An earthquake is brought about by an abrupt slip on a fault, much like what happens when you snap

your fingers. Going before the snap, you push your fingers together and sideways. Since you are pushing
them together, friction keeps them from moving to the side. At the point when you push sideways hard
enough to overcome this friction, your fingers move unexpectedly, discharging energy in a form of
sound waves that set the air vibrating and travel from your hand to your ear, where you hear the snap.

The same process goes on in an earthquake. Stress in the outer layer of the Earth pushes the sides of the
fault together. The grinding over the surface of the fault holds the rocks together so they do not slip
promptly when pushed sideways. In the long run, enough pressure develops, and the rocks slip suddenly
releasing energy in waves that make a travel through the rocks to cause the shaking that we feel during
an earthquake.

Earthquake is the shaking of the surface of the Earth resulting from the sudden release of energy in the
Earth’s lithosphere. The energy will eventually be released once the fault overcomes the friction
movement.

Faults are thin zones of crushed blocks of rocks. These are often in centimeters to thousands of
kilometers long. Their surfaces can be vertical or horizontal. These can expand into the earth and might
possibly reach out up to the earth's surface. These are also breaking in the Earth's crust where rocks on
either side of the crack have slid past each other. There are three kinds of faults: strike-slip, normal, and
thrust (reverse) faults. Each type is the outcome of different forces pushing or pulling on the crust,
causing rocks to slide up, down or past each other. The amount of ground displacement in an
earthquake is called the slip.

Strike-slip faults are rocks sliding past one another on a horizontal plane, with little to no vertical
movement. Examples to these are the San Andreas Fault and the Anatolian Fault.

Normal faults are two blocks of crust layer pulling apart, extending the crust into a valley thus, creating a
space. A normal fault has the upper side or hanging wall appears to have moved downward with respect
to the footwall. The Basin and Range Province in North America and the East African Rift Zone are two
notable districts where normal fault is spreading apart Earth's crust.

Reverse faults are also known as thrust faults, the slide one block of crust on top of another. These
faults are normally found in collision zones where tectonic plates push up mountain ranges, for example,
the Himalayas and Rocky Mountains.

Focus and Epicenter

A fault is a weak point in the tectonic plate where the pressure inside the crust is released. The area
inside the Earth where an earthquake starts is known as the focal point of the quake or the focus. It is
centered on the portion of the fault that has the greatest movement. The point at the Earth's surface
directly above the focus is known as the epicenter of the quake. During an earthquake, the strongest
shaking occurs at the epicenter. Sometimes, the ground surface breaks along the fault as shown in
Figure 3. There are also times the movement is deep underground and the surface does not break.
Scientists often name an earthquake after the region that is closest to its epicenter. Generally, if two
earthquakes of equal strength originate from the same epicenter, the one with the shallower focus
causes more destruction. Seismic waves from a deep-focus earthquake lose more of their energy as they
travel farther up to surface.

Magnitude and Intensity

The earthquake’s magnitude and intensity have different characteristics. Magnitude
measures the energy being released from the origin of the earthquake. It is
measured by an instrument called seismograph. The Richter Magnitude Scale
measures the quantity of seismic energy released by an earthquake. Intensity is the
strength of the trembling made by the earthquake at a place. The intensity of an
earthquake varies relying on where you are and is determined by the Mercalli Scale.
Active and Inactive Faults
Active faults are areas along in which displacement is expected to occur. Since a shallow earthquake
produces displacement across a fault, all shallow earthquakes occur on active faults. These are
considered to be geologic hazards. Inactive faults are areas that can be identified, but which do not have
earthquakes.

What causes Earthquake?

An earthquake occurs because of geologic forces inside the Earth. These inner
forces build up slowly and eventually become so strong that may cause
underground rocks to break.
When this happens, tremendous energy is released causing the ground to move and
shake. These waves of energy travel through the Earth are called seismic waves.
Seismic waves behave in different ways, depending on what they encounter along the way.

What are Seismic Waves?

Seismic waves are the waves of energy that travel either along or near the Earth’s
surface. This energy that travels through the Earth is recorded by seismographs.
Types of Earthquake Waves
1. body waves
2. surface waves

Body Waves
The body waves are seismic waves that travel through the interior of the Earth. These waves are of
higher frequency than surface waves. The two types of body waves are primary and secondary waves.

Primary Waves
The first type of body waves are the P waves or primary waves. These are the
fastest kind of seismic waves, and consequently, the first to arrive at a seismic
station and recorded in the seismograph. The P waves can move through solid rocks
and fluids, like water or the liquid layers of the Earth. They push and pull the rocks
as they move through just like the sound waves that push and pull the air.
P waves are also known as compressional or longitudinal waves because of the
pushing and pulling they do. P waves vibrate parallel to the direction and travelling
in a push-pull motion. Primary waves can travel at a velocity of about 4 to 6 km/s
depending on the nature of the material it passes through.
Secondary Waves
The second type of body waves are the S waves or secondary waves. These are
waves that arrive second, after P waves are being detected in the seismic station
and recorded in the seismograph. S waves are slower than P waves and can only
move through solid rocks, not through any liquid medium. This concludes that the
Earth’s outer core is liquid due to this property of the S wave. These waves move
rock particles up and down, or side-to-side perpendicular to the direction that the
waves are traveling in. S waves are also known as transverse or shear waves,
which create the shaking of the ground back and forth perpendicular to the
direction the waves are moving. S waves have a velocity of 3 to 4 km/s.
Surface Waves
Surface waves travel only through the crust. These are of lower frequency than
body waves, and are easily distinguished on a seismograph. Though they arrive
after body waves, it is the surface waves that are almost entirely responsible for the
destruction associated with earthquakes. There are two types of surface waves, the
Love wave and Rayleigh wave.
Love Waves
The first type of surface wave is called Love wave, named after Augustus Edward Hough Love, a British
mathematician who worked out the mathematical model for this kind of wave in 1911. This wave is the
fastest surface wave and moves the ground from side-to-side. Love waves produce entirely horizontal
motion. It can travel a velocity of 4 km/s and create more shaking.

Rayleigh Waves
The second type of surface wave is the Rayleigh wave, named after John William
Strutt, Lord Rayleigh, a British scientist who predicted the existence of this kind of
wave in 1885. This wave rolls along the ground just like a wave rolls across a lake or
an ocean. It moves the ground side-to-side and up and down in the same direction
that the wave is moving. Most of the trembling felt from an earthquake is due to the
Rayleigh wave, which can be much larger than the other waves.
What can seismic waves tell us?
The different types of seismic waves can tell us more about the nature of the
Earth’s interior.
For instance, seismologists use the seismic waves such as the P waves and S waves
to determine the distance from the source of an earthquake by getting the direction
and the difference in the time of arrival of the waves. When the seismic waves
travel deeper into the crust, the quake will speed up. This means that at depth, the
rocks are denser. When it reaches the upper part of the mantle, the waves will slow
down. This means that the rocks are partially molten.
As the waves reach the core, one kind of seismic waves called the secondary waves, will disappear thus,
the outer core is liquid. At a certain depth, the waves are reflected and refracted. This means that the
Earth must be layered.