Yuichiro Tanioka1 and Aditya Riadi Gusman2
1) Institute of Seismology and Volcanology, Hokkaido University
2) Earthquake Research Institute, University of Tokyo
Here we describe a new methodology for near-field tsunami inundation forecasting. We designed an algorithm that can produce high-resolution tsunami inundation maps of near-field sites before the actual tsunami hits the shore. This algorithm relies on a database of precomputed tsunami waveforms at several near-shore points and precomputed tsunami inundation maps from various earthquake fault model scenarios. By using numerical forward model, it takes several hours to simulate tsunami inundation in each site from each fault model. After information about a tsunami source is estimated, tsunami waveforms at near-shore points can be simulated in real-time. A scenario that gives the most similar tsunami waveforms is selected as the site-specific best scenario and the tsunami inundation from that scenario is selected as the tsunami inundation forecast. To test the algorithm, tsunami inundation along the Sanriku coast is forecasted by using source models for the 2011 Tohoku earthquake estimated from GPS, W phase, or offshore tsunami waveform data. The forecasting algorithm is capable of providing a tsunami inundation forecast that is similar to that obtained by numerical forward modeling, but with remarkably smaller CPU time. The time required to forecast tsunami inundation in 15 coastal sites from the Sendai Plain to Miyako City is approximately 3 minutes after information about the tsunami source is obtained. We found that the tsunami inundation forecasts from the 5-min GPS, 10-min W phase fault models, and 35-min tsunami source model are all reliable for tsunami early warning purposes and quantitatively match the observations well, although the latter model gives tsunami forecasts with highest overall accuracy. We evaluated the effectiveness of this algorithm in the real world by carrying out a tsunami evacuation drill in Kushiro City, Hokkaido, Japan, involving the city residents. The participants found that the use of the tsunami inundation forecast map produced by NearTIF was effective in helping them make better decisions with high confidence during the tsunami evacuation drill. This method can be useful in developing future tsunami forecasting systems with a capability of providing tsunami inundation forecasts for locations near the tsunami source area.