Understanding global security - Peter Hough

NATURAL THREATS TO SECURITY
humanity posed by earthquakes is almost entirely due to the secondary effects of
seismic waves destroying the man-made infrastructure built in such susceptible areas
rather than the event in itself.
Surface faulting
Direct death by earthquake is rare but possible if someone is killed by a fall into
a fault line, which has been widened or moved by seismic waves. More commonly,
though still a relatively minor form of earthquake-related fatality, people can be killed
by buildings being dislodged in this way.
Ground motion
Of far greater significance than faulting is the shaking effects of seismic waves on the
earth’s surface. A combination of the waves’ amplitude, frequency and duration will
determine how much ground motion they create. This is generally most pronounced
near the earthquake’s epicentre (the point on the surface directly above the source
of the seismic wave, the focus). Ground motion in itself is not hazardous to man but
the effects it has on the human environment can be devastating.
1 Falling buildings: The most common cause of death during an earthquake is
as a result of the collapse of dwellings or other constructions. The twentieth
century’s most calamitous earthquake, in Tang-shan China in 1976, killed
nearly a quarter of a million people in this way. Most of the city’s buildings
were destroyed during the principal earthquake and those that survived were
then toppled by the aftershocks that followed. Hence, the design and location
of buildings in earthquake prone areas is a critical factor in the scale of security
threat they represent. In some cities in locations vulnerable to earthquakes,
such as Tokyo and San Francisco, the security threat to citizens is significantly
diminished by the implementation of regulations requiring particular safetyconscious
engineering techniques in the construction of buildings.
2 Fire: The structural damage caused by earthquakes can prove lethal in ways
other than crushing victims with masonry or causing them to fall to their
deaths. A common knock-on effect is the spread of fire through a town hit by
an earth tremor. Most of the casualties of the famous earthquakes that hit San
Francisco in 1906 and Tokyo in 1923 were killed in fires instigated by damage
to cookers and heating equipment. In Tokyo fire swept through wooden
dwellings specifically designed to avoid the sorts of casualties associated with
the fall of stone buildings.
3 Liquefaction: Deaths may also result from earthquakes when geological
conditions permit ground water to seep to the surface due to seismic disturbance
in a process known as liquefaction. This can result in major land
subsidence or flooding. It is in this way that many of the victims of the 1985
Mexico City earthquake perished.
4 Landslides: Earthquakes can also pose a hazard by prompting the fall of stones
or soil from a hillside overlooking a town.
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