IMPLICATIONS OF AN EVOLVING REGIONAL STRESS FIELD ON FAULTING BEHAVIOR IN SOUTHERN CASCADIA

In southern Cascadia, two distinct and orthogonal tectonic processes deform the overlying crust: (1) subduction of the Juan de Fuca plate beneath North America, and (2) the northward migration of the Mendocino Triple Junction (MTJ) driving deformation via the Mendocino Crustal Conveyor (MCC) process. The cyclic accumulation and release of elastic strain from the Cascadia earthquake cycle occurs over a ~300-500 year period producing little permanent deformation of the upper plate. In contrast, deformation associated with the northward migration of the MTJ develops over million-year timescales and is recorded in the geologic record. The upper plate in southern Cascadia contains multiple active faults that record recent offsets. To understand the character of faulting in southern Cascadia we investigate how these two processes combine to produce an evolution in the upper plate stress field.

We decompose the observed GPS-derived velocity field for Cascadia to determine a subduction coupling model and MCC displacement model that can be used to assess the stress field evolution and its impact on upper plate faults. Because of its million-year duration, we assume that the MCC process can be represented by an approximately oriented static stress field, which is then combined with a stress regime generated by a cyclic subduction earthquake model. We find that the principal stresses in southern Cascadia rotate clockwise with time following a megathrust event as the subduction component increases. Coulomb stress analyses show that these evolving stresses change the likelihood of fault failure and also the most-favorable sense-of-slip on regional faults. Immediately following a large megathrust event, shallowly dipping NW striking faults (similar in geometry to the Little Salmon fault), favor right-lateral or oblique motion however by ~100-200 years following a megathrust event, a fault of this orientation is more likely to slip through reverse dip-slip motion.

Numerous (active) quaternary faults, with a range of orientations and kinematics, exist in southern Cascadia. These results indicate that the seismic potential of these faults may vary through the subduction cycle, which has important implications for assessing the hazard they pose to nearby communities.