Earthquake magnitude and intensity
Earthquake size can be defined using two parameters – magnitude M and intensity I. The earthquake magnitude defines energy released in the seismic source. The value of magnitude is determined analyzing amplitudes of vibrations registered by seismometers. Intensity of an earthquake defines the size of surface shaking in some area. In different places the intensity value is different, usually it reaches maximum in epicenter and decreases with distance. Although the relation between magnitude and intensity is quite ambiguous and complicated. Intensity in a particular site depends on earthquake magnitude, focal mechanism, hipocenter depth, epicenter distance and soil type in the area. Earthquake is defined by only one value of magnitude and several values of intensity depending on the geological conditions of the site.
There are used various earthquake magnitude and intensity scales. The magnitude scale is logarithmic one without lower or upper boundaries. The most known in the society is Richter‘s scale. The method to assess the size of an earthquake was suggested by Japanese scientist K. Wadati in 1931, and later the method was developed by Ch. Richter. According to Richter‘s method, the earthquake magnitude is determined from seismograms recorded by Wood-Anderson seismometr placed 100 km away from the epicenter. In case of different epicenter distance the corrections into the calculations are included. Also, other corrections due to different geological conditions are included.
The Richter‘s scale is not popular to use for scientific purposes, because it is quite complicated to use. The most common scales used for scientific studies are local, body-wave, surface-wave, moment, energy magnitude scales, etc. The magnitude scales are logarithmic, thus, an earthquake smaller of magnitude one is ~30 times weaker in energy (Table 1), for example, earthquake of M 6.0 is about 30 times as powerful as an earthquake of M 5.0; and earthquake of M 7.0 is 900 times as powerful as an earthquake of M 5.0.
The values in magnitude scales can be both negative or positive, and the scales are open (Table 1). The negative magnitude values determine small and negligible seismic events, like cracking in the rocks. Stronger events are evaluated with positive magnitude values. The magnitude sales are open but there is a physical limit for its highest value depending on the size and composition of the Earth. The magnitude of the largest known earthquake was 9.5, it took place in Chile in 1960; during the earthquake the fault of 1500 km length has formed. Supposedly the earthquake of M 10.0 would result in a fault of San-Andreas type around the globe.
Table 1. Relation between earthquake magnitude and released amount of energy. Notations: TNT – explosive material trinitrotoluene, EQ – earthquake, g – gramme, J – joule, t – tonne, k – kilo (103), M – mega (106), G – giga (109), T – tera (1012), P – peta (1015), E – eksa (1018), Z – zeta (1021), Y – yota (1024).
|
Magnitude M |
Energy equivalent of TNT mass |
Energy equivalent |
Example |
|
-0,5 |
3 g |
25 kJ |
Micro cracks in rocks due to hydrofracturing |
|
0,0 |
15 g |
130 kJ |
EQ in Spicbergen island, in the Arctic Ocean in 2004 |
|
0,5 |
85 g |
360 kJ |
Weak tectonic tremors |
|
1,0 |
480 g |
2 MJ |
Exploding in constructions |
|
1,5 |
2,7 kg |
11 MJ |
Exploding in seismic profiles |
|
2,0 |
15 kg |
63 MJ |
Quarry and mine blasting |
|
2,5 |
85 kg |
360 MJ |
Induced EQ in Poland due to mining activities |
|
3,0 |
480 kg |
2 GJ |
|
|
3,5 |
2,7 t |
11 GJ |
EQ in Dallas, USA in 2012 |
|
4,0 |
15 t |
63 GJ |
EQ in South Finland in 2004 |
|
4,5 |
85 t |
360 GJ |
EQ in Australia in 2013 |
|
5,0 |
