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Validation of Two New CyberShake Studies in California

Description: 

The SCEC CyberShake platform is a 3D physics-based ground motion simulation platform developed for seismic hazard analysis. Previous CyberShake studies predict higher median ground motions relative to NGA-West2 Ground Motion Models (GMMs). Since 2022, two new CyberShake studies have been completed: CS22.12 in Southern California and CS24.8 in Northern California. Compared to previous CyberShake studies, there are several changes in the two new studies aimed at producing more realistic ground motions: 1) the rupture generator is updated to reduce the level of coherency in rupture propagation; 2) a taper is applied to the top of the 3D velocity models to replicate firm-rock conditions; 3) the minimum V_S value in the simulation is reduced from 500 m/s to 400 m/s; and 4) V_S30 are now obtained from Thompson (2022).

To evaluate the effects of the implemented changes, we develop non-ergodic GMMs based on the two new CyberShake studies and compare them to NGA-West2 GMMs. The median predictions of CS22.12 show good agreement with the NGA-West2 GMMs at 2, 5 and 10s, supporting the implemented changes. However, the median predictions of CS24.8 are significantly larger than the NGA-West2 GMMs at 2s, and slightly larger at 5s. Our analysis finds that very strong directivity effects in CS24.8 are likely the primary cause for these large ground motions. Several factors contribute to the strong directivity in CS24.8: 1) All the major faults in CS24.8 rupture toward the sites, which are concentrated in the San Francisco Bay area; 2) the ruptures in CS24.8 tend to be narrower and longer, which is likely more effective in producing directivity effects; 3) many ruptures channel directivity toward the Santa Clara Valley in CS24.8, however only a fraction of events rupture toward the Los Angeles Basin in CS22.12. Moreover, differences in attenuation, crustal reflections and surface wave generation between the two 3D velocity models used in simulations may also play a role in affecting the ground motion level. These regional differences between the two CyberShake studies highlight the importance of non-ergodic GMM development.

Session: Challenges and Opportunities in Constraining Ground-motion Models from Physics-based Ground-motion Simulations [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Xiaofeng

Student Presenter: No

Invited Presentation: 

Poster Number: 44

Authors