GPS observations of the Slow Slip Events in northwestern Costa Rica, 2015-2020

Abstract:

Slow Slip Events (SSEs) affect the subduction zone earthquake cycle, releasing a significant amount of accumulated strain and perhaps triggering megathrust earthquakes by stress loading nearby sections of the fault. We analyzed all continuous GPS data collected from 2015 to 2020 to characterize SSEs in the Nicoya Peninsula, the northwestern section of the Costa Rica subduction zone, known to occur close to every 23 months. Data processing was done by the precise point positioning method using GipsyX software, with orbits and satellite clock estimates provided by the Jet Propulsion Laboratory (JPL). We used the resulting GPS time series to identify the SSEs and to calculate their surface displacements. We observed the occurrence of SSEs in 2015, 2017 and 2019. The event in 2015 is defined by deep and shallow slip patches at ~35 and ~10-15 km depth, respectively. The pattern of deep and shallow SSE in 2015 is similar to previous studies. The SSE in 2017 coincided in time with the November 13 M_W 6.5 Esterillos earthquake, southeast of the slipping area, suggesting a possible triggering relation between the two events. We model the 2017 SSE in two stages, with coseismic slip followed by accelerated slow slip. We apply Coulomb Failure Stress change (ΔCFS) models to evaluate how the earthquake and SSE changed stresses along the subduction zone following the earthquake. ΔCFS due to the earthquake was small at the SSE location, suggesting that the earthquake may have had little influence on SSE timing. Similarly the SSE imposed minimal stress change at the nucleation site of the earthquake. We hope to evaluate possible dynamic triggering effects caused by the earthquake, but at this point cannot preclude the possibility that the timing of the two events was coincidental.