Earthquakes – Aspects and Features

New Madrid Seismic Zone[2]
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The “size” of earthquakes is commonly expressed in two ways – magnitude and intensity.

Magnitude is a measure of the total energy released during an earthquake. It is determined from a seismogram, which plots the ground motion produced by seismic waves. As devised by C.F. Richter in 1935, the magnitude scale allows us to compare earthquakes in relative terms.

The important thing to remember about magnitude is that the scale is logarithmic, which means that each step in magnitude represents a tenfold increase in amplitude of wave motion. Therefore, an earthquake of magnitude 6.0 has ten times the wave amplitude of an earthquake of magnitude 5.0, a hundred times the wave amplitude of a magnitude 4.0 earthquake, and one thousand times the wave amplitude of a magnitude 3.0 earthquake.

Because magnitude does not describe the extent of the damage, its usefulness is limited to an approximation of whether the earthquake is large, small, or medium-sized. The destructiveness of an earthquake is a complex matter, related to the geology, population density, and cultural features of a specific area at a specific distance from the epicenter.

The most widely used intensity scale, the Modified Mercalli Scale (MM), is divided into 12 degrees, each identified by a Roman numeral. For example, an earthquake intense enough to be felt by a person standing nearby is said to have an intensity of Modified Mercalli Scale (MM) III.[1]

Seismic Waves

The shaking during an earthquake is caused by seismic waves. Seismic waves are generated when rock within the crust breaks, producing a tremendous amount of energy. The energy released moves out in all directions as waves, much like ripples radiating outward when you drop a pebble in a pond. Two types of seismic waves are generated at the earthquake focus (Body and Surface Waves):[3]

Body Wave (P wave)[4]

Body Wave (P Wave)
Body waves spread outward from the focus in all directions. The first type of body wave is the Primary Wave (P wave):

The P wave, or primary wave, is a compressional wave and travels in the same direction the waves move.

• Very fast (4 - 7 km/second)
• CAN pass through a fluid (gas or liquid)
• Arrives at recording station first


Body Wave (S wave)[5]

Body Wave (S wave)
Body waves spread outward from the focus in all directions. The second type of body wave is the Secondary Wave (S wave):

The S wave, or secondary wave, is a transverse wave and travels perpendicular to the wave movement.

• Slower moving (2 - 5 km/second)
• CANNOT pass through a fluid (gas or liquid)
• Caused by a shearing motion

Surface Wave[6]

Surface Wave

Surface waves spread outward from the epicenter to the Earth's surface, similar to ripples on a pond. They are produced when earthquake energy reaches the Earth's surface. These waves can move rock particles in a rolling motion that very few structures can withstand. These waves move slower than body waves, but are the most destructive for structures on earth.



How Scientists Determine Where an Earthquake Occurred

Earthquake Triangulation[8]
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Earthquake scientists, or seismologists, can locate the epicenter of an earthquake as long as the vibrations are felt at three different seismograph stations. Theses are the following steps:[8]

  • Scientist locate at least 3 stations on a map that recorded the seismic waves.
  • They then calculate the time difference between arrival of P – waves and arrival of S – waves from a seismogram. The time difference is proportional to the distance from the epicenter. Because the direction to the epicenter is unknown, the distance defines a circle around the receiving station. The radius of each circle equals that station‟s distance from the earthquake epicenter.
  • Finally, the epicenter is determined where the circles intersect.

Earthquake Magnitude – The Richter Scale

The Richter Scale is used to express earthquake magnitude on the basis of the height (amplitude) of the largest line (seismic wave, P or S) on a seismogram. The Richter scale was originally developed for earthquakes in Southern California. The utility of this scale was its ability to account for decreased wave amplitude with increased distance from the epicenter. The following chart demonstrates the Richter Scale levels and the earthquake effects associated with each magnitude level:[9]



Earthquake Effects

Estimated Number

Less than 2.0 Micro Not felt. ~8,000 per day
2.0 to 2.9 Minor Usually not felt, but can be recorded by seismograph. ~1,000 per day
3.0 to 3.9 Minor Often felt, but rarely causes damage. ~49,000 per year
4.0 to 4.9 Light Often felt, but only causes minor damage. ~6,200 per year
5.0 to 5.9 Moderate Slight damage to buildings and other structures. ~800 per year
6.0 to 6.9 Strong Can be destructive in areas up to about 100 miles across in populated areas. ~120 per year
7.0 to 7.9 Major Can cause serious damage over larger areas. ~20 per year
8.0 or 8.9 Great Can totally destroy communities near the epicenter and cause serious damage in areas several hundred miles across. 1 per year
9.0 to 9.9 Great Devastating in areas several thousand miles across 1 per 20 years
10.0 or greater Great Never recorded in history. Extreme devastating damage. Unknown

Earthquake Intensity – Modified Mercalli Intensity Scale (MMI)

The Modified Mercalli Intensity (MMI) scale depicts shaking severity and measures the effects of an earthquake. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

An earthquake has a single magnitude that indicates the overall size and energy released by the earthquake. However, the amount of shaking experienced at different locations varies based on not only that overall magnitude, how far you are from the fault that ruptured in the earthquake, and whether you are on rock or thick valley deposits that shake longer and harder than rock.

