Earthquake intensity scales
The effect of an earthquake is, generally, stronger near the epicenter and decreases gradually outwards. To have an idea about the intensity of an earthquake in different areas, intensity scales have been devised. The two commonly used intensity scales are : (i) Rossi Forel Scale, and (ii) Richter Scale
(1). Rossi Forel Scale:
The earthquake intensity scale devised by Rossi Forel is based on how people behave and react, and how much destruction or disturbance is caused to man-made structures and natural objects. The Rossi Forel Scale has nine divisions. Intensity I is given to the weakest earthquake, which is felt only by very sensitive people at rest. The highest number IX is assigned to the strongest earthquake that causes massive destruction to man-made structures and natural objects. Any intensity above IV can cause damage to property.
However, the destruction caused by earthquakes is not an adequate means for comparison. Many factors including distances from the epicenter, nature of surface material, and building design cause variations in the amount of damage.
(2). Richter Scale:
This earthquake intensity scale was devised by Charles F. Richter, an American seismologist. It is based on the total amount of energy released during an earthquake. This energy is called ”magnitude”. The magnitude is calculated mathematically using the amount and duration of ground vibrations as recorded by a seismograph.
Earthquake Magnitude Scale
| Magnitude | Earthquake Effects | Estimated Number Each Year |
| 2.5 or less | Usually not felt, but can be recorded by seismograph. | 900,000 |
| 2.5 to 5.4 | Often felt, but only causes minor damage. | 30,000 |
| 5.5 to 6.0 | Slight damage to buildings and other structures. | 500 |
| 6.1 to 6.9 | May cause a lot of damage in very populated areas. | 100 |
| 7.0 to 7.9 | Major earthquake. Serious damage. | 20 |
| 8.0 or greater | Great earthquake. Can totally destroy communities near the epicenter. | One every 5 to 10 years |
Earthquake Magnitude Classes
Earthquakes are also classified into categories ranging from minor to great, depending on their magnitude.
| Class | Magnitude |
| Great | 8 or more |
| Major | 7 – 7.9 |
| Strong | 6 – 6.9 |
| Moderate | 5 – 5.9 |
| Light | 4 – 4.9 |
| Minor | 3 -3.9 |
The Richter Scale is widely used to describe the magnitude of an earthquake. This scale is logarithmic, hence an increase in the magnitude of one unite corresponds to a tenfold increase in the size of seismic waves and above 30-fold increase in the energy released. Thus an earthquake with a magnitude of 6.5 releases 30 times more energy than one with a magnitude of 5.5. Damage to structures begins at magnitude 5 and increases to nearly total destruction at a magnitude greater than 8. Some of the world’s major earthquakes are San Francisco 1906, Japan 1923, Chile 1960, and Alaska 1964. All had Richter magnitudes greater than 8.2. An intensity scales modified by Mercalli is shown in the following table:
| Scale level | Acceleration cm/sec/sec, and (Ground conditions) |
|---|---|
| I. Not felt | Less than 1 cm/ (Not felt except by very few under especially favorable conditions). |
| II. Weak | Over 1 cm/sec/sec. (Felt only by a few people at rest, especially on upper floors of buildings). |
| III. Weak | Over 2.5 cm/sec/sec. (Felt quite noticeably by people indoors, especially on upper floors of buildings: Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations are similar to the passing of a truck, with duration estimated). |
| IV. Light | Over 5.0 cm/sec/sec. (Felt indoors by many, outdoors by few during the day: At night, some are awakened. Dishes, windows, and doors are disturbed; walls make cracking sounds. Sensations are like a heavy truck striking a building. Standing motor cars are rocked noticeably). |
| V. Moderate | Over 10.0 cm/sec/sed. (Felt by nearly everyone; many awakened: Some dishes and windows are broken. Unstable objects are overturned. Pendulum clocks may stop). |
| VI. Strong | Over 25.0 cm/sec/sec. (Felt by all, and many are frightened. Some heavy furniture is moved; a few instances of fallen plaster occur. Damage is slight). |
| VII. Very strong | Over 50.0 cm/sec/sed. (Damage is negligible in buildings of good design and construction, but slight to moderate in well-built ordinary structures; damage is considerable in poorly built or badly designed structures; some chimneys are broken). |
| VIII. Severe | Over 100 cm/sec/sec. (Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Examples include: 1925 Charlevoix–Kamouraska earthquake and the 2000 Nicaragua earthquake). |
