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

Abstract

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.

Parsimonious velocity inversion applied to the Los Angeles Basin, CA

“…We used the components of the CSN located within the northeast LA basin, which is the densest part of the array -the study area, including the locations of the stations, is shown in Figure 2. Various display of the Ridgecrest earthquake data are shown in Filippitzis et al (2021), along with a comparison of the data and predicted ground motions by several methods. For our study, data were first detrended, rotated into the ZRT frame, decimated to 5 Hz and then detrended once more.…”

“…In contrast, CVM-S is significantly smoother than CVM-H due to its reliance on wavefield-tomography during the final stages of construction, although several sharp resolution based artefacts are also evident. While many features of the seismic wavefield within the LA basin, such as phase arrival times and P-to-S amplitude ratios, are captured for local events at frequencies of up to 0.2 Hz (Taborda et al, 2016;Lai et al, 2020), excitations of the basin from the recent large regional Ridgecrest earthquake sequence in July 2019 have illustrated that ground motion amplification predictions from finite-difference wave propagation through the SCEC CVM-H and CVM-S models do not accurately model the observations even at the relatively low frequency 0.1-1Hz range that is relevant for tall buildings within downtown LA (Filippitzis et al, 2021), warranting continued close study of the LA basin velocity model. Seismic tomography offers the best opportunity for full spatial coverage of the basin at high resolution, especially when dense seismic arrays are utilized.…”

“…As a result, the instrument noise floor is above the amplitude of ground motions produced by smaller regional earthquakes, and is also above the ambient seismic noise level, which precludes the use of ambient-noise cross-correlation methods on CSN data as these methods rely -4-manuscript submitted to JGR: Solid Earth on coherent low-level energy propagation between sensors. However, both the Mw 6.4 and Mw 7.1 Ridgecrest, California earthquakes produced high quality records across the array, allowing for detailed analysis of ground amplification within the basin (Kohler et al, 2020;Filippitzis et al, 2021). The coherent surface-wave energy from these two events, recorded on the CSN, offers a unique opportunity to construct a high-resolution local tomographic model of the northeastern edge of the LA basin.…”

Parsimonious Velocity Inversion Applied to the Los Angeles Basin, CA

“…Many features of the seismic wavefield within the LA basin, such as phase arrival times and P-to-S amplitude ratios, are captured for local events at frequencies of up to 0.2 Hz (Taborda et al, 2016;Lai et al, 2020). However, excitations of the basin from the recent large regional Ridgecrest earthquake sequence in July 2019 have illustrated that ground motion amplification predictions from finite-difference wave propagation through the SCEC CVM-H and CVM-S models do not accurately predict the observations in the 0.1-1 Hz range that is relevant for tall buildings within downtown LA (Filippitzis et al, 2021) ing off-the-shelf components, and have a maximum observable acceleration of ±2g, in order to fulfil their primary goal of strong-ground-motion monitoring. As a result, the instrument noise floor is above the amplitude of ground motions produced by smaller regional earthquakes, and is also above the am- both the CVM-H and CVM-S models (Graves, 1996).…”

“…Many features of the seismic wavefield within the LA basin, such as phase arrival times and P‐to‐S amplitude ratios, are captured for local events at frequencies of up to 0.2 Hz (Lai et al., 2020; Taborda et al., 2016). However, excitations of the basin from the recent large regional Ridgecrest earthquake sequence in July 2019 have illustrated that ground motion amplification predictions from finite‐difference wave propagation through the SCEC CVM‐H and CVM‐S models do not accurately predict the observations in the 0.1–1 Hz range that is relevant for tall buildings within downtown LA (Filippitzis et al., 2021), warranting continued close study of the LA basin velocity model.…”