Topographic Regulators of the Seismic Cycle along the Subduction Megathrust

Abstract:

Although the subduction megathrust is generally represented as a flat dipping plane it is well known that substantial geometric heterogeneities exist composing the “topography” of the plate interface. These features may the subducted structure of prior seafloor bathymetry, or created during subduction due to heterogeneous stress, variances in material behavior, interactions with the overriding plate, or changes in megathrust orientation relative to convergence. Such geometric complexities along the seismogenic component of the subduction interface play the role of mechanical regulators during the interseismic, coseismic and postseismic phase producing a combined geometry-lithology effect. Here we examine the Nicoya peninsula of Costa Rica which has a rich dataset of GPS observation that includes 15 years of late interseismic coupling, strain release for the 2012 Nicoya moment magnitude 7.6 event, and 2 years of afterslip and postseismic recovery. Based on a newly developed 3D regional interface Finite Element (FE) model, generated from hypocenters of microseismicity recorded from regional seismic networks we : 1) identify the role of interface topography in modulating long-term interseismic locking; 2) quantify the effect of topographic obstacles; 3) define the extent that mechanical regulators determine the degree of interface coupling; 4) develop meaningful comparisons between interseismic locking and coseismic slip models for the 2012 Nicoya earthquake; and 5) we discuss what role these geometric features may have as the interface returns reenters the interseismic phase of the seismic cycle.