COLLEGE OF SCIENCES SCHOOL OF EARTH AND ATMOSPHERIC SCIENCES EAS Ph.D. Defense Tiegan Hobbs March 8, 2019 3:00 PM Earth and Atmospheric Sciences Ford Environmental Science & Technology (ES&T) 311 Ferst Drive, ES&T Atlanta, GA 30332-0340 Web: eas.gatech.edu ES&T L1255 Title: Postseismic Response Following the 2012 Mw 7.6 Nicoya, Costa Rica Earthquake Committee members: Dr. Andrew Newman (Advisor), Dr. Zhigang Peng, Dr. J. David Frost, Dr. Marino Protti, Dr. Timothy Dixon Abstract: Characterization of the surface deformation related to the 2012 moment magnitude (Mw) 7.6 Nicoya earthquake was undertaken using continuous and campaign Global Positioning System (GPS) observations. This location is uniquely situated to monitor megathrust conditions as the peninsula extends to within 60 km of the trench. The entirety of the postseismic and relocking period were considered, totaling 5 years. Seismic observations were also included, to cumulatively elucidate the timing and spatial extent of megathrust behavior. Afterslip with equivalent Mw of 7.5 was shown to exist in patches that were distinct from aftershocks, both of which were most abundant immediately updip of the coseismic rupture patch. This was an important, albeit failed, test of the applicability of using repeating aftershocks as a proxy for slip. As trench-normal afterslip waned, relocking was initiated but shortly interrupted by a period of exclusively trench-parallel motion across the Nicoya forearc. This was a novel observation. Combined with results of a backslip inversion, these findings suggest that slip partitioning may be controlled by megathrust coupling. By 2016 the surface velocities returned to preseismic levels, indicating a return to stable interseismic conditions. These results, and their impact on active tectonics and subduction physics, are discussed.