Collaborative Research: Measuring Strain along the Aleutian Subduction Zone Trench to better constrain seismic and tsunami hazard

Lead PI: Dr. Spahr C. Webb

Unit Affiliation: Seismology, Geology and Tectonophysics, Lamont-Doherty Earth Observatory (LDEO)

September 2017 - August 2020
North America ; Alaska
Project Type: Research Outreach

DESCRIPTION: 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. This project will train new engineers and researchers in these new seafloor geodetic techniques and will support a graduate student and undergraduate students in the research who will also participate in the at sea operations. The project also makes use of novel wave-glider technology that offers opportunities for broader outreach. Three GPS-A sites will be established near the Aleutian trench to measure deformation associated with strain accumulation along the Aleutian megathrust. Two sites will bridge the Shumagin seismic gap and the third site will lie within the Semidi segment, the source of the 1938 M8.2 earthquake. The sites will span a region where the inferred interplate coupling derived from onshore GPS observations increases from near 0% to 90%, representing a large variation in inferred seismic and tsunami hazard. Onshore observations are inadequate to resolve coupling near the trench and this is critical to assessing tsunami hazard. The GPS-A method uses acoustic travel time measurements to tie permanent benchmarks established on the seafloor to a sea surface platform that is geodetically tied into the global GPS network. A horizontal positional accuracy of order 1 cm can be obtained with this method. Using measurements two years apart, we can measure the deformation velocity at each site to an accuracy of order 7 mm/yr, (or about 10% of the plate convergence rate). The along strike observations will better constrain the variation in coupling across the Shumagin seismic gap region and the region of 1938 earthquake. Understanding how and why coupling varies between the subducting plate and the overlying plate within subduction zones is critical to understanding subduction processes and seismic and tsunami hazard from subduction megathrusts. The new data will be a crucial augmentation to observations obtained with a recent repeat of pre-1995 campaign GPS observations and with ongoing continuous GPS sites. Dislocation models of plate coupling show the observations should constrain coupling near the trench essential to understanding the potential tsunami hazard.