This project is a comprehensive interdisciplinary study of the March 11 initial M=9 and M=7.9 earthquakes, the resulting tsunami wave generation, propagation and coastal inundation along northern Honshu Island, and the initial pathways and changes in Cs-237 concentrations as it enters the coastal waters at the Fukushima Daiichi nuclear facility and spreads across the shelf to deeper water. The approach is to use a combination of advanced seismic and nested coupled atmospheric/3-D ocean circulation numerical models plus available field measurements to simulate these processes starting with the initial March 11 M=9 earthquake bottom movement through April 12. During this 33-day simulation, the Cs-137 source concentration levels peaked and decreased towards the increasing coastal and off-shelf concentration levels, indicative of cross-shelf transport and shelf-ocean exchange processes, with a potential sedimentation loss and biological accumulation in the near-shore region. Detailed descriptions of the different model simulations, the resulting ocean circulation and water property output fields and initial analysis will be uploaded to a project website on a frequent basis for use by others interested in coastal physical and bio/chemical processes in the study area and as initial conditions for studies of the long-term spread of Cs-137 and other radionuclides in the Pacific Ocean.
Intellectual Merit: The March 11 earthquakes, tsunami waves, coastal inundation, and initial release of Cs-137 into the ocean cover a wide range of time (from seconds to a month) and space (meters to 100's of km) scales. The multi-scale modeling approach with advanced models should produce a comprehensive and integrated description and understanding of the key physical processes involved and an independent assessment of the initial fate and spread of Cs-137 and its impact on the coastal ecosystem within the RAPID grant period.
Broader Impacts: This study will foster U.S-Japanese collaboration in several areas of ocean sciences (marine geophysics, physical oceanography, and bio-chemistry related to Cs-137 and other released radionuclides). The team includes one Japanese PhD student, and it is anticipated that the models and model results posted on a website will be used by researchers, students, and others as the study progresses. One outcome of this study will be a tested new combined earthquake/3-D ocean model system that can be used by researchers and coastal planners for assessing potential tsunami flooding from future earthquakes in the megathrust zones east of Japan. This system can be applied to other earthquake and or tsunami-prone areas.
A high-resolution nested global-Japan coastal FVCOM system was used to simulate the March 11, 2011 T?hoku M9 earthquake-induced tsunami waves and coastal inundation along the northeastern coast of Honshu Island in the western Pacific Ocean. Experiments were made with initial fields provided by five seismic rupture models under realistic conditions with inclusion of the Kuroshio, tides and wind forcing. Results show that the model-computed intensities and distributions of tsunami waves and subsequent coastal inundation could be significantly influenced by initial conditions, even though all five cases were capable of reproducing key features of the tsunami waves. Modeled tsunami waves featured a low dispersive, weakly nonlinear long wave controlled by hydrostatic dynamics. Non-hydrostatic effects only became significant when tsunami waves reached the inner shelf and the ratio of the amplitude of the leading tsunami wave to the local water depth is O(1). In both hydrostatic and non-hydrostatic cases, significant mixing occurred when the ratio of wave amplitude to local water depth grew to about 0.25 or greater. Model-predicted run-up was in good agreement with 2-D N-wave analytical solutions on the northern coast around South Iwate where inundation was small, but not in the central Sendai coastal region where inundation was large and 3-D wave dynamics became significant. The experiments suggest that once local bathymetry is accurately configured and the intensity and shape of the initial bottom movement can be predicted, this nested FVCOM system is capable of making accurate predictions of tsunami waves and coastal inundation. The March 11, 2011 earthquake-induced tsunami destroyed facilities at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) that led to a significant long-term flow of the radionuclide 137Cs into coastal waters. Based on the model’s success in reproducing the observed tsunami and coastal inundation, model experiments were then conducted with differing grid resolution to assess the initial spread of 137Cs over the east shelf of Japan. The 137Cs was tracked as conservative tracer in the three-dimensional model flow field over the period of March 26-August 31, 2011. The results clearly show that for the same 137Cs discharge, the model-predicted spreading of 137Cs was sensitive not only to model resolution but also the FDNPP seawall structure. A coarse-resolution (~2 km) model simulation led to an overestimation of lateral diffusion and thus faster dispersion of 137Cs from the coast to the deep ocean, while advective processes played a more significant role when the model resolution at and around the FNPP was refined to ~ 5 m. By resolving the pathways from the leaking source to the southern and northern discharge canals, the high-resolution model better predicted the 137Cs spreading in the inner shelf where in situ measurements were made at 30 km off the coast. The overestimation of 137Cs concentration near the coast is thought to be due to the omission of sedimentation and biogeochemical processes as well as uncertainties of leaking amounts from the sources in the model. The sedimentation was evident in sediment measurements taken after accident. A biogeochemical module could directly influence the fate of 137Cs in the ocean.
