The measurement of earthquakes

The history of scales

The ancient Greek already tried to measure the earthquakes. Jacopo Gastaldi, a mapmaker from Piemont, worked out the most formerly intensity scale in 1564. He did it to measure the big earthquake of Nizza's vicinity in 1594. In the XVII-XVIII. centuries, the scales generally had four grades, but in the XIX. century, the geologists described the destructive power of the earthquakes with ten grades. On the first place, Kitaibel and Tomcsányi made isoseismic map from among Hungarians in 1810.

One prototype of the earthquake-scales nowadays used is the scheme with 10 grades made by Rossi and Forel in 1883, and Mercalli's scale (also has 10 grades), which came into being in 1897. Cancani, who also connected the maximum values of ground movement's acceleration to each grade, expanded the latter to 12 grades in 1904. After that Sieberg continued the completion with the analysis of effects and with the description of building damages, which was accepted by an international scheme in 1917, and since then it was used all over the world, and called its full name: Mercalli-Cancani-Sieberg scale. Experts use the updated version of the Mercalli scale, the MSK '64, which was worked out in 1964.

The Japanese - who are often struck by earthquakes, because of the position of their country, and because of the geological conditions - made their own scale in 1900: the seven graded Omori scale, however they modified it also; by decreasing it to six grades. The Omori scale is suitable to process, measure especially destructive quakes. The first grade is for "perceptible", but harmless, the sixth is for "destructive quake".

While the Mercalli type grade and the preceding scales are all classified by environmental damages and losses caused by the earthquakes, until the Richter scale brought a completely different approach. In 1935, C. Richter American seismologist introduced the notion of extent and the magnitude, which are based on instrumental recordings, and he created his own scale on the basis of the earthquake's evolving energy. The earthquake's power in conformity with the Richter scale is determinable with a seismogram, whose recorded during the quake. In this way the largest earthquake till now had the magnitude of 8,9, which is equal to about 10¹⁸ joule. It is impossible to have a stronger earthquake than that, because the earth's crust isn't able to accumulate more energy than the Richter scale's 8,9th grade. (The nuclear bomb, which was dropped at Hiroshima, had the energy of 6x10¹³ joule. So, there was 16667 times more energy evolved in the largest earthquake than the detonation of Hiroshima's nuclear bomb.)

Intesity scales

The effects of the quakes are also estimated by the measure of occurrences and changes caused on the surface of the earth. The intensity scales are based upon this. These effects depend on evolved kinetic (total-)energy in the deep and also on:

• the focal depth,
• the distance of the location (establishment) and the epicentre,
• the frequency of the waves, which are bear the decisive part of the energy,
• the subsoil of the location (if it is loose, greater impulse is swallow up in given capacity), the geological structure of that, the terms of the ground,
• the direction of leaps, and finally on the structure and material condition of the establishment.

What's the difference between the intensity and the magnitude?

The intensity is the measure of the earthquake's effect on people and their buildings in a specific place, while the measure of the magnitude was determined by the evolving energy in the course of the quake. Recently the intensity is measured on the 12 degreed European Macroseismic Scale (EMS). This is the corrected version of the formerly used Mercalli scale and MSK scale. When the intensity-value is bigger than five, it is possible, that the quake damages the buildings. The Richter scale is for measuring the magnitude. Theoretically, it doesn't have lower and upper limits, but in practice, there is a limit value, and there is no earthquake, which is bigger than it. The known larger earthquakes, which have the magnitude around M=9, while the earthquake motion risen by a fallen brick is about M=-2. (The magnitude scale is logarithmic, so the value of the magnitude may be negative!) If a quake's magnitude is greater with one than an other, then it means 30 times bigger energy.

Charles F. Richter's scale

The Richter scale determinates the measurement of earthquakes, the so-called magnitude. Charles F. Richter, the American seismologist introduced the notion in 1935. The scale describes the earthquake's strength with numbers between 0 and 9. The scale also continued, but since they started to use it in the 1930s, fortunately there haven't been quakes larger than 8,9 magnitude. We get the earthquake's magnitude, if we measure the largest amplitude in micron (10^-6 m) on a seismogram, which created by a standard seismograph 100 kilometres away from the earthquake's epicentre, and we take the natural (ten based) logarithm of the quake.

Finally, let's see the observable effects of the earthquakes assigned to each magnitude:

Mercalli-Cancani-Sieberg's (MCS) scale

Mercalli-Cancani-Sieberg's 12-graded scale has come into general use in Europe. Besides the description of the occurrences (the effects on the buildings from the 6th grade) there are measurable acceleration-intervals, which are given in mms^-2 assigned to each degree.

