Natural Hazards: Causes and Effects - Disaster Management Center ...

Regional Prediction (long-term)
Systematic study of the distribution of the epicenters of major earthquakes and of their
aftershock zones along the boundary between the Pacific and American plates shows that the
zones of activity migrate from east to west along the Aleutian arc and from north to south along
the coasts of Chile. Between the zones of recent activity there are quiet regions. The map in
Fig. 2.7 shows a characteristic example in Central America: the six quiet zones (seismic gaps)
have not had an earthquake for 45 years or more.
Recent studies show that major destructive earthquakes do not recur in the same place along
faults until several decades or more have elapsed—the time needed for sufficient stress to build
up. In the main seismic regions, the present quiet zones present the greatest danger of future
earthquakes. In these quiet zones, seismic activity is very slight and not even micro-shocks are
observed. It is not yet known whether the quiet zones will become more active within years,
weeks or days before major earthquakes occur again. Monitoring these zones by the various
geophysical methods available therefore seems to be one of the tasks now required of
seismologists. It can be seen, however, that for the time being “prediction” is limited to relatively
large areas in which earthquakes are liable to occur within a period that it is not yet possible to
determine. 11
Short-Term Prediction
The most remarkable case of prediction is that relating to the Haicheng earthquake in China
(1975). The people were evacuated 5 1/2 hours before the heaviest shocks (magnitude 7.3),
thus avoiding considerable loss of life (90 percent of the houses were destroyed). Similarly, in
the province of Yunan (May 1976) two earthquakes of magnitude 7.6 and 7.5 were predicted,
which made it possible to give the alarm eight minutes before the first shock, evacuate the
population and shut down dangerous industries (electricity generation, etc.). These predictions
were based on observation of the water level in deep wells, the presence of radioactive gas
(radon) in the water, foreshocks and the unusual behavior of animals.
In the United States of America several earthquakes have been predicted, in particular in
California (November 1974), but their lesser magnitude (5) did not justify large movements of
the population, as was the case in China.
Artificial Earthquakes
For several years the attention of seismologists has been drawn to the fact that in some cases
human activities, such as the filling of certain lake reservoirs, the injection of water into deep
wells or the exploitation of oil and gas deposits are followed by seismic activity. In France, for
example, several earthquakes have occurred since 1969 at the Lacq oil and gas field.
The significant seismic activity accompanying the injection of waste fluid into a well sunk to a
depth of 3,700 meters (12,000 feet) near Denver, Colorado in the United States led scientists to
study this phenomenon in the laboratory and on the ground. The injection of fluid into a faulted
zone reduces the friction and thus diminishes the stresses in the fault. In simple terms, the
injection of fluid weakens a fault, whereas pumping out strengthens it. Where there is a
substantial stress in a fault, the injection of fluid will “release” the fault, thus causing an
earthquake.