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  • 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.

Click here for the Earthquake Tracker: Today’s biggest Earthquakes over the world 

Earthquake Watch: How many earthquakes happened today?….see here


Seismic waves: The energy released in an earthquake generates different kinds of seismic waves. These waves travels outward in widening circles, like the ripples produced when a stone is thrown into a pond, marked by diminishing amplitude with increasing distance from the focus.

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: 

The Haiti Earthquake: Seven Years Later

Deadliest Haiti earthquake analyzed

2004 Boxing day Tsunami

Tsunami risk at the Cascadia subduction zone


Haiti sits on a fault line between huge tectonic plates of the Earth’s crust, the North American Plate and the Caribbean plate. The Caribbean plate has been gliding past (transform boundary) about a quarter of an inch per year, relative to the North American plate. Stress builds up along faults and released in a sudden burst of energy, generating seismic weaves and sudden shaking of ground in the form of an earthquake.

Case study about the Hispaniola fault zone : Haiti Earthquake 2010 

Read from BBC Bitesize: The cases and impacts of the Haiti earthquake 

August 2021, Haiti earthquake. Read from Wikipedia about the deadliest natural disaster of 2021, also the worst disaster to strike Haiti since the 2010 earthquake.  

Haiti Earthquake August 2021

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

Spatial Distribution of Earthquakes

Earthquake prone regions:

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 basinShallow focus earthquakes are generally located along constructive and conservative boundaries.

Fun Learning

Movie Clip from San Andreas 🙂

Movie Clip 'The Quake' 🙂

The Impossible: on 2004 Tsunami

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