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Experiential Science 30—Freshwater Systems
Seismic Waves
Table 2.1 The Main Types of Seismic Waves
Two different types of seismic waves radiate out from
an earthquake. Body waves originate in the body of the
rock and are similar to sound waves. Surface waves travel
along the surface of the Earth and are similar in some
respects to ocean waves. The main types of seismic waves
are summarized in Table 2.1.
Wave type
Particle motion
Name
Body waves
longitudinal
P wave
transverse
S wave
horizontal transverse
L wave
vertical elliptical
R wave
Surface waves
Body waves
Earthquakes create two types of body waves. The first
is called a primary or P wave. P waves are longitudinal
waves, which means they move material back and forth
along the direction in which they travel. P waves can pass
through solids, liquids, or gases, so they go through rock,
magma, oceans, and the atmosphere. They are also called
compression waves because they squeeze and stretch the
material they pass through. When they reach the surface,
the waves are sometimes transmitted into the atmosphere
as sound waves that can be heard by people and animals.
Figure 2.1 As rocks are deformed, they store elastic energy.
Once their elastic limit is exceeded, they fail (fracture), rapidly
releasing the energy in the form of seismic waves that radiate
out in all directions. The rocks then rebound to their former
shapes but at new locations relative to each other.
Fault
Fence
The second type of body wave is called a secondary or S wave. S waves are transverse waves; they vibrate
from side to side. They move material at right angles to
the direction in which they are travelling. S waves can
travel only through solids. Also, S waves move at about
half the speed of P waves.
Surface waves
Surface waves shake the Earth’s surface and are responsible for most of the shaking and damage caused by
earthquakes. They travel more slowly than body waves
but have larger magnitude. They move around the Earth,
rather than through it, as P and S waves do.
The two most important types of surface waves are
Love waves (L waves) and Rayleigh waves (R waves),
named after the scientists who discovered them. An
L wave is a horizontal wave that moves the
ground from side to side and travels only
through solids, not liquids. It is the fastest type
of surface wave and is also one of the major
causes of damage during an earthquake.
SCIENTIFIC TERMS
Original position
Failure and release of energy
Deformation
Rocks rebound to original
undeformed shape
focus (hypocentre): the point under the Earth’s
surface where an earthquake originates.
Love (L) wave: a type of surface wave having a
horizontal motion that is shear or transverse
to the direction of propagation.
P wave: primary or fastest-travelling wave
moving away from a seismic event,
characterized by compressional vibration.
Rayleigh (R) wave: seismic surface wave
causing the ground to shake in an elliptical
motion.
S wave: secondary seismic wave produced by
a shearing motion that involves vibration
perpendicular to the direction in which the
wave is travelling. It does not travel through
liquids or the outer core of the Earth.
Chapter 2 Seismology79
An R wave moves elliptically and has both vertical
and horizontal motion, so it can move through solids
and liquids. It is the cause of most of the shaking that
people feel when they experience an earthquake. Refer
to Figure 2.2.
Behaviour of Seismic Waves
When an earthquake occurs, seismic waves radiate from
the focus in all directions. Some move to the surface
immediately, while others go deep into the Earth. By the
time they reach the surface, the waves have often taken
complicated paths as they move through different densities of rock.
Seismic waves change their speed as they travel
through different types of rock or sediment. They are
transmitted quickly through dense rocks such as basalt
but slow down in materials with low density such as clay
or sand. They also change direction as they are reflected and refracted at places where different types of rocks
meet. Waves can be bounced back, or reflected, when
they hit a rock boundary. They can also be refracted,
meaning their angle of travel is changed. More than just
changing direction, a wave’s energy is partly converted at
the rock boundary, separating into P and S waves.
When P and S waves hit the surface, most of their energy is reflected downwards, back into the crust. So the
surface experiences shaking from the waves that are both
coming and going, increasing the strength of shaking.
Other factors further influence how seismic waves
behave, particularly topography and soil composition.
Loosely compacted soils such as sand or old lakebeds
can liquefy, causing buildings to shake violently and subside, sinking into the ground. Variables such as direction
of travel further complicate the effects of a wave once it
reaches the surface. Earthquake damage is therefore usually related to the underlying geologic strata. That is why
in some earthquakes a city block can be completely destroyed while one block away the houses are seemingly
untouched.
DID YOU KNOW?
In 1855, Luigi Palmieri of Italy designed a mercury
seismometer. Palmieri’s seismometer had U-shaped tubes
filled with mercury and arranged along the compass
points. When an earthquake occurred, the mercury would
move and make electrical contact that stopped a clock
and started a recording drum on which the motion of a
float on the surface of mercury was recorded. This was
the first device that recorded the time of the earthquake
and the intensity and duration of any movement.
Expansions
Compressions
P wave
Undisturbed
medium
S wave
Wavelength
Love wave
Rayleigh wave
DID YOU KNOW?
If an earthquake ever reached a magnitude of 12.0 on the
Richter scale, it would have the equivalent energy of 1
trillion tons of dynamite. Yet that is the same amount of
energy that the Earth receives from sunlight every day.
Figure 2.2 Four types of seismic waves demonstrating the
effects of energy transfer during an earthquake.
Chapter 2 Seismology81
Activity 5
field activity
4 lab activity
library activity
classroom activity
chapter project
4 research team activity
Modelling Seismic Wave
Reflection
Purpose
3. Draw a diagram to show how the waves travelled.
To demonstrate how seismic waves are reflected in the Earth’s
crust.
4. Repeat the trial two or three more times, using the stick
to propagate the wave from a different corner of the tub.
Draw diagrams of these trials.
Materials and Equipment
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•
•
•
•
small clear plastic tub
rock (about 6–8 cm in diameter)
water
few drops of blue food colouring
stick or pencil
Procedure
1. Work in your research team. Place the rock in the centre of
the tub. Fill the tub with enough water so that about half
of the rock is submerged. Add a few drops of blue food
colouring to the water.
2. At one corner of the tub, tap the surface of the water once
with a stick to create a wave. Follow the path of the wave
as it moves across the surface and strikes the rock and the
edges of the tub.
Reflections and Conclusions
1. What happened to the wave when it hit the rock? What
happened when a wave hit the side of the tub?
2. Which seismic wave is the wave you created most like?
3. How does the demonstration model the behaviour of all
seismic waves? That is, what behaviours are common to
all waves when they strike a body of different density?
4. What do the rock, the water, and the sides of the tub
represent?
5. How might this type of experiment be useful in interpreting
types of strata deep within the Earth?
Rock
Stick
Waves in a tub of water can simulate
seismic waves travelling through the
Earth’s various density layers.