Crustal stress changes and characteristics of damage to geo-engineering structures induced by the Great East Japan Earthquake of 2011

T he Great 2011 Earthquake of East Japan, with a moment magnitude of 9.0, occurred off the shore of the Tohoku Region of Japan. Although seismologists anticipated an earthquake with a magnitude of 7.5 in the source area, the magnitude of this earthquake was much greater than anticipated. Furthermore, the level of the tsunami induced by this earthquake also exceeded expectations. Therefore, damage caused by the tsunami was tremendous and more than 18,000 people perished due to insufficient counter-measures. The author first presents characteristics of the earthquake and its associated tsunami and then describes damage to some geo-engineering structures. The final part of the article describes lessons learned from this earthquake and proposes recommendations. Changes in crustal stress occurred as forecasted by computations and in situ stress measurements. However, it is expected the orientation of the maximum principal stress would remain almost the same as those prior to the earthquake. The scale and number of slope failures were much larger on the hanging wall side of the earthquake fault as compared to those on the foot-wall side. It has also become clear that the scale of natural slope failures is much larger than that of cut-slopes (man-made) and, therefore, more attention should be given to potential natural rock slope failures. In addition, slope failures may also be induced by tsunamis, thus necessitating further studies on the effect of tsunamis on rock slopes, with consideration of their structural geological features. Strong ground shaking induced the collapse of semi-underground tunnel portals in the abandoned Oya quarries. In addition to potential caving of abandoned mines and quarries during earthquakes due to either pillar failure and/or roof failure, groundwater may present additional impacts on immersed mines. Sinkholes are thought to be due to ground rupture induced by the sloshing of groundwater in inclined shafts. Tsunami waves inundated many tunnels on the Onagawa, Minami-Sanriku and Kesennuma railways, although they did not cause any major structural damage. However, consideration should be given to the impact of high internal hydrostatic water pressure on conventional tunnels when they are located in tsunami-prone areas.