Determining a 3D Seismic Velocity Model Offshore Southern California Using Ambient Noise Data from the ALBACORE OBS array

The Pacific-North America plate boundary in Southern California extends far west of the coastline, and a 12-month ocean bottom seismometer (OBS) array spanned the western side of the plate boundary in order to image seismic velocities in the lithosphere. Crustal and uppermost mantle seismic velocities are modeled in the offshore region of Southern California through stacked cross correlations of ambient noise data. The offshore data come primarily from the OBS array that collected 12 months of continuous data during 2010-2011. The OBS data were combined with island and coastline Southern California Seismic Network (SCSN) station data for additional station-pair coverage. The cross correlations were stacked for noise correlation functions and examined using standard time- and frequency-domain methods to determine phase velocity and group velocity dispersion curves. We are simultaneously inverting all measureable dispersion curves to solve for 3D crustal velocity structure. Shear-wave velocities comprise the direct solution, and Vp/Vs ratios are constrained as much as the data allow. Calculations on data from 595 OBS-OBS, SCSN-SCSN, and OBS-SCSN pairs for the 10-33 s and 1-16.6 s bands show clear propagating waves traveling at group velocities of approximately 3.5 km/s, 3.0 km/s, 1.6 km/s and 1.2 km/s. These are the fundamental and second mode Rayleigh waves with sensitivity in the crust and in the water column. The outcome of this work comprises a 3D crustal and uppermost mantle velocity model with areal coverage not attainable before the deployment of the ocean bottom seismometers. The results define the transition in three dimensions from continental lithospheric structure in the near-shore region to oceanic structure west of the continental borderland, and will provide new constraints for determination of earthquake relocations and rupture styles, in particular the degree to which offshore faults produce dip-slip rupture.