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||"Geologic Studies and Seismic Source Model for a Proposed New Ocean Outfall, San Pedro Shelf, California"
||Stephen L. Varnell, P.G., C.E.G., ,
Project Geologist, Fugro West, Inc.
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Coauthors: Phillip J. Hogan, Fugro West Inc., Patrick Smith, Fugro West Inc., Thomas W. McNeilan, Fugro Atlantic, and Robert Dame, Fugro West, Inc.
The Sanitation Districts of Los Angeles County (Districts) are evaluating the feasibility of a new tunnel and ocean outfall for its Joint Outfall System. Fugro performed offshore geotechnical and seismic survey campaigns for the project between 2004 and 2008. Based on these new data, we describe structural and stratigraphic conditions on the Palos Verdes (PV) and San Pedro (SP) Shelves and adjacent slopes.
The offshore stratigraphy consists of a sequence of Cenozoic marine strata overlying Mesozoic basement rocks consisting of Catalina Schist. Soil and rock units present beneath portions of the shelf and slope in the survey area include: 1) Catalina Schist, 2) Monterey Formation, 3) Malaga Mudstone, 4) Fernando Formation, 5) Late Pleistocene sediments, and 6) Holocene sediments. Miocene basaltic intrusions are found within the lower Monterey Formation.
The PV Anticlinorium (PVA) dominates the structure of the Project Area. The PVA extends onshore and offshore for approximately 30 miles, from Santa Monica Bay to SP Bay, and is associated with a broad zone of uplift adjacent to the Palos Verdes Fault Zone (PVFZ). The PVFZ is an inverted Miocene normal fault that has been reactivated as a transpressional structure in the Plio-Pleistocene (Brankman and Shaw, 2009). We interpret the PVA as a broad hanging-wall anticline above the southwest-dipping PVFZ. Our current seismic source model, including segmentation of the PVFZ is presented.
Shorter faults, such as the Cabrillo fault and other faults on the continental shelf and slope to the southwest of the Project Area are considered secondary structures related to the PVFZ, PVA, and a deep basal decollement underlying the southern LA Basin and SP Shelf. The Cabrillo fault and numerous other short faults may have formed as bending moment faults as a consequence of folding during uplift of the PVA concurrent with shortening across the LA Basin.
Potentially active and inactive southwest-dipping reverse faults are present beneath the SP Slope, creating a stable buttress against slope failures. This appears to have resulted in relatively stable bedding conditions on the outer SP Shelf and Slope. Potential fault rupture and seismicity present geohazard challenges to the project. Geohazards will be mitigated through appropriate engineering design of the facilities.
Stephen Varnell is a project geologist in the Marine Geohazards Group in the Nearshore division of Fugro West, Inc. in Ventura, California. He specializes in marine geology, geohazards, and geophysical data interpretation for large infrastructure projects such as oil and gas facilities, pipelines, ports and harbors. He has a B.S. in Earth Sciences from U.C. San Diego, and an M.S. in Earth Sciences with an emphasis on marine geology from U.C. San Diego that was completed in conjunction with the Scripps Institution of Oceanography. He has over 8 years of geologic consulting experience, including over 4 years at Fugro West, and 4 years at LGC Valley, Inc., where he worked for former AEG Southern California Section Chair Matthew Hawley. Stephen is a licensed Professional Geologist in the state of California, and a Certified Engineering Geologist in California as well. He is also the Newsletter Editor for the Southern California Section of AEG.