Seismic Sleuths

Earthquakes

A Teachers Package for Grades 7-12

Unit Three
How Do People Learn about Earthquakes?

In the first lesson, students will learn about the 
movement of earthquake waves in our Earth and on its 
surface. It was the study of this movement, by 
Mohorovicic, Lehmann, and others, that developed 
our current understanding of the interior of the Earth. 
Activities illustrate the movement of P and S waves. 
Background material on Love and Rayleigh waves is 
also provided.

The second lesson in Unit 3 is built around 
biographical sketches of three individuals who made 
major contributions to our knowledge of the Earths 
inner structure: the German meteorologist Alfred 
Wegener, who amassed evidence for the controversial 
theory of continental drift; the Croatian seismologist 
Andrija Mohorovicic, who demonstrated the existence 
of the discontinuity between crust and mantle that 
bears his name; and the Danish seismologist Inge 
Lehmann, who demonstrated the existence of the inner 
core. A timetable of other relevant discoveries is 
included as teacher background material.

Lesson 3 begins with a pair of activities that contrast 
the two systems most commonly used to characterize 
earthquakesthe Modified Mercalli scale, which 
assigns a number on the basis of observed effects, and 
the Richter scale, which assigns a number on the basis 
of instrument readings. Students draw isoseismals on 
a map in the first activity. In the second they compare 
Richter magnitudes from several seismograph stations 
for the same earthquake. In the third activity, they 
practice reading seismograms.

Lesson 4 has three parts, moving from the worldwide 
distribution of earthquakes to one seismically active 
area of Japan and then home to the United States. At 
all three stages, students plot actual earthquake data in 
order to gain an experiential understanding of where 
earthquakes occur. In the second activity they 
construct a plot in three dimensions. Most students 
will be surprised to learn that earthquakes are 
everyday occurrences on a global scale, and that they 
may originate from foci as deep as 700 km inside the 
Earth1/9 of the way to its center.

Like the previous unit, this one offers a variety of 
experiences in scientific observation, computation, 
and the application of social science principles. It also 
presents factual information that will stand students in 
good stead as consumers of media information and as 
future property owners.

Social studies teachers may be especially interested in 
the second lesson of this unit. Mathematics teachers 
will find applications of algebra and geometry in many 
of the others. The lessons in this unit can stand on 
their own, but taken together, they provide an 
excellent preparation for learning to mitigate the effect 
of earthquakes on the human environment, in Unit 4.

3.1 The Waves of Quakes
 
RATIONALE
To understand earthquakes and their effects on Earths surface, and to 
understand how scientists have learned about Earths interior, students 
must begin to understand the properties of waves.

FOCUS QUESTIONS
How does matter transmit energy in the form of waves?

OBJECTIVES
Students will:
1. Compare and contrast the two main types of earthquake waves and 
draw sketches to illustrate them.
2. Describe the manner in which each of the two types transfers 
energy and moves Earth particles.

TEACHING CLUES AND CUES
Metal Slinkies will be more effective than plastic ones for this 
activity. If you use plastic Slinkies, tape two together, with 
about 7.5 cm of overlap, for each pair of students.

It will be helpful to refer to Master 2.2b, Earth 
Cross Section, as you talk about waves moving 
through the Earth. You may want to project the transparency.

PART ONE
PRIMARY WAVES (P WAVES)

MATERIALS
 Student copies of Master 3.1a, Earthquake Wave Background
 Slinky toys (One for every two students is ideal.)
 White plastic tape
 String
 Unlined paper
 Pencils

PROCEDURE
Teacher Preparation
Put the Slinky toys and the tape in a central location where they will 
be available to students. Students will mark the coils so they can see 
the movement of energy along their length.

VOCABULARY
Body waves: waves that move through the body (rather than the 
surface) of the Earth. P waves and S waves are body waves.

Compression: squeezing, being made to occupy less space. P waves 
are also called compressional waves because they consist of alternating 
compressions and expansions.

Longitudinal waves: waves that move particles back and forth in the 
some line as the direction of the wave (P waves).

P waves: primary waves, arrive at a station first because they travel faster 
than S waves, or secondary waves. These waves carry energy through 
the Earth in the form of longitudinal waves.

S waves: secondary waves; waves that carry energy through the Earth in 
the form of very complex patterns of transverse waves. These waves 
move more slowly than P waves (in which the ground moves parallel to 
the direction of the wave). In an earthquake S waves are usually 
bigger than Ps.

Transverse waves: waves that vibrate particles in a direction 
perpendicular to the waves direction of motion (S waves).

