DAMAGE CHARACTERISTIC AND FIELD SURVEY OF THE 2011 GREAT EAST JAPAN TSUNAMI IN MIYAGI PREFECTURE

摘要

On March 11th, 2011, the Pacific coast of Japan was hit by a tsunami generated by the largest earthquake (M9.0) in the history of the country and causing a wide range of devastating damage. Using preliminary reported data from many sources, some topics such as tsunami fatality ratio and tsunami fragility curves for structural damage are discussed and compared with other countries. This paper aims to discuss the damage characteristics of this tsunami as well as its mechanism, as observed through field surveys conducted over the 4 months following the tsunami. The field survey covers 13 areas in the Miyagi prefecture from Kesennuma city in the northernmost region to Yamamoto town in the southernmost region. The arrival time of the first tsunami along the coastal areas in the Miyagi prefecture was confirmed by stopped clocks found during the survey. The damage mechanism of coastal structures such as breakwaters, seawalls, tsunami gates, and evacuation buildings was investigated and discussed. Damage characteristics for each area, i.e., urban areas, port, coastal structures, fisheries, and agricultural areas, were also summarized. The conclusions drawn from the data analysis suggest that experience and education (soft countermeasures) are important to reduce the loss of life, as shown for example in the Sanriku area. The field surveys indicate that wood and reinforced-concrete (RC) structures should be balanced to survive both earthquake and tsunami forces, and the structural design for buildings should be reconsidered after the example in Onagawa town. In addition, coastal structures for tsunami countermeasures (hard countermeasures) should be more properly designed for survival instead of becoming floating debris upon being overturned by a tsunami. The combination of both hard and soft measures is especially necessary for optimizing the outcomes following a great disaster. These recommendations should be taken into consideration in the reconstruction efforts for better tsunami countermeasures in the future.