Yoshinobu Tsuji

The Flores Island tsunamis

On December 12, 1992, at 5:30 A.M. GMT, an earthquake of magnitude Ms 7.5 struck the eastern region of Flores Island, Indonesia (Figure 1), a volcanic island located just at the transition between the Sunda and Banda Island arc systems. The local newspaper reported that 25-m high tsunamis struck the town of Maumere, causing substantial casualties and property damage. On December 16, television reports broadcast in Japan via satellite reported that 1000 people had been killed in Maumere and twothirds of the population of Babi Island had been swept away by the tsunamis. The current toll of the Flores earthquake is 2080 deaths and 2144 injuries, approximately 50% of which are attributed to the tsunamis. A tsunami survey plan was initiated within 3 days of the earthquake, and a cooperative international survey team was formed with four scientists from Indonesia, nine from Japan, three from the United States, one from the United Kingdom, and one from Korea.

Tsunami field survey of the 1992 Nicaragua earthquake

An earthquake with surface magnitude (Ms ) 7.0 occurred 100 km off the Nicaraguan coast on September 2, 1992 (GMT). Despite its moderate size, this earthquake generated a sizable tsunami, which caused extensive damage along the coast of Nicaragua. In late September, about 170 people, mostly children, were listed dead or missing; 500 were listed injured; and over 13,000 were listed homeless, with more than 1500 homes destroyed. Damage was the most significant since the 1983 Japan Sea earthquake tsunami, which killed 100 people in Japan. The Flores (Indonesia) earthquake and tsunami of December 12, 1992, were more destructive than the Nicaragua or Japan Sea events.

Field Survey of the East Java Earthquake and Tsunami of June 3, 1994

A field survey of the June 3, 1994 East Java earthquake tsunami was conducted within three weeks, and the distributions of the seismic intensities, tsunami heights, and human and house damages were surveyed. The seismic intensities on the south coasts of Java and Bali Islands were small for an earthquake with magnitude M 7.6. The earthquake caused no land damage. About 40 minutes after the main shock, a huge tsunami attacked the coasts, several villages in East Java Province were damaged severely, and 223 persons perished. At Pancer Village about 70 percent of the houses were swept away and 121 persons were killed by the tsunami. The relationship between tsunami heights and distances from the source shows that the Hatori’s tsunami magnitude was m = 3, which seems to be larger for the earthquake magnitude. But we should not consider this an extraordinary event because it was pointed out by Hatori (1994) that the magnitudes of tsunamis in the Indonesia-Philippine region generally exceed 1–2 grade larger than those of other regions.

Tsunami ascending in rivers as an undular bore

At time of the 1983 Japan Sea tsunami, waves in the form of a bore ascended many rivers. In some cases, bores had the form of one initial wave with a train of smaller waves, and in other cases, such a wave train did not appear and only a step with a flat water surface behind was observed. In the present study, it is clarified that both undular-type and nonundular-type bores can be recognized as solutions of the KdV-Burgers equation which was introduced by Johnson in 1972. Numerically obtained analytical solutions and results of laboratory experiments are compared.

Damage, conditions of East Java Tsunami of 1994 analyzed

On June 3, 1994, an Ms = 7.2 earthquake occured at a depth of 15 km near the east end of the Java trench in the Indian Ocean. The earthquake generated a large tsunami that violently struck southeast Java and extended to southwest Bali (Figures 1 and 2). Approximately 200 people were killed, 400 were injured, and 1000 houses were destroyed. Runup heights (Figure 2) ranged from 0–5 m in west Bali to 1–14 m in southeast Java. This unusual tsunami was generated about 250 km from the hardest hit area. Surprisingly, strong earthquake-induced ground shaking was not a precursor so local residents had no warning of the impending catastrophe. The long-period characteristics of the earthquake were incommensurate with the relatively weak high-frequency magnitude Mb=5.5, and the rate of seismic moment release grew monotonically up to at least 270s. The pattern resembled that of the Nicaragua earthquake of September 2, 1992, in which strong ground shaking did not occur. Most of the damage was concentrated in villages located in pocket beaches, unlike previous tsunami damage in west Nicaragua, Flores, Indonesia [Yeh et al., 1993], and Okushiri, Japan.