A. Introduction
Explain: It is the energy from earthquakes that puts human beings and 
human structures in danger. This energy is transmitted through the 
Earth in the form of waves. Distribute copies of Master 3.la, 
Earthquake Wave Background. Go over the Body Waves section of 
the background material with the class.

B. Lesson Development
1. Ask students:
 Where is the energy of an earthquake released? (along the fault 
plane where tectonic stress is released)
 In what form is the energy from an earthquake released? (in the 
form of waves)

2. Divide students into pairs or groups of even numbers, depending on 
the number of Slinkies available. Direct students to pick up their 
Slinkies and mark two spots on each, near the center of the coil, with 
white plastic tape. The two pieces of tape should be at the tops of 
adjacent loops.

3. Two students will hold each Slinky, one on either end. Instruct 
them to stretch it out to a length of approximately three meters on the 
floor or a long counter. Have students take turns compressing between 
10 and 20 coils and then releasing them rapidly while they continue to 
hold the Slinky.

TEACHING CLUES AND CUES
Activity 3 in lesson 2.4 deals with tsunami, destructive water waves 
often caused by earthquakes. If you have not taught this 
lesson, refer to it now.

4. After several repetitions, ask students to describe what they saw 
happen with the coil and the tape. Then ask:

 What kind of earthquake waves does this movement resemble? 
(longitudinal body waves, or P waves)

 How is the movement of the spring like the movement of the 
waves? (It moves by contracting and expandingby compression 
and expansion.)

 In what direction does the energy move? (It moves out from the 
point at which the energy is releasedthe focus.)

 What happens within the body of the Earth when energy moves 
through it in this way? (Energy is transmitted from any material 
near the focus to particles away from the source in the form of 
waves.)

TEACHING CLUES AND CUES
Point out that in a P wave the movement of each coil of the Slinky is 
parallel to the wave motion (the length of the Slinky). In 
an S wave, the movement is perpendicular to the length of the 
Slinky. This difference may be easier for students to see than compression 
and dilation.

C. Conclusion
Challenge pairs of students to decide what a P wave would look like 
and draw one on the top half of a sheet of paper. Tell them to use 
pencil, so if they change their minds later they can revise the drawing. 
(There are several acceptable ways to do this.)

ADAPTATIONS AND EXTENSIONS
1. With students who show special interest, assign all of the reading 
on Master 3.1a and add Rayleigh waves and Love waves to your 
discussion.

2. Research and discuss (a) the intermolecular forces that provide the 
means of transmitting energy in longitudinal waves and (b) the speed 
of P waves traveling in the Earths crust. Ask:

 How does the density of materials affect the speed of P waves? 
(The more dense the material, the faster the waves move.)

 As P waves travel deeper into the Earth, does their speed change? 
(P waves, like S waves, move faster in the mantle than in the crust, 
because the mantle is more dense. The rocks that compose the 
Earth at great depths, however, are plastic, so they slow down the 
waves.)

 Why can P waves travel through liquids, while S waves cannot? (P 
waves travel by compression, and S waves travel by shearing at 
right angles to the direction of motion. Water cannot spring back 
after it is sheared, so the S waves die out in water.)

 How are P waves similar to sound waves? (A sound wave in the 
ground is a P wave; a P wave in the air is a sound wave. Both are 
compressional waves.)

PART TWO
SECONDARY, OR SHEAR WAVES (S WAVES)

MATERIALS
 7.6 m (25 ft) of coiled telephone cord, available at electronics or 
phone stores and some discount stores
 Two different colors of plastic tape, about 1 cm (.5 in.) wide
 Masking tape
 Two empty soda cans

TEACHING CLUES AND CUES
Students may be confused by the array of synonyms for P waves 
and S waves. Help them to understand that each term provides additional 
information about 
one type of wave. P waves, or primary waves, are also called compressional waves 
and longitudinal waves. S waves, or secondary waves, are also called shear waves 
and 
transverse waves. Both P and S 
waves are body waves.

PROCEDURE

A. Introduction
Ask students:

 What type of body wave did the Slinky exercise illustrate? (P 
waves, also known as compressional waves or longitudinal waves)

 What is the other type of body wave? (S waves, also known as 
waves or transverse waves)

B. Lesson Development
1. Give these instructions for demonstrating single shear waves (S 
waves).
a. Place a band of bright colored tape halfway along the coiled 
telephone cord. Place another band close to one end. Lay the cord 
straight along a smooth surface. Have two students hold the ends of 
the cord firmly so it will not move.
b. With plain masking tape, mark a 50-cm line perpendicular to one 
end of the cord. Mark another line of the same length at the cords 
halfway point, directly under the center band of tape. This will 
provide you with a reference line.
c. Pick up the end of the rope at the center of the first perpendicular 
tape line, then move your hand back and forth quickly along the 
masking tape. As you expected, a wave travels down the rope (or 
transmitting medium). Observe the motion of the colored tape while 
waves are moving by. Students may take turns holding the ends of the 
cord.
d. Place two empty soda cans upright on either side of the cord on the 
center tape line. Both cans should be the same distance from the 
cordabout 30 cm (1 ft). When a wave of sufficient amplitude is sent 
along the cord between these cans, what will happen? (The same 
piece of cord, as indicated by the colored tape marker, will knock over 
both cans.)

