Large and primarily updip afterslip following the 2012 Mw 7.6 Nicoya, Costa Rica, earthquake

We present detailed surface measurements of the first 3.5 years of postseismic deformation following the 5 September 2012 moment magnitude (Mw) 7.6 Nicoya, Costa Rica earthquake. The dominant signal in the first 2.5 years is uniform horizontal trenchward motion totaling 7-26 cm across 40 stations. Trenchward velocity is strongly diminished by mid 2014 and appears by 2016 to have begun reversing. We invert the first 2.5 years to determine the corresponding afterslip on a detailed 3D interface. Results show significant afterslip both up and down dip of the main coseismic rupture zone, with as much as 1.7 m of offset in two patches at 15-20 km depth and immediately up-dip of the maximum coseismic slip. This updip slip represents an important mechanism to address unrelieved interseismic locking, although sufficient strain energy remains to generate up to a Mw 7.1 event near the coastline. The afterslip patches are anticorrelated with strongly-clustered aftershocks at the same depth, which is indicative of varying frictional behavior along strike. An additional patch of slip is colocated with reoccurring slow slip events beneath the Gulf of Nicoya. The magnitude of the observed slip, however, cannot be sufficiently explained by the known slow-slip events. Ongoing measurements will be crucial to understanding the relocking process in Nicoya