Earthquakes
Suddenly the ground beneath your feet begins to
move, trees and poles begin to sway, another thud moves under your feet
and the buildings near you start to shake and move. Earthquake!
Anyone who has experienced even a small tremor knows that the above
description cannot hope to describe the feeling when the solid dependable
Earth suddenly starts to move and fall apart. Fortunately, large Earthquakes
are rare in Australia, but as was discovered in Newcastle, New South
Wales, on 28th December 1989, they do not have to be large
to be devastating.
Characteristics of earthquakes
Earthquakes occur when energy stored deep in the
rock of the Earth's crust is suddenly released. The rock moves and the
energy is released to travel as waves of energy through the rock. When
this energy reaches the surface, we experience the waves of energy as
movement, both vertical and horizontal, in the surface.
Most earthquakes occur at depths of less than 30km where the rock is
brittle, and tends to snap when placed under stress. This break in the
rock creates the vibrations that we know as an earthquake. Deeper than
30km, the crust is warmer and tends to slowly bend and fold when it
is placed under stress. This produces folded and warped rocks, but doesn't
result in an earthquake.
Earthquakes can occur anywhere, but are most common near the edges
of the tectonic plates along deep cracks in the Earth called "Fault
Lines". Earthquakes are also associated with volcanic activity.
During earthquakes, the land surface can move up or down in relation
to surrounding ground forming raised blocks, move sideways or even split
to form steep sided rift valleys.
The
Cataract Gorge in Launceston, Tasmania is an example of a fault that
split to leave a deep gorge.
Earthquakes in Australia
Earthquakes occur in Australia along old faults and in areas still settling
down from long past seismic activity. The two maps below show the risk
of an earthquake occurring and the possibility of damage from earthquakes.
The two are different as many earthquakes can occur in unpopulated areas
or in regions of surface rock that allow the shock waves of the earthquake
to pass with little damage caused. On the other hand, cities built on
clay or other sediments can suffer badly as the ground starts to flow
under the pressure of the passing seismic waves.
Both maps are extracted from the pamphlet "Earthquake Awareness
for Australians" published by Emergency Management Australia.
Measuring
earthquakes
The most common way of rating an earthquake is by using the Richter
Scale. The Richter Scale rates earthquakes in terms of total energy
released on a logarithmic scale of magnitudes. Each increase in scale
of 1 represents a 10-fold increase in energy release. A magnitude 7
earthquake is 10 times more powerful than a magnitude 6.
Not all earthquakes with the same magnitude will do the same amount
of damage, this depends on the depth of the earthquake's focus, local
geology and population.
Note:- Where an earthquake occurs inside the Earth is called its focus,
while the surface directly above the focus of the quake is called the
epicentre.
Roughly the Richter Scale can be rated against effect as follows:
Richter Magnitude
|
Effects |
3
|
No damage and only
slightly felt near the epicentre. |
4
|
Can be felt for a few
kilometres, objects shaken and toppled. |
5
|
Extensively felt with
minor building damage. |
6
|
Moderately destructive
with damage to weaker buildings. |
7
|
A major quake with
widespread damage to buildings and infrastructure. |
8
|
Cities destroyed with
major civil damage and high death tolls in built up areas. |
The Newcastle
earthquake measured magnitude 5.6 on the Richter scale.
|