Structural asperity focusing locking and earthquake slip along the Nicoya megathrust, Costa Rica

Abstract

On 5 September 2012, a moment magnitude 7.6 earthquake occurred along the locked megathrust interface directly beneath the Nicoya Peninsula in Costa Rica. The event was anticipated given the history of frequent large earthquakes and recent GPS results. Strain accumulation observed before and coseismic slip observed during the earthquake create a novel and important data set, offering a unique look at the strain and locking behavior around a large megathrust environment. Given this geodetic data, and a newly developed regional 3-D subduction interface, we develop a regionally appropriate finite element model to evaluate the role of interface topography in generating interseismic locking and ultimately coseismic slip following the 2012 earthquake. We find that the interface is most strongly coupled in a patch immediately beneath the central portion of Nicoya in an area where a topographic high in the downgoing Cocos Plate is resisting further subduction. This zone is the dominant slip environment in the 2012 earthquake. Reevaluation of the interseismic locking over 62 years before the most recent event finds that the total accumulated moment potential (M0 = 3.48 × 1020 N m) well matches the total moment release in the 2012 rupture found here (M0 = 3.73 × 1020 N m, Mw 7.68), with rupture extending to the SE outside of the strongly locked region, possibly as early afterslip. In contrast to a prior study, little locking is required immediately offshore Nicoya—a conclusion important for understanding the rupture area of the 2012 event, regional tsunami potential, and relation with regionally observed shallow slow-slip events.