We have completed three-dimensional inversions for lithospheric structure using the com- bined LARSE93-Southern California Seismic Network (Fig. 1) teleseismic residual dataset. The data support the existence of thickened crust beneath the San Gabriel Mountains and the well- known high-velocity anomaly. Our study is the first documentation that a significant crustal root exists beneath the San Gabriel Mountains and directly overlies a 60-80 km-wide column of thickened, high-velocity, high-density subcrustal lithosphere (Fig. 2). The images suggest cou- pled deformation between the ductile lower crust and mantle that is produced by gravitational instability of subcrustal lithospheric material and horizontal compression. Posters on this work have been presented at the 1997 IRIS Annual Workshop, the 1997 SCEC Annual Meeting, and the 1997 Fall AGU Meeting.
The following is a summary of specific tasks completed during 1996:
LARSE93 teleseismic residuals were combined with several thousand Southern California Seismic Network teleseismic residuals in 3-D tomographic inversions for lithospheric velocity structures below Southern California. The final grid space corresponding to the final solution consists of 8580 boxes each with dimensions of 10 km parallel to the LARSE93 array 20 km perpendicular to the array 10 km in depth; the velocity in each block is constant. Inversion solutions were obtained by a damped least-squares conjugate gradient method using back-projected rays and ray tracing. The uppermost crust has been stripped off using the velocity modeling results of Fuis et al., EOS, 1996, and Hauksson and Haase, JGR, 1997.
Checkerboard pattern and vertical block resolution tests indicate that lateral resolution is about 20 km and vertical resolution is 20-40 km, depending on raypath coverage.
Two-dimensional gravity perturbation calculations using sedimentary layers from the refraction line upper crustal velocity model (Fuis et al., EOS, 1996) illustrate that competing effects produce a complicated gravity profile. Theoretical gravity perturbations were calculated individually for the sediments, Moho depth variations, upper mantle velocity (using 1% density) anomaly, and all structures combined. The result shows that the sediments offset the thinned crust effect in the Los Angeles basin, and that the upper mantle high-density anomaly has a small but noticeable effect on the total gravity. Bouguer gravity data is from V. Langenheim, USGS.
We have also just completed a 9-month seismic experiment in which a high-density array of short-period SCEC seismometers (L4C3D sensors with Reftek Data Acquisition Systems) was installed across the entire Los Angeles basin to image high-resolution crustal and upper mantle structures (Fig. 1). The goals of the experiment were to 1) investigate crustal thinning in the ba- sin as suggested by LARSE93 teleseismic residuals, 2) relate the tectonic extensional history to thermal models of basin subsidence and stretching, and 3) obtain images of subcrustal litho- sphere beneath the basin to determine the lateral extent of mantle high-velocity anomalies. The local and teleseismic earthquake data will be available through the SCEC Data Center in 1998.
Resulting publications
Kohler, M. D., and P. M. Davis, Crustal thickness variations in Southern California from Los Angeles Region Seismic Experiment passive phase teleseismic travel times, Bull. Seis. Soc. Am., 1997.
Kohler, M. D., J. E. Vidale, and P. M. Davis, Complex scattering within D" observed on the very dense Los Angeles Region Seismic Experiment passive array, Geophys. Res. Lett., 1997.
Kohler, M. D., and P. M. Davis, Active, coupled crust-mantle deformation in the Transverse Ranges, California, in preparation, to be submitted to J. Geophys. Res., 1998.