The scale quantifies the effects of an earthquake on the Earth’s surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction). The Mercalli scale is not defined in terms of more rigorous, objectively quantifiable measurements such as shake amplitude, shake frequency, peak velocity, or peak acceleration. Human perceived shaking and building damages are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events.

The Modified Mercalli Intensity (MMI) value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place. The following chart identifies the levels and the intensity or effects felt relative to that level:[10] [11] [12]

MMI Value




Detailed Description

I Under 2.0 Instrumental Not felt Generally not felt by people unless in favorable conditions.
II 2.0 to 2.9 Weak Not felt by many Felt only by a couple people that are sensitive, especially on the upper floors of buildings. Delicately suspended objects (including chandeliers) may swing slightly.
III 3.0 to 3.9 Slight Felt slightly Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing automobiles may rock slightly. Vibration similar to the passing of a truck. Duration can be estimated. Indoor objects (including chandeliers) may shake.
IV 3.0 to 3.9 Moderate Felt by most Felt indoors by many to all people, and outdoors by few people. Some awakened. Dishes, windows, and doors disturbed, and walls make cracking sounds. Chandeliers and indoor objects shake noticeably. The sensation is more like a heavy truck striking building. Standing automobiles rock noticeably. Dishes and windows rattle alarmingly. Damage none.
V 4.0 to 4.9 Rather Strong Pictures Move Felt inside by most or all, and outside. Dishes and windows may break and bells will ring. Vibrations are more like a large train passing close to a house. Possible slight damage to buildings. Liquids may spill out of glasses or open containers. Small unstable objects displaced or upset. Doors swing. Pictures move. Pendulum clocks stop. None to a few people are frightened and run outdoors.
VI 4.0 to 4.9 Strong Objects Fall Felt by everyone, outside or inside; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Pictures off walls. Trees and bushes shake visibly. Waves on ponds. Damage slight to moderate to poorly designed buildings, all others receive none to slight damage.
VII 5.0 to 5.9 Very Strong Nonstructural Damage Difficult to stand or walk. Noticed by drivers of cars. Furniture broken. Damage light in building of good design and construction; slight to moderate in ordinarily built structures; considerable damage in poorly built or badly designed structures; some chimneys broken or heavily damaged. Noticed by people driving automobiles.
VIII 5.0 to 5.9 Destructive Moderate Damage Damage slight in structures of good design, considerable in normal buildings with a possible partial collapse. Damage great in poorly built structures. Brick buildings easily receive moderate to extremely heavy damage. Possible fall of chimneys, factory stacks, columns, monuments, walls, etc. Heavy furniture moved. Tree branches broken.
IX 6.0 to 6.9 Violent Heavy Damage General panic. Damage slight to moderate (possibly heavy) in well-designed structures. Well-designed structures thrown out of plumb. Damage moderate to great in substantial buildings, with a possible partial collapse. Some buildings may be shifted off foundations. Walls can fall down or collapse. Underground pipes broken.
X 6.0 to 6.9 Intense Extreme Damage Poorly built structures destroyed with their foundations. Many well-built structures destroyed, collapsed, or moderately to severely damaged. Most other structures destroyed, possibly shifted off foundation. Large landslides. Water thrown on banks of canals, rivers, lakes, etc.
XI 7.0 to 7.9 Extreme Very Extreme Damage Few, if any structures remain standing. Numerous landslides, cracks and deformation of the ground. Rails bent greatly. Underground pipelines completely out of service.
XII 8.0 or higher Catastrophic Total Destruction Total destruction – everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. Even the routes of rivers can be changed.



  1. Missouri Dept. of Natural Resources – The Relationship Between Richter Magnitude and Modified Mercalli Intensity:
  2. Image Source: [Accessed: January 8, 2014]
  3. South Carolina Geological Survey – Earthquakes and Siesmic Waves:
  4. Image Source: [Accessed: January 8, 2014]
  5. Image Source: [Accessed: January 8, 2014]
  6. Image Source: [Accessed: January 8, 2014]
  7. USGS The Science of Earthquakes:
  8. Image Source: [Accessed: January 8, 2014]
  9. USGS Measuring the Size of an Earthquake:
  10. Missouri Dept. of Natural Resources – The Relationship Between Richter Magnitude and Modified Mercalli Intensity:
  11. USGS The Modified Mercalli Intensity Scale:
  12. California Earthquake and Hazards Program – Modified Mercalli Intensity Scale (MMI):