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U.S. Geological Survey
Professional Paper 1687

Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

By Michael A. Fisher, William R. Normark, Victoria E. Langenheim, Andrew J. Calvert, and Ray Sliter


Oblique map looking from the sea towards the Los Angeles Basin showing faults
Oblique image, looking north-northeast, of the Los Angeles region, California, and the adjacent Pacific Ocean, showing major geographic features and locations of main faults. Width of image is about 70 km, center of image located at about 34°N, 118.2°W. Image consists of an enhanced Landsat 5 satellite image (Bands 3, 2, and 1 as red, green, and blue, respectively) draped over topography from Space Shuttle radar data, with vertical exaggeration of 1.5 times. From National Aeronautical and Space Administration (NASA), available (without annotations) from:

High-resolution seismic-reflection data have been combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro Shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington Graben, the Palos Verdes Fault Zone, various faults below the western part of the shelf and slope, and the deep-water San Pedro Basin. The structure of the Palos Verdes Fault Zone changes markedly southeastward across the San Pedro Shelf and slope. Under the northern part of the shelf, this fault zone includes several strands, but the main strand dips west and is probably an oblique-slip fault. Under the slope, this fault zone consists of several fault strands having normal separation, most of which dip moderately east. To the southeast near Lasuen Knoll, the Palos Verdes Fault Zone locally is a low angle fault that dips east, but elsewhere near this knoll the fault appears to dip steeply. Fresh sea-floor scarps near Lasuen Knoll indicate recent fault movement. The observed regional structural variation along the Palos Verdes Fault Zone is explained as the result of changes in strike and fault geometry along a master strike-slip fault at depth. The shallow summit and possible wavecut terraces on Lasuen knoll indicate subaerial exposure during the last sea-level lowstand. Modeling of aeromagnetic data indicates the presence of a large magnetic body under the western part of the San Pedro Shelf and upper slope. This is interpreted to be a thick body of basalt of Miocene(?) age. Reflective sedimentary rocks overlying the basalt are tightly folded, whereas folds in sedimentary rocks east of the basalt have longer wavelengths. This difference might mean that the basalt was more competent during folding than the encasing sedimentary rocks. West of the Palos Verdes Fault Zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because age dates on deformed or offset sediment are lacking.

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Maintained by: Michael Diggles
Created: March 22, 2005
Last modified: March 22, 2005 (mfd)