Evaluating the Influence of Short-term Cryospheric Fluctuations on the Earthquake Cycle in Southern, Coastal Alaska

Abstract:

In southern coastal Alaska, a strong seasonal cycle of snow accumulation and melt is superimposed on a high rate of interannual glacier mass wastage. These mass changes occur directly atop the southern Alaska plate boundary zone of primarily upper-crustal thrusting in the St. Elias region and subduction in the eastern Alaska-Aleutian zone. We calculated the annual 3D stresses and Coulomb stress changes associated with tectonic loading to place the stresses from seasonal and annual cryospheric fluctuations in context. We used the Pylith 3-D finite element model (FEM) of a subduction zone that consisted of a slab and a subduction interface of which the shallow portion (<40 km) was locked and the deeper portion creeps at the rate of Pacific-North American plate convergence (Aagaard et al., 2017). For the St. Elias region, we used updated and relocated seismicity data to evaluate the seasonal modulation of seismic energy release, tested for a cryospheric influence on hypocenter depth, and used well-constrained focal mechanisms to estimate stress orientations as a function of time. The eastern Alaska-Aleutian subduction zone features modest interannual (~9 cm/yr w.e.) and seasonal (22 cm w.e.) cryospheric changes but also ruptures in great earthquakes including the 1964 M=9.2 Alaska earthquake, as well as multiannual slow-slip events (e.g., Li & Freymueller, 2018). In this region we calculated the stress changes associated with the Harding Icefield mass changes and explored the influence on the initiation of slow-slip events in Kenai Peninsula.