For those, who shudder at these recondite mathematical calculations, there is the Mercalli-Cancani-Sieberg scale based on earthquakes' external effect, with the following degrees:

• Not perceptible. Only perceptible with devices.
• Very weak. Only certain people notice in houses, mainly upstairs.
• Weak. Smaller parts of those, who in the houses notice it. Generally, it is similar to the effect of a proceeding vehicle.
• Moderated. A lot of people notice it in houses and a few outdoors during the day. Some people awake at night. The plates, doors, windows give clanking sounds. The wall crackles. It has a similar effect to the building joilting of heavier vehicles. It swings the parking cars.
• Quite strong. Almost everyone notice it. A lot of people awake. The windows break. Certain objects overturn, the objects hanging from the ceiling swing. The pendulum clock could stop. The trees could sway.
• Strong. Everyone notices it. Many people get frightened and run out of the houses. One or more weighty pieces of furniture move from their places. Several chimneys could collapse.
• Very strong. Everyone runs from the house in their alarm. Does small damage to well-built buildings, and does more serious damage in not-well-built houses. Many chimneys collapse. Drivers notice it while driving.
• Quite destructive. The quarter of the buildings, suffer heavy losses. Some collapse, many become inhabitable. The chimneys of the dwellings fall down, factory chimneys fall apart, monuments, statues collapse, move on. Muddy water is impressed from the wet ground. Greatly prevents drivers from driving.
• Destroying. The half of the dwellings heavily damages. Relatively many collapse, the most become inhabitable. Clefts occur in the ground, the buried transmission lines break.
• Very destroying. Heavy damages occur in the ¾ part of the buildings. The most collapse. The well-built buildings are suffered from heavy damages, too. Considerable landslides happen, colossal clefts occur in the ground.
• Catastrophic. All stone building collapse, the bridges give away, the transmission lines get unusable, and the rails bend.
• Totally catastrophic. Every human structure gets deteriorate. The waves appear on the surface, certain objects are tossed into the air from the ground.

Medvegyev-Sponhauer-Karnik (MSK) scale

In the socialist countries, the MSK-64 scale was accepted. The individual grades' - which number are also 12 - characterisation is extended to the phenomena of nature, to the buildings and to the senses and the environment of people. Strongest earthquake in Hungary until now, had the grade of 8-9.

European Macroseismic Scale (EMS)

Editor:
• G. Grünthal, Chairman of the ESC Working Group, GeoForschungsZentrum Potsdam, Germany


Assistant editors:
• R.M.W. Musson, British Geological Survey, Edinburgh, Great Britain
• J. Schwarz, Bauhaus University Weimar, Germany
• M. Stucchi, Istituto di Ricerca sul Rischio Sismico, C.N.R, Milan, Italy

First of all the sudden loosening of the accumulated tensions in the course of larger mass' reorganisation (mountains and hills are ascending, basins are descending) causes quakes. The Working Party of European Seismological Board worked out the European Macroseismic Scale (EMS) based on many previous experiences, which first version was made in 1992, in 1998 they modified it in Luxemburg and since then they use it in the European countries.
The scaling uses the 12 grades of intensity (strength) based on ensued illustrative occurrence:

Intensity	Short description	Characteristic effect
I	Not perceptible	Not perceptible among the best conditions.
II	Hardly perceptible	Mainly reclining person feels it, especially on the upper floors of high buildings.
III	Weak	The quake is weak; several people feel it, mainly inside the buildings. The reclining persons feel swinging or weak shaking.
IV	Widely perceptible	Many people feel it inside the buildings, a few in the open air. Several people awake. The measure of the quake is not frightening. Windows, doors, pots clank, objects hang from the ceeling are swinging.
V	Strong	Most people feel it inside the buildings, only a few feel it in the open air. Many sleeping people awake, a few escape to the open air. The whole building is shakes; the pentended objects swing. Plates, glasses clink. The vibration is strong. Heavy objects on the top overturn. Doors, windows open or close.
VI	Does small amount of damage	Almost everybody feel it inside the building, many people feel it in the open air. Several of those, who staying in buildings are frightened, and escape to the open air. Smaller objects fall down. Smaller damage occurs in some traditional buildings, hair-crack in the plasterwork, smaller pieces of plasterwork fall down.
VII	Does damage	Most people are scared, and escape to the open air. Furniture leaves its place, many object falls down from shelves. Many traditional building suffer moderate damage: smaller gaps are produced in the walls, chimneys collapse.
VIII	Does heavy damage	Furniture overturns. Many traditional building are damaged: chimneys collapse, big gaps produced in the walls, several buildings partially collapse.
IX	Destructive	Pillars, monuments collapse or twist. Many traditional building partially, several totally fall to ruin.
X	Very destructive	Many traditional building collapse.
XI	Overwhelming	Most buildings collapse.
XII	Completely overwhelming	Practically all structure is destroyed.