Download Section 9-2

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