Geoscience Reference
In-Depth Information
give a long enough lead time in which to evacuate the
coast. Project THRUST (Tsunami Hazards Reduction
Utilizing Systems Technology) was established offshore
from Valparaiso, Chile, in 1986 to provide advanced
warning (within two minutes) of locally generated
tsunami along this coastline. When a sensor placed on
the seabed detects a seismic wave above a certain
threshold, it transmits a signal to the GEOS geostation-
ary satellite, which then relays a message to ground
stations. The signal is processed and another signal
is transmitted via satellite to a low-cost receiver and
antenna operating twenty-four hours a day, located
along a threatened coastline. This designated station
can be pre-programmed to activate lights and acoustic
alarms, and to dial telephones and other emergency
response apparatus when it receives a signal. For a cost
of $US15 000, a life-saving tsunami warning can be
issued to a remote location within two minutes of a
tsunamigenic earthquake. The THRUST system has
the potential to be used over a wide area of the Pacific
(Figure 10.16).
The Pacific Tsunami Warning System is not flawless.
The risk still exists in many island archipelagos along
the western rim of the Pacific, that local earthquakes
could generate tsunami too close to shore to permit
advanced warning. The 7.8 magnitude (M
s
scale)
Philippines earthquake of 16 August 1976, in the Moro
Gulf on the south-west part of the island of Mindanao,
generated a local tsunami 3.0-4.5 m high that report-
edly killed 8000 people. The event was virtually un-
predictable because the earthquake occurred within
20 km of a populated coastline. Similarly, the PNG
tsunami of 17 July 1978, which killed 2200 people, also
escaped the notice of the Pacific Tsunami Warning
System. Finally, our knowledge of tsunami is rudimen-
tary for many countries and regions in the Pacific
Ocean. While travel-time maps have been drawn up
for source regions historically generating tsunami - for
example, the coasts of South America, Alaska and
Kamchatka Peninsula - the entire Pacific Rim coast-
line has not been studied. This was made apparent on
25 March 1998 when an earthquake with a surface
magnitude, M
s
, of 8.8 occurred in the Balleny Islands
region of the Antarctic directly south of Tasmania,
Australia. Because of the size of the earthquake, a
tsunami warning was issued; but no one knew what
the consequences would be. The closest tide gauges
were located on the south coasts of New Zealand and
Australia. The Pacific Tsunami Warning Center in
Hawaii had to wait to see if any of these gauges
reported a tsunami before they issued warnings further
afield. While that may have helped residents in the
United States or Japan, it certainly was little comfort to
residents living along coastlines facing the Antarctic in
the antipodes. In cities such as Adelaide, Melbourne,
Hobart and Sydney, emergency hazard personnel
knew they were the 'mine canaries' in the warning
system. Fortunately, the Antarctic earthquake was not
conducive to tsunami and no major wave propagated
into the Pacific Ocean.
CONCLUDING COMMENTS
The seismic risk for many locations in the world can
now be assessed to the point that many buildings and
structures can be designed to withstand damage
from tremors. However, the seismic risk in many
locations, even in developed countries or ones where
earthquakes have been recorded for centuries,
cannot be assessed completely. For example, the
seismic risk for Australia is very much a product of
that country's distribution of cities (Figure 10.2). In
North America, the assessment of seismic risk in the
eastern part of the continent is still deficient because
major earthquakes occur so infrequently that, even
after centuries of settlement, the full extent of the
hazard is not known (Figure 10.2). This chapter has
not highlighted recent earthquakes but concentrated
upon two large events, the Alaskan earthquake of
1964 and the Tokyo earthquake of 1923, to illustrate
the force that can be released in a tremor and the
destruction that can result. The Alaskan earthquake
was one of the largest events in the twentieth
century. Even though it occurred in a relatively
isolated part of the world, its magnitude and sub-
sequent tsunami attracted world attention. The Tokyo
earthquake is noteworthy because of the appalling
death toll and the manner in which those deaths
occurred. The descriptions of these events could
easily have been set within southern California, a
region where earthquakes with the same magnitude
as the Alaskan one can easily occur, and a region
where the effects upon dense human settlement
might be as dramatic. In discussing the Californian
earthquake hazard, it was hoped to highlight the
extent of the hazard and some of the inadequacies in
people's attempts to minimize future damage and
loss of life. Despite decades of retro-fitting of
structures to withstand earthquakes, engineering