480 t |
2 TJ |
EQ in the Kaliningrad District of Russia in 2004 |
|
5,5 |
2,7 kt |
11 TJ |
Chino Hills EQ in South California, USA in 2008 |
|
6,0 |
15 kt |
63 TJ |
Explosion of nuclear bomb "Little Boy" in Hiroshima in 1945 |
|
6,5 |
85 kt |
360 TJ |
Zumpango del Rio EQ in Mexico in 2011 |
|
7,0 |
480 kt |
2 PJ |
Haiti EQ in Haiti in 2010 |
|
7,5 |
2,7 Mt |
11 PJ |
Cashmere EQ in Pakistan in 2005 |
|
8,0 |
15 Mt |
63 PJ |
San Francisco EQ in California, USA in 1906 |
|
8,5 |
85 Mt |
360 PJ |
Sumatra EQ in Indonesia in 2007 |
|
9,0 |
480 Mt |
2 EJ |
Tohoku EQ in Japan in 2011 |
|
9,5 |
2,7 Gt |
11 EJ |
The strongest ever recorded Valdivia (The Big Chilean) EQ in Chile in 1960 |
|
10,0 |
15 Gt |
63 EJ |
The magnitude would be equal generation of a fault of San-Andreas type which would circle the globe |
|
12,55 |
100 Tt |
420 ZJ |
Formation of Chicxulub crater in Jukatan peninsulart after meteorite impact 65 Ga ago |
|
22,88-32 |
310 Yt |
1,3*1039 J |
Starquake recorded in neutron star SGR1806-20 in 2004 |
The intensity of the resultant ground shaking is evaluated in a scale of 12 levels, from negligible I to devastating XII. There are a lot of intensity scales nowadays. From the middle of XX century in Europe the most popular was Medvedev-Sponheuer-Karnik scale (MSK-64), which was later updated and called European Macroseismic Scale (EMS-98) (Table 2) and used since 1998. In the USA there is usually used Modified Merkalli scale, in Japan there is used ten levels intensity scale compiled by Japan Meteorological Agency (JMA).
Table 2. European Macroseismic Scale EMS-98.
|
EMS-98 scale |
Description
|
Effects
|
|
I |
Not felt |
Not felt by anyone. |
|
II |
Scarcely felt |
Vibration is felt only by individual people at rest in houses, especially on upper floors of buildings. |
|
III |
Weak |
The vibration is weak and is felt indoors by a few people. People at rest feel swaying or light trembling. Noticeable shaking of many objects. |
|
IV |
Largely observed |
The earthquake is felt indoors by many people, outdoorsby few. A few people are awakened. The level of vibration is possibly frightening. Windows, doors and dishes rattle. Hanging objects swing. No damage to buildings. |
|
V |
Strong |
The earthquake is felt indoors by most, outdoors by many. Many sleeping people awake. A few run outdoors. Entire sections of all buildings tremble. Most objects swing considerably. China and glasses clatter together. The vibration is strong. Topheavy objects topple over. Doors and windows swing open or shut. |
|
VI |
Slightly damaging |
Felt by everyone indoors and by many to most outdoors. Many people in buildings are frightened and run outdoors. Objects on walls fall. Slight damage to buildings; for example, fine cracks in plaster and small pieces of plaster fall. |
|
VII |
Damaging |
Most people are frightened and run outdoors. Furniture is shifted and many objects fall from shelves. Many buildings suffer slight to moderate damage. Cracks in walls; partial collapse of chimneys. |
|
VIII |
Heavily damaging |
Furniture may be overturned. Many to most buildings suffer damage: chimneys fall; large cracks appear in walls and a few buildings may partially collapse. Can be noticed by people driving cars. |
|
IX |
Destructive |
Monuments and columns fall or are twisted. Many ordinary buildings partially collapse and a few collapse completely. Windows shatter. |
|
X |
Very destructive |
Many buildings collapse. Cracks and landslides can be seen. |
|
XI |
Devastating |
Most buildings collapse. |
|
XII |
Completely devastating |
All structures are destroyed. The ground changes |
Information prepared by Vilma Stankevičienė
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