Recent Tomographic Results in Southern California and their Tectonic Implications

The Broadband Seismic Network in Southern California, developed in the last fifteen years, represents one of the most dense, permanently-instrumented network in the world. Along with its high-quality broadband data comes an opportunity for various waveform analyses for retrieval of seismic parameters in the crust and mantle. We will review the state of the tomographic studies in this region with special reference to various tectonic features that have been identified. Review will be on various body wave tomography including both P- and S-wave tomographic results, shear-wave splitting data and surface wave results for S-wave velocity and anisotropy. In this paper, we will be concerned with the large-scale tectonic framework in this region and thus focus on larger-scale features in tomographic results. We will not go into details of small-scale features, such as shallow crustal structures, that are important for ground motion prediction and hazard mitigation.

Some of the tectonic features that we will discuss are: (1) Slow S-wave velocity mantle anomaly under the Southern Sierra: this was confirmed by body wave study (e.g. Savage et al., 2003) and surface wave study (Prindle-Sheldrake and Tanimoto 2004, hereafter PT). (2) Deep velocity contrast across San Andreas. This was seen in body wave results (Helmberger et al., 2001) and surface wave results (PT) (3) Deep fast velocity anomaly in the Transverse Range. This was reported first by the well-known paper by Humphereys et al. (1990) and confirmed by others (e.g., Kohler et al. 1998) using P-wave travel time data. Surface wave study found similar features in S-wave velocity model but the location of the fast velocity root is not necessarily the same. (4) Deep root-like structure seems to exist under the Western Transverse Range, that have rotated 90 degrees approximately in the last 15 Ma. There seems to be a hint for the existence of similar root-like structure also under the Peninsular Ranges from which the Western Transverse range broke off and rotated 90 degrees. (5) S-wave splitting data and surface wave data generally match on the pacific plate side of this region but differences seem to exist on the eastern side. Surface wave results seem to indicate a much stronger anisotropic structure on the Pacific plate side. On the Pacific plate side, the fast direction of azimuthal anisotropy is along the direction of the Pacific plate motion or (equivalently) parallel to the faults in general. (6) Under the Salton Sea region, longer period Rayleigh wave results at about 20 mHz indicate the fast direction perpendicular to the fault. If there is spreading in this region, the fast direction coincides with the spreading direction. This direction rotates 90 degrees for higher frequency maps. This result seems to stand our various resolution tests and indicates an interesting situation that the fast direction of S-waves differ for the crust and the underlying mantle.