Earthquakes

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Aftershocks

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Earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1-2 fault lengths distance from the mainshock fault. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.

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body wave

Back 2

A seismic wave that moves through the interior of the earth, as opposed to surface waves that travel near the earth's surface. P and S waves are examples. Each type of wave shakes the ground in different ways.

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earthquakes

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This term is used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth

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epicenter

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The point on the earth's surface vertically above the point in the crust where a seismic rupture begins.

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Fault

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A fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture. Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral. Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse (or thrust). Oblique-slip faults have significant components of both slip styles

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Focus

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The point within the earth where an earthquake rupture starts

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seismometer or seismograph

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A seismometer is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium. This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground. Seismograph is a term that refers to the seismometer and its recording device as a single unit.

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P wave

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A seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

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S wave

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A seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving, also called a shear wave

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seismic gap

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A section of a fault that has produced earthquakes in the past but is now quiet. For some seismic gaps, no earthquakes have been observed historically, but it is believed that the fault segment is capable of producing earthquakes on some other basis, such as plate-motion information or strain measurements.

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seismogram

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A record written by a seismograph in response to ground motions produced by an earthquake, explosion, or other ground-motion sources