Geodetic Characterization of the Easternmost Alaska Subduction Zone

Non-Technical Summary

Subduction zones, where one tectonic plate descends beneath another, produce great earthquakes and thus pose significant seismic hazards. Understanding the characteristics of the plates, such as the geometry of the descending plate (or slab), and the processes by which they slip past each other is key to assessing the extent of the seismic risk. Parts or all of the two plates may be stuck together, accumulating strain that may be released in a large earthquake, or they may slide past each other more quietly, including in "slow slip events", where energy is released over days or years instead of seconds.

South central Alaska is an excellent laboratory for investigating links between slab and upper plate characteristics and slip behavior in subduction zones. The region generated the second-largest earthquake ever recorded (a magnitude 9.2 in 1964), but major questions remain about processes along this margin. Earthquake activity shows that the Alaska-Aleutian slab is subducting beneath part of southern Alaska.

This earthquake activity stops abruptly to the east of where the magnitude 9.2 earthquake occurred, leading to speculation that the slab may not be continuous throughout the area. Recent GPS studies have suggested that the slab is present and locked with the upper plate across much of the area and seismic investigations have found evidence that transient behavior may also occur, but sparse data has limited further work.

The project team, from Purdue University, is installing four new continuous GPS sites and five temporary GPS sites. The team is combining data from these sites with data from existing sites to investigate subduction processes in the area, including understanding the geometries of the slab and upper plate and determining where, and to what degree, the plates are locked.

The team is also searching for subtle evidence of transient events; if found, those events can be related to plate geometries and plate locking.

The project is also providing local K-12 teachers the opportunity to participate in fieldwork and learn about how GPS data is used in tectonic studies. Lesson plans developed with these teachers incorporate this project's research into a larger scale view of the region's tectonics and give students experience working with scientific data. The team is spreading its research results to a larger audience through interactions with National Park Service staff and public lectures.

Overall, this project is generating and communicating a more complete understanding of processes in the easternmost Alaska subduction zone, which will improve seismic hazard estimates. Technical Description

This proposal seeks to improve understanding of the relationship between complicated slab geometries (e.g. flat slab subduction, edges or kinks in slabs) and subduction-related processes such as upper plate deformation, coupling along the subduction interface, and transient events (e.g. slow-slip). Data from 4 new continuous GPS sites, 5 new campaign sites, and existing continuous and campaign sites in the region are being used to examine strain patterns, the geometry of the subducting slab, coupling along the subduction interface, upper plate motion, and transient processes along the easternmost Alaska subduction zone.

The project is addressing the following questions: 1) What is the geometry of the subduction interface? Where is the eastern edge of the active subduction interface? How does this relate to the Wrangell Volcanic field and observed seismicity? 2) How does coupling vary within the easternmost Alaska subduction zone?

Where does the transition from locked to creeping occur? How does this relate to the observed tremor, upper plate deformation, and seismicity? 3) Are there transient events resembling slow-slip events on the subduction interface? How do these transient events relate to the interface geometry and coupling distribution?

The new data are allowing an assessment of plate boundary characteristics on a much more spatially and temporally dense scale than currently possible, leading to more detailed and realistic models of the behavior of the Yakutat slab interface and its influence of the region's tectonics.