Evaluating Bias in Simulated Ground Motions for Moderate Magnitude Earthquakes in Southern California: A Study Using the Graves-Pitarka Method

Sajan K C, Chukwuebuka C. Nweke, & Robert W. Graves

Published September 8, 2024, SCEC Contribution #13664, 2024 SCEC Annual Meeting Poster #183

This study utilizes the Graves-Pitarka broadband ground motion simulation method, integrated within the SCEC Broadband Platform (BBP), to conduct finite fault simulations for 51 well-recorded moderate magnitude earthquakes in Southern California, with magnitudes ranging from M 3.95 to 5.55. The primary aim is to assess whether simulated ground motions exhibit bias when compared to observed data, focusing on Effective Amplitude Spectra (EAS) and Pseudo Spectral Acceleration (PSA). Building on previous findings by Nweke et al. (2022), which indicated systematic underprediction of low-frequency spectral accelerations, this study expands the scope by including additional events and broadband records (BBR) recordings from the Community Seismic Network and incorporating EAS in the analysis. Our results confirm the persistence of bias at frequencies below 1 Hz. Further examination of residuals indicated that while site and path-related biases do exist, they are relatively minor, leaving substantial bias unaccounted for even after these factors are considered. Therefore, we posit that the remaining bias is likely linked to earthquake source characteristics, particularly the empirical magnitude-rupture area scaling relationship used in the simulations, as proposed by Leonard (2010), which appears to falter for lower magnitude events. This hypothesis holds for the 2008 M 5.39 Chino Hills earthquake, though further validation is constrained by the lack of finite fault models to accurately establish rupture areas for other events in our dataset. Ongoing research is focused on understanding the effects of fault rupture area, stress drop, and average slip on the overall bias, and includes a sensitivity study of other earthquake source attributes, such as average rupture speed, on a case-by-case basis to explore potential solutions for the observed bias.

Citation
K C, S., Nweke, C. C., & Graves, R. W. (2024, 09). Evaluating Bias in Simulated Ground Motions for Moderate Magnitude Earthquakes in Southern California: A Study Using the Graves-Pitarka Method. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)