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Workshop on Earthquake Hazards and Water-Resource Issues in Offshore Southern California

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In February 1999, members of the Western Region Coastal and Marine Geology team convened a one-day workshop at the Minerals Management offices in Camarillo, California, to examine the status of current research in earthquake hazards and water-resource issues in offshore Southern California. Participants discussed their current work between the Santa Barbara Basin and San Diego. The workshop was organized by Bill Normark and Jane Reid of Menlo Park's Southern California Coastal and Continental Borderland Seismic Hazard Project, and was conducted in conjunction with Brian Edwards of the Geologic Framework of Coastal Aquifers Project. The workshop was designed to assess the state of current knowledge and the amount and quality of available data, as well as to promote collaborations. It brought together researchers and customers from the USGS (Coastal & Marine Geology and other Geologic Division teams), academia, federal, state, and local government agencies, as well as private companies.

The Los Angeles metropolitan area is one of the most populous coastal urban regions of the United States. The Southern California Coastal and Continental Borderland Seismic Hazard Project (suggestions for an appropriate acronym are respectfully solicited!) is responsible for mapping Holocene to late Pliocene faults and framework stratigraphy. Knowledge of the location and geometry of fault systems in the area, some of which extend onshore, is critical to estimating the location and severity of ground shaking during future earthquakes. Emphasis is given to providing appropriate information to land-use managers, planners, and others involved with mitigating losses from earthquakes. The results of this project likely will contribute to decisions involving land use, hazard zonation, property insurance premiums, and building codes.

Ground water is an important fresh-water resource in the Los Angeles basin. Each year, urban and agricultural users pump approximately 45,000 acre-feet of ground water from aquifers underlying the region. As a result of this pumping, salt water is intruding freshwater aquifers near Long Beach, California, degrading the quality of local ground water. The Geologic Framework of Coastal Aquifers project is designed to study the process of saltwater intrusion into these coastal aquifers. Our principal goals are to 1) develop a sequence stratigraphic model of San Pedro shelf sediment, 2) identify potential pathways of saltwater intrusion into aquifer units (e.g., beds cropping out on the shelf or mainland slope, leakage through fault zones), 3) link onshore and offshore stratigraphy, and 4) if possible, confirm flow into aquifers by direct measurement.

The Camarillo workshop provided an avenue for focused discussions and explored areas of possible collaborations. Whereas many researchers are studying onshore faults, few are working offshore, some with decades-old, deep-penetration data. The current USGS projects will provide a relatively close-spaced (1- to 2-km) high-resolution multi-channel and deep-tow Huntec seismic-reflection survey over the area from Santa Monica Basin to San Diego, and possibly north to the Santa Barbara Basin area.

During the workshop, it became clear to the USGS participants that useful data are available from the area, thereby enabling the two USGS projects to focus time and money on interpreting existing data, identifying the considerable data gaps, and establishing needs. Already, the contact with the City of Long Beach is producing a proposal for partnership in Seabeam seafloor mapping of the San Pedro shelf, work that will greatly assist interpretation of active Holocene structural features. Contact information was exchanged during the workshop, and those present expressed interest in periodic meetings to maintain coordination among the diverse groups.

We would like to thank the workshop participants (as well as those unable to attend who sent information) for giving their time to make the workshop a success. Special thanks both to Drew Mayerson of the Minerals Management Service (MMS) in Camarillo for his generous loan of the MMS conference room, and for his time setting up the meeting, and to Mark Legg of Legg Geophysical Consulting for his input during the planning of the workshop.

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