Near-field tsunami forecasting using offshore tsunami data from the 2011 off the Pacific coast of Tohoku Earthquake

Tsunami heights greater than 4 m were observed at several coastal tide-gauge stations during the tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake (M(w) 9.0), causing thousands of casualties and damaging infrastructure along the Pacific coast of Japan. We retrospectively applied an algorithm of near-field tsunami forecasting to tsunami data that were recorded at various offshore tsunami stations 5-10 min before the tsunami reached the coastal tide-gauge stations nearest to its source. We inverted the waveform data recorded offshore to estimate the distribution of the initial sea-surface height, and then tsunami waveforms were synthesized from the estimated source to forecast tsunami arrival times and amplitudes at coastal tide-gauge stations. As a result of a retrospective application made 20 min after the earthquake, tsunamis with heights of 6-14 m were forecasted at tide-gauge stations nearest to the source where the sea-level increase due to the actual tsunami began to exceed 1 m after an elapsed time of 25 min. The result suggests a possibility that the forecasting method we used could contribute to the issuing of reliable near-field tsunami warning for M(w) 9 earthquakes.