Earthquakes

Definition:

• Earthquakes are sudden, violent shaking or vibration of the earth crust produced by the shock waves resulting mainly from a sudden displacement along a fault.

Why does it happen?
Earthquakes are associated with all types of plate boundaries. Earthquakes occur when tension is released from inside the Earth. Plates do not always move smoothly alongside each other and may get stuck. When this happens pressure starts to build up. When the accumulated pressure (energy) is eventually or abruptly released, earthquake tends to occur. While the fault rupture can be visible at the surface, the actual displacement may occur at a considerable depth as deep as 500 km beneath the surface in case of a subduction zone. Earthquake may also develop from the movement of magma or due to sudden ground subsidence.
Quasi-natural earthquakes: Man made causes like nuclear testing, building of large dam or reservoir, oil drilling, coal mining may also cause earthquakes.

Related Terminologies
Focus or hypocenter: The place beneath the ground where the earthquake originates is called the focus. It is also the center of the fault motion where energy is released; originating different kinds of seismic waves.

Epicenter: The point on the Earth’s surface directly above the focus is called the epicenter. The strongest shocks and crustal vibration are often felt on at the epicenter.

Aftershock: These are the series of small earthquake that follow a major earthquake near the original earth movement. If the initial earthquake is strong, then the aftershocks can also be very strong. Aftershocks represent the redistribution of stress on the fault zone.

Earthquake magnitude: magnitude depicts the strength of an earthquake. It measures the relative amount of energy released during an earthquake. Magnitudes are calculated on a logarithmic scale means there is a 10-fold increase every time the scale increases by 1. A magnitude 5 earthquake releases 1000 times more energy than a magnitude 3 earthquake. The most commonly used magnitude scale is Richter scale, devised in 1935 by Charles Richter. The scale is easy to use but not appropriate for comparing very large earthquakes with magnitude 7 and higher. Moment magnitude scale is now widely used to describe the size of large earthquakes. It combines several parameters like movement on the fault, rock strength, size of the rapture etc. Because of different magnitude scale in use, slightly different magnitude numbers are often reported for the same earthquake. Many Earth tremors are of such low magnitude that they are often not noticed by us.

Earthquake Intensity: means the degree of surface shaking. Every earthquake generates a wide range of local ground shaking intensities causing earthquake damages. The shaking intensities of an earthquake is qualitatively measured by Mercalli scale, originally devised by Italian geologist Giuseppe Mercalli in 1902 and was later modified in 1956. The scale varies from 1 to 12 categories based on observed effects and damages.

Earthquake magnitude calculator here

See the parameters of the Modified Mercalli Intensity scale by USGS

Case Study: Haiti and Nepal Earthquake: 

Interactive map of the Major Earthquakes on the World: click here

Spatial Distribution of Earthquakes

Earthquake prone regions:

• Find out the 10 countries most prone to earthquake
• The 20 most Earthquake Vulnerable cities_Forbes

Earthquakes can be classified according to the depth of their focus. Three broad categories are recognized:

• Deep focus: 300– 700 km deep
• Intermediate focus: 70– 300 km deep
• Shallow focus: 0–70 km deep

Shallow focus earthquake causes the greatest degree of damage and account for approximately 75% of all earthquake types.
Deep focus earthquakes are generally associated with plate margins (Benioff zone) where the oceanic plate is forced under the continental plates in the process of subduction. 20 percent of the subduction zones dominate the circum-Pacific ocean basin. Shallow focus earthquakes are generally located along constructive and conservative boundaries.

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