September 10 - October 3, 2010
We will be exploring the Northern San Andreas Fault at scales from kilometers to millimeters, extending in time from the beginning of the Holocene, through the present and into the future. We will be using surface mapping and imaging technologies, and subsurface imaging to visualize the fault, its structure, to develop a full three dimensional view of the fault. We will then relate this 3D model to ongoing paleoseismic studies or past earthquakes, and geodetic studies that capture the buildup of energy for the next one. Leg 1 begins with a multibeam mapping survey of the surface fault off Northern California, collecting bathymetry and backscatter data at ~ 0.2-0.5 m. During Leg 2, high resolution mini-sparker sub-bottom profiles will be acquired across the fault at close spacing to image subsurface structure around the submarine fault. Leg III will use the data from Legs I and II to explore the fault in detail, collecting high resolution photo mosaics and fine resolution multibeam sonar data along the fault in selected locations. Leg III will use an Autonomous Underwater Vehicle, the SeaBed AUV to collect the images. Legs II and III will take place aboard the Derek M. Baylis, a “Green” research vessel primarily powered by sail. An important element of the Exploration will be to track the carbon footprint of the Baylis as we conduct our work, and compare that to conventional vessels. While we will need to do much of our work under power, the vessel is very efficient, using much less fuel than typical, and will transit to and from the northern California study area under sail alone.
During re-charge periods for the AUV, multi-frequency sonar water column transects will be acquired to image fish distributions as well as gas venting along the fault. We will use the port of Fort Bragg for staging vehicles and personnel as needed during both legs. The best weather window is late Sept. and Oct. when fog and wind are at a minimum. One of the reasons the San Andreas is not well know offshore is perennial bad weather. The Exploration site is generally inshore of a perennial high wind region offshore as it is in the lee of Cape Mendocino for the most part, however the weather window is fairly narrow. Following the cruise, we will hold a workshop at the Fall AGU meeting in San Francisco to explore the mapping data and plan detailed strategy for data integration and publication.
Data Analysis and Publications
The analysis will consist of integrating the seismic profiles, multibeam sonar mapping and backscatter data, and existing regional multichannel seismic reflection data to develop a tectonic model for the NSAF. Numerous first order questions about this fault remain unanswered, in addition to the earthquake history. These include how the fault accommodates a 30 degree right bend, on the northern California shelf, and how the fault is seismically segmented. Song et al. (2008) demonstrated that the 1906 rupture had two distinct segments, yet nothing is known about these and what they relate to structurally. We will work closely with Carol Prentice of the USGS Earthquake Hazards team to integrate the land and marine data. Recent LIDAR coverage has revealed great details of the fault onshore, and now the marine side will be integrated with these new land data. Our long-term paleoseismic work offshore provides a framework of earthquake histories and segmentation along the NSAF, and ultimately, we hope to be able to understand how the fault segments tie into the earthquake history, a critical piece of the puzzle when it comes to earthquake hazards in San Francisco and northern California. Habitat related data will be integrated into the regional Essential Fish Habitat mapping that is used by the Pacific Fisheries Management Council to establish commercial fishing regulations. Beyond that, the AUV transect data ill be used to assess species distributions in the study area and relate them to their associated benthic habitats.
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