Depending
on the extent of the energy release during fault movement the earthquake region
can be affected in many ways. Effects of an earthquake can be classified,
broadly, into three categories, namely: primary effects, secondary effects and
tertiary effects. Primary effects
are those which are produced by the earthquake energy release directly, such as
ground shaking, ground failure, landslides, slope failures, dam breaks and the
like. These occur almost immediately after the occurrence of the earthquake.
Secondary earthquake effects are those, which result from the primary effects of
the earthquake. For instance, a dam break may lead to uncontrolled flow of water
causing floods, ground shaking may result in failure of electrical lines, which
in turn may result in short circuits followed by fires. Failed highways, bridges
and railroads may prove detrimental to traffic. Secondary effects, too, follow
the earthquake, almost immediately. Tertiary effects of an earthquake appear at
a comparatively slower rate. These include epidemics and water borne decease,
starvation and malnutrition, which result from the damage during the primary and
secondary effects and the subsequent disturbed environmental and ecological
conditions in the earthquake affected region. For a damaging earthquake actual
damage can be assessed only after several months, when life starts returning to
normal.
Technically,
release of the accumulated energy during an earthquake affects the earthquake
source zone and the surrounding areas in several ways. A large part of the
released energy is dissipated in close proximity of the earthquake source in
crushing the rock and heating it. The remaining portion is converted into
elastic energy, which is transmitted to far distances in the form of seismic
waves. Energy release during an earthquake is related to its magnitude (see
Earthquake magnitude,0
) through a logarithmic relationship of the type:
Log10
E = 11.8 +1.5 M
Where E is the energy release in ergs and M is the magnitude of the earthquake on the Richter scale. The energy released during an earthquake of magnitude M+1 is, thus, nearly 30 times higher than that for an earthquake of magnitude M. The observed effects of a strong earthquake arise either from the vibratory ground motions or from ground failure phenomena, e.g. surface faulting, liquefaction, collapse and subsidence. Foundations, which have suffered liquefaction, are incapable of supporting the load of the structures. If the structures, or their foundations, are not able to withstand the vibrations damage takes place. Other types of direct effects of an earthquake include cracks and fissures, local elevations and depressions on the surface, land slides, slope failure, earthquake induced flooding, seismic seiches and tsunamis .
Earthquake
energy is released within a very short time interval (a few seconds) at the
source. The direct (or primary) effects of a strong earthquake begin
immediately, reaching the peak. The secondary
and tertiary effects, which continue unabated.
After shocks prove more damaging because the structures have already
undergone some damage during the main shock, and earlier shocks. Preparedness to
face an earthquake by minimizing its damaging effects is the most sensible and
logical way of mitigating the hazardous effects of earthquakes, either by
avoiding earthquake prone areas altogether, or by designing dwellings and large
industrial structures sufficiently strong to withstand earthquakes.
For doing so earthquake occurrences in the region under consideration and
the damage potential arising from earthquakes are to be fully investigated to
determine the possible effects of future earthquakes.