Morphotectonic segmentation along the Nicoya Peninsula seismic gap, Costa Rica

The Nicoya Peninsula, Costa Rica forms a prominent morphologic high along the outer forearc of the erosive southern Middle America convergent margin. This emergent coastal landmass overlies the seismogenic zone and occupies a seismic gap that last ruptured in 1950 with a M7.7 megathrust earthquake. The edges of both the Nicoya seismic gap and the peninsula’s abrupt shorelines correspond with the aftershock limits of the more recent 1992 M7.2 Nicaragua tsunami earthquake to the north, and the 1990 M7.0 Cobano seamount rupture to the south. The coincidence of emergent topography and recent earthquake rupture zones suggests persistence of the Nicoya segment through multiple seismic cycles. Along the Nicoya Peninsula's seaward-facing coastline, net Quaternary uplift is recorded by emergent strandlines, marine terraces, and incised valley-fill alluvium. Ongoing field mapping, surveying, and isotopic dating reveal uplift variations along the Nicoya segment that coincide with three contrasting domains of subducting seafloor offshore (EPR, CNS-1, CNS-2). Uplift rates vary between 0.1-0.2 m/k.y. inboard of older EPR crust along the northern Nicoya coast; 0.2-0.3 m/k.y. inboard of younger CNS-1 crust along the southern Nicoya coast; and 1.0-2.0 m/k.y. inboard of CNS-2 seamounts impacting the peninsula’s southern tip. These results are consistent with geophysical observations that indicate finer scale segmentation of the Nicoya seismogenic zone related to along-strike changes in the characteristics of the subducting plate. Variable upper-plate uplift along the Nicoya segment may reflect differences in subducting-plate roughness, thermal structure, fluid flow, and seismogenic-zone locking (up-dip/down-dip limits). Based on the rapid convergence rate (9 cm/yr) and the frequency of historic seismicity, the recurrence interval for large Nicoya earthquakes is estimated at ~50 years. The most recent event, in 1950, generated >1m of coseismic uplift along the central Nicoya coast. Since then, most of this deformation has been recovered by gradual interseismic subsidence, reflecting strain accumulation toward the next earthquake. While elastic seismic-cycle strain produces high frequency shoreline fluctuations on the Nicoya Peninsula, long-term net uplift results in gradual coastal emergence and the growth of topographic relief. We suggest that net uplift along the Nicoya segment is the product of irrecoverable strain associated with the seismic cycle (e.g., upper plate shortening), coupled with tectonic erosion near the trench and subsequent underplating of eroded material at depth beneath the peninsula. The persistence of the Nicoya Peninsula segment may be the result of a feedback between subduction generated thickening of the upper plate and increased coupling along the plate interface due to isostatic loading.