DEFORMATION PROCESSES ALONG THE ALASKA-ALEUTIAN CONVERGENT MARGIN

The Alaska-Aleutian convergent margin stretches 3500 km from the collisional St. Elias range in southeast Alaska to the Russian far east. The margin exhibits an exceptional range of properties and behaviors including changes in slab dip, differences in the surface character of the incoming plate, significant variations in interseismic coupling, slip events from great earthquakes to multi-year transients, and contrasts in upper plate motion. Over the past two decades, a wealth of new geophysical and geologic data have provided new insights into the dynamics of the margin.

Based on geodetic estimates, coupling between the slab and upper plate varies widely along the margin and undergoes relatively sharp transitions in both the along-strike and down-dip directions. Some of these divisions appear to correlate with changes in the topography of the downgoing plate. The degree of coupling along the interface does not always correspond to the location of past earthquakes. Areas of the megathrust that experienced high slip during the 1964 M9.2 earthquake do appear to be strongly coupled in the present, suggesting persistent asperities. In contrast, the Sanak region to the east has hosted great earthquakes in the past but appears to be uncoupled now. The 2020 M7.8 Simeonof event ruptured an area that was moderately to weakly coupled. The change in geometry from flat slab to more steeply dipping crust may influence the location of transient slip and earthquakes. All of the geodetically detected slow slip events as well as two recent intraslab events bracket the transition. Over the flat slab, the upper plate moves northwest at a rate of up to 1 cm/yr. This motion extends as far inboard as the Denali fault and may drive deformation across the Alaska Range. To the west, upper plate motion undergoes a westerly rotation until the motion becomes arc parallel along the Alaska Peninsula.

A number of questions and research opportunities remain along the Alaska margin. Due to the lack of offshore geodetic data, it is not clear how the near-trench region slips during the interseismic or coseismic period. Limited data also means that the influence of splay faults on coupling patterns or slip during earthquakes is unclear. Geodetic coupling estimates need to be reconciled with the longer-term geologic record of the earthquake cycle.