Monitoring Potential Slope Failure in the Santa Barbara Basin Using Optical Fibers

Scripps Institution of Oceanography and BP America are engaged in a collaborative study of the northern flank of the Santa Barbara Basin. Multibeam bathymetry and subsequent seismic surveys show massive slope failures offshore Santa Barbara, California. The largest landslide complex, known as the Goleta slide, is 10.5 km wide and involved the displacement of 1.75 km3 during the Holocene. Approximately 8 km west of the Goleta complex lies the Gaviota slide, which involved the displacement of 0.02 km3. The Gaviota slide is estimated to have occurred between 1715 and 1840, bracketing two 1812 earthquakes of magnitudes 7.1 and 7.5. The Gaviota failure may be responsible for oral reports of a tsunami with roughly 1-2 m runup following the earthquakes, matching the modeled runup for a Gaviota slide source. Models indicate that an event similar to the Goleta slide would produce a 10 m high wave at the shoreline. The region between the two slides contains a highly gullied slope along with active seeps and pockmarks. A 10-50 m wide, 4 m deep crack prominently extends from the Gaviota slide head scarp 2 km east, and can be traced approximately 7 km nearly to the Goleta complex. Seismic surveys reveal that the Gaviota head scarp and crack are coincident with a subsurface anticline or thrust fault. Questions regarding the crack include is it a remnant from the Gaviota failure, did it occur during a subsequent geophysical event, or is it an actively propagating head scarp representing near future failure? To explore the possibility of active deformation, a group at Scripps is applying several new seafloor geodetic techniques that vary in spatial and temporal resolution to the region containing the potentially propagating head scarp. One technique is a fiber optic seafloor strainmeter (FOSS), which monitors the length of optical fibers (250 m to 750 m initial length) stretched and buried across the crack. FOSS can measure downslope deformation to a precision of 1.5 mm. A series of engineering tests were conducted offshore La Jolla, California to demonstrate performance of FOSS from 2005-September 2006, and the sensors will be deployed in the Santa Barbara Basin in November 2006 for a 2-3 year monitoring effort. Here we present the engineering performance results and the initial results from deployment offshore Santa Barbara for the FOSS sensor.