Ten Years of Slow Slip Events Observed by cGPS Network in the Nicoya Peninsula, Costa Rica

Abstract:

The Nicoya Peninsula in northern Costa Rica overrides the subduction zone where the Cocos plate subducts northeast beneath the western edge of the Caribbean plate along Middle American Trench. Historically, large subduction zone earthquakes (M > 7) strike this area approximately every 50 years [Protti et al., 1994]. A magnitude Mw 7.6 thrust earthquake occurred in September 2012 with strong aftershocks lasting for several months, following the historic pattern. The released moment was close to the accumulation estimate based on a simple recurrence model and GPS observations during 1996-2010 [Feng et al., 2012]. A continuous Global Positioning System (cGPS) network has operated in the Nicoya peninsula of northern Costa Rica since 2002. The network observes surface displacements above the subduction zone between Cocos and Caribbean plate, and has detected strain accumulation and Slow Slip Events (SSEs) along part of the plate boundary. We invert observed surface displacements induced by SSEs during 2002-2012 to estimate slip distribution on the plate interface, and assess their impact on the strain accumulation process. The maximum slips are distributed in areas lacking significant interseismic coupling. Three major slow slip patches are identified, with different slip magnitude and recurrence intervals. Both shallow and deep slip patches are observed, with a depth range of 10-50 km. Geodetic modeling shows that over ten years’ observation period, frequently occurring SSEs in northwestern Nicoya released 20% of accumulated elastic strain. Less frequent, more deeply occurring SSEs in southeastern Nicoya peninsula release all accumulated strain, downdip of the 1990 earthquake, assuming current behavior continues. Central Nicoya experienced a fully locked plate interface between 2007 and 2011, with the location and amount of slip roughly equivalent to that release in the September 2012 thrust earthquake, assuming constant behavior over the last 50 years. We observe an apparent relation between micro seismicity and SSEs, and, the aftershock area of the 1990 earthquake coincides with our deep SSE location.