Department of Ecosystem Engineering, University of Tokushima, Tokushima, JAPAN

sugimotot@nita.co.jp

　

In September 2001, Japanese government has evaluated the probability of occurrence of the next Nankaido earthquake in the Pacific Ocean to be 80% within 50 years the countermeasure of this disaster should be improved urgently.

　

In this study, the tsunami numerical simulation considering the effect of tide and flood gates on the inundation were first developed. The effect of tsunami inundation from these gates was estimated at U town of Kochi prefecture, Shikoku Island. These gates have been investigated if they will be able to close when tsunami comes.

　

As a result, it has been found that tsunami inundation decreased as degreased of the gaps of the gates, in U town. Especially, these structures, which shut out tsunami until the second wave, can make inundation time delay. Therefore, it is necessary to close these gates to have longer evacuation time for inhabitants as the tsunami prevention.

　

However, it was confirmed from investigation of control that some gates, which could not be closed because of the bad maintenance or take long time to close. The manager who closed gate cannot refuge safety, for the gates locate far from the evacuation place or closing gates take a long time. To be effectively useful for the regional tsunami countermeasure, some of these gates need improvement of the closing method and evacuation routes need management to keep good condition.

　

　

¹Earthquake Research Institute, University of Tokyo, Tokyo, JAPAN

tsuji@eri.u-tokyo.ac.jp

　

²Kochi University

　

A series of gigantic earthquakes accompanied with huge tsunamis occurred in the sea region off Tokai District, the Pacific side of the central part of Honshu Island periodically in the intervals of about one hundred years. Lagoons O-Ike in Owase city and Suwa-Ike in Kii-Nagashima town are both located on the southeast coastline of Kii peninsula, separated from the Pacific Ocean by sand dunes. Whenever a gigantic earthquake occurred in the open ocean area, seawater rushed into these lagoons by tsunami waves, and thin sand layers were formed. We gathered piston core sampling at several points for the bottom sedimentary layers of both of the lagoons, and found out more than ten thin sand layers formed by historical and pre-historical tsunamis. We made dating for several layers by the carbon 14 method. We found out three historical tsunami traces of the 1498 Meio, 1096 Kaho, and 684 Hakuho Tokai Earthquakes, and in addition that, we obtained seven pre-historical tsunami traces. It is clarifies that the oldest one shows the year of 6^th century BC.

　

　

¹Coastal Engineering Division, River and Coastal Engineering Department, NEWJEC Inc. Osaka, JAPAN

yoshidakz@osaka.newjec.co.jp

　

²Department of Construction Systems Engineering, Anan National College of Technology Anan, Tokushima, JAPAN

　

³Department of Ecosystem Engineering, University of Tokushima, Tokushima, JAPAN

　

It is predicted that, in the near future, a big earthquake and subsequent tsunami may occur along the Nankai Trough off the coast of Shikoku Island in the south west of Japan. In this study, characteristics of tsunami propagation in the Kii and Bungo channels leading into the Seto Inland Sea in Japan, which is called "Setonaikai" in Japanese, are clarified in the first stage of investigating tsunami characteristics in the Setonaikai.

　

Characteristics of tsunamis, which affect the Setonaikai, have been little investigated. This is because it is considered that much tsunami energy cannot enter the Setonaikai as a tsunami has to pass through narrow places such as the Kitan, Naruto, Akashi, Hyoyo straits and the Kii and Bungo channels and there are many islands of various sizes in the Seto Inland Sea that interfere with the penetration of a tsunami. However, as areas along the Setonaikai coast are densely populated and developed, the possibility that the coastal areas will suffer severe damage is high.

　

Characteristics of the resonance phenomenon and damping of tsunami in the Kii and Bungo channels are examined assuming a sinusoidal tsunami as an incident tsunami. Consequently, with both channels, it is found that a tsunami with a specific period increases amplification and properties of tsunamis occurring in the Setonaikai are described in this study.