| A | B |
|---|
| stress | forces per unit area that on a material compression, tension, and shear. |
| strain | deformation of materials in response to stress |
| 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). |
| 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. |
| 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 |
| focus | point of the initial fault rupture whre an earthquake originates that usually lies at leat several kilometers beneath Earth's surface |
| epicenter | point on the Earth's surface directly above the focus of an earthquake |
| seismometer | instrument used to measure horizontal or vertical motion during an earthquake |
| seismogram | record produced by seismometer that can provide individual tracking of each type of seismic wave |
| magnitude | measure of the energy released during an earthquake, which can be described using the Richter scale |
| richter scale | numerical scale used to measuer the magnitude of an earthquake, using values based on the size of the earthquake's largest seismic waves. |
| 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 |
| modified mercalli scale | measures earthquake intensity on a scale from 1 to 12; the higher the number, the greater the damage the earthquake has caused |
| 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 30m in height thacan damage coastal areas. |
| seismic gap | place along an active fault that has not experienced an earthquake for a long time. |