Damage to coastal villages due to the 1992 Flores Island earthquake tsunami

A field survey of the 1992 Flores Island earthquake tsunami was conducted during December 29, 1992 to January 5, 1993 along the north coast of the eastern part of Flores Island. We visited over 40 villages, measured tsunami heights, and interviewed the inhabitants. It was clarified that the first wave attacked the coast within five minutes at most of the surveyed villages. The crust was uplifted west of the Cape of Batumanuk, and subsided east of it. In the residential area of Wuring, which is located on a sand spit with ground height of 2 meters, most wooden houses built on stilts collapsed and 87 people were killed even though the tsunami height reached only 3.2 meters. In the two villages on Babi Island, the tsunami swept away all wooden houses and killed 263 of 1,093 inhabitants. Tsunami height at Riang-Kroko village on the northeastern end of Flores Island reached 26.2 meters and 137 of the 406 inhabitants were killed by the tsumani. Evidence of landslides was detected at a few points on the coast of Hading Bay, and the huge tsunami was probably formed by earthquake-induced landslides. The relationship between tsunami height and mortality was checked for seven villages. The efficiencies of trees arranged in front of coastal villages, and coral reefs in dissipating the tsunami energy are discussed.

Atmospheric pressure change associated with the 2003 Tokachi‐Oki earthquake

Clear atmospheric pressure changes associated with the 2003 Tokachi-Oki, Japan, earthquake with Mw 8.3 were recorded with the microbarographs distributed in Japan. The pressure change starts at the arrival of seismic waves and reaches its maximum amplitude at the arrival of Rayleigh waves, suggesting that the observed pressure change was driven by the ground motion of seismic waves passing by the site. We computed the seismic-to-pressure transfer function (i.e., the spectral ratio of the pressure change to the vertical ground motion velocity) for periods between 10 to 50 s from the co-located barograph and seismograph records. Comparison of the observed transfer function with the theoretical one including the finite frequency and wavelength effects for a gravitationally stratified isothermal atmosphere confirms that the observed amplitude and phase of the pressure change are explained by the acoustic coupling between the atmosphere and the ground just beneath the sensors.

Observation of Edge Waves Trapped on the Continental Shelf in the Vicinity of Makurazaki Harbor, Kyushu, Japan

Sea-bottom pressure gauges were used to measure sea levels at two points on the shelf off the southern coast of Satsuma Peninsula, Kyushu, Japan. Spectral analysis of the observed records and the tide-gauge record of Makurazaki Harbor revealed several predominant common peaks. At the same time, the eigenmodes for the trapped waves on the shelf and inside Makurazaki Bay were obtained numerically using a two-dimensional model, and the periods and the spatial distribution of amplitudes of the proper modes were obtained. A comparison of the calculated modes with the periods and phase patterns of the observed peaks clarified that peaks with periods of 19.5, 16, 13.3, and 12.2 minutes in the shelf region were the modes of standing-edge waves, and the peak with the period of 16 minutes in Makurazaki Harbor was the fundamental mode of the harbor. Among the modes of standing-edge waves, the mode of the period 16 minutes on the shelf had nearly the same period as that of the fundamental mode of Makurazaki Harbor. An analysis of changes of spectral densities of these two modes confirmed that the fundamental mode of the Makurazaki Harbor was induced by this standing-wave mode.

Response of Residents at the Moment of Tsunamis-The 1992 Flores Island Earthquake Tsunami, Indonesia-

The number of loss of lives due to the 1992 Flores island tsunamis was about 1,000. The maximum tsunami runup height 26.2m was measured at Kroko. At Babi island, reflected tsunami with the height of 5 to 7m attached two villages so that every structure was broken and swept away. At Waring, mooring fishing boats were easily carried to the dense-housing areas by tsunamis so that wooden-made houses were swept away by dominos. Through the questionnaire, tsunami characteristics and residents’ behavior are also described. Most residents have no information on tsunamis. After looking at tsunami coming, they run away to safe places. Therefore, the disaster weak such as children and old people might fell victim to the tsunami disaster. As disaster lessons, spread of tsunami information such as local tsunami characteristics and refuge programs are very important to mitigate the damages in tsunami-prone areas.

Flow Strength on Land and Damage of the 1998 Papua New Guinea Tsunami

Runup and inundation data on the sand spits of Sissano lagoon are described with discussions of flow state, current velocity, and degree of damage to houses. Sand erosion data on the sand spits are also described with discussions of their relation to the inundation depth and the current velocity. Laboratory experiments were carried out to confirm the flow state and to discuss effects of vegetation and so on.