MEGATHRUST EARTHQUAKES AND MORPHOTECTONIC EVOLUTION OF THE NICOYA PENINSULA FOREARC, COSTA RICA, CENTRAL AMERICA

The Nicoya Peninsula, Costa Rica forms a prominent morphologic high along the Middle America forearc. This emergent coastal landmass lies directly above the megathrust, along a seismogenic zone that produces frequent major (M≥7.5) earthquakes with a ~50-year repeat time. Along the Nicoya coast, Quaternary marine and fluvial terraces record net uplift in a pattern that shadows the peninsula's overall topographic form and morphotectonic structure. 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. These results are consistent with geophysical observations that indicate along-strike segmentation of the Nicoya seismogenic zone. Variable upper-plate uplift along the Nicoya margin may reflect differences in subducting-plate roughness, thermal structure, fluid flow, and seismogenic-zone geometry.

The most recent Nicoya earthquakes (1950 M_s7.7 & 2012 M_w7.6) generated decimeter-scale coseismic uplift along the central Nicoya coast. The 2012 uplift pattern (Marshall et al., 2013) coincides with the area of pre-event locking (Feng et al., 2012), mainshock slip (Protti et al., 2014), prior 1950 rupture (Guendel, 1986), and 1950 coseismic uplift (Marshall & Anderson, 1995). Most of the 1950 uplift was recovered by gradual interseismic subsidence during six decades of strain accumulation leading to the 2012 rupture. 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 Nicoya forearc 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. Long-term persistence of the Nicoya seismogenic zone may result from feedback between subduction generated upper-plate thickening and increased coupling along the plate interface due to isostatic loading.