Model for small-scale crustal heterogeneity in Los Angeles basin based on inversion of sonic log data

Abstract

High-frequency seismic ground motion (10+ Hz), as needed for earthquake engineering design purposes, is largely controlled by the metre-scale structure of the earth's crust. However, the state-of-the-art velocity models poorly resolve small-scale features of the subsurface velocity and density variation.We invert 35 sonic logs (up to 3000 m in depth) in and near Los Angeles basin, CA, to obtain a statistical description of the small-scale heterogeneities of the basin. Assuming a von Karman autocorrelation function, our analysis finds that Hurst numbers, ν, between 0.0 and 0.2, vertical correlation lengths, az , of 15-150 m and standard deviations of about 5 per cent characterize the variability in the borehole data.We report average parameters for Los Angeles basin of ν = 0.064 (0.058, 0.069) ± 0.01 (0.006, 0.012) and az = 54 (51.1, 57.6) ± 5.9 (1.79, 9.53)m with 95 per cent confidence intervals listed in the parentheses. Despite the large depth range of the logs, there is no significant variation of the statistical parameters with depth. Our analysis of 371 depth-averaged shear wave velocities in the upper 30 m, Vs30, provides only an upper bound of basin scale-length estimates due to the coarse sampling distance, with a Hurst number of about 0.3 and lateral correlation lengths, ax, of 5-10 km.