LAUSR

Abstract How does deformation after an earthquake affect megathrust stresses? Five years of surface velocities following the 2012 moment magnitude 7.6 Nicoya, Costa Rica earthquake uniquely capture the lithospheric recovery. During a four-year period, seaward afterslip transitions to relocked, landward interseismic motion that matches the velocity field seen before the 2012 event. Locking reinitiates temporarily but is interrupted by late 2014 and is followed soon by a period of never-before-described, exclusively trench-parallel motion associated with a slow slip event and no resolvable megathrust locking. We present a conceptual model in which low postseismic megathrust coupling (little locking) generates partitioned slip: trench-normal motion on the megathrust during afterslip and trench-parallel motion during this never before seen transient. High coupling (strong locking) during the interseismic period drives oblique, convergent surface motions. This challenges the paradigm that megathrusts are either always partitioned or always oblique, contradicts the tectonic escape hypothesis in Central American, and introduces a new time-dependent megathrust fault behavior. Given that most subduction zones are oblique, explaining these observations is critical to characterizing stress accumulation.