Abstract
A dense cabled observation network, called the seafloor observation network for earthquakes and tsunami along the Japan Trench (S-net), was installed in Japan. This study aimed to develop a near-real time tsunami source estimation technique using the ocean bottom pressure data observed at those sensors in S-net. Synthetic pressure waveforms at those sensors were computed for 64 earthquake tsunami scenarios with magnitude ranging between M8.0 and M8.8. The pressure waveforms within a time window of 500 s after an earthquake were classified into three types. Type 1 has the following pressure waveform characteristic: the pressure decreases and remains low; sensors exhibiting waveforms associated with Type 1 are located inside a co-seismic uplift area. The pressure waveform characteristic of Type 2 is that one up-pulse of a wave is within the time window; sensors exhibiting waveforms associated with Type 2 are located at the edge of the co-seismic uplift area. The other pressure waveforms are classified as Type 3. Subsequently, we developed a method to estimate the uplift area using those three classifications of pressure waveforms at sensors in S-net and a method to estimate earthquake magnitude from the estimated uplift area using a regression line. We systematically applied those methods for two cases of previous large earthquakes: the 1952 Tokachi-oki earthquake (Mw8.2) and the 1968 Tokachi-oki earthquake (Mw8.1). The locations of the large computed uplift areas of the earthquakes were well defined by the estimated ones. The estimated magnitudes of the 1952 and 1968 Tokachi-oki earthquakes from the estimated uplift area were 8.2 and 7.9, respectively; they are almost consistent with the moment magnitudes derived from the source models. Those results indicate that the tsunami source estimation method developed in this study can be used for near-real time tsunami forecasts.
Highlights
The Tohoku-oki earthquake (Mw9.0) that occurred on March 11, 2011 generated a large tsunami along the Pacific coast from the Tohoku region to the Kanto region [1]
We developed a near-real time tsunami source estimation technique using ocean bottom pressure data observed from the sensors in S-net
The pressure waveforms of the sensors in S-net were computed for 64 fault models with a magnitude ranging between M8.0 and M8.8
Summary
The Tohoku-oki earthquake (Mw9.0) that occurred on March 11, 2011 generated a large tsunami along the Pacific coast from the Tohoku region to the Kanto region [1]. Meteorological Agency (JMA) issued a major tsunami warning along the Pacific coast of Japan immediately after the earthquake [2], this tsunami caused catastrophic destruction along the coast with approximately 19,000 casualties [3]. The magnitude was subsequently used to issue the initial tsunami height forecast of. This initial tsunami warning underestimated the real tsunami heights that were larger than 10 m along 500 km of the Pacific coast in the Tohoku region [1]. The tsunami height forecast by the JMA was updated twice to 6–10 m at 28 min after the earthquake and to beyond 10 m at 44 min after the event [2]. A tsunami warning system that is more accurate and rapid has become highly demanded in Japan
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