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.
Earthquakes along subduction zones are the world's biggest and most deadly, yet a predictive understanding that relates the size and frequency of earthquakes to the properties of the subduction zones that produce them has not been well developed. We have investigated this general problem by examining the tectonics and recent history of earthquakes along the Middle America Trench offshore Costa Rica. This location has been the focus of intense research over the past decade as part of the GeoPRISMS (formerly MARGINS) program, such that the knowledge gained is highly relevant to a developing context of discovery. This project consists of two parts; collaborators from Cal State Pomona have investigated the long-term deformation and uplift of the coastline, while researchers from Virginia Tech have conducted a feasibility study of paleoseismic sites along the Nicoya Peninsula. The latter is the subject of this outcomes report. Paleoseismology is the study of ancient earthquakes, using recent geologic deposits (i.e. Holocene sediment) to document events that may indicate the timing, location, and size of earthquakes over the past few thousand years. In this case, we have attempted to use mud and sand deposits in estuaries along the coast to track relative sea level change that may relate to sudden (i.e. during the earthquake) uplift and subsequent (i.e. post-seismic) subsidence. We examined numerous estuaries along the Nicoya Peninsula, first mapping them and then coring the sediment to depths of 4 meters and describing the dating the resulting sedimentary layers. Our work reveals that paleoseismic records may be very diffcult to obtain from estuaries of the Nicoya Peninsula. Some estuaries consist of broad tidal flats and exhibit shallow stratigraphy that is rich in changes of sedimentary character that likely relate to sea level change, but the deposits themselves are too old to constrain the most recent seismic activity. We found that depositional rates in these estuaries has been too slow, such that the upper meter of sediment, which has been overprinted by numerous changes in sea level and bioturbation, is over 1000 years old. There have probably been 10-20 earthquakes during this period, but it is not possible to sort out their chronology from the complex stratigraphy over this limited depth. Other estuaries along the coastline are "cramped", in that they are confined between steep topography and the coastline and thus are heavily affected by shoreline processes. We found that many of these are dominated by prograding young sands that are overwashing the backshore and filling estuaries where coseismic subsidence is occurring. Other estuaries have more protected mangrove tidal flats, but we found the sediment in these to be consist of non-distinct thick muds with no internal stratigraphy due to bioturbation by plants and animals. None of the sites we investigated was deemed suitable for further paleoseismic investigation. Our primary conclusion is thus a negative result, in that we were unable to identify suitable sites for unraveling the history of recent earthquakes along the subduction zone offshore Costa Rica. The sites investigated here, however, represent only the outer (southwest) coast of the Nicoya Peninsula. It is yet possible that a record of seismic sea-level change may exist further inland, where the change in sea level would be smaller and of a different sense (coseismic subsidence, interseismic uplift). Future work in the large tidal zone of the Tempisque River in the Gulf of Nicoya may yet yield a depositional signal that relates recent relative sea level change and seismic ground motion.
