The Mercalli Scale
measures intensity

Introduction
The Mercalli scale is commonly used by scientists after large earthquakes. Did you know there can be more than one intensity of an earthquake but only one magnitude?

Intensity map of Chile, USGS

Intensity map of Chilean earthquake, USGS

Intensity scale
The intensity scale by Mercalli was the first earthquake scale invented. It measures the effects of the earthquake on people and buildings in different areas.

Observations by people
People fill out reports on what happened to them during an earthquake. They are asked if they were awakened by the earthquake, if furniture moved, was there damage to chimneys or to walls in their home.

Questions reflect criteria for intensity
Each of these questions are part of the criteria used to determine the intensity of the earthquake at their location. Engineers also examine structural damage to buildings and other man made structures. They file their reports with the earthquake center.

Creating an intensity map
The earthquake center uses the information from these observers to create an intensity map. The map has circles radiating out from the epicenter of the earthquake with Roman numerals showing the intensity of the earthquake in different areas.

Modified Mercalli scale
The modified Mercalli scale used for these maps divides the intensity of the earthquake into twelve groups. Roman numerals I to XII are used to show the affects of the earthquake.

Officials use intensity maps
City officials and insurance companies use the intensity map after an earthquake to assess the damage to a town or region. They also use these maps to determine future housing regulations in an area. Insurance companies use projections to determine earthquake insurance.

Major factors that affect the intensity of an earthquake.

  1. The greater the magnitude, the greater the intensity near the epicenter.
  2. A single earthquake can produce a range of intensities from I to XII.
  3. The intensity decreases as the distance from the epicenter increases until there is no affect felt by the earthquake.
  4. Earthquakes can have a range of intensities but only one magnitude.

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Earthquake Zones Find out where and why earthquakes occur and why there are earthquake zones.

Moment Magnitude Scale Find out why the moment magnitude scale is used to measure great earthquakes


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