VARIATIONS IN QUATERNARY COASTAL EMERGENCE ALONG THE NICOYA PENINSULA SEISMIC GAP, COSTA RICA

The Nicoya Peninsula, Costa Rica deforms in response to rapid subduction at the Middle America Trench (10 cm/yr). Located 60-70 km inboard of the trench axis, this fore arc peninsula lies directly above the seismogenic zone. Emergent marine terraces along the Nicoya coast record the net pattern of late Quaternary uplift related to the subduction cycle.

The last major Nicoya earthquake (Mw=7.7, 1950) produced widespread damage, liquefaction, landslides, and a local tsunami. This event also generated 0.5-1.0 m of coseismic uplift at the coast. With a large slip deficit since 1950, the Nicoya Peninsula is a high-potential seismic gap. Recent field mapping and GPS surveying of uplifted Quaternary shorelines provide new constraints on net emergence patterns and earthquake repeat times.

On the northern Nicoya Peninsula, between Tamarindo and Nosara, the “Iguanazul surface” includes three separate wave-cut treads with paleo-shorelines at 10-12 m, 18-22 m, and 26-32 m elevation. Correlations with Pleistocene sea level high stands at 80-330 ka (OIS 5-9) yield net uplift rates of 0.1-0.3 m/k.y. Radiocarbon ages for Holocene beach rock horizons are consistent with recent net uplift at <0.5 m/k.y.

In contrast, the “Cobano surface” at Cabo Blanco in the south includes at least four distinct Pleistocene terraces at 30-220 m elevation. Sea level correlations suggest net uplift at 1.0-2.0 m/k.y. Radiocarbon ages from an adjacent Holocene terrace (Cabuya surface) indicate recent emergence at 1.5-3.5 m/k.y.

The order-of-magnitude contrast in Quaternary uplift rates between the northern and southern Nicoya Peninsula may be linked to sharp changes in subducting plate roughness, thickness, and dip. While the north coast overrides the Cocos plate "smooth domain", the south coast overrides seamounts of the "rough domain". These distinct coastal segments may deform separately in response to rupture of discrete seismogenic zone asperities.

Based on historical seismicity, estimated slip per event, and a rapid convergence rate, the recurrence interval for large Nicoya earthquakes is 50-60 years. While these frequent events may produce meter-scale coseismic uplift along the Nicoya coast, a large fraction is likely recovered during interseismic subsidence. The net result is gradual Quaternary uplift at the observed rates.