Kuniaki Abe

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.

Dominant periods of the 2004 Sumatra tsunami and the estimated source size

Dominant periods determined from tide gauge records of the 2004 Sumatra tsunami were used to estimate the size of the tsunami source. A period of 40 minutes dominates the initial motions and spectra are consistent with a fault width of 110 km for a sea depth of 1000 m. Another dominant period of 140 minutes in the spectra observed at the direction of fault azimuth can be explained using a fault length of 1200 km. These values taken together are consistent with source formation on the continental slope with width of 110 km and length of 1200 km along Nicobar and Andaman Islands.

Distribution of maximum water levels due to the Japan sea tsunami on 26 May 1983

By measuring the maximum water level of the traces attained by the Japan Sea Tsunami on 26 May 1983, we obtained the distribution along the west coast of the northeast Japan. The level reaches a maximum at the coast eastward of the epicenter and decreases with the relationship 8.6e−0.017x(m) with distancex (km) measured from the coast nearest to the epicenter. A small increase of levels was observed at coasts to the south of the tsunami source having distance larger than 200 km. With the aid of tide gauge records we revealed an excitation of edge wave which brought about the small increase of levels at the southern coast. In comparison with the decrease with distance obtained on the coasts of the main islands of Japan, some noticeable peaks were observed at several small islands. It is suggested that the reason why a short period component is predominant for the initial wave motion of tsunami is that the source region has depth of 3, 000 meters. The feature of wave period is discussed in comparison with that of the 1964 Niigata Tsunami.

Source Model of Noto-Hanto-Oki Earthquake Tsunami of 7 February 1993

A source model was discussed for a small tsunami accompanied by the Noto-Hanto-Oki earthquake (Ms 6.6), striking Japan on 7 February, 1994. Assuming a fault model under the sea bottom, we estimated the focal parameters jointly, using synthesized tsunami source spectra as well as the tsunami numerical simulation. The fault proposed by this study consists of a plane sized 15×15 km, dipping N47°W with the dip angle of 42°, which is almost pure reverse fault (slip angle 87°) with a dislocation of 1 meter. The numerical simulation shows that the shallow sea in the source region caused a comparatively long recurring tsunami (the periods are 12–18 minutes) in spite of its small size. The model fault is corresponding to an aftershock area of this earthquake.

Phases Representing Source Lengths of Tsunami in Tide Gauge Records

We identified a phase representing the source length of tsunami’s in the tide gauge records around Japan. This phase was observed at tide stations, located in the direction of the long axis of the sources, for four large tsunamis: 1964 Niigata, 1968 Tokachi-oki, 1983 Nihonkaichubu, and 1993 Hokkaido-nanseioki. The phase consists of two continuous crests starting as the initial arrival and has a time length of 15–47 minutes. This is the time required to propagate across the source area along the long axis. Strong evidence that the phase is generated at the source is the good correlation between waveform observed at one side and time-inversed waveform at another side. The correlation results from the instantaneous generation of the source. The source lengths of 74–254 km were obtained under an assumption of sea depths at the sources and verified to coincide with ones within a relative error of 15% that were previously obtained by other methods.