Upper Plate Motion, Slab Geometry, and Slip Behavior Along the Eastern Alaska Subduction Zone

The eastern Alaska subduction zone is a multifaceted boundary that has generated a number of significant events over the last century, including the M9.2 Prince William Sound earthquake. While it has often been assumed to be the boundary between the Pacific and North America plates, a wealth of new data acquired through a variety of projects over the past decade has revealed a more complicated situation. We present an expanded and updated GPS dataset and use it to constrain a tectonic block model that allows us to explore upper plate motion, slab geometry, coupling variations along the subduction interface, and the relationship of these parameters to slip behavior in the region.

Rather than a single plate, the crust above the interface is comprised of multiple blocks. Counterclockwise rotation in the eastern Chugach Mountains gradually transitions to near arc-parallel motion in the Sanak region. Our results suggest that the flat Yakutat slab is locked or partially locked from the western edge of the Wrangell Volcanic field north to the Denali fault and west to the eastern end of the Kenai Peninsula. West of that, more steeply dipping Pacific plate provides the best fit to the data. Significant variations in coupling exist along the interface. Strong coupling is predicted beneath Prince William Sound and Kodiak Island while the interface under interior Alaska, the Kenai Peninsula, and the Shumigan Islands is partially coupled. The interface is freely slipping in the Sanak region.

Areas of high slip during the 1964 earthquake beneath Prince William Sound and Kodiak Island correspond to areas of strong interseismic coupling in our model, suggesting that persistent asperities may exist. While the 1964 event ruptured across both the Yakutat and Pacific interfaces, the transition between the slabs below the eastern Kenai Peninsula may influence some earthquakes and transient slip. Slow-slip events in 1998-2001 and 2009-2013 occurred along the Yakutat interface just east of the transition while a 2010-2011 slow-slip event took place to the west of the transition on the Pacific interface. Two recent intraslab events also bracketed the transitional area: the 2016 M7.1 Iniskin event occurred in the Pacific slab while the 2018 M7.1 Anchorage earthquake began in the Yakutat flat slab and may have ruptured into the interface.