| Term | Definition |
|---|
| epicenter | point on earth's surface directly above the focus of an earthquake |
| fault | fracture or system of fractures in earth's crust that occurs when stress is applied too quickly or stress is too great |
| focus | point of the initial rupture where an earthquake originates that usually lies at least several km beneath earth's surface |
| 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 |
| secondary wave | seismic wave that causes rock particles to move at right angles to the direction of the wave |
| strain | deformation of materials in response to stress |
| stress | forces per unit area that act on a material- compression, tension, and shear |
| 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 |
| seismogram | record produced by a seismometer that provide individual tracking of each type of seismic wave |
| 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 |
| 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 |
| 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 |
| seismic gap | place along an active fault that has not experienced an earthquake for a long time |
| 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 |
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