SEISMOGRAPH
Source: Bobbie Callison / Tony Kramer
EQ-14-15
Objectives:
Understand the seismic waves that travel through the earth and create earthquakes. Understand
how a seismograph records energy waves as they travel through the earth. Locate the epicenter of
an earthquake from seismographic readings.
Science Themes:
Energy
Patterns of Change
Scale and structure
Systems and interactions
Process Skills:
Observing
Inferring
Applying
Communicating
Grade Levels:
5- 8
New Words:
Seismograph; pressure or primary (P) waves; shear or secondary (S) waves; Richter
and Mercalli scales; magnitude; amplitude; epicenter; focus
Materials:
seismograph model with paper and pen or pencil millimeter rulers and drawing
compasses
slinky and rope
math exercise on how to find an epicenter
Background Information
Wave Types
The focus of an earthquake is that point below the surface of the earth where rocks first fracture
because the stress exceeds their strength. The epicenter is the place on the surface of the earth
directly above the focus. Waves of energy radiate outward from the focus and can be recorded by
a seismograph. These waves are types of very low frequency sound waves propagating through
the earth.
There are three wave types: pressure, or primary, (P), shear, or secondary, (S), and surface (L)
waves. Secondary and surface wave speeds are only about 60 and 40 percent of the primary
wave speed. Therefore in an earthquake the P wave is first (a sharp jolt), followed by a larger,
undulating S wave, then the still larger circular motion of the L wave.
The P waves are pressure waves that compress and expand the rocks the same way sound waves
travel through air. Pressure (P) waves travel through all materials, solids, liquids or gas, so
spread throughout the earth and quickly falloff in intensity.
In shear (S) waves, adjacent rock layers slide left and right' (or up and down) in a scissors- like
(shear) motion perpendicular to the direction the wave is traveling. Secondary (S) waves travel
only through soiids because in a liquid the molecules slide past each other with little resistance. S
waves spread throughout the mantle and quickly falloff in intensity. Moreover, because the
earth's outer core is liquid, S waves do not propagate in the earth's core.
In L waves the rocks move in a circular motion ( up- forward- down- back or left-forward- right- back)
just like ocean waves. This is possible only in a thin layer right at the surface. Because L wave
concentrate their energy on the surface they retain high amplitude for a much longer distance and cause
most of the damage to buildings.
A slinky can be used to demonstrate P and S waves. Alternately a rope can be used to demonstrate S
waves. L waves are not demonstrated easily.
Earthquake Strength
Earthquake strength can be assessed by two different approaches: magnitude and intensity.
Magnitude is determined by the energy released into seismic waves traveling through the earth.
Charles Richter of Cal Tech invented the Richter scale, which uses seismograms to measure the
energy released by an earthquake. It indicates the strength of the earthquake on a logarithmic
scale ranging from 1 to 10. On this scale the amplitude of the motion changes by a factor of 10
when the Richter magnitude changes by 1, but the energy released changes by the square root of
1000, or 31.62. Intensity is a measure of the local effects of an earthquake obtained from reports
or observed effects on structures. The Mercalli scale is used to indicate intensity, and is based on
a Roman Numeral scale from I to XII: it indicates the relative level of damage.
Seismographs and Seismograms
The instrument that measures seismic motion is called a seismograph, and the recorded signal is
called a seismogram.
To measure anything it is necessary to have something unchanging to use as a reference. In the
case of an earthquake, early instruments used springs to isolate a large mass, which was
stationary while the earth moved. Something like a pen on paper then tracked the motion.
Modern instruments still use this principle, though in a more sophisticated way.
A geophone is a device often used in modern seismic stations to measure earthquakes. It consists
of a coil of copper wire isolated by springs in a magnetic field. In an earthquake the coil lags
behind the earth's motion, thereby moving in the magnetic field. This generates an electric
current in the coil, the same way an electric generator at a power plant produces electricity. This
current is recorded and becomes a seismogram.
The model seismograph in the kit has a platform isolated on four springs and a pen to record
relative motion on a paper strip. In some classrooms it is hard to generate good Por Swaves by
pounding or shaking the table. Students find it easy to think of the seismograph inside out, so
that the earth is the platform on springs and the supporting structure is the non- moving
reference. Shaking the platform appropriately can generate S and P waves as required.
