The San Andreas Fault in California is a transform fault.
When energy builds up in the Earth's crust, the stress and strain lead to a sudden release of energy in the form of vibrations. Earthquakes are vibrations that radiate from the source of the released energy to the surface. Earthquakes tend to occur around certain types of landforms such as faults and volcanoes. Movement in the Earth's crust caused the formation of these landforms and the accumulation of stored energy that results in earthquakes.
Plate Tectonics
The theory of plate tectonics states that Earth's crust is composed of about twelve plates. These plates float atop a layer of molten rock and slide over, under and past each other. Plate movement causes displacement of portions of Earth's crust as areas of rock push above or below the surface or lock together. Energy builds as a result of this displacement. The release of built-up energy causes the crust to bend and break and sends vibrations to the surface.
Plate Boundaries
About 90 percent of earthquakes happen at the boundaries between plates. New landforms result from earthquakes at these boundaries. In spreading zones, molten rock pushes plates apart and forms ridges when it cools. Transform faults occur where plates slide past each other and form straight, shallow cracks; the San Andreas Fault is an example of a transform fault. One plate slides under the other in subduction zones and the sunken portion of rock melts. Deep ocean trenches and volcanic mountains form at subduction zones.
Faults
Earthquakes tend to recur in weakened zones of the crust called faults. A fault is a crack in Earth's surface where two areas of crust exert force on each other. Faults are classified according to the way they move. A normal fault occurs when tension between blocks causes one block to slip below the surface. Compression forces squeeze two blocks together to form a reverse fault where one block slips above the surface. Strike-slip faults result when either tension or compression causes blocks to slide past each other horizontally.
Volcanoes
Earthquakes have an association with volcanoes. Both earthquakes and volcanic eruptions occur near the edges of plates. A release in energy from Earth's crust can trigger both events. The vibrations from an earthquake in the vicinity of a volcano can cause the onset of an eruption. U.S. eruptions including Washington's Mount St. Helens, Hawaiian volcanoes Mauna Loa and Kilauea and Redoubt Volcano and Mount Spurr in Alaska followed an increase in earthquake activity.
Tags: Earth crust, each other, past each, past each other, Andreas Fault, below surface
