Various Earthquake Scales
Earthquake Magnitude and Earthquake Intensity are two terms often misunderstood. Earthquake magnitude is a measure of the size of the earthquake reflecting the elastic energy released by the earthquake. It is referred by a certain real number on the Richter scale (such as magnitude 6.5 earthquake).
On the other hand, earthquake intensity indicates the extent of shaking experienced at a given location due to a particular earthquake. It is referred by a Roman numeral ( such as VIII on MSK scale). Intensity of shaking at a location depends not only on the magnitude of the earthquake, but also on the distance of the site from the earthquake source and the geology / geography of the area. We note here that the Isoseismals are the contours of equal earthquake intensity. The area that suffers strong shaking and significant damage during an earthquake is termed as meizoseismal region.
Richter Magnitude Scale
The concept of earthquake magnitude was first developed by Richter and hence the term “Richter scale”. The value of magnitude is obtained on the basis of recordings of earthquake ground motion on seismographs.
Richter magnitude scale is a base-10 logarithmic scale obtained by calculating the logarithm of the shaking amplitude of the largest displacement from zero on Wood-Anderson torsion seismometer. It was developed in 1935 by Charles Richter in partnership with Beno Gutenberg, both of the California Institute of Technology.
Since in this scale, Earthquake magnitude is measured on a log scale, a small difference in earthquake recording on the instruments leads to a much smaller error in the magnitude. An increase of 1 in the Richter magnitude, there is a tenfold increase in the size of the waves also known as shaking amplitude. The Richter scale 5.0 is 10 times more shaking amplitude of 4.0. But there is a huge difference in energy. The energy release of an earthquake denotes the destructive power. It scales with 3/2 power of the shaking amplitude. A difference in magnitude of 1.0 is equivalent to a factor of 31.6. This is shown by the following equation:
A difference in magnitude of 2.0 is equivalent to a Factor of 1000. It is shown below:
With increase in magnitude by 1.0, the energy released by the earthquake goes up by a factor of about 31.6. Thus, a magnitude 8.0 earthquake releases about 31 times the energy released by a magnitude 7.0 earthquake, or about 1000 times the energy released by a magnitude 6.0 earthquake. There are no upper or lower bounds on earthquake magnitude. In fact, magnitude of a very small earthquake can be a negative number also. Usually, earthquakes of magnitude greater than 5.0 cause strong enough ground motion to be potentially damaging to structures. Earthquakes of magnitude greater than 8.0 are often termed as great earthquakes
Following table shows the exponential increase in earthquake energy on Richter scale:
Examples of the most devastating Earthquake recorded are Indian Ocean Earthquake 2004, which caused the 2004 Indian Ocean Tsunami and the Valdivia earthquake (Chile), 1960. The Indian Ocean Earthquake was of 9.3 intensity in Richter scale while the Valdivia earthquake of Chile was 9.5. An earthquake of 10.0 on Richter scale has never been recorded by Humankind. The 2010, Haiti Earthquake was 7.0 on the Richter scale. The undersea megathrust earthquake off the coast of Japan that occurred on 11 March 2011 was 9.0 on Moment Magnitude Scale.
Moment Magnitude Scale
The Richter scale is denoted by ML. This scale was replaced in 1970s by the new Moment magnitude scale which is denoted as Mw. The scale is almost same and media uses the same term “Richter Scale” for the new MMS also. This is because medium earthquakes such as 5.0 are equal on both the scales.
The Richter scale was based on the ground motion measured by a particular type of seismometer at a distance of 100 kilometers from the earthquake, and Richter scale has a highest measurable magnitude. The large earthquakes have a similar magnitude of around 7.0 on Richter scale. The Richter scale measurement is also unreliable for measurements taken at a distance of more than about 600 kilometers from the earthquake’s epicenter. This problem is solved by the MMS (Moment magnitude scale). The Moment magnitude scale does not uses the ground motion, but used the physical properties of the Earthquake such as seismic moment. The scale was introduced by Thomas C. Hanks and Hiroo Kanamori in 1979. The US Geological survey uses the Moment magnitude scale for all large earthquakes. Drawback: Moment magnitude scale deviates at the low scale Earthquakes.
Shindo scale is also known as Japan Meteorological Agency (JMA) seismic intensity scale. It is used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of Shindo which literally means degree of shaking. Unlike the moment magnitude scale, which measures the energy released by the earthquake, the JMA scale describes the degree of shaking at a point on the Earth’s surface. Thus it is similar to Mercalli intensity scale. The Shindo Scale ranges between Shindo-0 to Shindo-7. Shindo-0 quake is not felt by most people, while Shindo-7 is most devastating earthquake. However, note that same earthquake has different Shindo numbers at different places. For example, 2011 Great Earthquake of Japan registered Shindo-7 at Kurihara, Miyagi Prefecture, while Shindo-6 at Fukushima, Ibaraki and Tochigi and Shindo-7 in Tokyo.
Medvedev-Sponheuer-Karnik scale (MSK-64)
Prior to the development of ground motion recording instruments, earthquakes were studied by recording the description of shaking intensity. This lead to the development of intensity scales which describe the effects of earthquake motion in qualitative terms. An intensity scale usually provides ten or twelve grades of intensity starting with most feeble vibrations and going upto most violent (i.e., total destruction). The most commonly used intensity scales are: Modified Mercalli (MM) Intensity Scale and the Medvedev-Sponhener-Karnik (MSK) Intensity Scale. Both these scales are quite similar except that the MSK scale is more specific in its description of the earthquake effects. Medvedev-Sponheuer-Karnik scale denoted by MSK or MSK-64, is a macro seismic intensity scale which is used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence. It was proposed by Sergei Medvedev (USSR), Wilhelm Sponheuer (East Germany), and Vft Karnfk (Czechoslovakia) in 1964.
MSK-64 is used in India, Israel, Russia, and throughout the Commonwealth of Independent States. In India the seismic zoning has been done on the basis of this scale. This scale has 12 intensity degrees expressed in Roman numerals, which are shown in the below graphics.