A seismogram of a typical earthquake consists of the onset of a small primary (P) wave, an
abrupt increase in amplitude when the secondary (S) wave arrives, and a more or less distinct
further increase in amplitude when the surface (L) wave arrives. There are many variations in
amplitude and timing, depending on the strength of the earthquake, its distance from the
seismograph, and the path in the earth taken by the various wave components. Numerous
examples are in the kit.
Finding the Epicenter
The important data are the P and S wave arrival times from the seismogram, because they can be
used to' determine the distance from the seismograph to the epicenter of the earthquake. The
mathematics for how this relation is developed is available in the notebook. The result is as
follows:
Distance = (Constant) X (S-time minus P-time)
Here Distance is the distance from the epicenter to the seismograph, S-time and P-time are the
arrival times of the S and Pwaves respectively, and the constant depends on the velocity of the P
and S waves in the earth. For most rocks near the earth's surface, this constant is close to 8
kilometers per second. For example, if the Swave arrives at the seismograph 5 seconds after the
P wave then the epicenter is 5 seconds X 8 kilometers per second = 40 kilometers from the
seismograph. Put another way, the epicenter lies somewhere on a 40 kilometer radius circle that
is centered on the seismograph.
The epicenter of the earthquake can be found by establishing the distance to the epicenter as
above for three seismic stations, then drawing on a map the corresponding radius circles centered
on each station. Since the epicenter is somewhere on each circle, the circles should all meet at a
point, and that must be the epicenter.
Activity:
Orienting the Students
Ask what kind of energy is in an earthquake - sound- like energy with very low frequency
traveling through the earth. Indicate that sound in air has only one form, but sound in the earth
takes several forms. We will deal only with pressure (P) and shear (S) waves, but surface (L)
waves usually do the most damage.
Demonstrating Seismic Waves
P(pressure) waves can be illustrated by putting an ear to the table while pounding the table. S
waves (shear) can be illustrated by shaking the tale from side to side. It is usually clearer to the
students when you use the Slinky to show both kinds of waves. Have one student hold one end of
the Slinky while you move the other end to produce the desired type of wave. Show the P wave
by slapping the hand holding the Slinky along the axis of the coil with your empty hand. Show S
waves by shaking the Slinky up and down, then side to side.
The Meaning of Magnitude
(As time allows) Introduce the Richter scale by showing how earthquake motions can vary
widely: e.g., with the Slinky. Show the student some numbers with several different places, such
as 1, 10, 1000, 1,000,000, etc. Note that one way to relate these numbers is by the number of
places occupied. Now, starting with 1, assign it magnitude zero. The 10 has magnitude 1, 1000
has magnitude 3, and so on. The Richter scale is built like this. A magnitude 0 earthquake has
amplitude 1, a magnitude 2 has amplitude 100, and so on. But damage does not change the same
way as Richter magnitude, so the Mercalli scale was developed. It goes in steps from I to XII and
corresponds to the damage of an earthquake.
The Seismograph.
If time allows, introduce the focus and the epicenter. Introduce the word seismograph, and
indicate that it measures earthquakes. Ask how such a measurement can be made. Show that any
measurement needs a reference, something that serves as an unchanging standard. Ask how that
can be done if the earth is moving. Show an isolated mass, either the lead ball on a string or the
cut away geophone, to indicate that the mass does not follow the earth motion, but lags behind,
and that lagging creates a signal that we convert some way into a chart, a seismogram. Point out
that a seismogram is made by a seismograph.
Using the Geophone
(As time allows) Use the cut- away geophone to show the isolated mass and how it moves.
Demonstrate the response of the geophone to being moved parallel and perpendicular to its axis,
and note that it takes 3 geophones to cover motion in three directions. With the geophone
mounted in the hole in its protective box, have the students pound the table to generate P waves.
If possible, push the table to and fro for S waves. Note that this motion is not sensed by a
vertically mounted geophone. Alternately, let one or more student jiggle the geophone gently to
create oscilloscope traces.
Making Your Own Seismogram
One or more students can make a pseudo- seismogram in the following manner. One student
pulls the paper tape slowly and evenly while you or another student hold the pen so that it
records on the paper. Make a P wave by pounding on the table - "hen-peck line." Make an S
wave by pushing the table to and fro. Be sure to compare your seismogram(s) to one from a real
earthquake. Alternately, explain to the students that it is not easily possible to shake the earth, or
the table, to create both S and P waves big enough to get much response from the model
seismograph. Instead, think of the isolated platform as the earth, and the chart as the unmoving
mass. Then have a student pull the paper while you manipulate the platform to generate a typical
seismogram.
