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Section 9-2 • Define seismic waves and focus. • Explain the 3 types of seismic waves. • Define epicenter and briefly explain how to locate it. • Describe the structure of the Earth. • Define Moho discontinuity and shadow zone. • Seismic waves – the energy waves in an earthquake that move outward from the focus and make the ground quake • Focus – the point in Earth’s interior where earthquake energy is released Types of Seismic Waves 1. Primary waves – waves of energy that travel through Earth by causing particles in rocks to compress and stretch apart in the direction of the wave 2. Secondary waves – waves of energy that travel through Earth by causing particles in rocks to move at right angles to the direction of the wave 3. Surface waves – waves of energy that reach Earth’s surface and travel outward from the epicenter in all directions • These waves travel by giving rock particles an elliptical (curved) and side-to-side motion. • Surface waves cause most of the destruction during an earthquake. • Epicenter – the point on Earth’s surface directly above an earthquake’s focus Locating an Epicenter • Primary, secondary and surface waves do not travel through Earth at the same speed. • Primary waves are the fastest, surface waves are the slowest. • Scientists use the different speeds of seismic waves to determine the distance to the earthquake epicenter. Seismograph Stations • Because seismic waves arrive at seismic stations at different times, scientists can use the information to determine the distance to an earthquake. • The farther apart the waves (in time) the farther away the epicenter is. Epicenter Location • If seismic wave information is obtained at 3 seismograph stations, the location of the epicenter can be determined. • The point where all three circles intersect is the location of the earthquake epicenter. Epicenter Location Structure of Earth • Inner core – the solid and dense center of Earth • Outer core – the liquid layer surrounding the inner core • Mantle – the largest layer located above the outer core • Crust – the outermost layer Structure of Earth Moho discontinuity – the boundary between Earth’s crust and mantle; seismic waves travel faster below the Moho and slower above it • Shadow zone - the area on the other side of the Earth where waves are not detected • S waves are stopped completely • P waves are slowed and deflected by the liquid inner core