[S-CG45] Science of slow-to-fast earthquakes

[SCG45-19] Multiple controls on megathrust properties and behavior along the Alaska-Aleutian subduction zone

★Invited Papers

*Donna J Shillington¹, Valeria Cortés-Rivas¹, Anne Bécel², Mladen Nedimović³, Daniel Lizarralde⁴, Hannah Mark⁴, Joshua Burstein¹, Jacob Clarke^1,5 (1.Northern Arizona University, Flagstaff, AZ, USA, 2.Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA, 3.Dalhousie University, Halifax, NS, Canada, 4.Woods Hole Oceanographic Institution, Woods Hole, MA, USA, 5.Southern Methodist University, Dallas, TX, USA)

Keywords:subduction zone, earthquakes, active-source seismic imaging

The Alaska-Aleutian subduction zone exhibits profound variations in earthquake history and coupling, making it an ideal place to evaluate controls on megathrust properties and behavior. Here we synthesize constraints from active-source seismic imaging and other studies in the continental subduction zone off the Alaska Peninsula and in the oceanic subduction zone in the Andreanof segment to evaluate the contributions of the incoming and overriding plates to this variability.

Off the Alaska Peninsula, coupling decreases to the west, with high coupling in the Kodiak and eastern Semidi segments and reduced coupling in the western Semidi and Shumagin segments. The Semidi segment regularly ruptures in great earthquakes, unlike the Shumagin segment. All recent large earthquakes occurred at depths of 20-40 km. Both the overriding and subducting plates appear to influence these patterns of locking and slip. A westward decrease in sediment thickness and increase in bending faulting and hydration of the incoming plate are interpreted to result in a more heterogeneous megathrust in the Shumagin segment, which could contribute to higher levels of seismicity and lower coupling. The overriding plate is built from a series of accretionary complexes that increase in age and metamorphic grade away from the trench. Wide-angle seismic data show landward increases in P-wave velocity and rigidity between complexes, which may influence downdip changes megathrust properties and slip behavior.

In the Andreanof segment of the Aleutian subduction zone, the region off Adak has higher coupling and experiences more frequent large earthquakes than off Atka. The incoming plate in both regions includes bend faulting, thin pelagic sediments and terrigenous trench fill; thus, changes in subduction zone inputs cannot account for variations in megathrust behavior. However, we observe large changes in forearc deformation, with more compression and faulting of the Adak forearc than the Atka forearc. Increased forearc faulting off Adak appears to allow more dewatering of the megathrust than in the Atka section; more retained fluids on the Atka megathrust could contribute to lower coupling and less upper plate strain.

These regions showcase the suite of factors that control subduction zone plate boundary slip behavior; no single variable can explain everything.