Some stretches of a subduction zone lock up and build toward great earthquakes; neighboring stretches creep along quietly. Why? SES PhD student Valeria Cortés-Rivas went looking for the answer in the Andreanof segment of the Aleutian Subduction Zone offshore Alaska, and published what she found in Geology with SES professor Donna Shillington and co-authors at Woods Hole Oceanographic Institution.
The Andreanof segment makes a clean natural experiment: it’s a geologically young, relatively simple subduction zone where the forearc above the strongly coupled Adak region shows more uplift and compression than the weakly coupled Atka region next door. Using marine multichannel seismic reflection data, the team first ruled out the usual suspect: the incoming Pacific plate looks essentially the same along the whole segment, so its properties can’t explain why one region sticks and the other slips.
The difference, instead, appears to lie in fluids. Variations in the thickness of the methane hydrate stability zone point to more heat advection, and more dewatering, in the locked Adak region, where a better-developed fault network above the megathrust can drain fluids away. Lower pore pressure on the fault promotes coupling; coupling drives the seismic stress cycling that keeps those drainage faults active. The result is a feedback loop between deformation and locking that, the authors suggest, may be at work but hidden in older, more complicated subduction zones around the world.
The work is part of Cortés-Rivas’s dissertation research on the structure and behavior of subduction margins.
Reported by SES Research Highlights for the School of Earth and Sustainability.