The Dynamic Crust Chapter 3 Vocabulary (p. 65) Earthquake: Natural vibrations, sometimes destructive, that radiate from a sudden movement along a fault zone within Earth or from sudden movements of magma under a volcano. Fault: Break in the rock of Earth’s lithosphere along which there has been displacement (movement). Seismic Waves: Vibrational energy that radiates through Earth from an earthquake. Focus: One of two fixed points that determine the shape and position of an ellipse; an earthquake’s point of origin within Earth (aka hypocenter). Epicenter: Location along Earth’s surface that is directly above the focus of an earthquake. What Causes an Earthquake? Most earthquakes occur when stress on earth’s crust becomes too great and the crust shifts and breaks releasing energy in waves. The focus is the place underground where the break first occurs. The epicenter is the location at Earth’s surface just above the focus. Vibrations begin at the epicenter and move outward. Vocabulary (p. 66) Intensity: Strength Seismograph: Instrument designed to measure and record the magnitude of an earthquake (ground vibrations). Seismologist: Scientist who studies earthquakes. Seismometer: Instrument that detects vibrations of the ground. Earthquakes are measured using intensity and magnitude scales. The Mercalli scale measures intensity based on the reports of people who experience the earthquake or observe the damage caused. The intensity of an earthquake depends upon: The observer's distance from the epicenter The nature of the ground The ability of the ground to transmit energy Seismographs are used to make the most dependable measures of earthquakes. Magnitude: Total energy released by an earthquake, measured the Richter Scale.