An earthquake is when the surface of the earth is shaking because of rapid movement of the rocky outer layer of the earth.  If energy that is stored inside the earth (strain in rocks is the usual form) is released suddenly, an earthquake occurs.  When this happens, earthquake waves transfer the released energy to the surface of the earth.  Seismology is the study of earthquakes, and the waves that they create.

The focus (sometimes referred to as the hypocenter) is the point within a geological fault that is rupturing where the earthquake begins.  The epicenter is the point on the surface of the earth that is directly above the focus.  When the energy is released, the earthquake waves start at the focus, and then radiate out from there along the part of the fault that has ruptured.  Shallow-focus earthquakes are created if the focus is near the surface of the earth (between zero and forty miles deep) while deep-focus earthquakes are created if the focus is deep within the earth (between forty and four hundred miles deep).  Shallow-focus earthquakes are much more common than deep-focus ones, and they are usually larger, which in turn makes them more dangerous.  Shallow-focus earthquakes usually begin near an area where the crustal plates of the earth are moving against one another. Deep-focus earthquakes usually begin in places where one tectonic plate moves under another one, or subducts.

Faults, which are places in the crust of the earth where rocks have moved (if those rocks slip, an earthquake occurs), are mainly created when the crust becomes stressed and slips suddenly, as the result of tectonic plates having moved.  The movement of tectonic plates is very slow, and pressure within the earth's crust builds up as  they collide or slide past each other.  Over hundreds of years, the pressure builds up, and  when it finally becomes stronger than the rocks, all that energy is suddenly released, creating an earthquake.  Although a lot of earthquakes occur naturally, human activities such as filling reservoirs increases the amount of stress that is in the earth's crust and helps an earthquake to occur sooner than it otherwise would have.  Other examples of things that humans do to speed up the start of earthquakes include: injecting fluids into deep wells for waste disposal, and firing underground nuclear test blasts.

The surface that is between the two sides of a fault is in a plane, the plane usually slants into the earth at an angle rather than going in vertically.  The fault is called a normal fault if the rock that is hanging over the slanted fault plane slips downwards, but if the rock slips upwards, then it is a reverse fault (sometimes called a thrust fault).  Vertical displacements (when one side of a fault moves upward above the other side) that at the surface look like steep slopes, are created by both reverse and normal faults.  Another type of fault is a strike-slip fault, but in a strike-slip fault the displacement is horizontal (when the fault is moving from side to side) instead of vertical.  One strike-slip fault is the San Andreas fault in California.

When the rocks along a fault move suddenly, vibrations occur and waves transmit energy through the earth.  There are two broad categories of earthquake waves, that can each then be classified into smaller types. Body waves are waves that travel below the surface of the earth in rocks.  Primary (P) waves and secondary are the two different types of body waves.  The most damage to the earth is done by S waves, which are sometimes called sheering waves because they move the ground back and forth.  Surface waves are the other type of waves, and there are two types of them: Rayleigh waves (which are named after the British scientist Lord Rayleigh), and Love waves (which are named after the British geophysicist A.E.H. Love). 

Beginning at the focus of the earthquake, body waves will radiate out from a fault that is rupturing. All of the rock that is in the path of a P wave  is compressed and expanded alternately, which is why they are sometimes called compression waves.  P waves are also the fastest earthquake wave, in strong rocks, they can travel at speed of up to four miles per second.  Once the P waves have begun radiating out from the focus of the rupturing fault, S waves follow.  The S waves sheer or twist the rocks (by moving up and down or side to side, perpendicular to the direction that the waves are traveling in), rather than compress it, and they don't travel as fast P waves, at their fastest moving at about two miles per second.  A Rayleigh wave travels on the surface of the earth and causes the rock pieces to move in a path that goes up, down, forward, and backward, and follows the direction the wave is moving. The rock only moves from side to side perpendicular to the direction that the wave is traveling, and no vertical displacement occurs that results from Love waves.  Both Rayleigh and Love waves always travel more slowly than P waves do, and although they can move faster, usually they travel more slowly than S waves too.

An American seismologist named Harry Fielding Reid began  to study the effects of an earthquake that occurred in California in April of 1906, in 1911.  In order to explain the "generation of earthquakes" that occur near the boundaries of different tectonic plates, and in tectonic areas, he proposed the elastic rebound theory.  This theory says that "during an earthquake, the rocks under strain suddenly break, creating a fracture along a fault.  When a fault slips, movement in crustal rock causes vibrations.  The slip changes the local strain out into the surrounding rock.  The change in strain leads to aftershocks (smaller earthquakes that occur after the initial earthquake), which are produced by further slips of the main fault or adjacent faults in the strained region.  The slip begins at the focus and travels along the plane of the fault, radiating waves out along the rupture surface.  On each side of
the fault, the rock shifts in opposite directions.  The fault rupture travels in irregular steps along the fault; these sudden stops and
starts of the moving rupture give rise to the vibrations that propagate as seismic waves."

The magnitude (size), how long an earthquake lasts, and how much shaking actually occurs are factors that determine how much damage a particular earthquake causes.  Large earthquakes can easily crack the ground, cause buildings to collapse, and create tsunamis that are capable of inflicting incredible damage.  The Ritcher scale, developed by Charles Ritcher, an American seismologist in 1925, was the first seismic magnitude scale that was widely used.  The amplitude (height) of the surface waves are measured by the Ritcher scale, which is a logarithmic scale (each successive unit of magnitude measure represents ten times
the amount of the previous number) because earthquake waves can cause ground displacement between less than a millimeter and several meters. This was taken care of by using the logmaritic recorded wave heights, so that a magnitude of three on the Ritcher scale is ten times more than a magnitude two, and one hundred times more than a magnitude one.

Although the Ritcher scale is still commonly used, seismologists now prefer to use a new kind of magnitude scale that is called the moment magnitude scale.  The moment magnitude is calculated by multiplying the area that the earthquake occurred in, and the amount of displacement that occurred in the slip.  Seismologists believe that the moment magnitude scale is more reliable for measuring earthquakes, especially those that measure above a seven on other scales because the other scales only take into account part of the seismic waves, and the moment magnitude measures the total size of the earthquake.  By using Global Positioning Satellites (GPS), seismologists can measure how much a fault moves; they have found that most faults move about a few centimeters per year.  They also figured out the four main regions of the interior of the earth (the curst, the mantle, the inner core, and the outer core) by using earthquake waves.

Several hundred seismic tremors (very small earthquakes) occur around the world every day, and about one million occur every year.  Luckily, earthquakes that are extremely large only occur once every few years anywhere in the world, but moderate earthquakes (earthquakes with a magnitude of around 7.0 on the Ritcher scale) usually occur around twenty times a year.  At least several million people have been killed around the world within the last five hundred years.  In Táng-Shan, China an earthquake that occurred in 1976 killed over two hundred and forty thousand people.

The first picture is of a fault, courtesy of http://www.esc.cam.ac.uk/new/v10/research/!images/fault.jpg The next picture is of a building that has been destroyed by an earthquake, courtesy of http://www.riverdeep.net/current/1999/08/081999_images/ earthquake.jpg The next picture is of a road that has been cracked by an earthquake, courtesy of http://www.dot.state.ak.us/stwddes/rrowdys/assets/ images/img_0127.jpg