HR: 15:10h
AN: S52H-07 [PDF]
TI: Estimating Earthquake Hazards in the San Pedro Shelf Region, Southern California
AU: * Baher, S
EM: sbaher@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd
MS 977, Menlo Park, CA 94025 United States
AU: Fuis, G
EM: fuis@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd
MS 977, Menlo Park, CA 94025 United States
AU: Normark, W R
EM: wnormark@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd
MS 977, Menlo Park, CA 94025 United States
AU: Sliter, R
EM: rsliter@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd
MS 977, Menlo Park, CA 94025 United States
AB:
The San Pedro Shelf (SPS) region of the inner California Borderland offshore southern California poses a significant seismic
hazard to the contiguous Los Angeles Area, as a consequence of late Cenozoic compressional reactivation of mid-Cenozoic
extensional faults. The extent of the hazard, however, is poorly understood because of the complexity of fault geometries and
uncertainties in earthquake locations. The major faults in the region include the Palos Verdes, THUMS Huntington Beach and
the Newport-Inglewood fault zones.
We report here the analysis and interpretation of wide-angle seismic-reflection and refraction data recorded as part of the
Los Angeles Region Seismic Experiment line 1 (LARSE 1), multichannel seismic (MCS) reflection data obtained by the USGS
(1998-2000) and industry borehole stratigraphy. The onshore-offshore velocity model, which is based on forward modeling of
the refracted P-wave arrival times, is used to depth migrate the LARSE 1 section. Borehole stratigraphy allows correlation of
the onshore and offshore velocity models because state regulations prevent collection of deep-penetration acoustic data
nearshore (within 3 mi.).
Our refraction study is an extension of ten Brink et al., 2000 tomographic inversion of LARSE I data. They found high
velocities ($>$ 6 km/sec) at about ~3.5 km depth from the Catalina Fault (CF) to the SPS. We find these velocities,
shallower (around 2 km depth) beneath the Catalina Ridge (CR) and SPS, but at a depth 2.5-3.0 km elsewhere in the study
region. This change in velocity structure can provide additional constraints for the tectonic processes of this region.
The structural horizons observed in the LARSE 1 reflection data are tied to adjacent MCS lines. We find localized folding
and faulting at depth (~2 km) southwest of the CR and on the SPS slope. Quasi-laminar beds, possible of pelagic origin follow
the contours of earlier folded (wavelength ~1 km) and faulted Cenozoic sedimentary and volcanic rocks. Depth to basement,
where observed, is approx. 1.7 km. beneath the base then shallows to approx. 1 km at the top of the SPS. This corresponds
to the results obtained by Fisher et al. (in press) and Wright (1991).
The pattern of faulting changes from southwest to the northeast. West of CF, faulting is confined to the pelagic and older
units. Closely spaced faulting (~0.75 km) is prominent between CF and Avalon Knoll (AV), while generally more widely spaced
faults (~5 km) with localized fracture zones is observed from AV to the SPS. The SPS is dominated by major faults such as
the Cabrillo, Palos Verdes, THUMS Huntington Beach and Newport-Inglewood fault zones. The Cabrillo and Palos Verdes fault
are major stratigraphic discontinuity with laminar beds (~30 cm) adjacent to gently folded sediments (wavelength ~1.5 km).
There is evidence of recent displacement on the Cabrillo fault.
DE: 3025 Marine seismics (0935)
DE: 7205 Continental crust (1242)
DE: 7220 Oceanic crust
DE: 7223 Seismic hazard assessment and prediction
DE: 8105 Continental margins and sedimentary basins
SC: Seismology [S]
MN: 2003 Fall Meeting