| IX. Violent | 250 cm/sec/sec. (Damage is considerable in specially designed structures; well-designed frame structures are thrown out of plumb. Damage is great in substantial buildings, with partial collapse. Buildings are shifted off foundations. Liquefaction occurs. Examples include: 2004 Indian Ocean earthquake and tsunami and the 2011 Tōhoku earthquake and tsunami) |
| X. Extreme | Over 500 cm/sec/sec. (Some well-built wooden structures are destroyed; most masonry and frame structures are destroyed with foundations. Rails are bent. Examples include 1939 Chillán earthquake and the 1960 Agadir earthquake). |
| XI. Extreme | 750 cm/sec/sec. (Few, if any, (masonry) structures remain standing. Bridges are destroyed. Broad fissures erupt in the ground. Underground pipelines are rendered completely out of service. Earth slumps and landslips in soft ground. Rails are bent greatly. Examples include the 1819 Rann of Kutch earthquake, 1964 Alaska earthquake, and the 1976 Tangshan earthquake). |
| XII. Extreme | Over 980 cm/sec/sec. (Damage is total. Waves are seen on ground surfaces. Lines of sight and level are distorted. Objects are thrown upward into the air. Examples include 1960 Valdivia earthquake and the 1920 Haiyuan earthquake). |
What is the difference between earthquake magnitude and earthquake intensity?
- The magnitude of an Earthquake: It is the most common measurement of an earthquake’s size. It is a measurement of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like. These days, the Richter Scale has become an outdated device for measuring the magnitude of an earthquake. The Richter Scale is no longer being used by the United States Geological Survey for large, and teleseismic earthquakes. The Richter scale measures the largest wiggle (amplitude) on the recording, but other magnitude scales measure different parts of the earthquake. The United States Geological Survey currently reports earthquake magnitudes using the Moment Magnitude scale, though many other magnitudes are calculated for research and comparison purposes.
- The intensity of an Earthquake: The value of the intensity of an earthquake changes from location to location. The intensity of the earthquake is a measurement of shaking and destruction caused by an earthquake. Earthquake size can be judged through its intensity which reflects its impact on the structures, people, and their activities. The intensity of an earthquake is measured on Mercalli Scale which was developed in 1905 by Italian geologist Giuseppe Mercalli. Later on, it was modified in 1931. This scale assigns a number to an earthquake ranging from I to XII Roman numbers. For example, an earthquake of intensity I is negligible and can be detected by instruments only. Similarly, an earthquake of intensity II is also feeble, an earthquake of the intensity of III is slight, IX is ruinous. X is disastrous. XI is very disastrous, and an earthquake of intensity XII is catastrophic.
Measuring Scales of Earthquakes:
- Magnitude Scale: It is a scale, which is used to measure the size of an earthquake at its source. An earthquake has only one magnitude at all places. The magnitude does not change from place to place and does depend on where the measurement is made. But, still, there is slightly different magnitudes are recorded at varied places, because of the complexity of the relationship between seismic measurement and magnitude. Thus, different procedures will often give slightly different magnitudes for the same earthquake.
- Intensity Scale: Intensity scales generally measure the intensity of an earthquake at different locations. Modified Mercalli Scale and the Rossi-Forel Scales are very commonly used scales to measure the intensity of earthquakes. An earthquake causes many different intensities of shaking in the area of the epicenter where it occurs. So the intensity of an earthquake will vary depending on where you are. Sometimes earthquakes are referred to by the maximum intensity they produce.
- What is the Modified Mercalli Scale?: In the United States, Modified Mercalli (MMI) Scale is used to measure the intensity of the Earthquakes. The Mercalli Scale is based on observable earthquake damage. From a scientific standpoint, the magnitude scale is based on seismic records while the Mercalli is based on observable data which can be subjective. Thus, the magnitude scale is considered scientifically more objective and therefore more accurate. For example, a level I-V on the Mercalli scale would represent a small amount of observable damage. At this level, doors would rattle, dishes break and weak or poor plaster would crack. As the level rises toward the larger numbers, the amount of damage increases considerably. Intensity X (10) is the highest value on the MMI.
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