RAPID: Nicoya Earthquake After-event Response (NEAR)

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On September 5th, 2012, a moment magnitude (MW) 7.6 earthquake occurred along thelocked segment of the Nicoya Peninsula in Costa Rica. This event ruptured in, or immediatelybelow a region that previously ruptured in similarly sized earthquakes in 1853, 1900, and 1950.Because of the known repeat risk, and the near-ideal local geometry, Nicoya has been the site of numerous studies to evaluate the subduction megathrust interface. Most recently, two projects were funded which allowed for the deployment of seismometers and continuous GPS instruments, and a more extensive GPS campaign to image the interseismic coupling and associated microseismicity.Given the recent event, this RAPID grant is designed to expand the scope of this researchfrom detailed imaging of a mature and mostly locked subduction megathrust, to characterizingearthquake rupture and early afterslip processes, including aftershocks. Because land is directlyover the rupture zone, unlike most every other subduction environment, this new data will giveunprecedented images of the extent and magnitude of seismic and postseismic failure. This data will be compared with published models of locking to evaluate whether geodetic methods can adequately forecast regions that will fail in large earthquakes. It will require detailed on-theground efforts to image the true extent of failure, and to determine whether the upper locked patch is still present, and primed for failure. The project will allow for the capture of critical and time-sensitive ground deformation and early aftershock seismicity that will be necessary to characterize the spatial extent and magnitude of earthquake rupture along the shallow subduction interface. The data, along with pre-event data recorded by our networks there will become a first-of-its-kind dataset for evaluating the transition from a mature and locked subduction interface, to a large earthquake, and early recovery as the seismic cycle begins anew.Using RAPID funds, the researchers will capture coseismic and early post-earthquake grounddeformation and aftershock occurrence. Because these data are highly ephemeral, rapid action is necessary. The large displacements that occur during the earthquake, up to ~1 meter in this case, can change dramatically in the days/weeks/months that follow as poroelastic rocks relax, and momentum-driven mantle continues to drive more slip along the fault. Across the NicoyaPeninsula, we plan to: perform field repairs and collect data from about 20 continuous GPS andseismometers that may have been damaged in the earthquake; perform a GPS campaign of anapproximate 30-site survey network; and deploy a semi-continuous network of GPS stations tocapture post-seismic deformation across the rupture area.The research will fund fieldwork for five student and postdoctoral researchers. The experience will improve their scientific understanding of field-based geophysical research, and give first-hand experience of the impact a large earthquake can have on people and the land. The scientific results from this project will be directly beneficial to society, as it will give someof the first detailed images of coseismic rupture in comparison to previously imaged lockedzones. This information is invaluable for assessing the potential to map faults that are likely tofail in future earthquakes.