Improved Scenario ShakeMaps Using 3D Physics-based Ground Motion Simulations

Ke Xu, & Kim B. Olsen

Submitted September 7, 2025, SCEC Contribution #14934, 2025 SCEC Annual Meeting Poster #TBD

The US Geological Survey ShakeMap tool for rapid characterization of the extent and distribution of strong ground shaking following significant earthquakes worldwide has the option to generate maps for scenario earthquakes to anticipate seismic hazards in future damaging events. Scenario ShakeMaps are typically generated using intensity measures from empirical ergodic Ground Motion Models (GMMs). However, the ground motion estimates from GMMs suffer from a wealth of limitations in source, path and site effects, rendering the resulting scenario ShakeMaps less useful for seismic planning and engineering applications.

Alternatively, ShakeMaps may be generated based on physics-based ground motion simulations, or hybrid methods combining physics-based ground motion simulations at low frequencies (LF, typically up to 1 Hz, dictated primarily by computational resources), typically calculated in a 1D velocity model with stochastic signals for higher frequencies (e.g., the SCEC Broadband Platform, BBP). Here, we first explore the benefit of replacing the 1D model with a well-constrained 3D model as well as increasing the maximum frequency of the physics-based LF seismograms from 1 Hz to 2 Hz or higher. Despite bias from the source models, preliminary 0-2 Hz 3D synthetics improve the Fourier Amplitude Spectral (FAS) bias against recorded data from 124 stations in total for the 1989 M6.9 Loma Prieta, 1994 M6.7 Northridge, and 2019 M7.1 Ridgecrest, CA, earthquakes by up to 23% and >60% for the horizontal and vertical components, respectively. The mean Goodness-of-fit combining 5 metrics (peak ground velocity, peak ground acceleration, cumulative absolute velocity, duration, and FAS amplitude) in these validation events show a general improvement up to 16% from the 3D BBP with 2 Hz merging frequency.

We then show that ShakeMaps based on GMMs are considerably different from those generated from hybrid physics-based/stochastic simulations of large scenario earthquakes in CA with a merging frequency of 2 Hz. For example, the GMM-based ShakeMaps fail to accurately reproduce source directivity and some basin amplification effects present in maps based on the broadband synthetics. These results suggest that with accurate velocity models and sufficient computational resources available, scenario ShakeMaps can be considerably improved using physics-based synthetics, potentially leading to better planning for and response to future damaging earthquakes.

Key Words
Ground motion, ShakeMap, Physics-based simulation, Broadband Platform, Scenario earthquake

Citation
Xu, K., & Olsen, K. B. (2025, 09). Improved Scenario ShakeMaps Using 3D Physics-based Ground Motion Simulations. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)