Geoscience Reference
In-Depth Information
regions have a 20 per cent probability in 100 years of a
destructive earthquake corresponding to an intensity
of 8 or greater on the Mercalli scale, and 6.5 or greater
on the M s scale. The seismic risk in Turkey, Iran, and
the region bounding the Tibetan Plateau exceeds that
in North America. Seismic risk, however, does not
equate to damage or loss of life. These latter factors
depend upon the establishment of building codes by
planners and engineers, and their enforcement, espe-
cially in urban areas, by government. For example, the
17 August 1999 Izmit earthquake (M s = 7.4) in Turkey,
where codes are lax, killed 25 000 people compared to
the similarly sized, 17 October 1989, earthquake
(M s = 7.1) in San Francisco - where codes are strictly
enforced - which killed only 62 people.
(Keller, 1982; Giardini et al., 1999)
Maps can be constructed, based upon the historical
record of seismic waves, to give probability estimates
of the peak ground wave acceleration possible for any
region over a fixed period. These accelerations are
usually expressed as the 10 per cent probability over a
fifty-year period. In terms of recurrence interval, they
represent a 1:475 year event. Engineers can use such
maps to assess the lateral forces acting on objects, such
as rigid buildings, to determine the risk of their
collapse during earthquakes. Figure 10.2 presents the
seismic risk for the world's landmasses based upon
historical data. Obviously, the estimated risk is a
function of the historical record and spatial detection
of earthquakes. For example, much of Australia (see
Figure 10.3 for the location of all major placenames
mentioned in this chapter), which lies in the middle of
a continental plate, has a higher risk than Africa, which
contains the Great Rift Valley and a historically seismic
region in Algeria. In Australia, seismic detection is
better developed than in most of Africa. In North
America, publicity is overexaggerated regarding the
seismic risk in southern California around the San
Andreas Fault zone. The northern Rocky Mountains
and the lower St Lawrence estuary are equally at risk.
Large urban centers such as Salt Lake City, Seattle,
Montreal, and Quebec City are all seismic. These latter
Gen eral
(Holmes, 1965; Cornell 1976; Whittow, 1980; Ritchie &
Gates, 2001)
Each year on average, earthquakes kill 10 000 people
and cause $US400 million property damage. In the
period 1964-1978, over 445 000 people lost their
lives from earthquakes or earthquake-related hazards.
Table 10.1 lists significant earthquakes in terms of loss of
life since 800 AD. As with tropical cyclones, there have
been some notable events. In 1290, in China, two earth-
quakes - in the Gulf of Chihli, and at Beijing - took
Peak ground acceleration m s -2
Global seismic risk map. Shading defines peak horizontal accelerations with a probability of exceedence of 10 per cent in 50 years or a
recurrence interval of 475 years (based on Giardini et al., 1999).
Fig. 10.2
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