How are Earthquakes Measured?

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When we hear that an earthquake has happened we want to know how big it was and how many were affected.  Although only eye-witnesses and aid workers can tell how many were killed or injured, there are several methods used to measure and describe earthquakes.

The Seismograph

The first seismometer was invented around 132 AD in China, and was formed of a vase with dragons balancing balls. When the ground shakes, some of the balls fall into the mouths of frogs below.  The differences in the balls that drop give information about the direction and strength of the tremor.

Chinese Seismograph image by Deror avi

The modern seismograph works a little differently and produces a readout based on tremor activity.  A stable object holds a writing surface, with a pen suspended above in such a way that the paper moves and the pen is stationary. This provides a record of how the earth is moving under the static pen.  The amount of movement in the readings demonstrate how severe the earthquake is.  The seismograph can also provide information on the different kinds of energy released.

 

At least three seismograph readings are used to pinpoint the location of each recorded earthquake.  The seismograph is the basis for most of the measurement scales now used.

Seismograph image by Yamaguchi先生

The seismogram shown below is the output of a seismograph,  which shows the movement in three directions as an earthquake occurs.

  • The P wave comes first, this is the Primary wave, moving through and down into the earth, pushing and pulling the rock back and forth.
  • The S wave is next, S stands for Secondary, as the rocks within the earth are whipped one way and then the other in a vertical snakelike movement.
  • The slowest wave is not highlighted on this seismogram.  It is the L wave, which causes the most damage to the surface of the earth.  L waves can be seen if you happen to be looking at open land – they produce a rippling effect.

Seismogram showing P and S waves. Image By Crickett

Mercalli Scale

The Mercalli Scale was devised by Guiseppe Mercalli in 1902, and uses the observations of people who felt the earthquake to gauge the intensity of the tremors.   The limitations of the Mercalli Scale are that witnesses are required, and that their reports should be objective, free from emotion or exaggeration.  After an earthquake these aspects are difficult to control.  Furthermore, if the earthquake occurs in a sparsely populated region there will be few or no structures to base criteria upon.  Today, the Modified Mercalli Scale is used. It was developed in 1931 by Harry Woods and Frank Neumann.  It takes time to work out the results, as the scale is based on collected viewpoints, so it is usually used in conjunction with other measuring systems. The Modified Mercalli Scale uses the following criteria.

I Not felt except by a very few under especially favourable circumstances.
II Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
III Felt quite noticeably indoors, especially on upper floors, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing truck.
IV During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V Felt by nearly everyone; many awakened. Some dishes, windows, etc. broken; a few instances of cracked plaster; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.
VI Felt by all, many frightened and run outdoors. Some heavy fumiture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.
VII Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly or badly designed structures; some chimneys broken. Noticed by persons driving motor cars.
VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well-water levels. Disturbs persons driving motor cars.
IX Damage considerable in specially designed structures, well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.
X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed over banks.
XI Few, if any, masonry structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.
XII Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.

Abbreviated Version of Modified Mercalli Scale from the United States Geological Survey

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