USING DENSELY RECORDED TELESEISMIC DATA TO IMAGE LITHOSPHERIC STRUCTURES BELOW THE TRANSVERSE RANGES: RESULTS FROM THE LARSE II PASSIVE EXPERIMENT

Upper mantle deformation processes below the plate boundary in Southern California have their signatures in seismic data. Densely recorded waveforms now afford us the resolution to determine whether those processes are diffuse within a broad ductilely deforming region, or whether they are influenced by block motion on deep fault zones. We present tomographic P-wave images and SKS-splitting anistropy measurements from teleseismic data collected during the 1998-1999 passive LARSE II experiment. The 83-station, 100-km long linear array spanned the Santa Monica Mountains, San Fernando Valley, Western Transverse Ranges, San Andreas fault, and western Mojave Desert. We use the imaging results to address key questions regarding plate boundary structures and dynamics, such as the degree to which the crust and mantle lithosphere are thermally and mechanically coupled, and block motion versus ductile deformation in the mantle lithosphere. Previous images of lithosphere showed that the crust below the actively deforming, converging segment of the plate boundary has thickened by 8-10 km and that the crustal root is directly underlain by a high-density mantle lithospheric root whose downwelling geometry can be predicted from theoretical dynamical experiments. The addition of three-component, broadband array data from LARSE II provides a test of whether the lithospheric root is one localized part of a multiple downwelling process, or whether it is a single large downwelling characteristic of the convergent region as a whole. We use SKS/SKKS splitting measurements at stations of the LARSE II, Los Angeles Basin Passive Seismic Array, and TriNet stations in southern California to characterize the seismic anisotropy.