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Measurement of Earthquake

Measurement of Earthquakes:

Earthquakes range from tremours so small that they are bearly detectable to great shocks that can destroy entire cities. This reflects their magnitude, the amount of the shaking of the ground as the quake passes, as measured by a seismograph. Magnitude is assessed on the Richter Scale, which assigns a number to an earthquake based on the severity of that ground motion. This open-ended scale, developed in 1935 by geophysicists Charles Richter, ranges from 0 to 8+. It is logarithmic, so that the earthquake of magnitude 4 causes 10 times as much ground motion as one of magnitude 3 and 100 times as much as a quake of magnitude 2.
Another measure of an earthquake’s size is its intensity. This measure reflects the impact of an earthquake on the cultural landscape, on the people, their activities, and structures. Intensity is reported on the Mercalli Scale, which was first developed by the Italian Geologist Giuseppe Mercalli in 1905 and modernized in 1931. It assigns a number ranging from I to XII to an earthquake (Roman numerals are always used). For instance, an earthquake of intensity of IV is felt outdoors, and hanging objects swing. At intensity XII damage is total, and even heavy objects are thrown into the air. Following is detailed description of Mercalli Scale:

  • Instrumental: Detected only by instruments
  • Very Feeble: Noticed only by people at rest
  • Slight: Felt by people at rest, like passing of a truck.
  • Moderate: Generally perceptible by people in motion. Loose objects disturbed.
  • Rather Strong: Dishes broken, bells rung, pendulum clocks stopped. People awakened.
  • Strong: Felt by all, some people frightened. Damage slight, some plaster cracked.
  • Very Strong: Noticed by people in autos. Damage caused to poor construction.
  • Destructive: Chimenies fall, much damage in substantial buildings, heavy furniture overturned.
  • Ruinous: Great damage to substantial structures. Ground cracked, pipes broken.
  • Disastrous: Many buildings destroyed.
  • Very Disastrous: Few structures left standing.
  • Catastrophic: Total destruction.

At the earth’s surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake epicenter is located affshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger land slides and occasionally volcanic activity.
An earthquake’s point of initial rapture is called its focus or hypocentre (The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth. Also commonly termed the focus.). The term epicenter means the point at ground level directly above this.

Note:

The Richter Scale is not commonly used anymore, except for small earthquakes recorded locally, for which ML and Mblg are the only magnitudes that can be measured. For all other earthquakes, the moment magnitude scale is a more accurate measure of the earthquake size.

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