Neotectonism of the Nicoya Peninsula, Costa Rica: geomorphology and earthquake relocations along the Nicoya Seismic Gap

Includes bibliographical references (pages 73-83)The Pacific margin of Costa Rica forms the leading edge of the Caribbean plate along the southern Middle America Trench, where the Cocos plate subducts northeastward at ~9 cm/yr. Steep subduction of older, smooth Cocos seafloor beneath northern Costa Rica transitions abruptly into shallow subduction of younger, rough, hotspot-thickened crust beneath southern Costa Rica. This transition in seafloor morphology originates at the Cocos-Nazca-Pacific Triple Junction and intersects the trench at the southern end of Costa Rica's Nicoya Peninsula, profoundly influencing upper plate morphotectonics and regional seismicity. South of this transition, the subducting Cocos plate exhibits rugged vertical relief where a series of seamounts along the flank of the aseismic Cocos Ridge collides with the overriding plate resulting in frequent moderate magnitude earthquakes (Mw ??? 6.5). North of the transition, the Nicoya Peninsula forms a broad emergent portion of the northern Costa Rica forearc located <60 km inboard of the trench, overlying a locked segment of the seismogenic zone with a historic record of less frequent major earthquakes (Mw ??? 7.5). The Nicoya Peninsula's trench-parallel coast exhibits abundant late Quaternary emergent strandlines, marine terraces, and incised valley-fill alluvium, attesting to active tectonic uplift. Mapping, surveying, and isotopic dating of these landforms provides important constraints on patterns of vertical tectonism along the Nicoya coast. Correlation of terrace sequences with eustatic sealevel curves reveal differences in uplift rate along the peninsula's coastline corresponding to three contrasting domains of subducting seafloor offshore. The northern Iguanazul Surface exhibits slow uplift rates (0.1-0.2 m/ky) directly inboard of steeply subducted, older and smoother seafloor generated at the East Pacific Rise (EPR). The Carrillo and Camaronal Surfaces along the central coast reveal slightly higher uplift rates (0.2-0.5 m/ky) inboard of younger, more buoyant crust formed at the Cocos-Nazca Spreading Center (CNS-1). Finally, emergent landforms of the Cobano Surface in the south exhibit fast uplift rates (1.5-2.5 m/ky) where seamounts of the Galapagos hotspot (CNS-2) impact the peninsula's southern tip. Radiocarbon ages for uplifted Holocene beachrock horizons yield short-term uplift rates that are consistent with longer-term rates determined for late Pleistocene terraces. Pronounced uplift along the Nicoya coast is the result of close proximity to the Middle America Trench, the source of large subduction earthquakes. The Seismic Strong Motion Array Project (SSMAP) for the Nicoya Peninsula is poised to record earthquakes produced by strain release along the Nicoya seismogenic zone. The network is composed of 11 sites including Geotech A900/A800 accelerographs (three-component), Ref-Teks (three-component velocity), and Kinemetric Episensors. The purpose of the network is to record strong subduction zone earthquakes and moderate to strong upper plate earthquakes. Data from these instruments were used in conjunction with data recorded by the Volcanological and Seismological Observatory of Costa Rica (OVSICORI) to refine locations for four events (Mw > 4.0). Historic seismicity and convergence rates (9 cm/yr) suggest a recurrence interval for large earthquakes estimated at 50 years for the Nicoya Seismic Gap. The last major event (Mw = 7.7) occurred in 1950 and produced >1m of coseismic uplift. This sudden uplift is recovered gradually with interseismic subsidence interpreted as strain accumulation towards the next event. Long-term emergence and uplift of the Nicoya Peninsula suggests that a fraction of coseismic uplift is never recovered and lost to thickening of the overriding plate. The recurrence of large magnitude earthquakes along this stretch of forearc may stem from feedback between thickening of the overriding plate and increased coupling from isostatic loading