Nobuo Shuto

Numerical Simulation of Tsunamis — Its Present and Near Future

Hindcasting of a tsunami by numerical simulations is a process of lengthy and complicated deductions, knowing only the final results such as run-up heights and tide records, both of which are possibly biased due to an insufficient number of records and due to hydraulic and mechanical limitation of tide gauges. There are many sources of error. The initial profile, determined with seismic data, can even be different from the actual tsunami profile. The numerical scheme introduces errors. Nonlinearity near and on land requires an appropriate selection of equations. Taking these facts into account, it should be noted that numerical simulations produce satisfactory information for practical use, because the final error is usually within 15% as far as the maximum run-up height is concerned.The state-of-the-art of tsunami numerical simulations is critically summarized from generation to run-up. Problems in the near future are also stated. Fruitful application of computer graphics is suggested.

Coastal sedimentation associated with the June 2nd and 3rd, 1994 tsunami in Rajegwesi, Java

Abstract This paper presents the second detailed study of sediments deposited by modern tusnamis, the first being that of the Flores (Indonesia) tsunami of December 1992 (Shi et al., 1995). Sediment cores were collected from areas in which eyewitnesses reported sediment deposition. Grain size analysis shows pronounced vertical variations in grain size as well as changes in standard deviation, skewness and kurtosis that appear to be indicative of complex tsunami flooding. Vertical variations in grain size in individual cores are greater than spatial variations between cores taken along a transect completed perpendicular to the coastline. The Java tsunami-deposited sediments do not show unequivocal evidence of local erosion but instead evidence for sediment transport and deposition is clear and is characterised by dominantly unimodal sediments with fine-tailed distributions.

Sequence of sedimentation processes caused by the 1992 Flores tsunami: Evidence from Babi Island

Sedimentation processes caused by a modern tsunami have been discussed from the point of view of geologic and numerical investigations of the 1992 Flores tsunami in Indonesia. Geologic evidence on Babi Island shows an invasion of two waves of different direction and magnitude, which resulted in widespread deposition of marine sand on the north and south-southwest shores. On the latter, coarse and well-sorted carbonate sand containing molluscan shells suggests that much more destructive waves swept across the southern coast, as compared with the northern coast, where the deposit included medium carbonate sand with a silty component. A physical explanation for such destructive waves on the southern coast of Babi is provided by a numerical simulation of the tsunami generation and propagation. The geologic and numerical results indicate that an isolated island surrounded by a circular shoreline or reef edge will be subject to the most destructive waves on the coast on the back side of the island relative to the tsunami source.

Nonlinear Long Waves in a Channel of Variable Section

ABSTRACTThe Kakutani equation is extended to include the effect of the variable width of channel, which is important in such a practical problem as refraction.Assuming that the wave profile is that of cnoidal waves and that this profile is maintained throughout the travel, a law of shoaling is derived.A comparison with experimental data shows that the present theory describes very well the shoaling for gHT2/d2>30, while for gHT2/d2<30 the theory of linear surface waves gives good agreement.For 30< gHT2/d2<50, is Hd2/7=const. and for 50< gHT2/d2, it is Hd3/2[≠ gHT2-2≠3]=const. in case of variable depth only.Due to the lack of experimental data, the theoretical result for variable width could not be examined for the present.

Estimate of the tsunami source of the 1992 Nicaraguan Earthquake from tsunami data

The Nicaraguan earthquake of September 2, 1992 excited large tsunamis which caused significant damage along the Nicaraguan coasts. Some of the inhabitants felt only minimal shock before the tsunami arrived, indicating that excitation of short-period seismic waves was very small. Numerical simulation with the initial tsunami source estimated by the seismic data is carried out to study the characteristics of the tsunami source by comparing the calculated data with the measured data along the Nicaraguan coast. Finding of the comparison shows that the dislocation of the fault estimated from the measured data is 5.6 to 10.0 times larger than that from seismic data. The tsunami source area, which is 200km in length × 100km in width, is used to explain the distribution of measured tsunami heights along the coast and the wave period as shown in the tide record at Corinto. The effect of rise time on tsunami excitation indicates a slow process, which corresponds with the seismic waves. This event falls under the tsunami earthquake category to produces anomalously large tsunamis relative to earthquake magnitude.

