Along-strike variations of earthquake apparent stress at the Nicoya Peninsula, Costa Rica, subduction zone

Abstract

Oceanic plates vary in temperature, topography, and sediment load as they enter subduction zones. These variations persist along the subduction interface causing perturbations in coupling and earthquake rupture processes. We explore the effects of variable subducting plate structure on microseismicity rupture characteristics along the Nicoya Peninsula, Costa Rica. The subducting Cocos Plate has low relief along the northern and central portion of the peninsula, with seamounts present at the southern tip of the peninsula. We compute apparent stresses for 94 ML 2.5–4.2 earthquakes along the plate interface using waveform coda and find along-strike variations that mimic bathymetric variability. Median stress values are higher (3.2 MPa) in the smooth northern region, with lower values in the central (2.1 MPa) and southern (0.7 MPa) segments. Higher apparent stresses along a zone of little suspected subduction topography imply increased coupling or higher friction along the interface. These results agree with geodetic and other seismic studies that suggest variable plate coupling along the Nicoya Peninsula.