To understand faults, it is helpful to understand plate tectonics. Earth's crust is not a solid skin. Instead, it is made up of huge blocks of rock that fit together to form the entire surface of the planet, including the continents or land masses and the floors of the oceans. Scientists believe the crust is composed of about 12 of these plates. Each plate is relatively rigid, and, where the plates meet, they can spread apart, grind against each other, or ride one over the other in a process called subduction. Spreading plates most commonly occur in the oceans in the phenomenon known as sea-floor spreading; when plates spread within land masses, they create huge valleys called rifts. The process of plates grinding together causes near-surface earthquakes, and the collision and subduction of plates causes the most intense earthquakes much deeper in the crust.
The engine driving the movement of the plates originates deep in the earth. The mantle, a zone underlying the crust, is very dense rock that is almost liquid. Deeper still is Earth's core, which is molten rock. Because it is fluid, the core moves constantly. The mantle responds to this, as well as to centrifugal force caused by the rotation of Earth on its axis and to the force of gravity. The slower motions of the mantle pulse through the thin crust, causing earthquakes, volcanic activity, and the movement of tectonic plates. Together, the pulses caused by the heat engine inside Earth result in over a million earthquakes per year that can be detected by instruments. Only one third of these can be felt by humans, most of which are very small and do not cause any damage. About 100–200 earthquakes per year cause some damage, and one or two per year are catastrophic.
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