Forearc Morphotectonics and Megathrust Earthquakes Along the Middle America Convergent Margin, Nicoya Peninsula, Costa Rica

Abstract:

The Nicoya Peninsula, Costa Rica forms a prominent morphologic high along the Middle America forearc where the Cocos plate subducts beneath the Caribbean plate at 8.5 cm/yr. This emergent coastal landmass lies directly above the megathrust along a seismogenic zone that produces frequent major earthquakes. Along the Nicoya coast, Quaternary marine and fluvial terraces record net uplift in a pattern that shadows the peninsula's overall topographic form. Terrace mapping, surveying, and age dating (14C, OSL, TCN) reveal uplift variations that coincide with three contrasting domains of subducting seafloor (EPR, CNS-1, CNS-2). Uplift rates vary between 0.1-0.2 m/ky inboard of older EPR crust in the north; 0.2-0.5 m/ky inboard of younger CNS-1 crust along the central coast; and 1.5-2.5 m/ky inboard of CNS-2 seamounts impacting the peninsula’s southern tip. GIS digital terrain analysis reveals a deformation pattern consistent with field geomorphic and geologic observations. The two largest Nicoya earthquakes in the past century (1950 Mw7.8; 2012 Mw7.6) each generated decimeter-scale coseismic uplift along the central coast. The 2012 uplift pattern coincides with the area of pre-event locking, mainshock slip, prior 1950 rupture, and 1950 coseismic uplift. Most of the 1950 uplift was recovered by gradual interseismic subsidence during six decades of strain accumulation leading to the 2012 rupture. Paleoseismic sediment coring in Nicoya coastal wetlands reveals fragmentary stratigraphic evidence consistent with earlier Holocene earthquake induced changes in land level. While elastic strain accumulation and release produce short-term cycles of uplift and subsidence, long-term net uplift results in gradual coastal emergence and the growth of topographic relief. Net uplift along the central Nicoya segment may be the product of irrecoverable seismic cycle strain (shortening), coupled with tectonic erosion near the trench and subsequent underplating of eroded material at depth beneath the peninsula. Our results are consistent with geophysical observations that indicate along-strike segmentation of the Nicoya seismogenic zone and the presence of three principal source areas for large earthquakes: 1) Papagayo (1916, M>7.0), 2) Nicoya (1950, Mw7.8; 2012 Mw7.6), and 3) Cobano (1990 Mw7.3).