Determination of Shallow Structure of Los Angeles Basin based on ambient noise correlations with dense seismic arrays

The shallow velocity structure of Los Angeles (LA) Basin can amplify strong ground motion, and hence it is important to understand the lateral variations in sediment structure. Existing sediment models of the LA Basin are generally derived from sonic logs and industry reflection profiles, but these models are limited by spatial coverage of well sites and seismic lines. There have been several dense seismic arrays deployed by oil companies that can be used to determine the shallow velocity structure with ambient noise cross correlations. In this study, we show cross-correlations between five dense arrays and 21 continuous broadband stations of the Southern California Seismic Network (SCSN) in LA Basin. Each SCSN station is regarded as a virtual source, and all dense array stations are receivers. We observed clear fundamental and higher mode Rayleigh waves of 0.2 2.0 Hz from the noise correlation functions. Surface waves in this frequency range are sensitive to structure of depth less than 2 km, and their rays cover a large portion of LA Basin. However, Rayleigh wave dispersion curves show modal complexity and make automatic identification of particular mode branches challenging. We measured Rayleigh wave group velocities by fitting the peaks in frequency-time analysis, and retrieve shear-wave velocity structure with seismic tomography.