Measuring Earthquakes

Seismograph Is an instrument used to detect and record seismic waves.

The Mercalli Scale The Mercalli scale was the first to be used to measure earthquakes. It rates an earthquake according to the damage it causes at a given place. It uses roman numerals to rank the earthquake from I to XII (1 to 12). Scientists create the rating by using eyewitness accounts and observations of areas that the quake hit. As a result, the Mercalli scale is not always very accurate. Also, the same earthquake can have different Mercalli ratings because it causes different amounts of shaking in different locations. Even though it isn’t perfect, the Mercalli scale can still give us valuable information about an earthquake. This chart describes each rating on the Mercalli Scale

The Richter Scale The most widely known scale for rating earthquakes is the Richter Scale. The Richter scale rates earthquakes on a scale of 1 to 10 based on the size of the seismic waves created. Seismic waves are measured by an instrument called a seismograph (look at the picture on the right). The bigger the waves created by the earthquake, the higher the number on the scale. Each time you move up a number on the Richter Scale the shaking of the earthquake gets 10 times stronger. That means if you move up two numbers, the shaking gets 100 times stronger! Even though it is the most famous scale, the Richter scale isn’t actually used much anymore because it isn’t very accurate past about a 7.0. A diagram of a seismograph used to measure earthquake waves.

This diagram illustrates how the Richter Scale works. Read carefully to understand how strong each measurement is. The Richter Scale

The Moment Magnitude Scale This scale rates earthquakes based on the amount of energy they release. To determine the energy, seismologists evaluate three things: 1) how big the waves are on the seismogram, 2) how far the land moved along the fault and 3) how strong the rocks are that broke at the fault during the earthquake. Each time you move up a number on the Moment Magnitude Scale, the earthquake releases 32 times more energy! So, if you move up two numbers it releases almost 1000 times more energy! This is the scale that is most widely used today. It is based on more information than the Richter scale which makes it much more accurate. It also gives a better overall picture of the size of the quake. Even if a news reporter talks about an earthquake being a 5.0 on the Richter scale, it is very likely that the rating is actually the moment magnitude. This figure illustrates the relative strength of different earthquakes

Take a look at how this diagram compares moment magnitude to earthquake frequency and energy equivalents.