There are two ways to categorize the power of earthquakes. One way is perceived by humans in the form of intensity and the second way is magnitude measured by instruments.
B) Earthquake Magnitude
It is a quantity to measure the size of an earthquake and is independent of the place of the observation. It is a quantitative measure of the strength of an earthquake. Magnitude is calculated from ground motion as measured by seismograph and incorporates the distance of the seismograph from the earthquake epicenter so that, theoretically, the magnitude calculated for an earthquake would be the same from any seismograph station recording that earthquake. The magnitude of most earthquakes is measured on the Richter scale, invented by Charles F. Richter in 1934. The Richter magnitude is calculated from the amplitude of the largest seismic wave recorded for the earthquake, no matter what type of wave was the strongest. The Richter magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number we go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. The record of actual ground motion amplitude provides a relatively precise method for representing the size of an earthquake. It is closely related to the energy released in an earthquake. Only a few percent of the released energy is radiated in the form of seismic waves. But since these waves are responsible for the ground motion and for the resulting damage to buildings and structures, this radiated energy is referred as the seismic energy of the earthquake. Using this scale, an earthquake of magnitude 5 would result in ten times the level of ground shaking as an earthquake of magnitude 4 (and 32 times much energy would be released). Except in special circumstances, earthquakes of magnitude below 2.5 are not generally felt by humans.
Modified Mercalli Intensity Scale
I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.
III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.
V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.
VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.
VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in wellbuilt buildings; considerable in poorly built buildings.
VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.
IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.
X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.
A) Earthquake Intensity
It is a measure of the effects of an earthquake at a particular place on humans and/or structures. It is a qualitative assessment of the kinds of damage. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the epicenter and the local geology at that point. The damage reduces with increasing distance from the epicenter for the same earthquake. Earthquake intensity is the violence of an earthquake felt in a particular locality. Intensity is assessed in terms of the associated effects and is dependent on:
1. distance from the epicenter
2. local geological condition
3. type and quality of buildings
4. human observation influenced by panic and state of shock after an
earthquake
Because earthquake intensity assessments do not depend on instruments but on the actual observation of effects in the meizoseismal zone, intensities can be assigned even to historical earthquakes. In this way, the historical record becomes of utmost importance in modern estimates of seismological risk.
Several intensity scales have been used and generally provide ten or twelve grades, denoted by Roman numerals, spanning from the feeblest to the most severe. Intensity scales are established on the basis of visible phenomena and human feelings.
Therefore, they bear no specific relation to the maximum acceleration of ground motion. One of the most widely used earthquake intensity scale is the Mercalli scale (1902) in the USA and modified version now on use. Other scale for intensity meausrement is European Macro seismic Scale (EMS-98) in Europe, the Shindo scale in Japan.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
B) Earthquake MagnitudeIt is a quantity to measure the size of an earthquake and is independent of the place of the observation. It is a quantitative measure of the strength of an earthquake. Magnitude is calculated from ground motion as measured by seismograph and incorporates the distance of the seismograph from the earthquake epicenter so that, theoretically, the magnitude calculated for an earthquake would be the same from any seismograph station recording that earthquake. The magnitude of most earthquakes is measured on the Richter scale, invented by Charles F. Richter in 1934. The Richter magnitude is calculated from the amplitude of the largest seismic wave recorded for the earthquake, no matter what type of wave was the strongest. The Richter magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number we go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. The record of actual ground motion amplitude provides a relatively precise method for representing the size of an earthquake. It is closely related to the energy released in an earthquake. Only a few percent of the released energy is radiated in the form of seismic waves. But since these waves are responsible for the ground motion and for the resulting damage to buildings and structures, this radiated energy is referred as the seismic energy of the earthquake. Using this scale, an earthquake of magnitude 5 would result in ten times the level of ground shaking as an earthquake of magnitude 4 (and 32 times much energy would be released). Except in special circumstances, earthquakes of magnitude below 2.5 are not generally felt by humans.
Modified Mercalli Intensity ScaleI. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.
III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.
V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.
VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.
VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in wellbuilt buildings; considerable in poorly built buildings.
VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.
IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.
X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.
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