Recovering the Full Afterslip Following the 2012 Mw 7.6 Nicoya, Costa Rica Earthquake

The nearfield deformation before, during and after major megathrust events, though they generate destructive earthquake shaking and tsunami waves, has proven difficult to observe due to the prohibitive cost of sea-floor geodesy. The Nicoya Peninsula in Costa Rica, however, is ideal for study as the seismogenic zone sits directly below the peninsula allowing for dense proximal instrumentation (18 continuous and 22 campaign GPS sites). Furthermore, rapid convergence of the Cocos and Caribbean plates results in M≥7 earthquakes approximately every 50-60 years, including the 2012 Mw 7.6 Nicoya event. By combining all available continuous GPS data with 3 postseismic GPS campaigns, we captured postseismic surface deformation on and around the Nicoya Peninsula. The main signal is nearly fault-normal trenchward motion of between 6 and 24 cm amongst 40 stations in the 3.5 years following the earthquake. By mid-2014 this signal diminishes, and by 2016 appears to have reversed. We invert the first 2.5 years to determine corresponding slip on a 3D interface that includes detailed microseismic structure [Kyriakopoulos et al., 2015]. Results show significant and well-resolved up- and downdip afterslip that terminates at the periphery of the 2012 coseismic rupture zone. The updip portion (up to 1.7 m) corresponds to about 1/3 the maximum coseismic slip and may be an important mechanism to address unrelieved interseismic locking [Feng et al., 2012]. Updip slip is concentrated in two patches at 15-25 km depth and correlates well with repeating aftershocks, which represent an independent measurement of continued slip activity in the updip afterslip region. Curiously, the downdip afterslip zone is devoid of repeating aftershocks, warranting further study of the relation between these supposedly linked phenomena [i.e. Uchida & Matsuzawa, 2013]. In coming years we expect to record accelerating landward motion as the subduction zone relocks. We intend to use these measurements to differentiate afterslip from the time-dependent, postseismic, viscous response of the mantle. Understanding nuanced postseismic behavior of the Nicoya peninsula may aid interpretation of more fragmented observations in areas with less dense instrumentation.