UPPER PLATE DEFORMATION ABOVE AN UNCOUPLED MEGATHRUST: INVESTIGATING THE ROLE OF SPATIALLY VARYING COUPLING IN PRODUCING PATTERNS OF SUBDUCTION ZONE STRAIN

Recent analyses of global subduction zones indicate that many of these plate boundaries have spatially varying inter-seismic coupling, with areas of high coupling (“asperities”) separated by regions of low coupling. This coupling variability has been interpreted to affect the locations and geometries of seismically active faults within both the upper and lower plates in the Japan, Chile, and the Aleutian subduction zones. To better understand how plate boundary coupling affects upper plate deformation, we use a generalized 3D subduction zone model incorporating plausible asperity distributions, with plate motions, megathrust slip, and intraplate stresses in balance. The stress field in the upper plate associated with locked asperities on the plate interface is broadly compressive. However, the transition from an asperity edge to a zone of lower coupling produces a significant shear stress parallel to the plate motion. This shear stress is the dominant local contributor to the stress field, with an associated substantial rotation of the principal stress axes favoring slip on strike-slip faults oriented parallel to the plate motion. An example of slip on such a fault was the October 2020 Shumagin Islands intraslab earthquake, which occurred at a coupling transition in the Aleutian subduction zone. As the along-trench length of the uncoupled area increases, the footprint of shearing extends farther into the upper plate. We apply these lessons in the Cascadia subduction zone, using a newly developed inter-seismic inversion approach that allows us to locate locked asperities. The input data are GPS velocities with the effects of plate interface coupling isolated from other tectonic processes. Although GPS observations in Cascadia have been interpreted to indicate a high degree of plate coupling along most of the megathrust, our inversion indicates that less than 50% of the areal extent of the plate interface needs to be locked to explain the observations. Resolution of the exact locations of these asperities is limited; nevertheless, the low coupling area suggests that there should be numerous coupled-uncoupled transitions at the edges of asperities. The associated perturbations to the subduction zone stress field may favor the development of upper plate faults oriented at a high angle to the trench.