Geoscience Reference
In-Depth Information
One of the most striking historical examples of volcanogenic tsunamis is repre-
sented by the waves caused by the activity of the Krakatau volcano in August of
1883 [Choi et al. (2003)]. On August 26, at 17:00, local time, a series of loud ex-
plosions took place, and the volcano ejected an ash cloud to a height of up to 25 km.
A small tsunami 1-2 m high formed. In the morning of August 27 three colossal
explosions took place. The first explosion (at 5 o'clock 28 min) destroyed mountain
Perboewatan on Krakatau island, which was 130 m high. The caldera produced was
immediately filled up with sea water, leading to the generation of a small tsunami.
At 6.36 mountain Danan, which was 500 m high, exploded and collapsed, which
gave rise to a tsunami wave up to 10 m high. The main (third) explosion took place
at 9.58. It literally blew apart what remained of Krakatau island (Rakata island). The
volcano threw out 9-10 km
3
of tephra (solid material) and 18-21 km
3
of pyroclastic
deposits that were distributed over an area of about 300 km
2
with an average thick-
10
6
km
2
. In the place
of the island there emerged a caldera 6 km in diameter and 270 m deep. The third
explosion came with the most strongest noise ever heard by mankind. Air-blasts cir-
cumvented the globe seven times. The energy released during the main eruption of
the Krakatau volcano amounted to 8
.
4
ness of 40 m. Ash covered a territory of approximately 2
.
8
·
10
17
J. The waves that resulted from the third
most strong explosion were 42 m high; they arrived 5 km inland. The average height
of waves on the coast of the Sunda straits (separating islands Java and Sumatra)
was about 15 m. At least 36,000 people died. About 300 villages were destroyed.
The tsunami caused by explosion of the Krakatau volcano was noticed everywhere.
Waves were recorded by many mareographs not only in the Indian Ocean, but also
in the Pacific and Atlantic. Far from the coasts of Indonesia wave amplitudes were
relatively small.
On the basis of the diameter and depth of the caldera formed as a result of the ex-
plosion of the Krakatau volcano, it is not difficult to estimate the volume of the ini-
tial perturbation—the 'local depression' of the ocean level. It amounts to
·
7km
3
.
It is interesting to note that this volume approximately corresponds to the volume
of water ousted by ocean bottom deformations in the case of strong earthquakes
(100
∼
1m = 10 km
3
). The potential energy corresponding to the initial
perturbation that can be estimated by formula (2.2) amounts to
100 km
2
×
×
10
15
J, which is
∼
6
×
of the order of 1% of the eruption energy.
Another frequently discussed event took place in the Bronze Age (around 35
centuries ago) in the Aegean sea. There exists a hypothesis that explosion of the vol-
canic Thera island (the Santorini volcano) and the resulting tsunami caused the death
of mythical Atlantis, while the explosive eruption itself contributed to destruction of
the Cretan-Mycenaean culture. At any rate, geological traces of this tsunami have
been found along the coastlines of Greece and Turkey [Minoura et al. (2003)].
Of the 933 volcanoes, known to be active on Earth, 195 are underwater volca-
noes. Numerous works are devoted to the study of tsunami generation, related to
volcanic eruptions [Basov et al. (1981); Egorov (1990), (2007); Pelinovsky (1996);
Waythomas, Neal (1998); Belousov et al., (2000); Tinti et al., (2003); Ward, Day
(2001), (2003); Kurkin, Pelinovsky (2004); Mader, Gittings (2006)]. The main phys-
ical mechanisms of volcanogenic tsunami generation comprise the following:
