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

Natural Precondition for Disaster Occurrence
The regions of the world where earthquakes occur are characterized by certain geological
aspects. The earth’s crust is broken into a series of blocks or “plates” that are separated by
deep fractures called faults. Faults form lines of weakness in the masses of rock at the earth’s
surface. Pressures that build up below the surface eventually force a sudden shift between two
of these blocks. This sudden shift is the earthquake.
Why these pressures build up and cause the movements is explained by the theory of
continental drift or “plate tectonics.” Briefly, this theory holds that all the earth’s land area once
was a single mass. This mass broke apart, and the pieces began drifting. Wherever plates
meet there is a high degree of earthquake activity or “seismicity.”
The meeting of these plates can be classified as three distinct types of borders: divergent rifts,
where new ocean floor is created by basaltic magma (the molten rock from beneath the earth’s
crust) that rises and spreads out from the earth’s interior; convergent zones, where two plates
meet, either colliding head on or one being forced under the other; and shear borders, where
the plates grind slowly past each other. At each of these borders, or faults, strain builds up,
eventually resulting in earthquakes.
The fault movement, at these meetings of plates, is the result of elastic rebound, the slow buildup
and sudden release of strain within masses of rock. At the place where stresses are
released, mechanical energy is initiated in the form of waves that radiate in all directions
through the earth. When this energy arrives at the earth’s surface, it forms secondary surface
waves. The frequency and amplitude of the vibrations produced at the surface, and thus the
severity of the earthquake, depend upon the amount of mechanical energy released at the
focus, the distance and depth of the focus, and the structural properties of the rock or soil on or
near the surface. 5
Disaster Event
The earthquake event is a complex set of movements in the earth’s crust. It is necessary to
describe the physical character of these movements to understand their impact on the natural
and built environment. (See Fig. 2.2 and 2.3.)
Earthquake Magnitude and Moment
There are three identifiable types of earthquake movements or shock waves. The P wave is the
fastest moving wave, traveling at about five kilometers a second, (three miles a second).
Having the characteristic of sound waves, it moves longitudinally, creating a “push-pull” effect
on the rock as it passes.
The S, or shear wave, travels about three kilometers a second (two miles a second) near the
surface, causing the earth to move in right angles to the direction of the wave. An example of
this kind of motion is a rope snapped like a whip. The waves move the length of the rope, but
the actual motion is at right angles.
The L, or long wave, is a slow surface wave. These long-period waves cause swaying of tall
buildings and slight wave motion in bodies of water at great distances from the earthquake
center.