Improved High‐Resolution 3D Vs Model of Long Beach, CA: Inversion of Multimodal Dispersion Curves From Ambient Noise of a Dense Array

Ambient noise recorded by a dense seismic array provides an opportunity to resolve detailed 3D shear wave velocity (Vs) structures. We utilize the frequency-Bessel transform (F-J) method to compute the dispersion spectrogram for ambient noise recorded by the Long Beach seismic array, extract multimodal dispersion curves with a deep learning approach, and invert multimodal dispersion curves with a gradient algorithm. The addition of higher-mode dispersion curves reduces nonuniqueness and provides an improved high-resolution 3D Vs model that shows good correlations with the known stratigraphic sequence and geologic features in the study area. Furthermore, the Richard fault between the Pacific Coast Highway and Newport-Inglewood faults, which have never been discussed in previous studies, is resolved. Our study demonstrates the potential of using the F-J method to extract multimodal dispersion curves and consequently retrieve an improved high-resolution 3D Vs model from a dense seismic array. Plain Language Summary A seismometer not only records earthquake events but also records weak vibration signals in the environment. Researchers use the seemingly irregular ambient vibration recorded by a dense seismic array to obtain the shear wave velocity (Vs) structure underground. In this study, the ambient vibration recorded by the Long Beach seismic array is analyzed with a recently developed frequency-Bessel transform method, which can effectively extract higher-mode dispersion curves. For each subarray, we measure the dispersion curves with this recently developed method and simultaneously invert the fundamental and higher-mode dispersion curves to obtain a 1D Vs model. Then, we merge all 1D Vs models at each subarray to construct a 3D Vs model. Our model shows high-resolution features in both lateral and vertical directions, which correlate well with known geological backgrounds. In addition, the Pacific Coast Highway fault, Newport-Inglewood fault, and Richfield fault never discussed in the previous velocity model, are clearly resolved in this study.