SEISMIC CYCLE DEFORMATION AND NET COASTAL UPLIFT AT SAN JUANILLO, NICOYA PENINSULA, COSTA RICA

The Nicoya Peninsula, Costa Rica lies ~60 km inboard of the Middle America Trench where the Cocos Plate subducts under the Caribbean Plate at ~9 cm/yr. This outer fore arc peninsula overlies the seismogenic zone along a mature seismic gap, with an estimated recurrence interval of ~50 yr for large megathrust earthquakes. The last major event (M 7.7, 1950) resulted in 1.0-1.5 m of coseismic uplift along the peninsula’s central coast. Since then, gradual coastal subsidence has been observed as the plate interface accumulates interseismic strain. While elastic seismic-cycle deformation produces short-term shoreline fluctuations on the Nicoya Peninsula, net tectonic uplift throughout the Quaternary results in long-term coastal emergence and the formation of marine terraces.

At San Juanillo, north of Punta Guiones, emergent carbonate beachrock horizons mark former Holocene shorelines with calibrated radiocarbon ages of 0.9-1.8 ka. The position of these deposits on the upper beach face is consistent with net uplift at ~0.5 m/k.y. Tidal erosion and the undermining of beachrock and adjacent cliffs may reflect coastal inundation associated with the current phase of elastic interseismic subsidence.

Inland of the beach, two uplifted marine terraces mark former late Quaternary shorelines with inner edge elevations of ~30 m and ~55 m above mean sea level. The ages of these surfaces are estimated at 80 ka and 125 ka respectively, based on correlation with OSL dated terraces ~80 km to the south and the timing of late Pleistocene sea level high stands (Oxygen Isotope Stages 5a-5e). Long-term net uplift at San Juanillo therefore occurs at a rate of 0.3-0.5 m/k.y. This is the highest uplift rate recorded among seven marine terrace study sites along the Nicoya coast, with the exception of the peninsula’s southern tip (Cabo Blanco) where accelerated uplift occurs in response to seamount subduction.

Since the 1950 Nicoya earthquake, Costa Rica’s population and tourism industry have grown substantially. It is therefore critical to develop a better understanding of the megathrust earthquake hazard through both geophysical studies of the seismogenic zone and geomorphic/geologic investigations of fore arc deformation patterns.