A methodology for near-field tsunami inundation forecasting: Application to the 2011 Tohoku tsunami

Existing tsunami early warning systems in the world can give either one or a combination of estimated tsunami arrival times, heights, or qualitative tsunami forecasts before the tsunami hits near-field coastlines. A future tsunami early warning system should be able to provide a reliable near-field tsunami inundation forecast on high-resolution topography within a short time period. Here we describe a new methodology for near-field tsunami inundation forecasting. In this method, a precomputed tsunami inundation and precomputed tsunami waveform database is required. After information about a tsunami source is estimated, tsunami waveforms at nearshore 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 coastal sites from the Sendai Plain to Miyako City is approximately 3min after information about the tsunami source is obtained. We found that the tsunami inundation forecasts from the 5min GPS, 5minW phase, 10minW phase fault models, and 35min 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. The required times to obtain tsunami forecast from the above four models are 8min, 9min, 14min, and 39min after the earthquake, respectively, or in other words 3min after receiving the source model. 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.