Ground motion variability observed in the 2019 Ridgecrest, California earthquake sequence

Elizabeth S. Cochran, Grace A. Parker, Sarah E. Minson, & Annemarie S. Baltay

Submitted September 7, 2025, SCEC Contribution #14452, 2025 SCEC Annual Meeting Poster #TBD

We estimate ground motion variability and spatial correlations in the region near the 2019 Ridgecrest, California earthquake sequence. Ground motions are known to be highly variable about their median expected values, yet to accurately estimate hazard across distributed systems we must understand how ground motions correlate with inter-station distance. Ground motion spatial correlations have been reported for multiple regions, but data are often lacking for closely spaced station pairs. The 2019 Ridgecrest earthquake sequence presents a unique opportunity to examine ground motion variability and spatial correlations due to dense temporary station spacing. Following the Ridgecrest earthquake, the permanent seismic network (Hauksson et al., 2020) was augmented with a set of temporary velocity and accelerometer sensors (Cochran et al., 2020). Additionally, nodal sensors were deployed at 585 recording sites including fault-crossing arrays with nominal station spacing of 100 m (Catchings et al, 2020). Using the gmprocess software package (Thompson et al., 2024), we determine peak ground velocity (PGV) and acceleration (PGA) in bandpass filtered data (0.3-100 Hz) for 16,416 recordings from 120 earthquakes (M3.3-M5.0). We determine the between- and within-event residuals (dWij) from the fit for each event (i) and station (j) to the median ground motion model of Boore et al. (2014), considering local site conditions (Rekoske et al., 2020; Thompson 2020). We compare dWij between station pairs that record an event to measure the semivariance of the ground motion versus interstation distance. We fit an exponential model (Baker and Chen, 2020) to the empirical semivariograms and find a correlation distance of approximately 25 km. Smaller-scale variability in the semivariances are associated with station pairs located near or across the Ridgecrest fault traces whose ground motions are highly variable. Additionally, large positive median dWij with relative values 2-3 times larger than nearby stations are observed for stations near the fault traces. Near-fault amplification and larger ground motion variability may delineate fault damage zones and provide insights into non-ergodic contributions to the ground motions near faults that may locally increase the hazard.

Key Words
Ridgecrest earthquake sequence, ground motion variability, spatial correlation, fault zone amplification

Citation
Cochran, E. S., Parker, G. A., Minson, S. E., & Baltay, A. S. (2025, 09). Ground motion variability observed in the 2019 Ridgecrest, California earthquake sequence. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology