PRELIMINARY SEISMIC IMAGES FROM THE LOS ANGELES REGION SEISMIC EXPERIMENT, PHASE II (LARSE II), SOUTHERN CA

In the second phase of the Los Angeles Region Seismic Experiment (LARSE II, October 1999), a refraction/ low-fold reflection data set was collected along a 160-km-long corridor extending from Santa Monica Bay northward to the southern Sierra Nevada. Chief imaging targets include the Santa Monica, San Gabriel, and San Andreas (SAF) faults, blind thrust faults (including the Northridge (NRF) fault and Northridge Hills (NHF) fault), and the depths and shapes of the San Fernando Valley and other sedimentary basins. Preliminary velocity models of this transect show that (a) the sedimentary basin underlying the northern San Fernando Valley/ Santa Susana Mts is perhaps as deep as 5 km; (b) the crust underlying the crystalline northwestern Transverse Ranges is low in velocity (~5.75 km/s on average to 10-km depth); (c) the crust underlying the Mojave Desert is intermediate in velocity (reaching ~6.5 km/s at 7-8-km depth); (d) the SAF, separating these latter two blocks, dips steeply; and (e) a moderately north-dipping low-velocity zone that is several km wide and located beneath the Santa Susana Mts is centered on the aftershock zone of the 1971 San Fernando earthquake and may project to the NHF in the northern San Fernando Valley. Preliminary single- and low-fold reflection images of the middle and lower crust reveal the following reflectivity pattern: (a) generally low reflectivity southward of the crystalline northwestern Transverse Ranges; (b) generally high reflectivity, with criss-crossing reflections, in the northwestern Transverse Ranges and Mojave Desert; and (c) no unusually bright reflective zones as found beneath the San Gabriel Mts in LARSE I. Automatic and manual line drawings followed by migration reveal (d) weak reflectivity in the vicinity of the NRF; (e) a reflective, moderately north-dipping zone that is centered on the deeper aftershocks of the 1971 earthquake and projects upward to the low-velocity zone noted above; and (f) highly reflective, moderately south-dipping zones in the Mojave Desert.