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Seafloor Mapping for Earthquake, Tsunami Hazard Assessments
Geophysical and Seismic Surveys Conducted Offshore A California Nuclear Power Plant for Preventive Safety

AUTHOR:
Dr. Stuart Nishenko
Senior Seismologist
Pacific Gas and Electric Co.
San Francisco, California

Dr. Phillip Hogan
Principal Marine Engineering Geologist
Fugro Consultants Inc.
Ventura, California

Dr. Rikk Kvitek
Director
Sea Floor Mapping Lab
California State University, Monterey Bay
Monterey, California


The 3D cube image illustrates structural relationships in deformed sedimentary rock both in cross section and plan view (time slices), allowing for identification of folds and faults represented by the seismic reflections.
The global dialogue on the safety and reliability of nuclear power plants has changed since the 2011 magnitude-9 earthquake and tsunami in Tohoku, Japan. While coastal central California is tectonically different than eastern Japan (a strike-slip versus subduction plate margin), critical facilities located near active fault zones in coastal environments, like Pacific Gas and Electricís (PG&E) Diablo Canyon Power Plant (DCPP), have been the focus of increased attention regarding earthquake and tsunami hazards. Extensive geological and geophysical investigations underway at DCPP are designed to improve hazard assessments. DCPP geosciences staff monitors regional earthquakes and studies global seismic events as part of the plantís safety program. Earthquake hazard information from the program is used to confirm that DCPP can withstand all credible ground motions from nearby faults.

After the 2003 magnitude-6.5 San Simeon, California, earthquake, PG&E began a multiyear geological and geophysical evaluation of earthquake and tsunami hazards for the south central coastal area. PG&E funded U.S. Geological Survey (USGS) research that reevaluated more than 20 years of earthquake data, which led to the 2008 discovery of the Shoreline fault offshore DCPP. The Central Coastal California Seismic Imaging Project (CCCSIP) now conducts multibeam echosounding (MBES) mapping, potential field (gravity and high-resolution marine and aeromagnetic) mapping, high-resolution high and low energy 2D and 3D seismic-reflection profiling, and ocean-bottom seismic monitoring.


Transition Zone Mapping
For much of its length along the coast, the Shoreline fault zone is located in the transition zone (also known as the white zone) in water less than 25 meters deep, with intervening kelp beds, wash rocks and pinnacles. Combined with steep coastal bluffs, the transition zone around DCPP presented a challenge for geophysical data collection.

PG&E sponsored an MBES survey offshore DCPP in 2009 to document possible surface expression of the Shoreline fault. The survey was conducted by the California State University, Monterey Bay (CSUMB), Seafloor Mapping Lab as part of the California Seafloor Mapping Project, a statewide initiative to map all state waters out to the 3-mile territorial sea boundary. The CSUMB 34-foot aluminum RV Ven Tresca was used to map much of this area using RESON A/S (Slangerup, Denmark) 8101 and 7125 MBES systems. MBES data clearly identified a series of linear seafloor features offshore DCPP coinciding with the earthquake epicenter lineations.

Areas within the shallow subtidal and intertidal zones that could not be safely mapped with the vessel required a new approach. Rikk Kvitek developed the RV Kelpfly, an armored airboat/jet-ski rigid inflatable boat hybrid equipped with fully integrated hydrographic and topographic mapping instrumentation, including an Applanix Corp. (Richmond Hill, Canada) POS MV Wavemaster for recording sensor position and attitude at 100 hertz, a SEA (Beckington, England) SWATHplus 468-kilohertz interferometric side scan sonar for ultrawide (15:1) swath bathymetry, a back≠scatter Riegl USA (Orlando, Florida) LMS-Z420i topographic lidar used in line scan mode for intertidal and shoreline mapping to port or starboard, and a YSI Inc. (Yellow Springs, Ohio) Castaway CTD sound velocity profiler for bathymetry refraction correction.

The ability to work in the surf zone, bounce off rocks in as little as 50 centimeters of water and skim the sea surface over eel grass and kelp beds allows the Kelpfly to operate and map in most shallow coastal and estuarine areas off-limits to conventional mapping platforms, including airborne bathymetric lidar often limited by kelp cover, fog and water clarity along the California coast.


3D Subsurface Mapping
Marine seismic reflection surveys were conducted using a nested survey strategy. High-resolution single-channel data were collected at 800-meter intervals perpendicular to the coast aboard the 34-foot aluminum RV Parke Snavely by the USGS Pacific Coastal and Marine Science Center in Menlo Park and Santa Cruz, California in 2008 and 2009, using a SIG 2 Mille mini-sparker. Using these data, specific target areas were identified for higher-resolution multichannel 2D and 3D surveys. These studies were conducted beyond the Transition Zone, in waters more than 20 meters deep outside the kelp canopy, where there was less risk of fouling or snagging equipment.

The 2010 and 2011 surveys focused on the northern end of the Shoreline fault zone, offshore Point Buchon, where it projects towards the larger Hosgri fault zone. Mobile sand sheets of variable thickness obscure seafloor basement features that may be related to the fault zone in this northern area. PG&E contracted Fugro Consultants Inc. (Ventura, California) to conduct a series of low-energy (less than 2 kilojoules, the power limit in California state waters) high-resolution 2D and 3D seismic-reflection surveys. To continue this article please click here.



Dr. Stuart Nishenko is a senior seismologist in the geosciences department of the Pacific Gas and Electric Co. in San Francisco, California, and serves as technical coordinator for the Central Coastal California Seismic Imaging Project.

Dr. Phillip Hogan has 25 years of experience in marine geohazard surveys for coastal and offshore infrastructure and energy projects. Since 2004, he has been a principal marine engineering geologist with Fugro Consultants, following his more than 15-year career with Dames & Moore and URS Corp.

Dr. Rikk Kvitek is a professor and the director of the Seafloor Mapping Lab within the Division of Science and Environmental Policy at California State University, Monterey Bay.





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