Analysis of small earthquake source parameters along the Nicoya Peninsula: Probing changes following the 2012 Mw=7.6 earthquake and within slow slip and tremor zones

The subduction zone along the western Costa Rica margin has been host to a wide range of slip behaviors, covering the range from typical earthquakes to slow slip events (SSE) and non-volcanic tremor (NVT). Because of the unique geometry of the Nicoya Peninsula extending close to the Middle America Trench, the area has been substantially instrumented with seismic and geodetic networks to capture these various slip events. Several distinct tectonic features affect the region as well, such as seamount subduction, tectonic erosion, and along-strike variations in plate origin and temperature. These features can also impact the seismic behavior of the megathrust, thus comparisons between the slip behaviors and these tectonic variations can be an important step towards understanding the generation of slip over different timescales. Here we focus on comparing source characteristics of small earthquakes recorded on the local networks with spatial patterns observed in the slow slip and tremor regions as well as with the tectonic changes. We determine source parameters using source spectra of the S-wave coda, from which we estimate the seismic moment, corner frequency, and apparent stress. Our dataset focuses on aftershocks from the 5 September 2012 Mw=7.6 megathrust event, the largest event in the area since 1950. This event had a concentrated zone of high slip at depths of ~15-20 km in an area of strong geodetic coupling, adjacent to previously detected slow slip zones. We focus on events that occurred in high slip area of the 2012 mainshock, updip of the mainshock slip at the transition to the 2008 SSE, and deeper along the megathrust in the area of the 2007 SSE. These event groups are important to help understand the nature of the transition along the fault from seismic to aseismic behavior. We also examine events in the region of seamount subduction and in the central Nicoya region where the oceanic crust transitions from East Pacific Rise (EPR) origin to Cocos Nazca Spreading Center (CNSC) origin, as these are the most significant along-strike tectonic transitions. Our previous results for apparent stress from small magnitude earthquakes recorded in the area from 1999-2001 suggested higher apparent stress for events along the smoother EPR-origin crust in the north relative to the rougher CNSC-origin crust in the south. Finally we examine the possibility of temporal changes in earthquake behavior before and after a large mainshock using the 2012 aftershocks and the 1999-2001 dataset.