Numerical simulation of the 1992 Flores tsunami: Interpretation of tsunami phenomena in northeastern Flores Island and damage at Babi Island

Numerical analysis of the 1992 Flores Island, Indonesia earthquake tsunami is carried out with the composite fault model consisting of two different slip values. Computed results show good agreement with the measured runup heights in the northeastern part of Flores Island, except for those in the southern shore of Hading Bay and at Riangkroko. The landslides in the southern part of Hading Bay could generate local tsunamis of more than 10 m. The circular-arc slip model proposed in this study for wave generation due to landslides shows better results than the subsidence model, It is, however, difficult to reproduce the tsunami runup height of 26.2 m at Riangkroko, which was extraordinarily high compared to other places. The wave propagation process on a sea bottom with a steep slope, as well as landslides, may be the cause of the amplification of tsunami at Riangkroko. The simulation model demonstrates that the reflected wave along the northeastern shore of Flores Island, accompanying a high hydraulic pressure, could be the main cause of severe damage in the southern coast of Babi Island.

Numerical Simulation as a Means of Warning for Near-Field Tsunamis

ABSTRACTThe feasibility of quantitatively forecasting a near-field tsunami prior to its arrival is examined, provided that the initial tsunami profile can be determined from fault parameters calculated using a method similar to that of Izutani and Hirasawa. Examination of basic equations, boundary conditions and grid lengths has led to the conclusion that the following combination is the best to perform rapid, accurate, and detailed numerical forecasting; the linear long wave theory discretized with the staggered leap-frog scheme, perfect reflection at the land boundary, and a grid length varying from 5.4 km out at deep sea to 0.2 km at the shoreline. With the aid of a super computer, tsunami heights along every 200 m of Japans Sanriku coast (250 km long) can be obtained within 7 minutes after the occurrence of an earthquake. This method gives enough time for warning transmission and for evacuation of residents because the standard arrival time of tsunamis in this district is 25 to 30 minutes.

Field Survey of the 1993 Hokkaido Nansei-Oki Earthquake Tsunami

Runup data in Hokkaido and in three prefectures in the Tohoku District are described with a few witnessed arrival times and with comments of tide records. The highest runup of 31.7 m was found at the bottom of a narrow valley on the west coast of Okushiri Island. In order to explain high runups of 20 m at Hamatsumae in the sheltered area, roles of edge waves, refraction of the Okushiri Spur and tsunami generation by causes other than the major fault motion should be understood. An early arrival of the tsunami on the west coast of Hokkaido suggests another tsunami generation mechanism in addition to the major fault motion.

A Numerical Model for Far-Field Tsunamis and Its Application to Predict Damages Done to Aquaculture

The 1960 Chilean tsunami which traveled the Pacific Ocean and caused much damages to Japan is simulated from its generation to the terminal effects on coastal areas. In the computation of ocean propagation by the linear longwave theory, a new technique is introduced to keep the same accuracy as the linear Boussinesq equation and reduce the CPU time as well as the computer memory. In the coastal transformation computation, the energy dissipation due to sea-bottom scouring is suggested to be included, particularly in the case of long bays. To obtain accurate results, the current velocity requires finer spatial grids than the water surface elevation. Damage done to pearl culture rafts are explained in terms of the computed current velocity.

A short history of tsunami research and countermeasures in Japan

The tsunami science and engineering began in Japan, the country the most frequently hit by local and distant tsunamis. The gate to the tsunami science was opened in 1896 by a giant local tsunami of the highest run-up height of 38 m that claimed 22,000 lives. The crucial key was a tide record to conclude that this tsunami was generated by a “tsunami earthquake”. In 1933, the same area was hit again by another giant tsunami. A total system of tsunami disaster mitigation including 10 “hard” and “soft” countermeasures was proposed. Relocation of dwelling houses to high ground was the major countermeasures. The tsunami forecasting began in 1941. In 1960, the Chilean Tsunami damaged the whole Japanese Pacific coast. The height of this tsunami was 5–6 m at most. The countermeasures were the construction of structures including the tsunami breakwater which was the first one in the world. Since the late 1970s, tsunami numerical simulation was developed in Japan and refined to become the UNESCO standard scheme that was transformed to 22 different countries. In 1983, photos and videos of a tsunami in the Japan Sea revealed many faces of tsunami such as soliton fission and edge bores. The 1993 tsunami devastated a town protected by seawalls 4.5 m high. This experience introduced again the idea of comprehensive countermeasures, consisted of defense structure, tsunami-resistant town development and evacuation based on warning.