Thermal state of the Explorer segment of the Cascadia subduction zone: Implications for seismic and tsunami hazards

The Explorer segment of northernmost Cascadia is an end-member “warm” subduction zone with very young incoming plate and slow-convergence rate. Understanding the megathrust earthquake potential of this type of subduction zone is of both geodynamic and societal importance. Available geodetic observations indicate that the subduction megathrust of the Explorer segment is currently locked to some degree, but the downdip extent of the fault area that is potentially seismogenic is not known. Here we construct finite-element models to estimate the thermally allowed megathrust seismogenic zone, using available knowledge of regional plate kinematics, structural data, and heat flow observations as constraints. Despite ambiguities in plate interface geometry constrained by hypocenter locations of low-frequency earthquakes beneath Vancouver Island, the thermal models suggest a potential rupture zone of ∼60 km downdip width located fully offshore. Using dislocation modeling, we further illustrate that a rupture zone of this size, even with a conservative assumption of ∼100 km strike length, can cause significant tsunami-genic deformation. Future seismic and tsunami hazard assessment in northern Cascadia must take the Explorer segment into account.