Energy in Earthquakes

You may know that anything kept at a height has potential energy. For
instance, if you hold a ball on your terrace it has potential energy
P = mgh where m is its mass, h the height above the ground and g the
acceleration due to Earth's gravity. If you drop the ball this potential
energy is recovered as the kinetic energy of the ball which falls with
velocity v.

Now if the Earth were a perfect sphere, so that all the material inside
it is arranged in concentric layers, the potential energy would be the
same at any point on the surface. We are of course assuming that the
density of the material is the same throughout in each layer. (The
density will increase with depth since inner layers are more
compressed). Now if the
density were different at different places, the relative masses of the
layers would be different and so there would be an excess gravitational
energy in this place above the minimum value.

Our Earth is not an exact sphere nor does it have uniform density. These
density variations inside the Earth are called density structures. Since
every body wants to decrease (minimise) its energy, the masses of the
Earth must also move to reduce the excess gravitational potential
energy. This movement, or tendency of movement causes earthquake
stresses in the Earth. These stresses (or pressures) deform the Earth
matter and try to bring it to the uniform distribution. The stress is
called gravitation tectonic stress.

In the period between earthquakes, the land mass is moving and
resettling. Because of this there is a continuous deformation (called
tectonic deformation) that gives rise to a strain. The strain is
released in the earthquake when the land mass shifts: it either slips or
slides between the boundary at which the strain exists. This boundary is
called a fault. Somtimes the
gravitational energy of the density structure directly transfers into
the energy of the earthquake.

The gravitational potential energy released at the time of the earthquake
is split betrween the energy released by the earthquake (including work
done in the fault zone), and an increase in the stored elastic strain
energy. The stress associated with this elastic strian should oppose
further fault slip.

It is estimated that only 10 percent or less of an earthquake's total
energy is radiated as seismic energy. Most of the earthquake's energy
is used to power the earthquake fracture growth or is converted into
heat generated by friction. Therefore, earthquakes lower the Earth's
available potential energy and raise its temperature, though
these changes are negligible compared to the amount of
heat flowing out from the Earth's deep interior.

So we see where the large energy comes from. How large is the energy
release? One way is to compare earthquakes with the effect of a nuclear
bomb.

The energy released by one ton of TNT (trinitrotoluene) which is a
chemical explosive is about 4.18 X 10^9 Joules. The energy released
by the Hiroshima nuclear bomb is equivalent to 13,000 tons of TNT.


Richter	Number of
Magnitude	Hiroshima bombs
               
5.0	2
6.0	67
7.0	2120
8.0	67100
8.5	377000
9.0	2120000
9.5	11900000

Richter earthquake magnitude

The Richter Magnitude Scale is a measure of the amplitude of the
seismic waves produced by an earthquake.  An increase of one unit
on the Richter Scale, for example from magnitude 6.0 to 7.0,
corresponds to a 10-fold increase in the amplitude of the seismic
waves that shake the ground.  Magnitude is related to the energy
radiated from the earthquake source as seismic waves.  An increase
of one unit on the Richter Scale corresponds to approximately a
30-fold increase in the total energy released.