IRIDeS NEWs

2017.1.29

Emergency Investigation and Analysis of the Fukushima Prefecture Offshore Earthquake and Tsunami

An earthquake with a maximum seismic intensity of 5-lower (M7.4), having its hypocenter in the ocean of Fukushima Prefecture, occurred on November 22, 2016. It was accompanied by a tsunami. The researchers at IRIDeS took immediate action following the earthquake and conducted a survey and analysis whose findings were released at a briefing session held one month later on December 22.

 

  • Professor Fumihiko Imamura summarized the distinctive features of the occurrence and the dissemination of this tsunami, explaining the mechanism by which the tsunami was amplified in Sendai Bay in particular, based on results obtained from tsunami numerical analysis (simulation). Professor Imamura pointed out that it was difficult to reproduce the wave-height distribution at this instance by carrying out simulations while changing the direction of the fault strike to angles such as 90 degrees and 180 degrees, and that there were areas such as Sendai Bay where the tsunami this time was high. He recommended that the spatial grid for the analysis be set at a finer mesh than 500 m.

 

  • Associate Professor Suppasri Anawat and his team reported the findings of on-the-ground tsunami and damage surveys undertaken in areas for which tide-level measurement records were unavailable. At Tsukihama Public Beach (Higashimatsushima City, Miyagi Prefecture), the tsunami reached 4.0 m, exceeding the estimated tide level (T.P. +0.3 m). While the relationship between the external force of the tsunami and the damage inflicted by it matched that observed in the past, further improvement in damage estimates will continue to be an issue in the future.

 

  • Professor Shinji Toda explained that “There are numerous faults in the land-side plate of the offshore area of Fukushima due to seamount collisions and subduction. Stimulated by [the Tohoku Earthquake of] March 11 [2011], the earthquake activity has intensified since then. Though five and a half years have passed, this earthquake is another example of the same.” At the same time, Professor Toda pointed out that “This earthquake should not simply be explained away as an aftershock of the 2011 Tohoku Earthquake. Even though it occurred in the ocean, it can be characterized as an inland earthquake caused by active faults existing in the land-side plate, and it will receive ongoing attention in the future.”

 

  • Research Associate Yoshi Abe laid out the issues from evacuation perspective. While positively noting that most people began evacuating promptly, he also pointed out that there were evacuation bottlenecks and cases where evacuation was not accomplished. He also focused on the fact that during this earthquake, the public tsunami advisory in Miyagi Prefecture had been switched to a stronger tsunami warning, resulting in discrepancies in local authorities’ ways of handling of tsunami warnings. Research Associate Abe identified the greater practical usage of real-time tsunami measurement information as one area for future improvement.

 

This earthquake and tsunami, with their hypocenter in the ocean of Fukushima Prefecture, saw their first warnings being issued after four years in the areas struck by the Tohoku 2011 tsunami. This briefing session illuminated that there still are issues to be resolved in a wide range of areas, including measurement and evacuation.