Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 114

Tsunamis have been responsible for nearly a quarter of people killed during
volcanic eruptions (Dudley and Lee, 1998). Historically, there have been 92 major
tsunamis generated by volcanic eruptions. The tsunamis result from pyroclas-
tic flows entering the water, submarine explosions, earthquakes associated with
theeruption, caldera collapse or volcanic landslides. One of the best known
tsunamis associated with a volcano occurred during the Krakatau eruption of
1883. Numerous tsunamis were produced, probably by pyroclastic flows enter-
ing the sea, the day before the major eruption. The highest of these tsunamis
wasabout 10 m. At 9.58 am on the 27th August 1883, the major explosion
of Krakatau occurred generating a 30 m high tsunami that killed many thou-
sands of people. The entire population of one island approximately 13 km away
from the centre of the eruption was killed. In all, nearly 40 000 people died due
to the series of tsunamis that occurred from the afternoon of the 26th August
till approximately midday on the 27th. Tsunamis from this eruption rounded
theCape of Good Hope and entered the Atlantic Ocean to be recorded as small
wavesinFrance. Strange tidal oscillations also occurred in Sydney Harbour fol-
lowing the eruption. Whether these were part of a tsunami that ran along the
east coast of Australia, probably south to north, or were due to the atmospheric
shock wave that circled the globe following the eruption, is not known. A small
tsunami was even observed on Lake Taupo on the north island of New Zealand,
which being unconnected to the sea must have been generated by the shock
wave.Itisuncertain which elements of the eruption caused the major tsunami
along the coasts of Java and the Sunda Strait. The tsunami could have resulted
from the collapse of the caldera, pyroclastic flows entering the sea or submarine
explosions, or indeed a combination of all of these may have been responsible.
Meteotsunamis
Meteorological tsunamis are long-period waves generated by either
shock waves in the atmosphere or sudden changes in atmospheric pressure asso-
ciated with squall lines and/or tropical cyclones. They have been recognised the
world over and are given a variety of local names such as 'Rissaga' (Mediter-
ranean Sea), 'Abiki' and 'Yota' (Japan), 'Marubbio' (Sicily), 'Seebir' (Baltic Sea) and
'Stigazzi' (Gulf of Fiume). The absence of a seismic generating source suggests
to some that they are not true tsunamis, but their physical form and behaviour
is identical. Meteotsunamis have periods of many minutes (from several to tens)
and will build in height as they approach the shore. Their formation is thought
to be related to the formation of an inverted barometer wave on the ocean
that can resonate (through Proudman resonance) and amplify in height. This
occurs when the atmospheric pressure jump and coupled ocean wave travel at