The Role and Spatiotemporal Occurrence of Off-megathrust Earthquakes: Insights from Seismo-Thermo-Mechanical models

Abstract:

Shallow off-megathrust subduction earthquakes are important in terms of hazard assessment and coseismic energy budget. A recent global study shows that between 1976 and 2007 about 55% of the seismic moment released within subduction zones was actually not released at the megathrust interface. Their role and spatiotemporal occurrence, however, remain poorly understood. We for the first time simulate their spontaneous activation and propagation using a newly developed 2D, physically consistent, continuum, visco-elasto-plastic seismo-thermo-mechanical (STM) modeling approach. The characteristics of simulated normal events within the outerrise and splay and normal antithetic events within the wedge resemble seismic and seismological observations in terms of location, geometry, and timing. Their occurrence agrees reasonably well with both long-term analytical predictions based on dynamic Coulomb wedge theory and short-term quasi-static stress changes resulting from the typically triggering megathrust event. The impact of off-megathrust faulting on the megathrust cycle is distinct, as more both shallower and slower megathrust events arise due to occasional off-megathrust triggering and increased updip locking. This also enhances tsunami hazards, which are amplified due to the steeply dipping fault planes of especially outerrise events.