NEOTECTONICS OF THE NICOYA PENINSULA, COSTA RICA

The Pacific margin of Costa Rica sits atop the leading edge of the Middle America Trench where steep subduction of old, dense crust in the northern region transitions to shallow subduction of hotspot thickened crust and ridge collision in the south. This transition, initiated by an acute bathymetric change in the incoming Cocos Plate, profoundly influences upper plate vertical tectonism and regional seismicity. The Nicoya Peninsula represents an emergent portion of the outer forearc that lies 50 km inboard of the trench. It sits above a locked section of the seismogenic zone responsible for past earthquakes in excess of Mw ~ 7.5 with the last event in 1950 and an estimated recurrence interval of 50 years. Immediately south of the peninsula, the subducting Cocos Plate exhibits extreme changes in vertical relief as a series of seamounts and aseismic ridges collide with the overriding Caribbean Plate resulting in a greater earthquake frequency. The peninsula’s trench-parallel coast exhibits multiple suites of uplifted late Quaternary marine terraces revealing varying rates of vertical tectonism in response to differing subduction styles. The Nicoya Peninsula is an ideal location to study the Caribbean-Cocos Plate interaction through the harvest of seismic data and examination of uplifted geomorphic surfaces. Marine and fluvial terrace correlation from the northern, central, and southern coastlines of the peninsula reveal uplift rates an order of magnitude greater in the southern region (1.0-2.0 m/ky) than along the northern pacific beaches (0.1-0.5 m/ky). The uplift rates correspond to the differences in character of the incoming plate. Radiocarbon ages of uplifted beachrock horizons confirm uplift rates for several locations. In addition, the seismic strong motion array project (SSMAP) for the Nicoya Peninsula is composed of 10 - 13 sites including Geotech A900/A800 accelerographs (three-component), Ref-Teks (three-component velocity), and Kinemetric Episensors. This network records strong subduction zone earthquakes and moderate to strong upper plate earthquakes. Data from these instruments were used in conjunction with data recorded by the Earthquake and Volcano Observatory (OVSICORI) to refine locations for several large magnitude (Mw>4.0) events.