Ground motions in urban Los Angeles from the 2019 Ridgecrest earthquake sequence

We study ground-motion response in urban Los Angeles during the two largest events (M7.1 and M6.4) of the 2019 Ridgecrest earthquake sequence using recordings from multiple regional seismic networks as well as a subset of 350 stations from the much denser Community Seismic Network. In the first part of our study, we examine the observed response spectral (pseudo) accelerations for a selection of periods of engineering significance (1, 3, 6, and 8 s). Significant ground-motion amplification is present and reproducible between the two events. For the longer periods, coherent spectral acceleration patterns are visible throughout the Los Angeles Basin, while for the shorter periods, the motions are less spatially coherent. However, coherence is still observable at smaller length scales due to the high spatial density of the measurements. Examining possible correlations of the computed response spectral accelerations with basement depth and Vs30, we find the correlations to be stronger for the longer periods. In the second part of the study, we test the performance of two state-of-the-art methods for estimating ground motions for the largest event of the Ridgecrest earthquake sequence, namely three-dimensional (3D) finite-difference simulations and ground motion prediction equations. For the simulations, we are interested in the performance of the two Southern California Earthquake Center 3D community velocity models (CVM-S and CVM-H). For the ground motion prediction equations, we consider four of the 2014 Next Generation Attenuation-West2 Project equations. For some cases, the methods match the observations reasonably well; however, neither approach is able to reproduce the specific locations of the maximum response spectral accelerations or match the details of the observed amplification patterns.

Automated summary

The study reveals significant ground-motion amplification in urban Los Angeles during the two largest events of the 2019 Ridgecrest earthquake sequence. Coherent spectral acceleration patterns are visible in the Los Angeles Basin for longer periods, while coherence is less for shorter periods but still observable at smaller length scales due to high spatial density of measurements. Correlations of computed response spectral accelerations with basement depth and Vs30 are stronger for longer periods. The performance of state-of-the-art methods for estimating ground motions is tested, with both methods showing reasonable match with observations but unable to reproduce details of observed amplification patterns.