LINKS BETWEEN SUBDUCTION MEGATHRUST EARTHQUAKES AND UPPER-PLATE BRITTLE DEFORMATION IN THE PACIFIC NORTHWEST

The link between brittle deformation in the upper plate and subduction megathrust faulting is poorly constrained. This process was highlighted by the November 13^th, 2016 M7.8 earthquake in the North Canterbury region of South Island, New Zealand. In that event, the largest moment release occurred as slip on the megathrust associated with oblique subduction of the Pacific plate beneath the Australian plate. Contemporaneous with the slip on the subduction megathrust were significant fault displacements (brittle deformation) in the overriding continental plate; both the Kekerengu and Papatea faults had strike-slip displacements of up to 10 m. A similar tectonic situation may exist in Cascadia. Unlike in the New Zealand case, where upper crustal faulting was concentrated in sparsely populated regions, several major upper-plate crustal faults in Cascadia lie in the vicinity of densely populated metropolitan centers, such as Portland and Seattle.

In this study, we are investigating mechanisms that can link subduction megathrust faulting to upper plate brittle deformation (as in the M7.8 event in New Zealand). The insight from this analysis may help better constrain whether an equivalent event may potentially occur in Cascadia. We explore how loading of the upper plate changes through the earthquake cycle, and what faults may be appropriately oriented to be triggered as a result of slip of the subduction megathrust. Key parameters controlling upper-plate fault behavior include the orientation of upper crustal faults, the depth to the subduction interface, and the nature of slip partitioning in an obliquely convergent subduction system. We resolve the stresses (and stress changes) on the upper-plate crustal faults at different points during the earthquake cycle, in order to determine when, if and how these stresses are likely to promote slip. Results will be applicable beyond Cascadia, in that this mode of linked faulting may occur at subduction megathrusts globally.