4
7AVE #HARACTERISTICS
T
he first wave to reach the surface in an earthquake is the primary
wave (P-wave), which is a type of compression or longitudinal wave.
A longitudinal wave is one that transfers energy through compressions
and rarefactions in the medium through which the wave travels. A sound
wave is an example of a longitudinal wave. The secondary wave (S-wave),
which is a type of shearing or transverse wave, reaches the surface after the
P-wave. A transverse wave is one that causes the medium (or in case of
electromagnetic waves, the electric field) to move perpendicular to the
direction of propagation.
P-waves are an example of
a longitudinal wave (top).
S-waves are an example of
a transverse wave (bottom).
longitudinal wave
transverse wave
3158 SEPUP SGI Waves SE
Figure: SGI Wv SE 04.01a
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CHALLENGE
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SGI Waves SE
What
characteristics
do all waves have in common?
Figure: SGI Wv SE 04.01b
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MATERIALS
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7AVE #HARACTERISTICS s !CTIVITY READING
Complete Student Sheet 4.1, “Anticipation Guide: Wave Characteristics,” to help
prepare for the following reading.
Wave Energy Is Everywhere
The transfer of energy by waves can be observed from natural and human
sources. Waves can carry small amounts of energy, such as the waves you
observed in a slinky on the floor, sound waves, or water waves. Waves can
also transfer tremendous amounts of energy, such as the energy transferred
in earthquakes, tsunami waves, or gamma waves. The table below summarizes the characteristics associated with various types of waves.
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3158 SEPUP SGI Waves SE
Figure: SGI Wv SE 03.01 no label
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!CTIVITY s 7AVE #HARACTERISTICS
Types of Waves Zfek`el\[]ifdgi\m`fljgX^\
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Figure: SGI Wv SE 04.02
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3158 SEPUP SGI Waves SE
Figure: SGI Wv SE 04.03
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7AVE #HARACTERISTICS s !CTIVITY Transmission of Waves through Different Media
Regardless of the kind of wave or the amount of energy transmitted, it is
important to note that when waves are transmitted through a medium; the
medium itself is not transferred. In all cases, the energy is transmitted, not
the individual molecules or particles in the medium. When you observed the
piece of tape on the slinky in Activity 2, “Transmitting Wave Energy,” the
tape moved back and forth but was not transmitted to the end of the slinky
with the energy. Similarly, a boat will bob on a lake as the water waves
transmit energy underneath it.
Mechanical waves such as earthquakes, sound waves, or waves in a slinky
will travel differently depending on the medium. The same wave will travel
at different speeds through two different substances. In general, waves travel
faster through materials that are denser and have “springier” molecules. So
sound generally moves faster through solids than liquids and faster through
liquids than gases. For example, sound travels about fifteen times faster
through metal than through air.
Some waves, such as light, are not mechanical and do not require a medium
to be transmitted. For example, light can travel through the vacuum of outer
space, whereas sound cannot. Another major difference between sound and
light is that light travels much faster—at a rate of about 300,000,000 m/s
compared to about 340 m/s for sound.
Fundamental Characteristics
Every wave has four fundamental characteristics: wavelength, frequency,
amplitude, and speed. A wave’s wavelength is the length of one complete
cycle, or the distance between any two successive identical parts of the wave.
As you learned in the previous activity, the frequency is the rate the medium
is displaced, or the number of full cycles of the wave per unit time. In sound
waves, the frequency is a measurement of the pitch of the sound. A higher
frequency sound has a higher pitch than a lower one. The amplitude is the
wave’s displacement from its state of rest. In sound waves, the amplitude
is a measurement of the loudness of the sound. A larger amplitude sound
wave is louder than a similar sound wave with smaller amplitude. A diagram
of wavelength and amplitude is shown below.
wavelength
amplitude
3158 SEPUP SGI Waves SE
Figure: SGI Wv SE 04.04
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!CTIVITY s 7AVE #HARACTERISTICS
The speed of a wave is the distance it travels per unit time. Another way to
determine a wave’s speed is by its frequency and wavelength using the
formula,
speed (s) frequency (f) wavelength (L)
s fL
For example, a tsunami traveling across the deep ocean could have a
wavelength of 400 km (400,000 m) and a frequency of 1 wave every 40 min
(2400 sec). The speed would be,
s 1⁄2400 sec 400,000 m
167 m/s (374 miles per hour)
This means tsunamis can quickly travel across the ocean to coastal communities. Additionally, tsunamis typically have an amplitude of less than
one meter when traveling in the deep ocean water. This can pose a danger
because they may not be noticed until they are close to shore, when the tsunami reaches shallower water and its amplitude grows tremendously.
For a wave at constant speed, like the speed of light, the frequency and
wavelength are inversely proportional. If the frequency decreases then the
wavelength increases and vice versa. The diagram below shows this relationship for a wave of constant speed.
short wavelength
high frequency
How Earthquakes Move
long wavelength
low frequency
3158 SEPUP SGI Waves SE
In an earthquake,
longitudinal P-waves move faster than the transverse
Figure:
SGI Wv SE 04.05
S-waves. For example,
it could take 9 seconds for a P-wave to arrive but 64
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seconds for the S-wave arrival, for a difference of 55 seconds. By timing how
long it takes for both waves to arrive at the surface, scientists can determine
the distance to the epicenter, or the point on the surface directly above the
origin of the earthquake. Measurements from three seismic stations are used
to find the location of the epicenter of an earthquake. The location of the
epicenter is usually the site of the greatest damage. In the example on the
next page, the seismic graphs on the left show the time interval between
the arrival of the P-waves and the S-waves. Using this time interval and the
wave speed, the distance from the epicenter for each seismometer is calculated and plotted on a map.
7AVE #HARACTERISTICS s !CTIVITY 60
seismograph 2
12
sec
Distance (km)
105
seismograph 3
150
17
sec
200
22
sec
P-wave
arrival
0
60 km
7
sec
seismometer 1
Arrival time interval S-P
seismograph 1
seismometer 2
epicenter
105 km
150 km
seismometer 3
S-wave
arrival
20
40
60
Arrival time (seconds)
The fact that waves travel through media at different speeds has helped
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seismologists understand the3158
earth’s
composition.
For example, by timing
Figure: SGI Wv SE 04.06b
Figure: SGI Wv SE 04.06a
earthquakes
carefully,
scientists
have
discovered
that
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LegacySansMedium 10/11.5waves slow down as
they travel through the mantle of the earth. This is strong evidence that the
core of the earth is denser than the crust of the earth. Similarly, the knowledge that S-waves cannot travel through liquids has helped determine that
the outer core of the Earth is composed of liquid iron and nickel. In this
way, seismic waves have provided a window into the earth’s interior.
ANALYSIS
1. How are seismic waves
a. LIKE OTHER KINDS OF WAVES
b. UNIQUE
2. Look at the surface waves diagram in the “Types of Waves” chart.
Examine the motion of molecules in a water wave. Does this kind of
WAVE HAVE AN AMPLITUDE %XPLAIN
!CTIVITY s 7AVE #HARACTERISTICS
3. Look at the following pictures of different kinds of waves.
a. 7HICH OF THE FOLLOWING CHARACTERISTICS DO THEY ALL SHARE
b. For those characteristics that are not shared, explain how you know
they are not.
amplitude
compression
epicenter
frequency
medium
seismic graphs
P-wave
speed
wavelength
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Figure: SGI Wv SE 04.08
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4. An earthquake with a frequency of about 50 cycles per second moves at
6000 m/second for the P-waves, and 3500 m/second for the S-waves.
What is the wavelength of
a. THE 0WAVE
b. THE 3WAVE