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Glossary - Size of an Earthquake

The size of an earthquake

There are three common terms to describe the size of an earthquake: intensity, magnitude and acceleration. The intensity of an earthquake is the apparent degree of shaking felt in different places. The intensity is determined in a specific place, taking into consideration the different effects caused by the earthquake's shaking in objects, buildings, people and terrain itself. The scale used for intensity in the western hemisphere is the Mercalli Modified Scale. This scale goes from I (not felt) up to XII (total destruction) and was prepared by Charles Richter in 1956. The first intensity scale was developed by Rossi from Italy and Forel from Switzerland in 1880, and was used to describe the effects of the 1918 Puerto Rico earthquake.

The magnitude of an earthquake is determined by taking the logarithm (in base 10) of the greatest movement of the soil recorded during the arrival of a type of seismic wave and applying the standard distance correction. As the scale is logarithmic, the magnitude increases by one unit with the increase of ten units in the amplitude or duration of the seismic wave record. However, in terms of energy released by an earthquake, an increase in unity in magnitude increases the amount of energy released by a factor of approximately 30. Although there are different scales of magnitude, based on different waves, most of them are reported on the Richter scale in honor of Dr. Charles F. Richter who developed the concept in 1935.

The size of the earthquake can also be expressed using its gravitational acceleration. This is the acceleration in which a ball falls in a vacuum (1.0g, where g is 980 cm/s2). In addition to the acceleration, the velocity, terrain displacement, duration and wave properties are important when designing earthquake-resistant buildings. In the following table we compare the intensity, magnitude and acceleration values.

Intensity Intensity Scale Mercalli Modified Magnitude (Richter Scale) Max. Terrain Acceleration (g)
I Not Felt < 2.3 < 0.002
II Felt only by some people in resting position, especially in high floors. Suspended objects oscillate a little. 2.3 - 2.9 0.002 - 0.003
III Felt indoors. Many people do not recognize it as an earthquake. Standing cars sway. Vibrations like the passage of a small truck. Duration appreciable. 3.0 - 4.1 0.004 - 0.007
IV Felt indoors by many, few on the outside. Windows, dishes, doors vibrate. The walls creaked. Vibrations like the passage of a big truck; Shaking sensation like a heavy ball. Stationary cars shaking noticeable. 3.7 - 4.2 0.015 - 0.02
V Felt by almost everyone. Many wake up. Dishes & windows may break. Some masonry houses crack. Unstable objects overturned. Doors swing in and out. 4.3 - 4.9 0.03 - 0.04
VI Trees and shrubs visibly shaken. Felt by everyone, many are frightened and run outside. It's hard to walk. Windows, dishes and glass objects break. Some heavy furniture moves; Some masonry houses fall. Light damage. 5.0 - 5.6 0.06 - 0.07
VII Everyone runs outside. Small damage to structures specially designed for earthquakes, slight to moderate damage to correctly built buildings and considerable damage to poorly built structures. Difficult to drive. 5.7 - 6.2 0.1 - 0.15
VIII Slight damage to structures specially designed for earthquakes, partial collapse may occur in current buildings and higher in poorly constructed structures. The panels of the walls fall off frames. Monuments, columns and walls fall. Heavy furniture overturn. Small slides of sand and mud. Changes in the flow of sources and wells. Difficult to drive. 6.3 - 6.9 0.25 - 0.3
IX Significant damage to structures of good design and construction, well-designed structures displaced from its foundations; Higher in current buildings with partial and total collapse. Wide cracks in the floor. Ejection of sand and mud in alluvial areas. Broken underground pipes. 7.0 - 7.6 0.5 - 0.55
X Some well-constructed structures in wood and bridges destroyed, most of the building and structures destroyed with their foundations. Large cracks in the floor. Landslides, water goes beyond the banks of channels of rivers, lakes, etc. Sand and mud displaced laterally. 7.7 - 8.2 > 0.6
XI Collapse of most cement and concrete structures. Bridges and other transport routes seriously affected 8.3- 9.0 > 0.6
XII Total loss in infrastructure. Large masses of displaced rocks. Heavy objects vertically thrown into the air with ease. >9.0 > 0.6