1. Quaking, Shaking, Earth
All about Earthquakes

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What is an earthquake?
Simply put:
An earthquake is the shaking of the earth.
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Many buildings in Charleston, South Carolina, were damaged or destroyed by the large earthquake that occurred August 31, 1886.
Picture from the United States Geological Service
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San Francisco, California, Earthquake April 18, East side of Howard Street near Seventeenth Street. All houses shifted toward the left. The tall house dropped from its south foundation wall and leaned against its neighbor
Picture from USGS.GOV
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San Fernando, California, Earthquake February Collapsed overpass connecting Foothill Boulevard and the Golden State Freeway. Feb 10, 1971.
Photo by R.E. Wallace, USGS.
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6. What causes an earthquake?
Earthquakes are the Earth's natural means of releasing stress.
Due to the constant motion of the Earth’ plates, this put stress on the edges of the plates.
To relieve this stress, the rocks tend to bend, compress, or stretch.
An aerial view of the San Andreas fault in the Carrizo Plain, Central California. Picture from
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Locatin' the Shakin'
Focus: the place on the Earth’s crust where the pressure was released.
Epicenter: the spot on the Earth’s surface directly above the focus.
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Faults
If the force is great enough, the rocks will break.
An earthquake is the vibrations produced by the breaking of rock.
Most earthquakes occur near plate boundaries.
The Hanshin expressway in Kobe, Japan collapsed due to an earthquake in 1995.
Picture from
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Normal Fault
Rock above the fault surface moves downward in relation to rock below the fault surface.
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Reverse Fault
Reverse faults result from compression forces that squeeze rock.
If rock breaks from forces pushing from opposite directions, rock above a reverse fault surface is forced up and over the rock below the fault surface.
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Strike-slip Fault
At a strike-slip fault, rocks on either side of the fault are moving past each other without much upward or downward movement.
The San Andreas Fault is the boundary between two of Earth’s plates that are moving sideways past each other.
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12. How does energy created by an earthquake move through the Earth?
Seismic waves are energy waves that travel outward from the source of the earthquake.
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Surfin the Waves
When earthquakes occur, three different types of seismic waves are produced.
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P Waves
Primary waves (P-waves) cause particles in rocks to move back and forth in the same direction that the wave is traveling.
P-Waves are the fastest waves and are felt first, usually as a bang or a thump.
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S Waves
Secondary waves (S-waves) move through Earth by causing particles in rocks to move at right angles to the direction of wave travel.
These waves are slower than P-Waves.
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L Waves
Surface waves cause most of the destruction resulting from earthquakes.
Surface waves (L-Waves) or land waves move rock particles in a backward, rolling motion and a side-to-side, swaying motion.
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17. How are seismic waves measured?
Seismic waves from earthquakes are measured with an instrument known as a seismograph.
Seismographs register the waves and record the time that each arrived.
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18. Measuring Earthquake Magnitude
Magnitude is a measure of the energy that is released during an earthquake.
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Richter Scale
The Richter magnitude scale is used to describe the strength of an earthquake and is based on the height of the lines on the seismogram.
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Richter Scale
For each increase of 1.0 on the Richter scale, the height of the line on a seismogram is ten times greater.
However, about 32 times as much energy is released for every increase of 1.0 on the scale.
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Earthquake Severity
Richter Earthquake Magnitudes Effects
Less than 3.5 Generally not felt, but recorded.
Often felt, but rarely causes damage.
Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.
Can be destructive in areas up to about 100 kilometers across where people live.
Major earthquake. Can cause serious damage over larger areas.
8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across.
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