wide earthquake drill as well as for estimation of seismic hazard and potential economic losses in Southern California. Previous ShakeOut studies predicted significant long-period ground motion amplification in the Los Angeles areas, caused by a waveguide from interconnected sedimentary basins (Olsen et al., 2008, 2009; Graves et al., 2008). However, the original ShakeOut simulations omitted surface topography, and subsequent studies have refined the velocity structure of southern California as well as the SSAF. In this study, we conduct 0-1 Hz 3D numerical wave propagation simulations for the ShakeOut scenario in the Statewide California Earthquake Center (SCEC) Community Velocity Model (CVM) version 4.26M01, including updated high-resolution velocity structure of the Imperial and Coachella Valleys from reflection seismic imaging (Ajala et al., 2019; Persaud et al., 2016) and the northern basins (San Gabriel, Chino and San Bernardino Basin) from ambient noise imaging (Li et al., 2023), and detailed geometry of the SSAF (Fuis et al., 2017). We find a reduction of long-period ground motion levels (measured by spectral accelerations at a period of 3s, 3s-SA) by up to 55% along the waveguide into the Los Angeles basin, caused by stronger scattering effects from spatial complexity introduced by the updated CVM in areas outside the basins. Topographic scattering further decreases the 3s-SA values by another 40% due to the loss of coherency in the surface wave train. The addition of high-resolution near-fault models slightly increases the long-period (6s and longer) ground motion in the basins. The combined effects of the updated velocity models and surface topography reduce the maximum peak ground velocities in the waveguide from 250 cm/s to 100 cm/s. The significantly different ground motion pattern in the densely populated Los Angeles metropolitan area implies the need to revise seismic risk accordingly.

Session: Physics-Based Ground Motion Modeling - I

Type: Oral

Date: 5/3/2024

Presentation Time: 08:15 AM (local time)

Presenting Author: Kim

Student Presenter: No

Invited Presentation: 

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