Earthquake: Overview

An earthquake is simply defined as an abrupt movement of the earth’s plates. Using the theory of plate tectonics, one can see that plates are constantly and gradually moving. At the boundaries of plates, there exists a great deal of friction. This friction occurring on the Earth’s upper crust is the impetus force behind earthquakes

Generally, earthquakes are motivated by the sudden release of the strains that accrue at plate boundaries on the Earth’s lithosphere. The classification of this action is termed the Elastic-Rebound theory. As stated earlier, plates are constantly moving; this process occurs due to the extreme heat and magma from the inner core and outer mantle. When an earthquake actually occurs, scientists are able to discern two key locational elements: the hypocenter and the epicenter. The hypocenter is the point where the earthquake’s fault line seemed to have occurred. The epicenter is the point on the earth’s surface directly above the hypocenter.

Earthquakes are classified by two systems: one measuring intensity and the other measuring damage. The intensity measuring classification system is termed the Mercalli scale.

The Mercalli Scale

The Mercalle Scale has several levels of earthquake intensity. Here is a list ordered by the level number on the left:

  1. Instrumental: Only felt by few.
  2. Feeble: Felt by a few more people; noticed to a greater degree on the upper floors of tall buildings.
  3. Slight: Noticeably felt by those indoors. However, many psychologiclaly dismiss these tremors as not earthquakes.
  4. Moderate: Felt indoors by many, negligibly felt outside. Secured items like dishes, books, will shake.
  5. Rather Strong: Nearly felt by everyone, irrespective of situation. Unsecured objects will shake.
  6. Very Strong: Standing is difficult because of the extreme vibrations. Superficial damage to foundations in wooden houses.
  7. Destructive: In poorly designed structures, massive destruction. Unsecured buildings will experience partial/full collapse. In developed structures, chimneys will be toppled and buildings can experience foundation shifting.
  8. RuinousGreat damage in high density, population wise, buildings. Partial collapse is expected, even in secured buildings.
  9. Disastrous: Wooden structures will be displaced and their foundations could crack. Rails and steel foundations could bend and give way.
  10. Very Disastrous: Bridges are in danger of collapsing. Buildings perilously close to toppling.
  11. Devastating: Very few masonry structures still standing. Bridges destroyed.
  12. Catastrophic: Complete destruction to buildings and secured structures. Immense upheaval of natural rock and trees that which in some cases is used as an external damaging agent.

The Richter Scale

The other Earthquake measuring system is the Richter scale which measures damage potential on a classification-based, numerical scale.

Descriptor Richter Magnitude Earthquake Effects Occurance Frequency
Minor Less than 2.0 Micro-earthquakes, not felt. About 8,000 per day
Very minor 2.0-2.9 Generally not felt, but recorded. About 1,000 per day
Minor 3.0-3.9 Often felt, but rarely causes damage. 49,000 per year (est.)
Light 4.0-4.9 Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.)
Moderate 5.0-5.9 Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800 per year
Strong 6.0-6.9 Can be destructive in areas up to about 100 miles across in populated areas. 120 per year
Major 7.0-7.9 Can cause serious damage over larger areas. 18 per year
Great 8.0-8.9 Can cause serious damage in areas several hundred miles across. 1 per year
Rare great 9.0 or Greater Devastating in areas several thousand miles across. 1 per 20 years

Earthquake Prediction

In most situations, earthquakes cannot be predicted until the very moment they are set to occur. Positioning of houses and communities and proximity to hills and mountains can be indicative though, of the likelihood of earthquakes occurring in your region.

Sources Consulted

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