Earthquakes are the most destructive of natural disasters.  More than one million occur each year, although most are too small to be noticed.  More than 3,000 of those, however, are strong enough to move a section of the Earth's crust, and about 20 cause severe changes.  Efforts to be able to predict earthquakes are under way, but so far, there is no sure way.
 

Causes and Effects of Earthquakes

    The major cause of earthquakes is the build up of stress along the boundary between two lithospheric plates.  The plates are constantly exerting friction on each other and creating pressure.  Finally, the pressure becomes so great that the plates suddenly move and cause an earthquake.  The plates then return to the shapes they had before they deformed, but at new locations relative to each other.  This is called the elastic-rebound theory.  Earthquakes also occur from the eruption of volcanoes. Other causes are the collapse of a cavern, or even from the impact of a meteor.
     Two things that often happen as a result of earthquakes are faulting and folding.  Faulting is a break or crack in the Earth's crust along which movement has occurred.  A normal fault is when one side of a rock strata slides down and forms a hanging wall.  There are also reverse faults and grabens.  Grabens occur when an area of strata slips down, but the strata on either side stays in place.
    Folding in rock layers occurs when the ground shifts and is compacted.  An upfold in rock layers is called an anticline and a downfold in rock layers is called a syncline.
 Link to Plate Tectonic
 

Waves

    An earthquake is any event that causes the earth to shake from a release of energy.  This energy is released in the form of three different waves.  The first type of wave is the P wave, also called the primary, or compressional wave.  The P wave can travel through any material, including solid rock, magma, ocean water, and air.  It causes a back and forth motion that squeezes and stretches the land.
    The second type of wave is the S wave, or the shear or secondary wave.  S waves can travel through solids, but not liquids or gases.  The S wave moves up and down causing the particles to move at right angles to the direction the waves are traveling.
    P waves always travel about twice as fast as S waves, although the speed varies depending on the density of the material.  These waves are considered body waves because they travel through the Earth's body.  When they reach the surface, they cause a third wave called an L wave.  These surface waves travel like a ripple on a pond at about 3 kilometers per second.

Moho
     In 1909, a Yugoslav seismologist named Andrija Mohovicic discovered the boundary between the mantle and the crust.  He discovered this by studying seismographs and seeing the change in velocity of P and S waves.  This boundary, called the Mohorovicic discontinuity, or Moho for short, was originally thought to be a depth of 50 kilometers every where.  Later, it was discovered that the Moho averages 32 kilometers under the continents and 8 kilometers under the oceans.
 
 

Shadow Zone
     Even though waves can move throughout the Earth's interior, not all seismographs receive information from all earthquakes.  Seismograph stations that receive no P or S waves are in the shadow zone of that earthquake.  The shadow zone is a wide belt around the Earth on the side opposite the focus of the earthquake.  The cause of the shadow zone is Earth's outer core.  The P waves go through the mantle and then refract back to the surface, or if it travels deep enough, it will refract twice.  The S waves cannot reach the shadow zone because the outer core is a liquid, and therefore, it cannot pass through.

Bending Waves
     As waves enter the inner earth, their path is often altered.  One type of alteration in a wave's path is called reflection.  This is when the wave bounces back up from the inner layers.  The angle it goes down at and the angle it comes up at are the same.  Refraction occurs when a wave passes through layers of different densities and the wave bends downward.  This helps to determine the substructure of Earth.
 

Measuring and Locating Earthquakes

Seismographs
    To detect and record earthquake waves, an instrument called a seismograph is used.  There are two types:  one that records vertical motion, and one that records horizontal motion.  The way in which they work is simple.
     A weight is attached to a base anchored in bedrock.  The weight stays almost perfectly still (due to inertia) even when the bedrock and base are being shaken by an earthquake.  A record sheet, called a seismograph, is placed on a drum attached to the base.  A laser attached to the weight is placed above the drum.  When an earthquake occurs, the laser stays stationary and the paper moves causing zig-zags to be drawn.
    Three stations must record how far away the epicenter is and from these, there is a common point.  This common point determines the epicenter.
 Link to Seismology Terms  and  Link to Drum Recorder Display

Richter Scale
     To measure the strength of earthquakes, a man named Charles F. Richter developed the Richter Scale in the 1940’s.  It is designed to be a measure of the amount of energy released by an earthquake.  It is based on a scale of 1 to 10, 10 being the strongest.  Each magnitude number represents an earthquake 32 times stronger than the next lower number.
 

Depth of Earthquakes

    The depth at which an earthquake occurs depends on the kind of plate boundary involved.  At spreading centers and sliding boundaries, most earthquakes are less than 30 kilometers deep.  Examples of where these might occur are the Mid Atlantic Ridge and the San Andreas Fault.  At subduction boundaries, earthquakes can be as much as 700 kilometers deep.
 

Focus and Epicenter

    The place inside Earth where the earthquake actually occurs is called the focus of the earthquake.  As the depth of the focus increases, the area damaged increases.  The point on the Earth's surface directly above the focus is the epicenter of the earthquake.
 

Examples of Earthquakes

New Madrid
    Three of the largest earthquakes ever in the history of the United States didn't occur on a plate boundary.  They occurred in New Madrid, Missouri in 1811 and 1812.  There magnitudes were 8.6, 8.4, and 8.7.  The quakes were said to ring church bells in Boston and were felt throughout most of the northeast.  The shaking caused two new waterfalls to form on the land and boats to be scattered all over.
       So why did an earthquake occur there if it wasn't on a plate boundary?  The cause is three faults that are buried deep in the Mississippi River.  These faults may have been inactive for millions of years before stress on the North American Plate caused them to start moving again.  There is still a threat of another major quake in that area and a few minor ones have occurred since the ones in the early 1800s.
 Link to New Madrid Information

Alaska
    The Alaskan earthquake of 1964 had one of the largest magnitudes of any from this century.  The quake lasted for 5 minutes and destroyed whole blocks of houses.  It was caused from movement on a subduction boundary.  All along the Aleutian Islands, the Pacific Plate is pushing under the North American Plate.  The reason the earthquake was so severe was because the break on the fault triggered other breaks.  It was so severe it caused buildings to shake in Seattle, Washington.  Most seismograph stations around the world weren't able to record it because it threw the pen off of the drum!  The earthquake cause tsunamis that destroyed many villages.
 Link to Seismicity in Alaska

San Andreas Fault
    The San Andreas Fault marks the boundary between the Pacific Plate and the North American Plate.  Pressure often builds up at this boundary and causes earthquakes.  It is estimated that a major earthquake occurs on the fault every 160 years.
    Two earthquakes occurred in southern California in 1971 and 1987.  The two earthquakes demolished thousands of buildings and caused millions of dollars in damage.  In 1857, an earthquake 120 kilometers away from Los Angeles shifted the fault 9 meters in that area.  In 1906, San Francisco was destroyed from an earthquake that shifted the fault 6 meters.
Link to California Earthquakes

Link to USGS
 

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