Joint Gravity and Full-wave Seismic Inversion Towards Self-Consistent Wave Speed and Density Models of Southern California

Comprehensive and self-consistent models of wave speed and density variations are essential for understanding the composition and physical properties of the crust and mantle and their seismic behavior. We develop a joint inversion technique using Bouguer gravity and the vertical-radial (Z-H) component-differential traveltimes and Z-H amplitude ratios of body and surface waves to image the subsurface density and velocity structures. The utilization of the Z-H differential observables is based on our recent study of full-wave component-differential sensitivity kernels. The application of this method to Southern California demonstrates that the joint inversion increases the model resolution by reducing the trade-off between shear wave speed and density and the trade-off between shallow and deep structures compared to seismic-only or gravity-only inversions. The preliminary model exhibits density structures in more detail than the starting model particularly in the area between the western Peninsular Ranges Batholith and the San Andreas Fault in the east. The area with relatively low density in the upper crust corresponds to the geological observation of granitic distribution, yet relatively high density in the subsurface beneath the western Peninsular Range consist with Mesozoic plutons. The low-density anomalies beneath and close to major faults may suggest their subsurface extensions and compositions, as well as relations to seismic activity.