The San Andreas Fault (SAF) is a transform boundary between the North American and the Pacific tectonic plates. The SAF extends from the Gulf of California to the Mendocino Triple Junction (MTJ) in northern California. Most of the SAF is located on land and has been extensively studied over the last hundred years. The SAF posses one of the largest seismic hazards to the west coast of the United States. The ~7.9 Mw 1906 earthquake that destroyed most of San Francisco was caused by a rupture of the northern section of the SAF. This rupture is estimated to have occurred from San Francisco, CA to Shelter Cover, CA, roughly 477 km. A large portion of this segment is located offshore, from Pt. Arena to Shelter Cove, and even though over 100 years has passed since the 1906 earthquake this section has received little attention from the scientific community. Based on turbidite paleoseismology, conducted by the Active Tectonics and Seafloor Mapping lab (ATSML), it is believed that this section of the SAF ruptures roughly every ~200 years (Goldfinger 2006). Paleoseismology also suggests that the previous 10 ruptures likely extended from Cape Mendocino, CA to at least the vinity of San Francisco, CA (Goldfinger et al. 2006).
New Geophysical Surveys
In an attempt to constrain and characterize this section of the SAF, the ATSML collected ~572 km2 of high resolution multibeam bathymetry and, jointly with the USGS, also collected ~592 km of mini sparker seismic reflection profiles in 2010(See Map). The data sets collected in 2010 are being combined with other multibeam and seismic data, between Pt. Arena, CA and Fort Bragg, CA, collected by the California State Waters Mapping Project and the USGS. The ATSML has an ongoing collaboration with the USGS to process, interpret and publish findings from these datasets.
The data collection was performed aboard the R/V Derek M. Baylis, a sailing vessel, in an effort to conduct this project in an efficient and 'green' way, as well as to explore the benfits of a relatively quiet vessel for geophysical data collection. The R/V Baylis consumes on average 1.6 gallons of fuel per hour while surveying under motor power and virtually zero while surveying under sail. During the 2010 cruise all fuel consumption was monitored and recorded, i.e. travel, mobilization, survey, demobilization (Link). The Baylis consumed a total of 681 gallons of fuel, many times more efficient that using more common research vessels.
Based on preliminary interpretations of the bathymetry and the seismic profiles it is obvious that the SAF represents a major feature on the seafloor from Pt. Arena, CA to Shelter Cove, CA. In some areas the bathymetry shows that the SAF has vertical scarps greater than 20 meters. Also, based on multibeam bathymetry and seismic profiles, we observe the SAF taking up to 10° bends in its strike. These bends create basins and uplifted areas all along the fault. In addition to the SAF we have, using seismic profiles, mapped small parallel faults and splay faults striking northwest from the SAF. A couple of our goals are to create a detailed geologic map of the area and to determine how much slip this section of the SAF and these other faults are accommodating along the North American/Pacific Plate boundary. This project has and will continue to expand our understanding of this, poorly understood, seismically hazardous area of the United States.
Figure 1. Sailing research vessel Derek M. Baylis, operated by the Sealife Conservation Society. the baylis was used for collection of mini-sparker seismic reflection data, as well as multibeam bathymetry using a Reson 8101 multibeam sonar. The retracted multibeam pole is visible amidships.
Figure 2. Left: Baylis gets some underbody work done prior to the 2010 cruise. Center: grad student Morgan Erhardt monitors multibeam data collection. Right: Baylis stopped for a CTD cast during hte multibeam survey.
|Figure 3. Perspective veiw along the NSAF with 2010 multibeam data.
||Figure 4. Trackline of the OSU-USGS Mini-Sparker reflection profile survey. (Click for larger image)
Figure 5. Monitor record of line ms-89A across the NSAF.
NOAA Ocean Explorer NSAF Expedition Website
Goldfinger, C., Morey, A.E., Nelson, C.H., Gutiérrez-Pastor, J., Johnson, J.E., Karabanov, E., Chaytor, J., Ericsson, A., and shipboard scientific party, 2007, Rupture lengths and temporal history of significant earthquakes on the Offshore and Northcoast segments of the Northern San Andreas Fault based on turbidite stratigraphy, Earth and Planetary Science Letters, v. 254, p. 9-27.
Goldfinger, C., Grijalva, K., Burgmann, R., Morey, A.E., Johnson, J.E., Nelson, C.H., Gutierrez-Pastor, J., Karabanov, E., Chaytor, J.D., Patton, J., and Gracia, E., 2008, Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone, Bulletin of the Seismological Society of America, v. 98, p. 861-889.
Goldfinger, C., Patton, J.R., Morey, A.M., 2009, Reply to comment on "Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone, Goldfinger, C., Grijalva, K., Burgmann, R., Morey, A., Johnson, J.E., Nelson, C.H., Ericsson, A., Gutiérrez-Pastor, J., Patton, J., Karabanov, E., Gracia, E.", Bulletin of the Seismological Society of America, v. 98, p. 861-889, 8 pp.
Goldfinger, C., 2009, Subaqueous Paleoseismology, in Mcalpin, J., ed., Paleoseismology, 2nd edition, Elsevier, p. 119-169.
Goldfinger, C., 2011, Submarine Paleoseismology Based on Turbidite Records, Annual Reviews of Marine Science, v. 3, p. 35-66.