Special Issue on the 2011 off the Pacific Coast of Tohoku Earthquake Tsunami
Tomoyuki Takahashi and Nobuo Shuto
An unprecedented M9.0 earthquake occurring at 14:46 local time on March 11, 2011, off of northeast Japan’s Pacific Ocean generated a huge tsunami which had a run-up of over 40 m at the highest point and nearly 20,000 lives were lost. The tsunami demonstrated the need to drastically readdress current tsunami countermeasures.
“Guidebook for Tsunami Preparedness in Local Hazard Mitigation Planning” published prior to the March 11 tsunami had already estimated, as one of the cases of tsunami assumptions, that the tsunami could be generated by the largest earthquake near off the Sanriku Coast predicted by the recent seismology. The seismotectonics had predicted that off the Sanriku Coast consisted of three independent blocks, which could conceivably cause an M8.6 earthquake at the largest. However, three blocks were not independent and they moved continuously to yield an earthquake of M9.0.
The Guidebook had recommended a combination of three approaches for handling such a tsunami; Construction of defense structures, Tsunami-resilient town development, and Disaster prevention systems – defense structures were not expected to completely prevent every tsunami but only reduce its effect. Caissons forming part of Kamaishi Port’s tsunami breakwaters and registered in Guinness World Records, were overturned but reduced the tsunami height from 14 m outside the port to 8 m inside. Many coastal dikes were also destroyed, even though three surfaces – fore slope, top slope, and rear slope – had been protected using concrete and other means. Such phenomena pinpoint the importance of toe protection against erosion.
Since 2004, tsunami inundation hazard maps have been distributed to communities in Japan as an aid to public education and as part of the country’s nationwide disaster prevention system. Unexpectedly, these maps had a negative effect in many places where residents living outside inundation areas mentioned on the hazard maps believed they were safe under all condition. Many did not in fact keep track of the actual tsunami rising in front of their very eyes and not evacuate, thus losing their lives.
The tsunami hitting the coast of the Fukushima Prefecture had a run-up height almost double that designed in defense plans. The Fukushima No.1 Nuclear Power Plants of the Tokyo Electric Power Company (TEPCO) located on ground 4.8 m above sea level were immerged and a concurrent electric system failure led to total plant shutdown.
The Fukushima nuclear disaster itself has become well known worldwide. The effects of the tsunami, however, are less so, despite damage such as fires, railroad destruction and drifting ships caused by the tsunami. With the nuclear incident overshadowing such effects, we are concerned that these results might be overlooked.
To better prepare against potential future tsunami disasters, we must understand clearly what sort and how such diverse damage has been generated by the 2011 tsunami. This special issue focuses on the various types of tsunami-induced damages, emphasizing the valuable data and modeling obtained from field investigations in the tsunami-devastated areas. It will be more than worth publication if this special issue contributes in whatever way to furthering tsunami disaster research.
Finally, we extend our sincere thanks to all of the contributors and reviewers involved with these articles.
(written by Nobuo Shuto and Tomoyuki Takahashi)