Earthquakes are extremely common. As many as 8,000 earthquakes take place every day. Most earthquakes are so small they can only be felt by the most sensitive instruments.

Earthquakes take place along faults. A fault is a plane that separates blocks of rock that are moving in opposite directions.

There are several types of faults, identifiable by which direction the blocks of rock on either side move and the angle of the fault plane. The main types of faults are dip-slip and strike slip.

The focal mechanism of an earthquake is the amount of motion that the rocks along the fault undergo and the orientation of the fault.

Dip-slip faults. A dip-slip fault has vertical motion.

There are two types: In a normal fault, the hanging wall goes down relative to the footwall. In a reverse fault, the hanging wall goes up relative to the footwall. The footwall is the block of rock that you would stand on if you were standing on the fault plane; the hanging wall is above you. A thrust fault is a reverse fault in which the fault plane is at an angle less than 45 degrees.

Dip-slip fault. A fault in which the hanging wall goes down relative to the footwall (normal fault) or the reverse (reverse fault).

Strike-slip fault. A strike-slip fault has only horizontal motion.

If you were to stand with one foot on each side of the fault and the block moving toward you is under your left foot, the fault is a left-lateral (sinistral) strike slip fault. If the block moving toward you is under your right foot, the fault is a right-lateral (dextral) strike slip fault.

The San Andreas Fault is a strike-slip fault. Since the fault separates two different tectonic plates, the San Andreas is a type of strike-slip fault known as a transform fault. Transform faults are one of the three types of plate boundaries.

Earthquake energy travels as waves. The energy released during by an earthquake travels as seismic waves.

There are two types of seismic waves: body waves and surface waves.

Earthquake waves are detected by seismographs. A seismograph records the when each of the waves arrives and the magnitude of the wave.

A seismograph recording seismic waves on a paper known as a seismogram.

Body waves. The two types of body waves are P waves and S waves.

P waves, or primary waves, are the fastest seismic waves. They are the first waves to arrive at a seismic station. P waves can travel through any substance, solids, liquids, and gases.

P waves are compressional waves, meaning that the medium they travel through is compressed and dilated in the direction the wave is moving. When a P wave passes through a structure, such as a building, the structure compresses and expands, causing it to weaken and even collapse.

P waves. Imagine taking a spring toy and bunching up one end. When the bunched coils are released, the spring toy compresses and dilates as the energy moves through it.

S waves, or secondary waves, travel in a side-to-side motion. S waves shake buildings from side to side.

S waves are the second waves to arrive at a seismic station. They can only travel through solids.

S waves. The same spring toy mentioned above, held onto at both ends, and moved from side to side, simulates S waves.

Surface waves travel at the interface between two different substances. There are two types of surface waves, Rayleigh and Love waves.

Surface waves are much slower than S waves and P waves. They roll along the surface like the ripples on a pond and also move side-to-side.

Surface waves do significant damage to buildings and structures because they move along the surface. Also, structures that have already been weakened by the P and S waves may be knocked down by surface waves.

Surface waves. Surface waves travel in a rolling motion and from side to side.

Seismologists determine the epicenter of an earthquake using seismographs. Seismic stations are set up all over the world and can detect seismic waves as they travel around and through the planet.

P waves arrive first, followed by S waves. To locate the epicenter, subtract the arrival time of the first P wave from the arrival time of the first S wave. Find that time on the time-travel graph. Use that number to find the distance that seismic station is from the epicenter.

All the points that are that distance from the seismic station, can be plotted as a circle around the seismic station.

Time-travel graph. In this time-travel graph, the difference in the arrival time of the first P and S wave can be used to find the distance of the earthquake epicenter to the seismic station. For example, if the difference in arrival times at a station is 18 minutes the distance to the epicenter is 6,600 km.

Locating the epicenter. At least three seismic stations are needed to find the earthquake epicenter.

If there is information from only one station, the epicenter of the earthquake could at any point on that circle. If there is information from two stations, the circles intersect at two points, so there are two possible epicenters. With three stations, the circles intercept at only one point, which must be the epicenter.

A circle is drawn around each seismic station connecting all the points that are the distance from that station to the earthquake epicenter. Circles around three seismic stations meet at only one point - the epicenter.

Earthquake intensity. Scientists can try to determine the intensity of an earthquake by asking people what they saw and felt during and after the event.

The Modified Mercalli Intensity Scale allows people to choose a number to rate earthquake intensity. The United States Geological Survey (USGS) has 12 levels of the Modified Mercalli intensity scale.

Not felt except by a very few under especially favorable conditions.
Felt only by a few persons at rest, especially on upper floors of buildings.
Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
Damage total. Lines of sight and level are distorted. Objects thrown into the air.

[source: http://earthquake.usgs.gov/learn/topics/mercalli.php]

Earthquake magnitude. The magnitude of an earthquake is the total amount of energy released by the earthquake.

Magnitude scales for measuring earthquakes are often logarithmic. A one point increase on a log scale means an increase of a factor of ten in wave amplitude and a factor of thirty in energy released.

The Richter Scale measures the amplitude of the largest seismic wave released by an earthquake. The effects of different Richter magnitudes are as follows:

Richter Magnitudes	Earthquake Effects
Less than 3.5	Generally not felt, but recorded.
3.5-5.4	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.
6.1-6.9	Can be destructive in areas up to about 100 kilometers across where people live.
7.0-7.9	Major earthquake. Can cause serious damage over larger areas.
8 or greater	Great earthquake. Can cause serious damage in areas several hundred kilometers across.

Moment Magnitude Scale. The Moment Magnitude Scale measures the total energy released by an earthquake.

This scale is thought to be a better measure than the Richter scale since it takes into account earthquakes that release a tremendous amount of energy but do not release a single extremely high amplitude seismic wave.

To see the difference between the Richter Scale and the Moment Magnitude Scale, consider these earthquakes. The 1906 San Francisco earthquake measured 8.3 on the Richter Scale, but only 7.8 on the Moment Magnitude Scale. This quake produced an exceptional jolt.

The 1964 earthquake in Alaska also measured 8.3 on the Richter Scale, but measured 9.2 on the Moment Magnitude Scale. The largest jolt this quake produced was the same as in San Francisco, but the amount of energy it released was enormously higher.

Damage from Earthquakes. Many factors determine how much damage an earthquake will cause.

The strength and amplitude of the seismic waves are the main factors as is the distance of a location to the epicenter. The duration of shaking has an effect since the longer a quake shakes the more damage it is likely to cause.

Earthquake damage also depends on the geology of an area. Solid rock moves less than sediment or landfill and so will allow less damage.

Fun fact: Astronauts placed seismographs on the moon! This enables Earth-based scientists to measure "moon quakes." Most moon quakes are caused by meteorite impacts.

The Good Friday Earthquake in Anchorage in 1964 was enormous, yet caused only 131 deaths because the area was sparsely populated at the time.