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Aftershock: a smaller earthquake that occurs after a larger earthquake.
Compression: a type of stress that squeezes rock, causing it to break or fold.
Earthquake: movement of the ground caused by the release of energy from a sudden shift of rocks in Earth's crust.
Epicenter: the point on Earth's surface directly above the focus of an earthquake.
Fault: a break in Earth's crust where movement of rock occurs.
Fault zone: a place along plate boundaries where many faults are located.
Focus: the point below Earth's surface where movement of rock produces an earthquake.
Foreshock: a small earthquake that precedes a larger earthquake.
Magnitude: the measurement of the total strength or amount of energy released by an earthquake.
Mercalli scale: a measurement of an earthquake's intensity based on how much damage it causes. The Mercalli scale ranges from Level I (not felt except by very few under favorable conditions) to Level XII, (causing almost total destruction.)
Moment magnitude scale: a measurement of an earthquake's magnitude based on the amount of movement of the rock along a fault line.
Normal fault: a type of fault where forces of tension are pulling rock apart.
P (Primary) wave: the fastest moving type of seismic wave, which expands and compresses rock, like the movement of a slinky. Also known as pressure waves. P waves can travel through both liquids and solids.
Reverse fault: a type of fault where compression pushes rock together. Also known as a thrust fault.
Richter scale: a measurement of the magnitude of an earthquake based on the readings of a seismograph. The Richter scale is a logarithmic scale ranging from 0 to 9, with each number representing a 10-fold increase in ground motion, and a 30-fold increase in energy released.
S (Secondary) wave: the second-fastest moving type of seismic wave, which moves rock horizontally from side to side. Also known as shear waves. S waves cannot pass through liquids, and therefore cannot pass through Earth's liquid outer core.
Seismic wave: a vibration that travels through Earth carrying the energy released during an earthquake.
Seismograph: an instrument that records seismic waves.
Shearing: a type of stress that pushes two adjacent areas of rock in opposite directions.
Stress: a force that causes rock to change shape.
Strike-slip fault: a type of fault where rocks slide horizontally past each other in opposite directions, with little up or down motion. The San Andreas fault in California and the North Anatolian fault in Turkey are examples of strike-slip faults.
Surface waves: seismic waves that move along Earth's surface. They can have an up-and-down motion or a horizontal motion. Surface waves travel slower than P or S waves and usually cause the most damage.
Tension: a type of stress that stretches rock and makes it thinner.
Tsunami: a giant, fast-moving wave that is caused by an undersea earthquake. Also known as a seismic sea wave.