A methodology for near-field tsunami inundation forecasting and its application to the 2011 Tohoku tsunami

Aditya Riadi Gusman
Institute of Seismology and Volcanology, Hokkaido University
Kita 10 Nishi 8 Kitaku, Sapporo, Hokkaido, Japan

Yuichiro Tanioka
Institute of Seismology and Volcanology, Hokkaido University
Kita 10 Nishi 8 Kitaku, Sapporo, Hokkaido, Japan

Japan Geoscience Union Meeting, Yokohama, Japan, 28 April – 2 May 2014. Website

Abstract

We develop a new methodology for near-field tsunami inundation forecasting (NearTIF). This method required site-specific pre-computed tsunami inundation and pre-computed tsunami waveform database. Information about tsunami source of an event is required as an input for the method to work. By this method, we will not attempt to obtain a reliable earthquake source model for an event. Instead, any available information about tsunami source such as earthquake’s moment magnitude, earthquake fault model, or tsunami source model will be used. After information about the tsunami source is obtained, tsunami waveforms at near-shore points can be simulated in real-time during an event. Simulating tsunami waveforms by solving the linear shallow water equation on low-resolution bathymetric data does not take long time, therefore it is suitable to be used in real-time. By using root mean square analysis, a scenario that gives the most similar tsunami waveforms in the database is selected as the best-fit site-specific scenario. Then the corresponding pre-computed tsunami inundation of the best scenario is selected as the tsunami inundation forecast.

The pre-computed tsunami database is built from thrust earthquake scenarios of simple rectangular fault models with moment magnitude ranged from Mw 8.0 to 9.0. We arrange a total of 56 reference points along the subduction zone off the east coast of Honshu, Japan as the center top of the fault planes. The points are grouped into four depth categories of shallowest, upper intermediate, lower intermediate, and deepest plate interface. The earthquake scenarios for each depth category have moment magnitude range of Mw 8.0 to 9.0, Mw 8.0 to 8.9, Mw 8.0 to 8.8, and Mw 8.0 to 8.7, respectively, from the shallowest to the deepest plate interface, making a total of 532 scenarios.

Sites are chosen based on their coastal geomorphology (i.e. bay, lagoon, isthmus) or location of coastal community. Virtual observation points at which tsunami waveforms is computed are placed strategically near-shore, around a bay at depth of deeper than 30 or 50 m depending on the bathymetry.

We test the algorithm to hindcast tsunami inundation along the Sanriku coast that was generated by the 2011 Tohoku earthquake. To produce accurate tsunami inundation map, accurate information about tsunami source is required. We used source models for the 2011 Tohoku earthquake previously estimated from GPS, W phase, or offshore tsunami waveform data. These source models could be available before tsunami hits the shore. The forecasting algorithm is capable of providing a tsunami inundation map that is similar to that obtained by numerical forward modeling, but with remarkably faster speed. Using a regular laptop computer, the time required to forecast tsunami inundation in coastal sites from the Sendai Plain to Miyako City is approximately 3 min after information about the tsunami source is obtained. We found that the tsunami inundation forecasts from the GPS (5 min), W phase (5 min and 10 min) fault models, and tsunami source model (35 min) are reliable for tsunami early warning purposes and considerably similar to the observation. This method can be used to develop a future tsunami forecasting systems with a capability of providing tsunami inundation forecasts in the near field locations.

Keywords: near-field tsunami inundation forecast, pre-computed tsunami database, tsunami early warning.

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