Validation of NEOWAVE with Measurements from the 2011 Tohoku Tsunami

An accurate and reliable numerical model is essential in mapping tsunami hazards for mitigation and preparedness. The model NEOWAVE (Non-hydrostatic Evolution of Ocean WAVEs) is being used for tsunami inundation mapping in Hawaii, American Samoa, the Gulf coast states, and Puerto Rico. In addition to the benchmarks established by the National Tsunami Hazard Mitigation Program, we have been conducting a thorough investigation of NEOWAVE's capability in reproducing the 2011 Tohoku tsunami and its impact across the Pacific. The shock-capturing non-hydrostatic model is well suited to handle tsunami conditions in a variety of coastal environments in the near and far field. It describes dispersive waves through non-hydrostatic pressure and vertical velocity, which also account for tsunami generation from time histories of seafloor deformation. The semi-implicit, staggered finite difference model captures flow discontinuities associated with bores or hydraulic jumps through a momentum conservation scheme. The model supports up to five levels of two-way nested grids in spherical coordinates to describe tsunami processes of varying time and spatial scales from the open ocean to the coast. We first define the source mechanism through forward modeling of the near-field tsunami recorded by coastal and deep-ocean buoys. A finite-fault solution based on teleseismic P-wave inversion serves as the starting point of the iterative process, in which the source parameters are systematically adjusted to achieve convergence of the computed tsunami with the near-field records. The capability of NEOWAVE in modeling propagation of the tsunami is evaluated with DART data across the Pacific as well as water-level and current measurements in Hawaii. These far-field water-level records, which are not considered in the forward modeling, also provide an independently assessment of the source model. The computed runup and inundation are compared with measurements along Northeastern Japan coasts and the Hawaiian Island chain. These coastlines include shallow embayments with open plains, narrow estuaries with steep cliffs, and volcanic insular slopes with fringing reefs for full validation of the model in a single event. The Tohoku tsunami caused persistent oscillations and hazardous currents in coastal waters around Hawaii. Analysis of the computed surface elevation reveals complex resonance modes along the Hawaiian Island chain. Standing waves with period 16 min or shorter are able to form a series of nodes and antinodes over the reefs that results in strong currents and large drawdown responsible for the damage in harbors and marinas. The results provide insights into effects of fringing reefs, which are present along 70% of Hawaii's coastlines, on tsunami transformation and runup processes. The case study improves our understanding on tsunamis in tropical island environments and validates the modeling capability to predict their impacts for hazard mitigation and emergency management.