The purpose of the study is to determine the optimum placement of seafloor geodetic monuments along the Cascadia Subduction Zone and the frequency and duration of horizontal and vertical seafloor geodetic measurements required to resolve the character of slip along the offshore portion of the thrust fault. Presently, onshore geodesy has determined that the locked region lies almost entirely offshore, however these data lack proximity and poorly resolve details of the stick-slip behavior near the deformation front and the location of the boundary from full stick slip to some component of stable sliding. This one-year project to assimilate existing models of fault geometry, locking behavior along the fault, onshore GPS data, and field-proven precisions of horizontal and vertical seafloor geodesy into an elastic/visco-elastic model. Using this model construction, various placements of seafloor geodetic monuments will be simulated and the resolving power estimated to determine the minimum required array configuration along Cascadia to constrain regional-scale slip behavior on the thrust fault. The project contributes directly to the first two science goals in the GeoPRISMS Science plan on Subduction Cycles and Deformation: What governs the size, location and frequency of great subduction zone earthquakes and how is this related to the spatial and temporal variation of slip behaviors observed along subduction faults? And how does deformation across the subduction plate boundary evolve in space and time, through the seismic cycle and beyond? The broadest impact of this study will be to provide a guide towards using seafloor geodesy to better quantify the earthquake and tsunami risk associated with a large rupture of the thrust fault within the Cascadia subduction zone. Seafloor geodetic measurements could be collected all along the CSZ as a needed constraint to models of megathrust slip that are mostly constrained by the sub-aerial GPS vectors from the Plate Boundary Observatory, a part of Earthscope. Results of this study will help guide this data collection.
The Cascadia subduction thrust fault lays just offshore northern California, Oregon and Washington. The fault is formed where the Juan de Fuca tectonic plate, an oceanic plate covered by the Pacific Ocean, dips and moves under the North American plate, a continental plate which supports the land mass occupied by humans. The contact between the two plates is frictional, i.e., rough and sticky, allowing some of the motion of the down-going Juan de Fuca plate to be transferred to the overriding continental plate. This builds up elastic energy just like squeezing a rubber ball. Periodically, every few hundred years, the fault slips releasing the stored energy causing a major earthquake and tsunami like in 2010 offshore Chile and in 2011 offshore Japan. The last great earthquake in Cascadia occurred in January 1700. We have developed a technique to measure the slow accumulation of strain build up on the sea floor overlying the fault where the Juan de Fuca and North America plates interact. The technique combines precise position determination of the Global Positioning System with under water ranging to the sea floor. By measuring the deformation at the sea floor the location where the fault sticks can be estimated. This information can be used to calculate possible amounts of shift between the plates when the next great earthquake occurs. The purpose of this study was to calculate where best to make deformation measurements on the sea floor. We determined there should be measurements made offshore northern Washington/southern Canada, central Washington, central Oregon, and northern California. At each of these locations measurements should include one site on the incoming Juan de Fuca plate to measure its speed and two sites on the submerged continental slope of the North American plate to measure the strain build up.
