# Section 9-2 Define seismic waves and focus.

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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 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 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

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 mainly composed of iron and nickel Outer core – the liquid layer surrounding the inner core and mainly composed of iron and nickel Mantle – the largest layer located above the outer core and mainly composed of silicon, oxygen, magnesium, and iron 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