Earthquakes
An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of
energy in the Earth's crust that creates seismic waves.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the
ground. Earthquakes occur due to faults in the Earth's crust, the uppermost layer of the Earth. This
happens when there is discontinuity in the rock, so it appears as if there are two plates sliding or
slipping against each other.
The basic reason for these movements is the movement of the land itself. The earth's crust and
upper part of the mantle are together called the lithosphere. While the lithosphere is hard and rigid,
it is supported on the softer lower mantle. Since this support layer can flow, it causes the lithosphere
to break up into tectonic plates (See JM issue Jan-Feb 2010).
These tectonic plates move against each other, so that stresses get built up at the plate boundaries.
The abrupt release of these stresses causes earthquakes to occur.
Types of earthquakes
There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and
strike-slip. These are named depending upon how the plates move against each other: horizontally
or vertically.
Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each
other. Strike-slip faults, particularly continental transforms can produce major earthquakes up to
about magnitude 8. Earthquakes associated with normal faults are generally less than magnitude 7.
The hypocentre of an earthquake is the location beneath the earth's surface where the rupture of the
fault begins. The epicentre of an earthquake is the location directly above the hypocentre on the
surface of the earth.
The magnitude of an earthquake is a measured value of the earthquake size. The magnitude is the
same no matter where you are, or how strong or weak the shaking was in various locations.
The intensity of an earthquake is a measure of the shaking created by the earthquake, and this value
does vary with location.
Seismic waves
Earthquakes can be recorded by seismometers up to great distances, because the disturbance caused
by the earthquake travels through the whole Earth's interior in the form of seismic waves.
Every tremor produces different types of seismic waves, which travel through rock with different
velocities: P and S waves are both body waves. Body waves travel through the interior of the Earth,
and get refracted by the changing matter density in the Earth's interior, just like light waves.
P waves (primary waves) are compressional waves that are longitudinal in nature. P waves are
pressure waves that are the initial set of waves produced by an earthquake.
S waves (secondary waves) are shear waves that are transverse in nature. These waves typically
follows P waves during an earthquake and displaces the ground perpendicular to the direction of
propagation. For example, in the case of horizontally polarized S waves, the ground moves
alternately to one side and then the other. S waves can travel only through solids, as fluids (liquids
and gases) do not support shear stresses. They are slower than P waves.
Surface waves are like water waves and travel along the Earth's surface. They travel slower than
body waves. Because of their low frequency, long duration, and large amplitude, they can be the
most destructive type of seismic wave. There are two types of surface waves: Rayleigh waves and
Love waves.
Measuring an earthquake
The absolute magnitude of a quake is conventionally reported by numbers on the Moment
magnitude scale (formerly Richter scale, magnitude 7 causing serious damage over large areas).
The first concept here is that of stresses and resulting strains. Consider a land mass with a surface
area A. When a force acts on A, stresses are felt by the land. This stress is the force divided by area.
This stress causes the area to shear, or move obliquely, by an amount x. Since x depends on the
thickness l of the land mass, usually the quantity x/l, called the strain, is considered. The ratio of
stress to strain is a characteristic of the rock material and is called the shear modulus, .
Seismic moment is a quantity used by earthquake seismologists to measure the size of an
earthquake. It provides an empirical estimate of the amount of energy radiated.
The scalar seismic moment M0 is defined by the equation M0 = AD, where is the shear
modulus of the rocks involved in the earthquake, A is the area of the rupture along the fault where
the earthquake occurred, and D is the average displacement on A.
The moment magnitude is given in terms of the seismic moment by
2
M w= log 10 M 0 − 10.7.
3
The constants have been adjusted to match this scale with the old Richter scale for intermediate size
earthquakes (of magnitude 6 or so).
As with the Richter scale, an increase of one step on this logarithmic scale corresponds to a 101.5 =
32 times increase in the amount of energy released, so an increase of two steps corresponds to a 103
= 1000 times increase in energy.
For instance, a magnitude 4 earthquake is equaivalent to the energy released by exploding 15 tons
of TNT, a chemical explosive. A magnitude 9 earthquake corresponds to the equivalent of
475063712 tons of TNT!
It is estimated that there are 500,000 detectable earthquakes in the world each year. 100,000 of
those can be felt, and 100 of them cause damage.
The largest recorded earthquake in the world was a magnitude 9.5 (Mw) in Chile on May 22, 1960.
When the Chilean earthquake occurred in 1960, seismographs recorded seismic waves that traveled
all around the Earth. These seismic waves shook the entire earth for many days! This phenomenon
is called the free oscillation of the Earth.