Collaborative Research: Near-Field Observations of Preseismic, Coseismic, and Postseismic Slip on the Northern Costa Rica Megathrust

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The plate boundary at convergent margins produces most of the world?s largest earthquakes, threatening local inhabitants and global populations through destructive shaking and tsunami generation. An Mw 7.6 earthquake occurred on 5 September 2012 in Nicoya Peninsula, northern Costa Rica directly beneath a network of seismic and continuous GPS stations. It provides a unique opportunity to study the cycle of a megathrust earthquake in unprecedented detail. This project seeks to better understand the preparation process of megathrust earthquakes by identifying slow preslip and/or patterns in foreshock activity, and evolutions of aftershocks and afterslip induced by the mainshock. Our project has important implications for predictability of large earthquakes, the potential size of the megathrust earthquake, the strong ground motions and associated damage, and tectonic strain accumulation and release during an earthquake cycle. Major research questions to be addressed include whether there is discernible preparation activity before the mainshock, if distinct regions of the plate boundary host coseismic versus slow and after slip, if slip in the mainshock occurs in regions accumulating strain during the interseismic period, whether aftershocks are driven by afterslip or other processes, and what effect mainshock rupture has on seismic velocities and properties of the plate interface. Because we have been measuring surface deformation and seismicity directly above the megathrust for over a decade preceding the 2012 Nicoya earthquake, we can compare the distribution of coseismic slip with the interseismic strain accumulation pattern, slow slip behavior, and precursory seismicity and address all of these questions in a single location. To ascertain whether any accelerating foreshock activity and aftershock expansion/migration occurs, a complete catalog of seismic activity before and after the mainshock is required. We will use all earthquakes in our existing catalog as templates and scan through continuous records to identify additional events easily missed by automated detection algorithms and visual inspection. Event pairs with the highest correlation coefficients will be grouped into repeating earthquake clusters and used to infer slow-slip and afterslip, and track temporal changes of subduction zone interface properties. We will also detect tectonic tremor long before and after the mainshock and use it as a proxy for slow slip and investigate whether tremor/slow slip events occur before major earthquakes. Finally, we will compare high-resolution co-seismic slip with aseismic slip during postseismic and interseismic period as well as microseismicity to understand different frictional properties of the plate interface. Because of the high-quality data collected over the past decade, and the close proximity to the megathrust, our project could lead to significant advances on how subduction zone properties evolve immediate before and after a major megathrust event.