December 16, 1975 Tsunami: Fiji Islands

Shortly after the minor (MS 5.2) earthquake was felt in 1975 fluctuations in sea level were observed over a period of about an hour and a half at Laucala Bay (Fig. 3) and for an unknown period off' northeastern Kadavu at Ono (Everingham, 1983b). In Laucala Bay waters the amplitude, measured from crest to trough, may have been about 0.5 m (i.e. the order of half the tidal range) because sand banks appeared and disappeared and the change in level on the barrier reef was sufficient to have a noticeable effect on appearance of the ocean waves breaking on the reef edge, according to reliable observers at the MRD office which is about 2 km from Laucala Bay shoreline and 8 km from the reef bounding the seaward side of the bay (Fig. 3). Unfortunately the only available tide-gauge data in Suva are smoothed 15-minute mean plots and mean hourly values of the Suva Harbour sea level. These data 'effectively smooth out the tsunami waves and a tsunami in Suva Harbour could not be detected. The occurrence of the waves is of particular interest because earthquakes with such a small magnitude do not generally cause tsunamis (Iida, 1970). The tsunami was probably generated by a minor submarine slump caused by the shaking (intensity MM 4-5) in the Suva-Beqa outer reef zone in a manner similar to that of the 1953 tsunami (Everingham, 1983b), which illustrates .the ease with which a slump can be initiated. If slumping can be started by relatively low earthquake intensities even a minor shallow tremor in the southeastern Viti Levu seismic zone (Fig. 4) could start a submarine slump should the epicentre be close to a suitable source. (reference #3817)