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Seismic Waves
Today’s digital seismic monitoring instruments use waveforms to record and analyze seismic data. They
give information not only on local seismic activity, but also earthquakes at far distant locations. When
the Chilean earthquake took place in 1960, seismographs recorded the seismic waves that traveled all
around the Earth. Seismic waves rattled the Earth for days. This occurrence is known as the free
oscillation of the Earth.
Body Waves
There are two types of seismic waves, body waves and surface waves . Body waves travel through the
interior of the Earth, while surface waves travel only within the top surface layers. Most earthquakes take
place at depths of less than 80km below the Earth’s surface. There are two types of body waves, P
waves and S waves . P waves bunch together and then spread apart when they move. It is a bit like the
movement of an inch worm or a slinky. S waves are like rolling ocean waves or like when you snap a
rope. The oscillations are in a waveform. Figure 12-7 gives you a rough idea of how P waves and S
waves look.
Primary Waves
P waves are the fastest seismic waves and the first to
arrive at any monitoring station. P waves travel at
about 5km/s (14 times faster than sound waves
travel through air). For this reason, they are
called primary waves or P waves of an earthquake.
P waves are longitudinal compressional waves .
Like sound waves, they travel through rock or
buildings causing squeezing and expanding, parallel
to the direction of transmission. The speed that P
waves travel depends on the types of matter through which they move. Generally, in the case of P waves,
the denser the matter, the faster the wave travels.
Because P waves are the first seismic waves to reach any given location after an earthquake takes place,
their arrival can easily be identified on a seismogram. The tracing is flat during zero or background
vibration, then jumps into action as the first P waves arrive.
Secondary Waves
S waves arrive at a monitoring location second, since they only travel around 60% of the speed of P
waves. For this reason, they are called secondary or S waves .
S waves are transverse shear waves . They cause a shearing, side-to-side motion transverse
(perpendicular) to their direction of travel. Because of this, they can only travel through a substance like
rock that has shear strength. Liquids and gases have no shear strength and S waves cannot travel through
water, air, or even molten rock.
Since the seismogram is already jumping wildly with the action of the P waves, it can be tricky to figure
out when S waves arrive. However, seismologists study the amplitude and wavelength of a seismic
recording. S waves are often lower in frequency and longer in wavelength. So a sudden increase in
wavelength tips off the arrival of an S wave. A better indicator of an S wave’s arrival is a sudden jump in
the amplitude. In cases where the earthquake is large and the source is nearby, however, this doesn’t
always work because the P wave movement has not yet slowed to the point where the S wave arrival
overwhelms it.
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Date
Enrichment
Class
LESSON 2
Analyzing Earthquake Waves
Scientists use the indirect method
of analyzing earthquake waves to study
the interior of Earth. In most sciences,
researchers use indirect methods of
gathering data; these methods extend our
senses in exploring natural phenomena.
For example, scientists use telescopes and
microscopes to examine phenomena that is
larger and smaller, respectively, than can be
seen with the unaided eye; they use radar
waves to detect the positions of airplanes
and ships; and we hear and see voices and
images from afar with telephone, radio,
and television.
The velocity of the two kinds of seismic
waves depends upon the composition and
state of the material through which the
waves travel. Researchers have found that
some waves cannot travel through liquid
and that the velocity of a wave changes
when it reaches a boundary between two
types of material. This graph shows the
velocity of P-waves and S-waves at different
depths below Earth’s surface.
Earthquake Waves
During an earthquake, vibrations of the
moving crust make waves that travel
through Earth and that can be recorded on
instruments called seismographs. Wave
data provide information about the unseen
interior of Earth.
Experiments in labs have shown that
two types of seismic waves—P-waves and
S-waves—travel through Earth materials.
Applying Critical-Thinking Skills
Directions: Respond to each statement.
1. Compare the velocities of P-waves and S-waves as shown in the graph.
2. Relate the depth underground and the velocities of P-waves and S-waves.
3. Hypothesize which layers are in Earth between 2,000 km and 4,000 km.
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S+P Waves
The graph below shows travel time in minutes and distance traveled for primary and
secondary waves. Primary and secondary waves start at the same time but do not
travel at the same speed. Study the graph. Use the graph to help answer the
questions that
follow.
1. How long does it take for a primary wave to travel 2000 kilometers? _____________
2. How long does it take for a secondary wave to travel 2000 kilometers? __________
3. How far does a secondary wave travel in 10 minutes? _____________
4. How far does a primary wave travel in 10 minutes?_____________
5. What happens to the time difference between primary and secondary waves as the
distance traveled gets longer?
6. Suppose a primary and secondary wave both travel a distance of 4000 kilometers
before they are picked up by a seismograph. Which wave will arrive first? _________
7. How much time lag at 4000 km will there be between these two waves? __________
8. Suppose both a primary and secondary wave start together and travel for 5
minutes. Which wave will travel farther? ___________