Earthquakes, Faults, Plate Tectonics, Earth Structure

The magnitude of an earthquake is a measure of the energy it releases. This means that it doesn't matter that the earthquake might not "feel" as strong farther away from its source; the magnitude just depends on the earthquake's total energy.

A hotspot is an isolated region of the earth that produces molten silicate rock, called lava. At temperatures in excess of 1000°C (1800°F), lava can periodically erupt from the ground. Before it erupts onto the earth's surface, this material is called magma. Magma contains many types of other chemicals, including large amounts of gases like carbon dioxide. Many of these gases are toxic.

The simple answer is that the largest earthquakes occur at shallower depths in the earth's crust, but smaller earthquakes can and do occur at all depths down to about 700 km (400 mi).

Earthquakes occur in the earth's crust, the topmost layer of the earth, which is typically 7 to 30 km (4 to 18 mi) thick. The crust is the coldest and most brittle part of the earth, and has many fault systems on which earthquakes occur. These earthquakes are caused by the buildup of tectonic stress, and result in frictional sliding on the faults.

Earthquakes release energy in several forms:

Think of the Earth as an onion, composed of layers made of rocks of various temperatures, getting hotter as you go deeper. The outer layer, the crust, is pretty rigid and floats along on top of the mantle, which can flow (slowly). The mantle can flow because radioactive elements (uranium and thorium) left over from the formation of the earth provide the heat. The top layer, which we live on, is 7-35 km thick, out of a total of 6368 km, so the skin is 0.2% of the radius, really very thin.

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.

Also, the depths of earthquakes gives us important information

Earthquakes do occur occasionally in Antarctica, but not very often. See http://www.iris.edu/seismon/. 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). There is also a hint of a line of

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.