The proposed work would use onshore geologic evidence from the Nicoya Peninsula to address basic questions related to subduction along a 100-km-long part of the Middle America Trench. The work includes the following components: (1) Paleoseismology: This part of the project will investigate pre-historic earthquake size and recurrence using the stratigraphy and paleoecology of an estuary. (2) Paleogeodesy: As part of the paleoseismic investigation, the study would investigate relative sea level change on the Nicoya Peninsula to assess patterns of vertical ground motion during intervals between earthquakes. (3) Morphotectonics and long-term uplift: The study will test the relationship of long-term deformation to earthquake generation. All of these techniques will be combined to define the pre-historic rupture behavior along the subduction zone. Broader Impacts This project includes the training of a Ph.D. student. Undergraduate students will participate in the field program. There is a large component of international collaboration with colleagues in Costa Rica. A workshop for students from the US and Costa Rica will be organized, and a field school for the students will be held at the research site. The study is important for understanding seismic hazards in Costa Rica.
Overview: Megathrust earthquakes along subduction zones are among Earth's most powerful and deadly natural hazards. During the past decade alone, more than a quarter-million people have lost their lives to great subduction earthquakes and tsunami in Sumatra (2004, M9.3), Chile (2010, M8.8), and Japan (2011, M9.0). Such catastrophic events are also notable for sudden geomorphic changes that they bring to coastlines through either coseismic uplift or subsidence. Sudden earthquake-induced changes in land-level result in either emergence or submergence of the coast, shifting the relative position of the shoreline, and all subsequent tides. Evidence of past events is often preserved in the sedimentary record of beaches and coastal wetlands, and by such features as emerged tidal platforms and coral reefs. Geomorphic and stratigraphic analysis of these features is a powerful tool for unraveling the past earthquake history of convergent margin coastlines. Understanding how earthquake induced changes in land-level affect the long-term growth and decay of coastal topography is also a fundamental question in the field of tectonic geomorphology. An excellent place to study these processes is the Nicoya Peninsula on the Pacific coast of Costa Rica, Central America (Figs. 1-3). The Nicoya Peninsula is unique because it is one of the few landmasses along the Pacific Rim located directly above the seismogenic zone of a subduction megathrust fault. Due to its proximity to the subduction zone, the peninsula is particularly sensitive to vertical movements related to the earthquake cycle. Costa Rica is part of the Central American convergent plate margin, where the Cocos oceanic plate subducts beneath the Caribbean plate at the Middle America Trench. The two plates converge at a rapid rate (8.5 cm/yr) along the Nicoya Peninsula, resulting in a high seismic potential, as demonstrated by repeated large magnitude (>M 7.5) earthquakes over the past few centuries (Fig. 4), including events in 1853 (M≥7.5), 1900 (M≥7.5), 1950 (Ms=7.7), and 2012 (Mw=7.6). Project Outcomes: Geomorphic, paleo-geodetic, and paleoseismic studies conducted as part of this NSF-funded research project resulted in substantial improvements in our understanding of fore-arc deformation associated with the Nicoya seismogenic zone. This research reveals upper plate deformation patterns that provide important clues about seismogenic zone geometry and the periodicity of megathrust earthquakes. Our new model for margin segmentation and coastal uplift along the Nicoya Peninsula (Fig. 5) is consistent with a range of other independent observations from seismologic, geodetic, and geologic investigations. The recent 2012 Mw7.6 Nicoya earthquake further validated this model by generating a coseismic uplift pattern (Fig. 6) consistent with the long-term deformation measured in this study. Along the Nicoya Peninsula coastline, net Quaternary uplift is recorded by emergent marine terraces (ancient shorelines) and by incised valley-fill alluvium (ancient river deposits). Field mapping, surveying, and isotopic dating (Fig. 5) 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/ky inboard of older EPR crust along the northern Nicoya coast; 0.2-0.5 m/ky inboard of younger CNS-1 crust along the central Nicoya 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 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 (8.5 cm/yr) and the frequency of historic seismicity, the recurrence interval for large Nicoya earthquakes is ~50 years. The most recent events, in 1950 and 2012, generated >0.5m of coseismic uplift along the central Nicoya coast. The coseismic uplift pattern of the 2012 Mw7.6 earthquake (Fig. 6) coincides with the area of GPS-modeled pre-event locking, the estimated rupture area for the prior 1950 Ms7.7 earthquake, and the area of coseismic coastal uplift produced by the 1950 event. Most of the 1950 uplift was recovered during several decades of gradual interseismic subsidence leading to the 2012 rupture. As shown in this investigation, Quaternary marine and fluvial terraces record net uplift along the Nicoya coast in a pattern that shadows the peninsula's overall morphotectonic structure and topographic form (Fig. 5). While elastic strain accumulation and release produce short-term cycles of uplift and subsidence (Fig. 6), long-term net uplift results in gradual coastal emergence and the growth of topographic relief. Net uplift along the Nicoya segment may be the product of irrecoverable strain during 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. Long-term persistence of the Nicoya fore arc segment and seismogenic zone 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.
