An earthquake is a result of a sudden release of energy in the earth’s crust that creates seismic waves
Earthquakes are recorded with a seismograph and are reported on a magnitude on the Richter scale.

In general, earthquakes of magnitude less than 3 are imperceptible, and more than 7 cause serious damage
The intensity of an earthquake can also be measured on the Modified Mercalli (MM) scale. The MM scale quantifies the effect an earthquake has on humans, natural objects and man-made structures

The shaking caused by earthquakes can result in landslides, and in volcanic activity as well. When a large earthquake occurs in the oceans, the ocean floor can suffer sufficient displacement to cause a tsunami

Earthquakes are usually caused by rupture of geological faults, but can also be caused by volcanic activity, landslides, mine blasts and nuclear experiments
The point of initial rupture of an earthquake is called its hypocentre, while the point on the surface directly above it is called the epicentre

Earthquakes that occur under the ocean and of high magnitude can generate tsunamis (eg 2004 Indian Ocean tsunami)
The most powerful earthquake ever recorded is the Valdivia earthquake in Chile in 1960. It measured 9.5 on the Richter scale
Mechanism of action
Earthquakes can occur anywhere within the earth where there is stored elastic energy sufficient enough to drive fault propagation along a fault plane

Tectonic plates move past each other smoothly only if there are no irregularities and asperities. Most plate boundaries do have asperities and this leads to stick-slip behaviour
Once the boundary has locked into a relative stable position, continued relative motion between the plates leads to increased stress and stored strain energy
This continues until the stress rises sufficiently to break through the relative stable position, suddenly sliding over the locked position of the fault and thereby releasing the stored energy
The energy is released as a combination of elastic seismic waves, frictional heating of the surface and cracking of rock, thereby causing an earthquake

This process of gradual build up of stress and sudden release of energy in the form of earthquakes is called elastic-rebound theory
It is estimated that less than 10 % of the total energy of an earthquake is radiated as seismic energy. Most of the earthquake’s energy is used to power fracture growth or is converted as heat generated by friction
Occurrence of earthquakes

Minor earthquakes occur nearly constantly. Most of these happen in places like California and Alaska in the US, as well as in Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, Turkey, Greece, Italy, Japan and New Zealand. Larger earthquakes occur less frequently
However, in general, earthquakes can occur almost anywhere (even away from plate boundaries)
The relationship between frequency and intensity of earthquakes is roughly exponential i.e. for instance, there are roughly 10 times as many earthquakes of magnitude 4 as of magnitude 5
Most of the world’s earthquakes occur in Pacific Ring of Fire seismic belt. Massive earthquakes occur along other plate boundaries too, such as the Himalayas
Induced seismicity
While most earthquakes occur due to natural movement of the earth’s tectonic plates, human activity can produce earthquakes as well

Four main human activities that contribute to earthquakes include
Large dams
Drilling and injecting liquids into wells
Coal mining
Oil drilling

For instance, the 2008 Sichuan earthquake in China is believed to have been caused by the Zipingpu dam which caused the pressure of a nearby fault to fluctuate, increasing the movement of the fault and the magnitude of the earthquake
Earthquakes and volcanic activity
Earthquakes often occur in volcanic areas
They are caused both by tectonic faults and the movement of magma in volcanoes
Such earthquakes can serve as early warning of impending volcanic eruptions. Eg: Mount St Helens eruption of 1980 (USA)

*Seismic waves*

Seismic waves are waves of force that travel through the earth
Earthquakes produces different types of seismic waves that travel through the earth at different velocities:
P waves (Pressure or Primary waves): they are longitudinal waves that travel fastest through solids, and are therefore the first waves to appear on a seismogram
S waves (shear or secondary waves): transverse waves that travel slower than P waves.

They do not exist in fluids such as air or water
Surface waves (Rayleigh waves and Love waves): slower than P and S waves, but have much larger amplitude. These surface waves cause most damage during an earthquake
The propagation velocity of the seismic waves depends on density and elasticity of the medium
In solid rock, P waves travel at about 6-7 km/s (within the mantle about 13 km/s), while S waves travel at about 2-3 km/s (mantle 9 km/s)

Global tectonic plate movement
Earthquakes can be recorded at great distances, since seismic waves travel through the whole of the earth’s interior
The absolute magnitude of a quake is reported on the Moment Magnitude scale, while perceived magnitude is reported on the Modified Mercalli (MM) scale. The Richter scale is another scale that measures the absolute magnitude – it is no longer used in academic circles but is still used in popular parlance.
As a rule of thumb, the distance to the earthquake epicentre is the number of seconds between the P and S waves multiplied by 8