Hiroaki NARITA
The Behavior Analysis of a Tsunami Generated by the Niigataken Chuetsu Offing Earthquake
Tokuzo HOSOYAMADA
The Niigataken Chuetsu offing earthquake occurred on July 16, 2007. The seismic intensity at the maximum recorded over 6, and magnitude 6.8.This earthquake brought the serious damage to Chuetsu district of Niigata, Kashiwazaki-shi in particular. Small scale tsunami which is about 30 cm greatest wave height was generated. Tsunami by this earthquake was observed in the coast of Akita and Ishikawa The damage by this tsunami was not seen in particular.Because it is once in a period 10 years that a big tsunami brings the serious damage , to grasp the behavior of this small tsunami can help disaster prevention.
This study investigated grasping the change of the tsunami in a shallow sea area which was located between the land. The tsunami generated by the Chuetsu offing earthquake was reproduced by numerical value simulation. In addition, the spread situation of the tsunami and an intensive point was examined by a figure of refraction of the tsunami. The numerical simulation of the tsunami was run with the technique of the motion equation and continuous equation for a difference for a positive. The refraction of tsunami calculated with the refraction calculation model of the wave which used the equation of wavy lines.
The quantity of change of the water level to cause a tsunami calculated with dislocation model. On this occasion, water level was raised 1.5m in 20 seconds in a range of 5*10km around the seismic center. The tsunami simulation made it visible every 10 seconds from the earthquake outbreak time and examined the spread situation. About the refraction calculation, the refraction course of the wave determindby tracing 360 wavy lines every once from the seismic center. The calculation domain divided north latitude 37-39 degrees, a range of east longitude 137-140 degrees (about 267,000*222,000m) into about 500m, East-West courses in the north and south direction in about 420m mesh space. The position of the seismic center is north latitude 37 degrees 33¡ì 24¡í, 138 degrees 36¡ì 24¡í.
The calculation made the animation of the tsunami, a wave height distribution map, a figure of refraction, a figure of refraction of the reflection wave from Kashiwazaki and Ogi. The result shows the tsunami vibration by a reflection and refraction between Honshu and Sado in the sea area of the Niigata coast, because of the shape of the bottom of the sea and the shore is complex. In addition, the tsunami is concentrated around Kashiwazaki and Ogi which recorded the greatest wave height. This means that results of the numerical computation could reproduce behavior of the true tsunami well. Furthermore, when the tsunami occurred on shallow sea area that located between the land such a Sado channel, it is dangerous that a tsunami come many times.
A future problem, it is necessary to examine the tsunami generating range and the changing water level that decided from dislocation model for improve accuracy. In addition, to apply the tsunami generated other area is needed to inspect the versatility.
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