ASSESSING POTENTIAL TSUNAMI SOURCES FOR EXTREME WAVE DEPOSITS ON SOUTHWEST ISLA DE MONA, PUERTO RICO, USING NUMERICAL SIMULATIONS AND HYDRODYNAMIC BOULDER TRANSPORT EQUATIONS

Extreme waves on coastlines pose a threat to human life, habitats, and critical coastal infrastructure. Geological evidence of extreme waves can provide valuable information on the magnitude, frequency, wave characteristics and source of past events, thus improving coastal hazard assessment. Reef-rock boulders, as much as 5m in diameter, are found up to 500 m inland on the southwestern coast of Isla de Mona, Puerto Rico. These boulders were emplaced ~4000 years ago based on age dates from encrusting corals (Taggart et al., 1993). This study aims to identify an event capable of forming these deposits. For this, a numerical model of the 1918 Mona Passage tsunami was constructed using the New Evolution of Ocean Wave (NEOWAVE) model with three nested grids of 3, 1 and 1/3 arc-second resolution, respectively. A second simulation of a submarine landslide (1km³ volume) located ~300m from the southwestern Mona shoreline was run using 3D Tsunami Solution Using Navier–Stokes Algorithm with Multiple Interfaces (TSUNAMI3D). The resulting inundation and wave heights at the shoreline are compared to minimum wave heights required to initiate transport (sub-aerial and submerged) of measured boulders and idealized cubic boulders with varying volumes. The 1918 Mona Passage tsunami simulation shows no significant inundation on the SSW Mona coast and a maximum wave height of 1.3m, which is below the minimum wave height required to initiate transport of a ~1m diameter boulder. This result suggests that a tsunami like the one generated in 1918 is not capable of transporting even the smaller boulders. However, the submarine landslide generated extensive inundation on the SW coast with maximum wave height of 10m at the shoreline, 20m run-up, and 900m inundation distance. This is greater than the minimum wave height needed to initiate transport in both submerged and subaerial pre-transport settings; therefore, a submarine landslide with characteristics of the modeled landslide can form the boulder deposits observed. Marine geological surveys providing dates of landslides found in deep waters south of Mona Island will be required to validate this hypothesis.

Taggart, B.E. et al., 1993, Holocene reef-rock boulders on Isla de Mona, Puerto Rico, transported by a hurricane or seismic sea wave. GSA, Abstract with Programs v. 25(6), p. 61.