CH. 19 Earthquakes

Richter scale

Numerical scale used to measure the magnitude of an earthquake, using values based on the size of the earthquake's largest seismic waves.

Stress

(p. 495) Forces per unit area that act on a material—compression, tension, and shear.

Strain

Deformation of materials in response to stress.

Seismometer

Instrument used to measure horizontal or vertical motion during an earthquake.

Magnitude

Measure of the energy released during an earthquake, which can be described using the Richter scale.

modified Mercalli scale

Measures earthquake intensity on a scale from I to XII; the higher the number, the greater the damage the earthquake has caused.

seismic gap

Place along an active fault that has not experienced an earthquake for a long time.

Focus

Point of the initial fault rupture where an earthquake originates that usually lies at least several kilometers beneath Earth's surface.

Epicenter

Point on Earth's surface directly above the focus of an earthquake.

seismogram

Record produced by a seismometer that can provide individual tracking of each type of seismic wave.

Fault

Fracture or system of fractures in Earth's crust that occurs when stress is applied too quickly or stress is too great; can form as a result of horizontal compression (reverse fault), horizontal shear (strike-slip fault), or horizontal tension (normal fault).

tsunami

Large, powerful, ocean wave generated by the vertical motions of the seafloor during an earthquake; in shallow water, can form huge, fast-moving breakers exceeding 30 m in height that can damage coastal areas.

Moment magnitude scale

Scale used to measure earthquake magnitude—taking into account the size of the fault rupture, the rocks' stiffness, and amount of movement along the fault—using values that can be estimated from the size of several types of seismic waves.

Secondary wave

Seismic wave that causes rock particles to move at right angles to the direction of the wave.

Surface wave

Seismic wave that moves in two directions as it passes through rocks, causing the ground to move both up and down and from side to side.

Primary wave

Seismic wave that squeezes and pulls rocks in the same direction that the wave travels, causing rock particles to move back and forth.