Geoscience Reference
In-Depth Information
weight of air above the Earth's surface. The deloading
can be as much as 2-3 million tonnes km
-2
over a matter
of hours. In addition, storm surges about 6-7 m in height
can occur in shallow seas with a resulting increase in
pressure on the Earth's surface of 7 million tonnes km
-2
.
In total, the passage of a cyclone along a coast can induce
a change in load on the Earth's crust of 10 million tonnes
km
-2
. In areas where the Earth's
crust
is already under
strain, this pressure change may be sufficient to trigger
an earthquake. The classic example of a cyclone-induced
earthquake occurred with the Tokyo earthquake of 1923
(see Figure 3.2 for the location of major placenames
mentioned in this chapter). A typhoon swept through
the Tokyo area on 1 September, and an earthquake
followed that evening. The earthquake caused the
rupture of gas lines, setting off fires that were fanned by
cyclone-force winds through the city on 2 September. In
all, 143 000 people lost their lives, mainly through incin-
eration. The events of this tragedy will be described in
more detail in Chapter 10. There is also evidence that
tropical cyclones have triggered earthquakes in other
places along the western margin of the Pacific
Plate
and
along plate boundaries in the Caribbean Sea. In Central
America, the coincidence of earthquakes and cyclones
has a higher probability of occurrence than the joint
probability of each event separately.
The Tokyo earthquake is also unusual for another
reason. The typhoon winds blew the fires out of
control. This dichotomous occurrence of torrential
rain and uncontrolled fire is quite common in Japan.
Typhoons moving north-east in the Sea of Japan
produce very strong winds in the lee of mountains. In
September 1954 and again in October 1955, cyclone-
generated winds spread fires that destroyed over 3300
buildings in Hokkaido and 1100 buildings in Niigata,
respectively. Nor is the phenomenon of cyclone-fire
unique to Japan. The great hurricane of 1938 that dev-
astated New England, in the United States, started
fires in New London, Connecticut, that raged for six
hours, and would have destroyed the city of 30 000
people if it were not for the fact that the fires were
turned back on themselves as the winds reversed direc-
tion with the passage of the cyclone.
Storm surge
Wind
Rain
Flooding of coastal
lowland
Disruption to
transportation
Severe flooding
Contaminated
water supplies
Death, injuries
Saline intrusion
Spread of
disease
Urban fire
Earthquakes
Loss of crops
& livestock
Landslides
Coastal erosion
Damage to
structures
Death from drowning
& disease
Death from
drowning
Hazards related to occurrence of tropical cyclones.
Fig. 3.1
through drowning, inundation of low-lying coastal
areas, erosion of coastline, loss of soil fertility due to
intrusion by ocean saltwater, and damage to buildings
and transport networks. In the largest cyclones, winds
can exceed 300 km hr
-1
. High wind velocities can
directly cause substantial property damage and loss of
life, and constitute the main agent for crop destruction.
Surprisingly, strong winds - simply because they are so
strong - can also exacerbate the spread of fires in urban
and forested areas, even under heavy rainfall.
On average, a tropical cyclone can dump 100 mm
per day of rain within 200 km of the eye, and
30-40 mm per day at distances of 200-400 km. These
rates can vary tremendously depending upon local
topography, cyclone motion, and the availability of
moisture. In 1952, a tropical cyclone at Reunion Island
dropped 3240 mm of rain in three days. Rainfall is
responsible for loss of life, property damage, and crop
destruction from flooding - especially on densely pop-
ulated floodplains. Contamination of water supplies
can lead to serious disease outbreaks weeks after the
cyclone. Heavy rain in hilly or mountainous areas is
also responsible for landslides or mudflows as flood-
waters in stream and river channels mix with excess
sediment brought down slopes. The destruction of
crops and saline intrusion can also result in famine that
can kill more people than the actual cyclone event.
This was especially true on the Indian subcontinent
during the latter part of the nineteenth century.
Earthquakes are not an obvious consequence of
cyclones; however, there is substantial evidence for their
occurrence during cyclones. Pressure can vary dramati-
cally in a matter of hours with the passage of a cyclone,
bringing about a consequentially large decrease in the
Mec
hanics of cyclone generation
(Anthes, 1982; Nalivkin, 1983; Gray, 1984; Gray et al.,
1994; Chang et al., 2003; Simpson, 2003)
Tropical cyclones derive their energy from the evapo-
ration of water over oceans, particularly the western
