2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 22
Presentation Time: 8:00 AM-12:00 PM


MARSHALL, Jeffrey S.1, KHAW, Fookgiin1, PARRA, Julie G.1, ANNIS, Lauren K.1 and PROTTI, Marino2, (1)Geological Sciences Department, Cal Poly Univ, Pomona, CA 91768, (2)Ovsicori, Universidad Nacional, Heredia, Costa Rica, marshall@csupomona.edu

The Nicoya Peninsula lies within the outer Costa Rican forearc along the Middle America Trench where near-orthogonal convergence occurs at 9-10 cm/yr between the Cocos and Caribbean plates. On 5 October 1950, the peninsula was severely shaken by a ~M7.7 subduction earthquake. This event produced widespread damage and generated 0.5-1.0 m of coseismic uplift along the peninsula's coast. With a significant slip deficit since the 1950 earthquake, the Nicoya Peninsula is recognized as a high-potential seismic gap. Field study of uplifted Quaternary marine terraces provides an excellent opportunity to evaluate local deformation patterns and large earthquake repeat times.

Along the northern Nicoya coastline, a prominent Quaternary marine terrace (Iguanazul surface) extends from Tamarindo to Nosara. This terrace is a composite surface, consisting of at least three separate wave-cut treads that preserve paleo-shorelines at 10-12 m, 18-22 m, and 26-32 m elevation. Preliminary correlations with late Pleistocene sea level high stands at 80-330 ka (marine oxygen isotope stages 5-9) indicate net uplift rates of 0.1-0.3 m/ka. Holocene beach rock horizons along the upper shore face of the active beach yield calibrated radiocarbon ages of 1610 and 760 ybp (Playa Negra and Playa Lagarto). These ages are consistent with net Holocene uplift at <0.5 m/ka.

In contrast to the Iguanazul surface, the Cobano surface at the peninsula's southern tip (Cabo Blanco) consists of at least five distinct Pleistocene marine terrace treads separated by well-defined risers at 30-220 m elevation. Holocene uplift rates here range from 3.0-6.5 m/ka. While the northern Nicoya Peninsula lies onshore of the Cocos plate "smooth domain", the southern peninsula sits inboard of subducting seamounts of the "rough domain".  The order-of-magnitude difference in Quaternary uplift rates between the northern and southern Nicoya Peninsula may be linked to sharp contrasts in the roughness, thickness, and dip of the subducting Cocos plate offshore.

Plate motion and seismicity data suggest recurrence intervals of <100 years for large Nicoya earthquakes. While these frequent events may produce meter-scale coseismic uplift, a significant fraction of this is likely recovered by interseismic subsidence. The net result is gradual coastal uplift at the observed rates.