SUBSURFACE STRUCTURE OF THE NORTHERN LOS ANGELES BASIN SOUTH OF THE SANTA MONICA AND HOLLYWOOD FAULTS

In contrast to Ventura basin, where modern deformation rates have operated since the Miocene, northern Los Angeles (LA) is an evolving deformation system with modern rates and deformation styles based on structures younger than 1 Ma. The Las Cienegas blind reverse fault steps left and eastward to the Elysian Park blind fault, which is taking over the slip rate of 1.5-2 mm/yr formerly concentrated on the Las Cienegas fault. The 1987 Whittier Narrows earthquake uplifted the anticlinal Montebello and Santa Fe Springs oil fields and the La Habra syncline between them, consistent with the lack of thickening of the San Pedro Formation between these east-west structures. The dominant Pliocene structure of the Coyote Hills is an east-striking fault with 1.2 km left slip, the Stern fault of East Coyote and South Flank fault of West Coyote and Leffingwell oil fields. This fault was folded, along with Plio-Pleistocene strata, above a north-dipping blind reverse fault not penetrated by wells, uplifting the Coyote Hills. Inactive structures defined by isopachs of the underlying Plio-Pleistocene Fernando Formation trend northwest-southeast; these include the LA trough, the Anaheim nose, and the proposed Compton-Los Alamitos thrust. The Whittier fault is strike slip based on late Quaternary evidence but its earlier history is mainly dip slip, including a footwall anticline at Whittier, Sansinena, and Brea-Olinda oil fields. The change in tectonic style may be due to the northwest propagation of Peninsular Ranges right-lateral faults into the LA basin. Earthquakes generated on these northern LA structures are expected to be in the M 6-7 range, like the 1987 event, unless they rupture in a cascade, in which case larger magnitudes are possible.