Investigating Apparent Anticorrelation of Repeating Aftershocks and Afterslip in Nicoya, Costa Rica

The majority of the seismogenic portion of a megathrust is located underwater, which makes instrumentation expensive and logistically challenging. The Nicoya Peninsula in Costa Rica is ideally situated for studying the subduction interface as it extends to within 60 km of the trench, allowing for collection of both seismic and geodetic data directly above the rupture zone. This study utilizes the rare subduction geometry to illuminate the relationship between afterslip and repeating aftershocks following the 2012 magnitude 7.6 Nicoya, Costa Rica earthquake.

Analyst-picked aftershocks from the several months following the earthquake are used as templates in a waveform match filter technique (template matching). Detections with very high cross correlation are termed repeating aftershocks. These are thought to result from patches of slip-weakening material (asperities) within an otherwise stably-sliding region. Using magnitude-slip relations, the amount of slip on these asperities can be used as a proxy for the amount of afterslip occurring adjacent to the patches, resulting in a spatially variable afterslip map without geodetic data. This methodology has been broadly applied in subduction zones such as New Zealand and Japan.

However, repeating seismicity results from the first 3.5 months following the 2012 Nicoya earthquake appear to be spatially anticorrelated with the cumulative distribution of afterslip inverted directly from GPS. Such spatial anticorrelation indicates that slip inferred from repeating aftershocks may not equal to geodetically inverted afterslip. By expanding the template matching search through another full year (2013) we aim to further constrain the temporal evolution of repeaters with respect to afterslip and comment upon the observed applicability of empirical magnitude-slip relations compared to geodetically-determined slip. Implicitly this work also expands the catalog of detected aftershocks by including detections with lower cross correlation coefficients, allowing us to examine detailed spatial and temporal evolution of aftershock seismicity out to 1.3 years following the earthquake.