The devastating tsunami waves of March 11, 2011, along the northeastern coast of Japan caused severe damages to coastal communities. The unusual, very high-energy conditions achieved by the March 11, 2011, Tohuku Tsunami afford a unique opportunity to greatly advance our understanding of the geological effects of tsunamis in coastal areas. While much of the immediate post-tsunami research is focused on damage assessment, this particular project aims to understand the dynamics of tsunami waves and their impacts on natural landscapes. Prompt access to the field areas is crucial for this effort in order to obtain evidence of the tsunami wave effects. Current disaster response reconstruction efforts are quickly erasing the traces of the event in developed areas. Natural, undeveloped areas of interest in this project will also quickly lose evidence of high-water indicators and subtle sediment layers because of rainstorms, human disturbance, and other post-tsunami processes. This project will integrate numerical modeling with field measurements and remote sensing. A large group of collaborating Japanese scientists, unfunded by this project, will facilitate the operations in the field areas, and the quantitative modeling efforts by the U.S. team will be supplemented by collaborator from Finland, who will be funded by his own sources independent of the project funds.
Results obtained from this study will contribute to tsunami hazard assessment by greatly advancing the scientific capability to recognize and understand the effects of high-energy tsunami waves that may be preserved as geological evidence of ancient tsunamis on potentially hazardous coastlines around the world. This new understanding will prove particularly relevant to hazardous coastlines in the western U.S. In particular, the potential earthquake zone off coastal Washington and Oregon has many similarities to northeastern Japan in regard to tsunami hazards. Tsunami-vulnerable areas also occur in California, Alaska and Hawaii.
We discovered tsunami erosion and deposition effects from the March 11 Tohoku-oki Tsunami, northeastern Honsh, Japan, that are all remarkably similar to what is observed for catastrophic fluvial erosion and deposition in bedrock river channels. At Aneyoshi, Miyako-Shi, where wave run-up heights achieved 40.5 meters (among the highest recorded for the March 11 event), there was spectacular erosion of valley sides, transport and deposition of coarse gravel and boulders (Figure 1), and scour-hole generation around large boulders. The distinctive high-energy erosion/deposition features that we documented constitute definitive signs for exceptionally high-energy tsunami events. The high potential for long-term preservation of these features affords an opportunity to reconstruct the physics of the formative events through the application of hydraulic modeling codes. We employed this computerized hydraulic modeling to associate the causative tsunami velocities and unit stream powers with the observed Tohoku-oki tsunami erosion features. We found that peak flow velocities at the Aneyoshi site locally exceeded 10 meters per second. Preservation of similar, distinctive tsunami erosion features in other high-energy tsunami settings, such as those landward of the Cascadia Subduction Zone off the northwestern coast of the U.S., affords an opportunity to estimate tsunami magnitudes via the modeling procedures noted above. This will ultimately contribute to a better understanding of the tsunami hazard for coastal Washington, Oregon and northern California.
