Tsunami Coastal Hazard Along the Us East Coast From Coseismic Sources in the açores Convergence Zone and the Caribbean Arc Areas
Authors: Grilli Stephan Toni , Mohammadpour Maryam , Schambach Lauren , Grilli Annette 
Published: April 2021

Abstract
We model the coastal hazard caused by tsunamis along the US East Coast (USEC) for far-field coseismic sources originated in the A\c{c}ores Convergence Zone (ACZ), and the Puerto Rico Trench (PRT)/Caribbean Arc area. In earlier work, similar modeling was performed for probable maximum tsunamis (PMTs) resulting from coseismic, submarine mass failure and volcanic collapse sources in the Atlantic Ocean basin, based on which tsunami inundation maps were developed in high hazard areas of the USEC. Here, in preparation for a future Probabilistic Tsunami Hazard Analysis (PTHA), we model a collection of 18 coseismic sources with magnitude ranging from M8 to M9 and return periods estimated in the 100-2,000 year range. Most sources are hypothetical, based on the seismo-tectonic data known for the considered areas. However, the largest sources from the ACZ, which includes the region of the Madeira Tore Rise, are parameterized as repeats of the 1755 M8.6-9 (Lisbon) earthquake and tsunami using information from many studies published on this event, which is believed to have occurred east of the MTR. Many other large events have been documented to have occurred in this area in the past 2,000 years. There have also been many large historical coseismic tsunamis in and near the Puerto Rico Trench (PRT) area, triggered by earthquakes with the largest in the past 225 years having an estimated M8.1 magnitude. In this area, coseismic sources are parameterized based on information from a 2019 USGS Powell Center expert, attended by the first author, and a collection of SIFT subfaults for the area (Gica et al., 2008). For each source, regional tsunami hazard assessment is performed along the USEC at a coarse 450 m resolution by simulating tsunami propagation to the USEC with FUNWAVE-TVD (a nonlinear and dispersive (2D) Boussinesq model), in nested grids. Tsunami coastal hazard is represented by four metrics, computed along the 5 m isobath, which quantify inundation, navigation, structural, and evacuation hazards: (1) maximum surface elevation; (2) maximum current velocity; (3) maximum momentum force; and (4) tsunami arrival time. Overall, the first three factors are larger, the larger the source magnitude, and their alongshore variation shows similar patterns of higher and lower values, due to bathymetric control from the wide USEC shelf, causing similar wave refraction patterns of focusing/defocusing for each tsunami. The fourth factor differs mostly between sources from each area (ACZ and PRT), but less so among sources from the same area; its inverse is used as a measure of increased hazard associated with short warning/evacuation times. Finally, a new tsunami intensity index (TII) is computed, that attaches a score to each metric within 5 hazard intensity classes selected for each factor, reflecting low, medium low, medium, high and highest hazard, and is computed as a weighted average of these scores (weights can be selected to reinforce the effect of certain metrics). For each source, the TII provides an overall tsunami hazard intensity along the USEC coast that allows both a comparison among sources and a quantification of tsunami hazard as a function of the source return period. At the most impacted areas of the USEC (0.1 percentile), we find that tsunami hazard in the 100-500 year return period range is commensurate with that posed by category 3-5 tropical cyclones, taking into account the larger current velocities and forces caused by tsunami waves. Based on results of this work, high-resolution inundation PTHA maps will be developed in the future, similar to the PMT maps, in areas identified to have higher tsunami hazard, using more levels of nested grids, to achieve a 10-30 m resolution along the coast.