Geoscience Reference
In-Depth Information
25 years. The high number of tsunami in Oceania
includes those at Hawaii, which receives about
85 per cent of all tsunami affecting the Pacific. In fact,
Japan and Hawaii are the two most tsunami-prone
regions in the world, each accounting for 19 per cent
of all tsunami measured. For this reason, Hawaii
monitors closely all tsunami source regions in the
Pacific.
Peru and Chile are most affected by local earth-
quakes. This coastline is also the only source area to
affect the south-west Pacific region. Japan is mostly
affected by locally generated tsunami, and directs its
attention to predicting their occurrence locally. Since
684 AD, in the Japan region, 73 tsunami have resulted
in over 200 000 deaths. Indonesia and the Philippines
are also affected by locally originating tsunami. In the
Philippines, recorded tsunami have killed 50 000
people, mainly in two single events in 1863 and 1976.
Indonesia has experienced a similar death toll in
recorded times; however, half this total has been
caused by tsunami associated with local volcanism. The
Atlantic coastline is virtually devoid of tsunami;
however, the Lisbon earthquake of 1755 produced a
3-4 m wave that was felt on all sides of the Atlantic.
The continental slope off Newfoundland, Canada, is
seismically active and has produced tsunami that have
swept onto that coastline. One recorded tsunami from
this site reached Boston, with a height of 0.4 m, on
18 November 1929. One of the longest records of
tsunami occurrence exists in the eastern Mediter-
ranean. Between 479 BC and 1981 AD, 7 per cent of
the 249 known earthquakes produced damaging or
disastrous tsunami. Here, about 30 per cent of all
earthquakes produce a measurable seismic wave.
Tsunami generated close to shore are not so large as
those generated in deeper water. Tsunami have wave
periods in the open ocean of minutes rather than
seconds, and heights up to 0.5 m. Waves with this
period will travel at speeds of 600-900 km hr
-1
(166-250 m s
-1
) in the deepest part of the ocean and
100-300 km hr
-1
(28-83 m s
-1
) across the continental
shelf. In some cases in the Pacific Ocean, initial wave
periods of 60 minutes have been recorded. Wave
periods of this length in the open ocean will often
degrade to less than 2.5 minutes in shallow water.
Tsunami resonating in harbors typically have periods of
8-27 minutes. If the wave period is some harmonic of
the natural frequency of the harbor or bay, then the
amplitude of the tsunami wave can greatly increase
over time. This resonance may cause the period of the
wave to shift to higher wave periods (lower frequen-
cies). Figure 10.12 plots typical tide gauge records or
marigrams
of tsunami at various locations in the Pacific
Ocean. The concept that a tsunami consists of only one
or two waves is not borne out by the records. Some
wave periods range between 8 and 30 minutes and
persist for over six hours. Wave characteristics are
highly variable. In some cases, the waves consist of an
initial peak that then tapers off in height exponentially
over four to six hours. In other cases, the tsunami wave
train consists of a maximum peak well back in the wave
sequence.
The wavelength of an average tsunami of eight
minutes is about 360 km (refer to Equation 8.3). As
explained in Chapter 8, with this wavelength it is
possible for the tsunami to feel the ocean bottom at any
depth, and to undergo refraction effects. Refraction
diagrams of tsunami give accurate prediction of arrival
times across the Pacific, but not of run-up heights near
shore. It has been shown that the spread of tsunami
close to shore and around islands follows classic dif-
fraction theory (Figure 8.3c). Figure 10.13 illustrates
the movement of the Chilean tsunami wave of 23 May
1960, which wreaked havoc throughout the Pacific
region. It clearly shows that the wave radiated sym-
metrically away from the epicenter, and was unaffected
by bottom topography until islands or seamounts were
approached. The island chains in the west Pacific
clearly broke up the wavefront; however, much of
Japan received the wave unaffected by refraction.
It is a very noticeable phenomenon that ships,
anchored several kilometers out to sea, hardly notice
the arrival of a tsunami wave. However, once the wave
approaches shore, the wave very rapidly shoals and
reaches its extreme height. This run-up height controls
the extent of damage. Because most shorelines are
relatively steep compared to the wavelength of the
tsunami, the wave will not break or form a bore but will
surge up over the foreshore (Figure 10.14). For
example, the earthquake offshore from Gisborne,
New Zealand, on 26 March 1947, generated a tsunami
run-up 10 m high along a 13 km stretch of coast, while
the eruption of Krakatau in 1883 generated a wave
that reached elevations up to 40 m high along the
surrounding coastline. The largest recorded earth-
quake-generated tsunami wave occurred in 1737 on
the Kamchatka Peninsula when a 64 m high wave
washed across the southern tip of the peninsula. By far