Understanding a Seismogram
Hold up the seismogram you just made, and ask the students why the first amplitude is small,
then it gets bigger, then bigger still. Show the sample of an actual seismogram, with P, S and L
wave arrivals indicated. Point out that pressure (P) waves travel faster than shear (S) waves, and
that shear waves are faster than surface (L) waves. Because the P waves are faster, they get to the
seismograph first. Pressure motion does not cause large amplitude rock motion, so the
disturbance in the seismogram is small. When the first shear (S) wave arrives, it abruptly
increases the amplitude of the seismogram, and when the surface waves begin the amplitude gets
larger still. Point out that the arrival of the first P wave arrives after the earthquake actually
happened, because it takes time for the energy of the earthquake to propagate to the seismograph.
The earthquake actually occurred at some earlier time.
Locating the Epicenter of an Earthquake using S and P Waves
There are two ways to approach this activity, the long form or the short form. The long form is
explained in detail in the following paragraphs and will take the students about five to ten
minutes to complete, requiring use of a ruler, compass and graphing. Emphasize the importance
of taking accurate measurements. To use the short form, you as the teacher do steps 2 and 3 and
preset some compasses ahead of (see step 3). In this procedure students learn how the epicenter
is determined, but math skills are not emphasized.
Preparation
Explain that if we know how fast the P and S waves travel through the earth, we can work
backward to find out when the earthquake occurred. And if we know that, we can discover how
far away the earthquake was. Explain that there is a relation between the distance from the
epicenter to the seismic station and the time between the arrival of the S and Pwaves. Hence,
close earthquakes have a short time difference, more distant earthquakes have a longer time.
Geologists have found that multiplying the time difference by 8 kilometers per second will give
good answers.
As an example, if the difference in arrival time is 10 seconds, then multiplying by 8 means the
epicenter is 80 kilometers from the seismograph. Put another way, the epicenter is somewhere on
a circle that is centered on the seismic station and has an 80 kilometer radius.
Show the Example, Figure 3, to illustrate finding an epicenter. Now suppose we have three
seismic stations that each measure the distance to the same earthquake epicenter. Then the
epicenter is on a circle of radius D1 from station 1, a second circle of radius D2 from station 2,
and a third circle of radius D3 from station 3. To locate the epicenter, plot these circles on a map.
Since the epicenter is on each circle, they should all meet at a point and that point is the
epicenter.
Exercise Steps
Figure 1 is three seismograms of an earthquake that occurred in the Bay Area in March 1958.
The seismic stations were Palo Alto, San Francisco and Lick Observatory. The beginnings of the
S and P waves are marked on each seismogram. The seismograms are scaled so that 1 millimeter
represents 1 second.
1.Using a ruler, measure the distance in millimeters between the Pand Swaves for each
seismogram in Figure 1. Record the data on the appropriate line in Figure 2, Map of the Bay
Area.
2. For each seismic station, multiply the number of millimeters by 1 to get seconds, then by 8 to
get kilometers. Record this data on the table on Figure 2.
Typical Data:
(a) Lick Observatory 6.5 mm or 6.5 seconds x 8 equals 52 kilometers
(b) Palo Alto
5.0 mm or 5.0 seconds x 8 equals 40 kilometers
(c) San Francisco
5.5 mm or 5.5 seconds x 8 equals 44 kilometers
Note: You will need to explain how to use the millimeter scale on the rulers.
3. Using Figure 2, determine the epicenter of this earthquake. Starting with the Lick Observatory
distance, place the sharp end of the compass on the map's kilometer scale at zero and expand the
compass so that the pencil tip is at the number of kilometers from the epicenter determined for
Lick Observatory in step 2 (e.g., 52 kilometers). Without changing the compass setting, move the
sharp point of the compass to the dot locating the Lick Observatory station and draw a circle
around that station. Repeat this procedure for the San Francisco and Palo Alto stations, using
their distances from the epicenter.
4. By inspection, find the place where all three circles intersect. This is the epicenter of the
earthquake. If the lines do not perfectly intersect it means measurements were not 100%
accurate. Use the closest area of intersection as the epicenter. Why is it important to have
accurate measurements? Why might the original data be inaccurate?