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A 3D Seismic Study of the Pacific-North American Plate Boundary in Southern
California: The ALBACORE Experiment
The Pacific-North America plate boundary in southern California is one of the only examples of recorded subduction of an oceanic spreading center in geological history, but this process is likely to be a common component of the plate tectonic cycle on Earth. Studies in the Pacific Northwest have shown that obduction and underplating of oceanic crust along the plate margin is important (e.g., Trehu, 1991), contributing to continental growth and accretion along its margins. There is still much to be learned, however, about the details of the accretionary process and deformation at plate boundaries which contribute to continental evolution and growth. Almost half the diffuse, transpressional, 500-km Pacific-North America plate boundary in southern California lies offshore, but little is known about the physical properties of the oceanic plate on the western side of this boundary, fault structure, stresses, nature of plate fracture, its lithospheric and asthenospheric properties or its mode of deformation. As a result, both dynamic and kinematic models for plate boundary deformation and associated mantle flow lack physical data from fully half the plate boundary and its transition to oceanic Pacific plate.
Collaborators Dayanthie Weeraratne (CSUN) and Monica Kohler (UCLA/Caltech) are currently wrapping up a 12 month marine seismic deployment offshore southern California. The research cruise titled ALBACORE describes Asthenosphere and Lithosphere Broadband Architecture of the Calfornia Offshore Region Experiment. Student research is the backbone of this project. CSUN undergraduate students Natsumi Shintaku, Brian Clements, CSUN grad students Teddy Sotirov and Lennin Escobar, and Caltech undergrad students Emmet Cleary, Lei Huang, Connie Sun, Stephanie Tsang, Paige Logan, Jennifer Zhu, Curie, and IRIS intern Kelsey (New Jersey College) and Katie Booth (Rutgers/Brown) have participated on a 3 week deployment (August 2010) and/or the 2 week recovery cruise which we are currently conducting (Sept 6-Sept 16, 2011). Data collected from 34 seismometers recovered on this research cruise will be combined with seismic data from the permanent CISN network (California Integrated Seismic Network). Researcher groups at CSUN, Caltech, and UCLA will study wavefields from earthquakes recorded simultaneously by both land and marine seismometers to understand seismic and tectonic structure of the North American and Pacific plate at crustal, lithospheric, and asthenospheric depths. Offshore southern California seismic data to this date are notably absent but are of crucial importance in understanding the crustal and plate-scale deformation, onshore and offshore fault stresses, and possible oceanic sources for tectonic block rotations (Atwater, 1989; Luyendyk, 1991). Moreover, improved locations of offshore earthquakes are needed to provide offshore earthquake hazard information that is directly connected to the interaction of these two plates.
Location of deployed seismographs from Alex Hanna's webpage:http://www.csun.edu/~ach26571/fall2011-cruise.jpg
Dr. Dayanthie Weeraratne holding a tag line with Scripps Institute of Oceanography OBSIP (OBS instrument Pool) technician, Ernie Aaron, on the sensor ball of an ocean bottom seismometer (OBS) as it is hoisted out of the water and brought on board the New Horizon.
Lennin Escobar (CSUN master's student), Paige Logan (Caltech geophysics undergrad), and Kelsey Brenner (IRIS intern and undergrad at New Jersey College), stabilizing an OBS during a recovery operation on the New Horizon.
The New Horizon shoving off from the San Diego port to embark on the first major seismic array ocean bottom experiment conducted offshore Southern California. Dolphins and sealions followed the ship for severa miles.
Teddy Sotirov (CSUN maser's student) on a night recovery of an OBS with Phil (SIO OBSIP technician). Ship costs to NSF are exhorbitant so operations are conducted 24 hours around the clock. Students and scientists work on shifts to be available 24 hours when we come on sight or need to recover an instrument.
A wave brings water on the back deck of the ship. The New Horizon is a bit rocky for these unseasoned scientists!
Brian Clements (CSUN undergrad) and Dave (SIO OBSIP technician) dismantling a sensor ball for storage in specialized shipping containers (black box). Many of the sensor footings (metal base) were covered in mud from 12 months of sitting on the seafloor.
Seismic record from an OBS showing the 2011 Honshu, Japan earthquake arriving at the ALBACORE array offshore southern California. Also notice the signal from the tsunami which traveled across the Pacific to southern California in approximately 9 hours. The magnitude of the tsunami decreased during travel across the Pacific ocean seafloor.