Geoscience Reference
In-Depth Information
Tectonically generated tsunami account for the greatest
death toll, 84.5 per cent, with volcanic eruptions
accounting for 11.2 per cent - mainly during two
events, the Krakatau eruption of 26-27 August 1883
(36 417 deaths) and the Unzen, Japan, eruption of
21 May 1792 (14 524 deaths). The number of fatalities
has decreased over time and is slightly concentrated in
South-East Asia, including Japan. The biggest tsunami
of the twentieth century occurred in Moro Gulf,
Philippines, on 16 August 1976 where 8000 people
died. The largest total death toll is concentrated in the
Japanese Islands where 211 300 fatalities have
occurred. Two events affected the Nankaido region of
Japan on 28 October 1707 and 20 September 1498,
killing 30 000 and 26 000 people, respectively. The
Sanriku Coast of Japan has the misfortune of being
the heaviest populated, tsunami-prone, coast in the
world. About once per century, killer tsunami have
swept this coastline, with two events striking within a
forty-year time span. On 15 June 1896, a small earth-
quake on the ocean floor 120 km south east of the city
of Kamaishi sent a 30 m wall of water crashing into the
coastline and killing 27 122 people. The same tsunami
event was measured 10.5 hours later in San Francisco
on the other side of the Pacific Ocean. In 1933, disaster
struck again when a similarly positioned earthquake
sent ashore a wave that killed three thousand inhabi-
tants. Deadly tsunami have also affected Indonesia and
the South China Sea. In the South China Sea, recorded
tsunami have killed 77 105 people, mainly in two events
in 1762 and 1782. Indonesia has experienced a compa-
rable death toll (69 420 deaths) over this period with
the largest following the eruption of Krakatau in 1883.
While earthquakes are responsible for most
destructive tsunami, the Santorini and Krakatau
volcanic eruptions were probably the most devastat-
ing volcanic events. The Krakatau eruption of
27 August 1883, which will be discussed in more
detail in the next chapter, sent out a tsunami wave
measured around the world. The wave rounded the
Cape of Good Hope in South Africa 6000 km away,
and was recorded along the English Channel 37 hours
later. On the other side of the Pacific Ocean, water
levels were affected along the west coast of Panama
and in San Francisco Bay. The waves that rounded
the world were probably associated with a substantial
atmospheric shock wave, as some tsunami were
detected in bodies of water that were not connected
to each other. For instance, it is difficult to see how
the tsunami wave could have got through the island
archipelagos of the west Pacific to register at San
Francisco. In addition, a 0.5 m oscillation was measured
in Lake Taupo, in the center of New Zealand, coinci-
dentally with the passage of the atmospheric shock
wave. The Santorini eruption of 1470 BC generated a
tsunami that must have destroyed all coastal towns in
the eastern Mediterranean. 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 250 m above sea
level. The initial tsunami waves may have been 60 m in
height as they spread out from Santorini.
The Lisbon earthquake of 1 November 1755, which
is possibly the largest earthquake known (9.0 on the M
s
scale), resulted in the lower town being submerged
under a tsunami 15 m in height. The wave raised tide
levels 3-4 m above normal in Barbados and Antigua,
West Indies, on the other side of the Atlantic. Tsunami
also affected the west coast of Europe and the Atlantic
coast of Morocco. The Spanish port of Cadiz, as well as
Madeira in the Azores, was also hit by waves 15 m high,
while a 3-4 m high wave sank ships along the English
Channel. Water level oscillations were also noticed in
the North Sea. The Caribbean has also witnessed its
share of devastating tsunami. Noted events occurred
in 1842, 1907, 1918, and 1946. However, the worst
tsunami recorded destroyed Port Royal, Jamaica, in
June 1692. The earthquake that triggered the tsunami
collapsed the city, sending much of it sliding into the
sea because of liquefaction. The resulting tsunami
flung ships standing in the harbor inland over two-
storey buildings. Because of the small size of the town,
the death toll was fortunately only 2000.
The most active area seismically producing tsunami
is situated along the eastern edge of the Nazca Plate,
along the coastlines of Chile and Peru. This region
has been inundated by destructive tsunami at roughly
30-year intervals in recorded history - in 1562, 1570,
1575, 1604, 1657, 1730, 1751, 1819, 1835, 1868, 1877,
1906, 1922 and, most recently, 1960. The tsunami
events of 21-22 May 1960 were produced in Chile by
over four dozen earthquakes with magnitudes up to
8.5 and 9.6 on the M
s
and M
W
scales, respectively.
A series of tsunami waves, spread over a period of
18 hours, took over 2500 lives across the Pacific, and
produced property damage in such diverse places as
Hawaii, Pitcairn Island, New Guinea, New Zealand,
Japan, Okinawa, and the Philippines (see Figure 10.13).
