Temporal and Spatial Variations of Earthquake Source Parameters within the 2012 Nicoya, Costa Rica Mw=7.6 Earthquake Rupture Zone

Abstract:

Subduction zone megathrust faults produce the majority of seismic activity, as well as host a wide range of slip processes, including slow slip events and non-volcanic tremor. In a few regions, such as along the Middle America subduction zone where the Cocos Plate subducts beneath the Caribbean Plate, we are fortunate to have significant long-term seismic and geodetic networks situated above the seismogenic zone. This allows for areas of moment release and geodetic locking to be defined before, during, and after large magnitude earthquakes. These types of long-term observations allow us to explore possible spatial and temporal relationships between slip behavior and geodetic coupling. Here we use the local seismic network data to compute earthquake stress drops using S-wave coda recorded over a period of ~13 years, including for a large set of aftershocks of the 2012 M_w 7.6 earthquake along central Nicoya Peninsula. We use over 1000 earthquakes to test for possible changes in microearthquake behavior after the 2012 Nicoya earthquake, as well as spatial variations that might be linked to variations in mainshock slip and geodetic coupling. Preliminary results, focusing on underthrusting events, suggest spatial stress drop variations that correspond to areas that had significant slip during the mainshock. We also see differences of roughly a factor of 2 in stress drops for aftershocks within the high slip area relative to earthquakes that occurred in that same region within the previous 13 years, suggesting temporal variations as well. These observations have interesting implications for the nature of strong and weakly coupled fault zones, as well as how temporal changes in fault zones may manifest in earthquake behavior through the earthquake cycle.