Evaluating strain partitioning in the Pacific Northwest using block models

Abstract:

At the Cascadia subduction zone, the Juan de Fuca plate (JdF) subducts obliquely under, and deforms, the North American plate (NA). Subduction deformation dominates deformation on land, as observed by geodetic measurements such as GNSS, limiting the resolution of surface faults on the overriding North American plate. The goal of this project is to better understand how oblique JdF-NA convergence is partitioned between surface faults in the Pacific Northwest region and the subduction zone using block models. Specifically, we apply an 1 regularization known as Total Variation Regularization (TVR) to algorithmically select the most active faults from a dense block model. We constrain the subduction zone to accumulate only dip-slip motion and ask the question: “how many surface faults are necessary and how do they partition the oblique plate motion away from the subduction zone?” We find a tradeoff between megathrust activity and surface fault activity: A dip-slip-only megathrust requires more active surface faults than a freely slipping (oblique) megathrust. However, these faults do not concentrate as trench-parallel strike-slip faults as would be expected in a subduction zone with a forearc sliver. Furthermore, geologic slip rates appear to be more consistent with estimated fault slip rates in models with a freely slipping subduction zone than in models with a constrained dip-slip subduction zone. In other words, near trench strike-slip faults are not required to explain onshore GNSS observations or geologic slip rates.