Earthquakes occur at depths from near the Earth's surface to about 700 km deep. (See Determining the Depth of an Earthquake.) Below that depth, rocks are too hot and ductile, so they tend to bend and flow rather than break in a brittle manner. The strength of shaking from an earthquake diminishes with increasing distance from the earthquake's source, so the strength of shaking at the surface from an earthquake that occurs at 500km deep is considerably less than if the same earthquake had occurred at 20 km depth.

Earthquakes do occur occasionally in Antarctica, but not very often. See http://neic.usgs.gov/neis/general/seismicity/s_pole.html. There have been some big earthquakes--including one magnitude 8--in the Balleny Islands (look "south" from the Pole toward New Zealand between the coast and the plate boundary on the Pacific-Antarctic Ridge). The boundary between the Scotia Plate and the Antarctic Plate just grazes the north tip of the Antarctic Peninsula (again, look "northwest" from the Pole toward South America).

Often, people wonder if an earthquake in Alaska may have triggered an earthquake in California; or if an earthquake in Chile is related to an earthquake that occurred a week later in Mexico. Over these distances, the answer is no. Even the Earth's rocky crust is not rigid enough to transfer stress fields efficiently over thousands of miles.

The moon, sun, and other planets have an influence on the earth in the form of perturbations (small changes) to the gravitational field. The relative amount of influence is proportional to the objects mass, and inversely proportional to the third power of its distance from the earth.

The stresses induced in the earth by an extraterrestrial mass are proportional to the gravitational field gradient dg( r ) / dr and NOT to the gravitational field g( r ).

g( r ) = GMm / r^2

thus:

"Foreshock" and "aftershock" are relative terms. Foreshocks are earthquakes which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes which occur in the same general area during the days to years following a larger event or "mainshock", defined as within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The frequency of these aftershocks decreases with time.

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.

Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles).

An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.

Sometimes earthquakes leave surface evidence, such as surface ruptures or fault scarps (cliffs made by earthquakes).

Sometimes a large earthquake has broken the fault since instrumental recordings began in 1932.

Sometimes the fault produces small earthquakes that can be recorded with the denser seismographic network established in the 1970s.

Earthquakes occur on faults - strike-slip earthquakes occur on strike-slip faults, normal earthquakes occur on normal faults, and thrust earthquakes occur on thrust or reverse faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle.