Volcanic Eruptions - Tsunami - page 153

8

Volcanic Eruptions

8.1 Introduction

accompanied by seismic tremors. If these are substantial
enough and the volcano lies near or in the ocean, the
tremors can trigger tsunami. For example, the eruption of
Vesuvius in the southeast corner of the Bay of Naples on the
west coast of Italy, in August AD 79, was preceded by a
tsunami induced by seismic activity. Pliny the Elder, the
commander of the Roman fleet at Misenum, sailed to the
coast at the base of the mountain to rescue inhabitants
5 days before the final eruption. He could not get near the
shore because of a sudden retreat of the shoreline. Two of
the largest events due to seismic activity occurred on the
January 10, 1878 and January 8, 1933 with the eruptions of
Yasour Volcano in the New Hebrides and Severgin Volcano
in the Kuril Islands, respectively. The respective tsunami
reached 17 and 9 m above sea level.
Pyroclastic flows, or nuées ardentes, are generated by the
collapse under gravity of hot vertically ejected ash clouds
(Blong 1984 ). When these reach the ocean surface, they
spread out rapidly as density flows that can either displace
water or transfer energy to the ocean and generate a tsunami
(Latter 1981 ). The size of the resulting tsunami depends upon
the density of the flow. If the density is less than seawater,
then the ash cloud rides the surface of the ocean, generating a
small wave. However, if the flow is denser than seawater, the
cloud will sink to the bottom of the ocean and will displace
water piston-like in front of it. In some cases, these flows can
travel tens of kilometers along the seabed. Pyroclastic flows
have the potential to generate devastating tsunami remote
from the source of the eruption. For example, Tambora in
1815 generated a tsunami 2-4 m high in this manner despite
lying 15 km inland. The May 7, 1902 eruption of Mt. Pelée,
Martinique produced a nuée ardente that swept into the
harbor of St. Pierre and generated a tsunami that travelled as
far as Fort de France, 19 km away (Lockridge 1988 ). Two of
the largest of these types of events occurred in Indonesia
during the eruption of Ruang on March 5, 1871 and Krakatau
on August 26-27, 1883, producing tsunami 25 and 10 m
high, respectively (Latter 1981 ).

Historically, volcanoes cause 4.6 % of tsunami and 9.1 %
of the deaths attributable to this hazard, totaling 41,002
people (Intergovernmental Oceanographic Commission
1999 ; National Geophysical Data Center 2013 ). Two events
caused this disproportionately large death toll: the Krakatau
eruption of August 26-27, 1883 (36,000 deaths) (Fig. 8.1 )
and the Unzen, Japanese eruption of May 21, 1792 (4,300
deaths). Tsunami account for 20-25 % of the deaths
attributable to volcanic eruptions. The eruption of Santorini
around 1470 BC is not included in these statistics because
of a lack of written record. Santorini and the Krakatau
eruption of 1883 will be discussed in more detail subse-
quently in this chapter. The main locations of the 65 tsu-
nami linked to eruptions historically are plotted in Fig. 8.2
(Latter 1981 ; Intergovernmental Oceanographic Commis-
sion 1999 ). The vast majority of these are restricted to the
Japanese-Kuril Islands and the Philippine-Indonesian
Archipelagos. Both of these regions form island arcs where
one plate is being subducted beneath another. Explosive
volcanism with caldera formation is a common occurrence
in these regions. Other isolated cases of eruptions that have
generated tsunami are associated with hot spots beneath the
Pacific Plate. Unfortunately, volcano-induced tsunami nei-
ther have been recorded well nor described except for a few
events such Krakatau in 1883.

8.2 Causes of Volcano-Induced Tsunami

There are ten mechanisms whereby volcanic eruptions can
generate tsunami (Latter 1981 ). These together with their
major events are summarized in Table 8.1 . Submarine
landslides sloughed off from non-erupting volcanoes are not
included in this table because they were dealt with in the
preceding chapter. Many of the events listed in Table 8.1
were catastrophic. The majority of volcanic eruptions are