HR: 0800h
AN: T41F-1284    [Abstracts]
TI: Structural Inversion of the Palos Verdes Fault, Southern California, and its Implications for Seismic Hazards Assessment
AU: * Brankman, C M
EM: brankman@fas.harvard.edu
AF: Department of Earth and Planetary Sciences Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Shaw, J H
EM: shaw@eps.harvard.edu
AF: Department of Earth and Planetary Sciences Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AB: The Palos Verdes Fault (PVF) defines the western margin of the Los Angeles basin, and is regarded as a likely source of moderate to large earthquakes that would affect the coastal metropolitan regions of southern California. In most hazard compilations, the PVF is generally considered to be a vertical, predominantly right-lateral, strike-slip fault system that extends continuously from the Santa Monica thrust southward across Santa Monica Bay, crossing the Palos Verdes Peninsula and continuing southeast across the Inner Borderlands to the area of Coronado Banks. A restraining bend where the fault dips steeply to the southwest generates uplift and folding of the Palos Verdes Peninsula. However, previous studies documenting the activity, slip rate, and slip sense of the PVF have used shallow subsurface excavations and high-frequency seismic data, which have generally limited observations to the upper kilometer of the crust. We use an extensive grid of petroleum industry seismic reflection data and well logs to define the three-dimensional subsurface geometry of the PVF in the region south of the Palos Verdes Peninsula. Our seismic data cover the complete offshore extent of the fault, from Santa Monica Bay to the Coronado Banks, and provide direct constraints on the fault geometry extending down to about 5km depth. We use the shapes of folded strata imaged in the seismic data and penetrated by wells to invert for permissible geometries of the fault as it extends to the base of the seismogenic crust. Our data and structural analyses indicate that the PVF developed by Pliocene inversion of a Miocene normal fault system. The fault has a significant component of reverse slip and southwesterly dip at depth along its extent. Oblique displacement on the fault appears to be partitioned at shallow levels into nearly pure right-lateral strike slip on near-vertical faults and contractional folding above gently to moderately dipping blind-thrust fault splays. These observations are used to define a realistic 3D geometry of the PVF, to define the sizes, shapes and spatial relationships of fault segments that may rupture in earthquakes, and to extend shallow slip and slip rate estimates from previous studies to depth along the fault. This fault model will provide improved forecasts of the possible size and frequency of large earthquakes on the PVF, and will provide more accurate geometric fault representations that can be used to predict strong ground motions resulting from these events.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting