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

The different rates of travel between the P and S waves produce two separate shocks. The
farther from the center, the longer is the time lag between the different shocks.
A major earthquake is never an isolated phenomenon. The violent and destructive main shocks
may be preceded by preliminary tremors or foreshocks, which are less severe and few in
number, but important to study in order to predict the destructive shocks and take protective
measures. Sometimes seismic activity in a region increases gradually in intensity up to a
climax. Then there are aftershocks, which are belated shocks of decreasing intensity occurring
at increasing intervals. The disturbance may last for months or years, keeping the threatened
population in a state of anxiety. This may cause them to evacuate the area.
The main shock rarely lasts even a minute in any local area. Generally the duration is only
several seconds, although to people experiencing it the time seems much longer.
Strong shaking from a major shock frequently lasts only 30-60 seconds. The major shock of the
1906 San Francisco earthquake lasted only 40 seconds. However, the major shock of the
Alaska earthquake lasted 3-4 minutes.
The times of arrival of seismic waves at selected seismograph stations throughout the world
indicate where and when the earthquake occurred and, sometimes, its focal depth. The
recorded amplitudes of seismic waves indicate the amount of energy released by the quake.
The Triggering Mechanism: Slippage Along a Fault
Source: Bryce Walker, Planet Earth: Earthquake, Time-Life Books, Alexandria, VA, 1982, p. 92.
Figure 2.2