`Chapter 11 Section 2 Outline
Features of Earthquakes
I. Seismic Waves: A seismic wave is a wave generated by an earthquake that
travels through Earth. Seismic waves can move the ground forward and
backward, up and down, and side to side.
A. The Origin of Seismic Waves: (How and where they begin)
1. As rocks continue to move along faults, their irregular surfaces
catch each other.
2. Stress continues to build up until the elastic limit is passed.
3. When the rocks finally break past each other, energy is released in
the form of seismic waves.
4. The point where the energy is first released is called the focus. This
is where the earthquake starts and the movement of the rocks
begins. The focus point is below Earth’s surface.
B. Three Types of Seismic Waves: Earthquakes produce three different
types of seismic waves.
1. Primary Waves (P-waves): These waves cause particles to move
back and forth in the same direction that the wave is traveling. As
P-waves travel through the Earth, particles in the rocks compress
and stretch. P-waves start at the focus.
2. Secondary Waves (S-waves): These waves cause particles to move
at right angles in the direction that the wave is traveling. These
waves cause particles to move from side-to-side. S-waves start at
the focus.
3. Surface Waves: These waves cause rock particles to move in a
backward, rolling motion and a side-to-side, swaying motion. These
waves cause the most destruction on Earth’s surface. Surface
waves travel outward from the epicenter.
C. The earthquake epicenter is the point on Earth’s surface directly
above the earthquake focus. Surface waves begin when the energy
from the seismic wave reaches Earth’s surface.
II. Locating the Epicenter: The speed of seismic waves is used to locate the
epicenter of an earthquake.
A. The Speed of Seismic Waves:
1. Primary waves are the fastest waves that travel about 6.0 km/s.
2. Secondary waves are the next fastest and travel about 3.5 km/s.
3. Surface waves are the slowest and travel about 3.0 km/s.
B. Seismic waves are measured with an instrument called a seismograph.
1. The paper record of the seismic vibrations is called a seismogram.
2. Seismographs measure the intensity of the waves and the times the
waves arrive at a seismograph station.
3. Seismographs use to consist of a rotating drum of paper and a
stationary pendulum with attached pen. They measure horizontal
and vertical seismic wave movement within the Earth
C. Seismograph stations: Stations that measure and monitor seismic
waves are located throughout the world. Seismic waves reach stations
at different times based on the speeds of the waves and their distance
from the epicenter.
1. Primary waves arrive at a station first.
2. Secondary waves arrive next.
3. Surface waves arrive last.
4. The difference in arrival times is used to calculate how far a
particular seismograph station is located from the epicenter of an
earthquake.
5. The longer it takes for a secondary wave to reach the station, the
farther that station is from the epicenter. (The farther apart the
primary and secondary wave arrive, the greater the distance the
station is from the epicenter.)
6.
Circles are drawn around each station on a map and then the radius
of each circle is used to determine the station’s distance from the epicenter.
7. Seismic waves must reach at least 3 seismic stations in order to get an exact location of an
epicenter. The point where all 3 circles intersect is the location of the epicenter.
Distance (km)
1,500
2,250
2,750
3,000
4,000
7,000
9,000
Difference in Arrival Time
2 min 50 sec
3 min 30 sec
4 min 20 sec
4 min 40 sec
6 min
8 min 10 sec
9 min 40 sec
III. The Basic Structure of Earth: A seismic wave’s speed and direction change as the waves move through
different layers of Earth.
A. Scientists have identified boundaries between Earth’s layers by studying the speeds of seismic waves.
Changes in seismic wave speed have allowed them to map Earth’s internal structure.
1. Earth’s inner core is solid and made mostly of iron.
2. Earth’s outer core is liquid, which also made mainly of iron.
3. Earth’s mantle lies directly above the outer core and is the thickest layer.
a. The mantle is divided into an upper and lower part, based on the speed of seismic waves.
b. The upper mantle, called the asthenosphere, consists of weak rock that can flow easily.
4. The Earth’s crust is the outermost layer of Earth.
a. The crust and a part of the upper mantle, just beneath it, make up the lithosphere, which is
broken into plates.
b. The Earth’s crust is less dense than the mantle and the core beneath it.
B. Mapping Earth’s Internal Structure: Scientists have concluded that the mantle and outer core are
made of different materials by observing the changes in speeds of primary and secondary waves.
1. Scientists have discovered shadow zones, which are areas of Earth that do not receive seismic
waves during an earthquake.
2. Shadow zones exist because secondary waves do not pass through the liquid outer core. Primary
can pass through the liquid core, but are bent and slow down.
3. The bending of primary waves and the stopping of secondary waves create the shadow zone.
C. Boundaries Between Earth’s Layers: Seismic waves change speed as they pass through Earth’s layers.
1. Seismic waves speed up when they travel through the crust and upper mantle. This boundary is
between the crust and upper mantle is called the Moho discontinuity.
2. Primary and secondary waves slow down again when they reach the asthenosphere.
3. They then speed up again when they reach the solid region of the mantle.
4. Secondary waves stop, and do not travel through the liquid outer core.
5. Primary waves slow down when they reach the outer core, but they speed up again when they
reach the solid, inner core.