Introduction - Tsunami - page 8

1.4 Causes of Tsunami

atmospheric pressure pulse generated water oscillations that
were measured in the English Channel 37 h later; on the
other side of the Pacific Ocean at Panama and in San
Francisco Bay; and in Lake Taupo in the center of the North
Island of New Zealand (Choi et al. 2003 ). Probably the most
devastating event was the Santorini Island eruption around
1470 BC, which generated a tsunami that must have
destroyed all coastal towns in the eastern Mediterranean
(Menzies 2012 ). The Santorini crater is five times larger in
volume than that of Krakatau, and twice as deep. On
adjacent islands, there is evidence of pumice stranded at
elevations up to 50 m above sea level. The initial tsunami
waves may have been 90 m in height as they spread out
from Santorini (Kastens and Cita 1981 ). Volcanoes as a
cause of tsunami will be discussed in Chap. 8 .
There has been only one suspected historical occurrence
of tsunami produced by an asteroid/comet impact with the
ocean. This occurred on September 28, 1014 (Haslett and
Bryant 2008 ). However, this does not mean that they are an
inconsequential threat. Stony asteroids as small as 300 m in
diameter can generate tsunami over 2 m in height that can
devastate coastlines within a 1,000 km radius of the impact
site (Hills and Mader 1997 ). The probability of such an event
occurring in the next 50 years is just under 1 %. One of the
largest impact-induced tsunami occurred at Chicxulub,
Mexico, 65 million years ago at the Cretaceous-Tertiary
boundary (Alvarez 1997 ). While the impact was responsible
for the extinction of the dinosaurs, the resulting tsunami
swept hundreds of kilometers inland around the shore of the
early Gulf of Mexico. Impact events are ongoing. Astrono-
mers have compiled evidence that a large comet encroached
upon the inner solar system and broke up within the last
14000 years (Asher et al. 1994 ). The Earth has repetitively
intersected debris and fragments from this comet. However,
these encounters have been clustered in time. Earlier civi-
lizations in the Middle East were possibly destroyed by one
such impact around 2350 BC. The last rendezvous occurred
as recently as AD 1500; however, it occurred in the Southern
Hemisphere where historical records did not exist at the
time. Only in the last decade has evidence become available
to show that the Australian coastline preserves the signature
of mega-tsunami from this latest impact event. The geo-
morphic signatures of tsunami, including mega-tsunami,
will be presented in Chaps. 3 and 4 , while asteroids as a
cause of mega-tsunami will be discussed in Chap. 9 .
Finally, meteorological events can generate tsunami
(Monserrat et al. 2006 ). These tsunami are common at
temperate latitudes where variations in atmospheric pres-
sure over time are greatest. Such phenomena tend to occur
in lakes and embayments where resonance of wave motion
is possible. Resonance and the features of meteorological
tsunami will be described at the end of this chapter.

Most tsunami originate from submarine seismic distur-
bances. The displacement of the Earth's crust by several
meters during underwater earthquakes may cover tens of
thousands of square kilometers and impart tremendous
potential energy to the overlying water. Tsunami are rare
events, in that most submarine earthquakes do not generate
one. Between 1861 and 1948, as few as 124 tsunami were
recorded from 15,000 earthquakes (Iida 1963 ). Along the
west coast of South America, 1,098 offshore earthquakes
have generated only 20 tsunami. This low frequency of
occurrence may simply reflect the fact that most tsunami are
small in amplitude—and go unnoticed—or the fact that
most earthquake-induced tsunami require a shallow focus
seismic event with a moment magnitude, M w , greater than
6.3 (Roger and Gunnell 2011 ). Earthquakes as a cause of
tsunami will be discussed in fuller detail in Chap. 5 .
Submarine earthquakes have the potential to generate
landslides along the steep continental slope that flanks most
coastlines (Bryant 2005 ). In addition, steep slopes exist on
the sides of ocean trenches and around the thousands of
ocean volcanoes, seamounts, atolls, and guyots on the sea-
bed. Because such events are difficult to detect, submarine
landslides are considered a minor cause of tsunami. The
July 17, 1998 Papua New Guinea event renewed interest in
this potential mechanism (Tappin et al. 1999 ). A large
submarine landslide or even the coalescence of many
smaller slides has the potential to displace a large volume of
water. Geologically submarine slides involving up to
20,000 km 3 of material have been mapped (Moore 1978 ;
Masson et al. 2006 ). Tsunami arising from these events
would be much larger than earthquake-induced waves. Only
in the last 50 years has coastal evidence for these mega-
tsunami been uncovered. Submarine landslides as a cause of
tsunami and mega-tsunami will be discussed in Chap. 7 .
Tsunami can also have a volcanic origin. Of 92 docu-
mentable cases of tsunami generated by volcanoes, 16.5 %
resulted from tectonic earthquakes associated with the
eruption, 20 % from pyroclastic (ash) flows or surges hit-
ting the ocean, 14 % from submarine eruptions, and 7 %
from the collapse of the volcano (Wiegel 1964 ). A volcanic
eruption rarely produces a large tsunami, mainly because
the volcano must lie in the ocean. For example, the largest
explosive eruption of the past millennium was Tambora in
1815. It only produced a local tsunami 2-4 m high because
the volcano lay 15 km inland (Bryant 2005 ). In contrast, the
August 27, 1883 eruption of Krakatau, situated in the Sunda
Strait of Indonesia, produced a tsunami with nearby run-up
heights exceeding 40 m above sea level (Verbeek 1884 ;
Blong 1984 ; Myles 1985 ). The wave was detected at the
Cape of Good Hope in South Africa 6,000 km away. The