PALEOSEISMOLOGY OF THE NORTH PANAMA DEFORMED BELT FROM UPLIFTED CORAL PLATFORMS AT MOÍN AND LIMÓN, COSTA RICA

In 1991, the Caribbean coastal area between Moín and Limón was coseismically uplifted up to 1.8 m by a M_s7.6 earthquake on the western segment of the North Panamá Deformed Belt (NPDB), resulting in the emergence of a broad coral reef platform. Designers of a proposed new harbor asked the highly relevant question “when is the next one?” as part of the Probabilistic Seismic Hazard Analysis (PSHA) process. Review of documents showed that multiple large earthquakes have been felt in the region during its ~270 year historical record, with an event in 1822 described somewhat similarly to the 1991 event. Geologic reconnaissance revealed multiple older platforms that provided an opportunity to quantify the timing and rate of 1991-like uplift earthquakes on the Limón section of the NPDB. Mapping and hand surveying identified 6 pre-1991 coral platforms and wave cut notches preserved on the coastal-facing slope beneath a set of higher marine terraces that cap the coast between Moín and Limón. 48 coral samples were obtained but only 12 were suitable for U-series dating due to diagenetic alteration of the original aragonite to calcite. Assuming that these coral die offs were caused by coseismic uplift events, and using additional dated corals from published sources and one radiocarbon-dated shell, we have interpreted and temporally constrained as many as 12 events in the past ~7,000 years. Our preliminary work, indicates that the penultimate earthquake did occur in 1822, but was smaller than 1991. We calculate an average slip rate of 3.8±0.3 mm/yr on the NPDB using a geophysically-determined fault dip of 27°, and an average earthquake recurrence of ~600 years, but return periods are irregular and clustered, suggesting that the region experiences two types of events: large events with ~1400-year return periods that could reflect multi-segment ruptures of the NPDB, and smaller events with recurrence intervals in the 100s of years. These results, along with other regional fault sources, helped to populate the PSHA logic tree and develop realistic ground motions used for the facility design.