Geoscience Reference
In-Depth Information
föhn winds were desiccating. All descriptions of the
fires allude to the strong winds, which swept flames out
of control.
Since 1923, the Japanese economy has grown to be
a crucial component of the global economy. The world
has been waiting for the next large Tokyo earthquake.
The death toll will be as high as 200 000 with up to
3.5 million people left homeless. The 1923 earth-
quake cost Japan 37.5 per cent of its gross national
product (GNP). In 1987, a similar sized event would
have cost 23 per cent of GNP or $US500-850 billion.
This is more than Japan's entire overseas capital
investment. The global insurance industry could cover
only a small percentage of this loss and Japan would
begin retrieving international investments to pay for
reconstruction. Such a move would trigger a global
recession that could last over five years. On 17 January
1995, a strong earthquake, measuring 7.2 on the M
s
scale, struck the Kobe-Osaka region, the second most
populated area in Japan. The earthquake - now
known as the Great Hanshin Earthquake - killed 5500
people, injured 35 000 others and destroyed or badly
damaged 180 000 buildings. Liquefaction caused the
dockyards to sink below sea level and displaced
supports for bridges. Shearing of inadequately
designed or poorly constructed columns collapsed
expressways, railway lines, and commercial premises.
Building codes were designed to prevent structures
crumbling during such an earthquake; yet, 4 per cent
of buildings constructed to those codes suffered
extensive damage and 2.5 per cent collapsed. Over
25 per cent of buildings higher than eight stories
suffered severe damage. Fires consumed the
crowded, older residential section of the city. Fewer
than 5 per cent of homes were insured. Damage
totalled $US100-150 billion or 13 per cent of
the national budget, a staggering amount given the
limited size of the earthquake. More significantly,
the Nikkei index dropped 5.6 per cent, wiping
the same amount off the share market. The fall of the
share market spread globally. The Japanese economy,
already in recession, stagnated further. Bureaucratic
rigidity and archaic chains of command paralyzed
relief efforts for several days. The response by the
Self-Defense Forces was tardy, hampered by politics,
and ineffectual. Everything about the Kobe earth-
quake indicates that a bigger event hitting Tokyo, as it
inevitably must in the near future, will cause an even
worse disaster scenario.
LIQUEFACTION OR
THIXOTROPY
(Hansen, 1965; Scheidegger, 1975; Hays, 1981; Lomnitz,
1988; Bolt, 1993; Pinter et al., 2003)
Liquefaction is a process whereby relatively firm clay-
free sands and silts can become liquefied and flow as a
fluid. This can lead to the sinking of objects that they
have been supporting. The mechanics of liquefaction
will be discussed in more detail in Chapter 13. For
now, liquefaction will be discussed qualitatively as an
earthquake-induced hazard. The weight of a soil is
supported by the grains, which touch each other.
Below the watertable, spaces or
voids
in the soil fill
with water. As long as the soil weight is borne by grain-
to-grain contacts, the soil behaves as a rigid solid.
However, if the pressure on water in the voids
increases to the point that it equals or exceeds the
weight of the soil, then individual grains may no longer
be in contact with each other. At this point, the soil par-
ticles are suspended in a dense slurry that behaves as a
fluid. It is this process, whereby a rigid soil becomes
a liquid, that is termed liquefaction or thixotropy.
Liquefaction can be induced by earthquake-
generated shear or compressional waves. Shear waves
distort the granular structure of soil causing some void
spaces to collapse. These collapses suddenly transfer the
ground-bearing load of the sediment from grain-
to-grain contacts to pore water. Compressional waves
increase the
pore water pressure
with each passage of a
shock wave. If pore water pressure does not have time
to return to normal before the arrival of the next shock
wave, then the pore water pressure increases incremen-
tally with the passage of each wave (Figure 10.9). The
process can be illustrated quite easily by going to the
wettest part of the beach foreshore at low tide. On flat
slopes, one can usually run easily along the sand. It is
even possible to drive vehicles along this zone on some
fine sand beaches. However, if you tap the sand in this
zone slowly with your foot, it will suddenly become soft
and mushy, and spread outwards. If the tapping is light
and continual, the bearing strength of the sand will
decrease and your foot will sink into the sand. All of the
damaging earthquakes towards the end of the twentieth
century - Loma Prieta, Northbridge, Kobe, and Izmit -
were characterized by liquefaction.
Pore water pressure can return to normal only if
there is free movement of water through the soil.
Smaller particles inhibit water movement because
