Natural Hazards: Causes and Effects - Disaster Management Center ...
Natural Hazards: Causes and Effects - Disaster Management Center ... 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.
Rupture Zones and Epicenters in Central America in the Twentieth Century The shaded areas are rupture zones since 1928: the triangles and circles show the epicenters of earthquakes, which occurred before and after 1928, respectively. Note the six gaps in which there has been no rupture for 45 years or more, and three areas which last ruptured between 30 and 45 years ago. Figure 2.7
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Rupture Zones <strong>and</strong> Epicenters in Central America in the Twentieth Century<br />
The shaded areas are rupture zones since 1928: the triangles <strong>and</strong> circles show the<br />
epicenters of earthquakes, which occurred before <strong>and</strong> after 1928, respectively. Note the<br />
six gaps in which there has been no rupture for 45 years or more, <strong>and</strong> three areas which<br />
last ruptured between 30 <strong>and</strong> 45 years ago.<br />
Figure 2.7