Aleutian GPSA

The largest earthquake ever recorded in US history occurred in 1964 on the Alaskan subduction zone fault where the Pacific plate slides steeply beneath Alaska on the North American plate. During this magnitude 9.2 event the Pacific plate slipped further beneath Alaska releasing built up strain, causing regions near Kodiak Island to suddenly move SE as much as 18 meters. This resulted in a large tsunami, much damage, and many deaths. The Pacific plate continues to move northwestward 6 cm/yr, compressing regions of Alaska, building up strain that would be released in a subsequent earthquake. GPS sites on Kodiak Island record northwestward motion and strain accumulation, but sites on the Shumagin Islands record little or no apparent motion suggesting two possibilities - either the Pacific plate in the Shumagin region is uncoupled from the overlying plate, such the plate is slipping quietly beneath the region, or else the Pacific plate is locked with the overlying plate near the trench. The first scenario suggests very little seismic and tsunami risk in western Alaska from subduction zone earthquakes, the second scenario poses significant risk and the potential for a large tsunami. The only way to determine where and if plate interface locking is occurring offshore is to measure the motion of the seafloor near the trench. This project will install three seafloor GPS-Acoustic (GPS-A) sites 60 km from the trench spanning the Shumagin Island to Kodiak region and the apparent transition from an unlocked to a locked interface on the subduction zone. If the fault is fully locked, the measurements should show all three sites moving roughly 12 cm NW during this interval. If the plate is unlocked in the Shumagin region, little or no motion of the two adjacent sites is expected. These observations are critical to assessing seismic and tsunami risk in the region and will provide a new understanding of the behavior of subduction zone faults near the trench.