Postseismic Slip Inferred from Repeating Earthquakes Following the 2012 Mw 7.6 Nicoya Earthquake in Costa Rica

Abstract:

The Nicoya Peninsula in Costa Rica is an ideal place to study megathrust processes due to its position immediately above the subduction interface. Continuous GPS and seismic recordings over the past decade provided a valuable dataset for better understanding the seismotectonics of Nicoya Peninsula and forecasting the size and location of the Sept. 5, 2012 Moment magnitude (MW) 7.6 Nicoya earthquake. Here we report on repeating earthquakes that rupture the same fault patch and generate nearly identical waveforms. To identify these events we first select event pairs with high mean cross correlation coefficients (CCC > 0.9) from newly detected events based on a waveform matching technique. Next, we group event pairs into clusters, and relocate them using a double-difference location algorithm (HypoDD) retaining only events with centroid locations that overlap by at least 50%. At the time of this submission we’ve found approximately 50 repeating sequences with at least three events during the first 3 months following the 2012 mainshock. The recurrence intervals of these clusters increase with time after the mainshock, consistent with the Omori's aftershock decay law. Despite clear evidence of foreshocks in the 10 days preceding the mainshock, we did not identify any repeating earthquakes during this period. Our next step is to estimate the inferred seismic moment of slip for all repeating events in each cluster, and use them to compare with models of postseismic slip along the megathrust using geodetic data. Our goal is to develop a robust approach to infer aseismic slip on the rupture zone from systematic analysis of repeating earthquakes.