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Earthquakes

Why does earthquake 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.

Terminologies Definition of the terms
EarthquakeEarthquakes are sudden, violent shaking or vibration of the earth crust produced by the shock waves resulting mainly from a sudden displacement along a fault.
Quasi-natural earthquakesMan made causes like nuclear testing, building of large dam or reservoir, oil drilling, coal mining may cause earthquakes.
Focus or hypocenterThe place beneath the ground where the earthquake originates due to sudden release of energy is called the focus.
EpicenterThe 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.
AftershocksThese are the series of small earthquakes that follow a major earthquake. If the initial earthquake is strong, then the aftershocks can also be very strong. Aftershocks redistribute stress at the fault zone.
Seismic wavesThe 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-Richter scale
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. There is no upper limit to this scale. The scale is logarithmic means that there is a 10 fold increase every time the scale increases by 1. So a scale of 2 earthquake on the richter scale is 10 times more powerful than scale 1 and a scale 3 is 10 times more powerful than scale 2 and 100 times more powerful than scale 1.
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.
Earthquake Intensity- Mercalli scaleMercalli 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.It uses subjective description of the resulting damages.
Isoseismal lines Lines of equal intensity drawn on a map around the epicenter of an earthquake.
The affect of an earthquake depends on numerous human and physical factorsThe depth of the earthquake focus and the magnitude of the earthquake (energy released)
The duration of the earthquake
The local geology (soft or hard rock, solid or cracked rock)
The location of the epicenter (in a rural or urban area)
The population density around the epicenter.
The building design and hazard mapping near the epicenter.
The time of day (earthquakes at night might trap people in their houses, earthquakes during dinner time might trigger more fires).
Economic condition and preparedness of the population (evacuation routes, emergency services, etc.)
Secondary hazards such as liquefaction or generation of a Tsunami.
Secondary hazards associated with EarthquakesSecondary hazards are caused or triggered by the primary hazard. Secondary hazards may sometimes be more dangerous than the primary hazard. Earthquakes can cause several secondary hazards, that can often be more damaging than the primary hazard.
Tsunamis
Dam failure
Landslides
Fires
Disease
Liquefaction - process by which water-saturated, unconsolidated sediments are transformed into viscose substance that acts like a liquid. Its effect on structures and buildings can be devastating.

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

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

Spatial Distribution of Earthquakes

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

Find out the 10 countries most prone to earthquake

The 20 most Earthquake Vulnerable cities_Forbes

Vertical Distribution

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

  • Deep focus: More than 300 km depth from the surface
  • Intermediate focus: 70– 300 km depth from the surface
  • Shallow focus: 0–70 km depth from the surface

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

Horizontal distribution

Earthquakes occur along the plate margins. The friction and pressure that build up where the plates meet are the causes of earthquake. Destructive margins: The pressure resulting from the subjecting plate and its subsequent melting can trigger strong earthquake as the pressure is released periodically. Conservative margins: Where the plates slide past each other tend to get trapped for a period of time causing stresses and pressure to build. Sudden quick release of the pressure and movement along the fault line may results in powerful earthquake. Constructive margins: Earthquake’s tends to be less severe as the friction and pressure caused by the plates moving apart is much less intense than the other two plate margins.

CASE STUDY

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 earthquake 

Deadliest Haiti earthquake analyzed

The Haiti Earthquake: Seven Years Later

2004 Boxing day Tsunami

Tsunami risk at the Cascadia subduction zone

Fun Learning

Movie Clip from San Andreas 🙂

Movie Clip 'The Quake' 🙂

The Impossible: on 2004 Tsunami

Japan Tsunami 2011

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