Eos, Transactions, American Geophysical Union, v. 85(47), Fall Meeting Supplement, Abs. T13C-1393, 2004.

 

 

Coastal Deformation Patterns Along the Nicoya Seismic Gap, Pacific Coast, Costa Rica, Central America

 

Jeffrey S. Marshall,1, Lauren K. Annis1, Fookgiin Khaw,1, Julie G. Parra1, and Marino Protti2

1Geological Sciences Department, Cal Poly Pomona University, Pomona CA 91768

2OVSICORI-UNA, Heredia 3000, Costa Rica

 

The Nicoya Peninsula, Costa Rica provides a unique setting for the study of upper plate deformation along the Middle America convergent margin. Located 60-70 km inboard of the trench axis, this outer fore arc peninsula sits directly above the seismogenic zone. A sequence of emergent marine terraces along the Nicoya coast records the net pattern of late Quaternary uplift associated with the subduction cycle. The last major earthquake centered beneath the peninsula (Mw=7.7, 1950) produced widespread damage and generated 0.5-1.0 m of coseismic uplift along the peninsula's coast. With a large slip deficit since 1950, the Nicoya Peninsula is recognized as a high-potential seismic gap. Ongoing field study of uplifted Quaternary shorelines provides an excellent opportunity to evaluate local deformation patterns and large earthquake repeat times. Recent mapping of late Pleistocene marine terraces along the peninsula’s northern coast allows for comparison with those previously mapped along the peninsula’s southern tip. The “Iguanazul surface” on the northern coast between Tamarindo and Nosara encompasses 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 (OIS 5-9) indicate net uplift rates of 0.1 - 0.3 m/k.y. Holocene beach rock horizons along the active beach yield calibrated radiocarbon ages of 1610 and 760 ybp (Playa Negra and Playa Lagarto), consistent with net Holocene uplift at < 0.5 m/k.y. In contrast, the “Cobano surface” at the peninsula's southern tip (Cabo Blanco) includes at least five distinct Pleistocene terrace treads separated by well-defined risers at 30-220 m elevation. Holocene uplift rates here range from <1.0 - 6.0 m/k.y. along an emergent Holocene terrace (Cabuya surface). 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. While the northern peninsula overrides the subducting Cocos plate’s "smooth domain", the peninsula’s southern tip overrides seamounts of the "rough domain".  These contrasting coastal segments may deform independently in response to rupture of different subduction zone asperities (Central Nicoya and Cabo Blanco). Based on historical seismicity data, estimated slip per event, and a rapid convergence rate (9 cm/yr), the recurrence interval for large Nicoya earthquakes is estimated at 50-100 years. While these frequent events may produce meter-scale coseismic uplift along the Nicoya coast, a significant fraction of this is likely recovered during interseismic subsidence. The net result is gradual coastal uplift during the Quaternary at rates ranging from 0.1 to 6.0 m/k.y.