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Recent Geologic Studies Along Portions of the Hollywood Fault in the Los Feliz District of Los Angeles, California

D. L. Perry1, M. Oborne2, and M. G. Olson3
1MACTEC Engineering and Consulting, Inc., Los Angeles, CA, [email protected]
2Bureau of Engineering, City of Los Angeles, CA, [email protected]
3Advanced Geoscience, Palos Verdes Estates, CA, [email protected]

Geomorphic features in Holocene–late Pleistocene alluvium in the Los Feliz and Hollywood Districts are the basis for inferred traces of the Hollywood fault on geologic maps. Prior subsurface studies in the Hollywood area have confirmed a north dipping fault zone and south-dipping antithetic faults. New data from a site in Los Feliz allows us to project the Hollywood fault eastward into the Los Angeles River floodplain.

The Hollywood fault extends from north of Beverly Hills to Los Feliz and has reverse-left lateral slip. Geologic mapping in the hills immediately to the west of the site indicates a fault juxtaposing granodiorite on the north against Puente Formation on the south. Possible fault traces to the north are suggested by subdued linear scarps in alluvial fans.

We conducted a subsurface investigation to identify potential traces of the Hollywood fault that may cross a planned sewer tunnel project. Trench excavations are unfeasible due to the dense urban environment, shallow groundwater, property ownership constraints, and underground utilities. Continuously cored borings with down-hole acoustic televiewer, installed monitoring wells, and seismic reflection/refraction surveys were used to investigate the fault. We identified a low-angle, sheared contact between Miocene sedimentary rock and Cretaceous granodiorite beneath Holocene alluvium in one of the core borings. This sheared contact is interpreted to be within the main fault zone and may be a major splay of the fault.

Seismic profile reflection patterns and sharp lateral velocity variations indicate a north-dipping fault zone, about 300 feet wide with south-dipping, secondary traces. A down-to-the-south groundwater elevation change in monitoring wells on either side of the main fault zone is consistent with studies in the Hollywood District. Changes in alluvial thickness across the fault may be related to either fault displacement or to irregularities associated with the fluvial/alluvial depositional environment and related erosion.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009