Next figure-- Table of Contents-- Pacific Northwest Earthquakes

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Figure 1. Block diagrams of fault types. (a) An earthquake is caused by the sudden fracturing of rock along part of a fault surface, shown here as a plane. If the fault reaches the surface, a visible ground fracture is created. The focus or hypocenter is the point on the fault plane where fracturing begins. The epicenter is the point on the ground surface directly over the focus. If the fault plane is inclined, the position of the epicenter will not coincide with the ground fracture. Simple fault motions are shown in (b), (c), and (d); directions of compressive stress are indicated. In a normal fault, (b), adjacent blocks of rock behave as if they were being pulled apart; the upper block slides downward along the fault relative to the other. In a thrust fault, (c), the blocks behave as if they were being pushed together; the upper block rides up the fault plane. In a strike-slip fault (d), one block moves horizontally past the other. Oblique motion of the blocks (not illustrated) combines thrust or normal fault motion with strike-slip motion. From an analysis of the seismic waves generated by an earthquake, called a fault-plane solution, scientists can determine the type of fault motion that occurred.

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