2. Point out that although students may only see motion in one 
direction, the transverse wave they have just observed vibrates in a 
direction perpendicular to its direction of motion. Remind students 
that S waves within the Earth are not just in one plane, and dont all 
have the same frequencies and amplitudes. In an earthquake, a jumble 
of S waves passes through a particular volume of Earth at the same 
time. Since S waves reach the surface from below, a special pendulum 
seismograph would show ground motion for S waves as a very 
complex, seemingly random pattern of horizontal (back and forth) 
motions.

C. Conclusion
Ask students:

 Which type of wave does this activity illustrate? (S waves)

 What are some differences between P waves and S waves? (P 
waves are longitudinal and S waves are transverse. P waves can be 
transmitted through solids, liquids, and gases, while S waves can 
only be transmitted through solids.)

Now ask students, working in the same pairs as for part one, to decide 
what a diagram of an S wave would look like and draw one on the 
other half of the page they used in part one. They may want to revise 
their drawings of a P wave at this time. When they have finished, ask 
each pair of students to exchange their drawings with another pair and 
discuss the similarities and differences.

ADAPTATIONS AND EXTENSIONS
1. If students have also discussed Rayleigh waves and Love waves, 
ask them to draw these on the backs of their pages at this time and 
follow the procedure for exchanging diagrams as above.

2. Invite students to research and discuss the intermolecular forces 
that provide the means of transmitting energy in transverse waves, and 
why S waves can not be transmitted through liquids, including Earths 
outer core.

The major types of seismic waves are classified as 
body waves and surface waves. The two have 
different shapes and properties. All waves in matter 
depend upon the interaction of forces among the 
particles of some material. These forces transmit 
movement of one particle to movement in adjacent 
particles.

Body Waves
Body waves, so called because they travel through 
the body of the Earth, consist of two types: primary 
(P) and secondary (S). S waves are also called shear 
waves and transverse waves.

Primary (P) waves consist of alternating 
compressions and expansions (dilations), so they are 
also referred to as compressional waves. P waves 
are longitudinal; they cause particle motion that is 
back and forth, in the same linear direction as 
energy transfer. These waves carry energy through 
the Earth, usually at the rate of 3.57.2 km/sec in the 
crust and 7.88.5 km/sec in the mantle.

Secondary (S) waves are transverse; the particle 
motion they cause is perpendicular to the direction 
of energy transfer. Their usual speed is 2.04.2 
km/sec in the crust and 4.54.9 km/sec in the 
mantle.

Longitudinal (P) waves can be transmitted through 
solids, liquids, and gases, while transverse (S) waves 
(with the exception of electromagnetic waves) can 
only be transmitted by solids. Waves can be 
reflected and refracted (bent) when they move from 
material of one density to that of another density. 
Wave energy can also be changed to other forms. As 
they move through the Earth, the waves decrease in 
strength, or attenuate. Waves attenuate more slowly 
in solid rocks than in the basins full of sediment so 
common in the West. Because of this, an earthquake 
in the crust of the eastern United States is felt over a 
wider area than a quake the same size in the rocks of 
the western states.

Surface Waves
Seismograph stations detect surface waves from 
many, but not all, quakes. Whether a station detects 
them or not depends on the strength of the quakes 
energy release, the depth of the quake, and the 
stations distance from the focus.

Unlike body waves, which travel through the Earth, 
surface waves travel around it. The two main types of 
surface waves are called Rayleigh waves and Love 
waves. These surface waves travel more slowly than S 
and P waves, and attenuate more quickly.

Within Rayleigh waves, Earth particles move in 
elliptical paths whose plane is vertical and set in the 
direction of energy transfer. When an Earth particle is 
at the top of the ellipse, it moves toward the energy 
source (seemingly backwards), then around, 
downward, and forward, away from the source. It then 
moves around and upwards back to its original 
position. This produces a ripple effect at the Earths 
surface that is similar to ripples on a pond. The orbits, 
or paths, of these particles become smaller and finally 
die out at a certain depth within the Earth.

Love waves move particles in a back and forth 
horizontal motion as the energy moves forward. If you 
could see a Love wave inside the Earth, you would 
notice a zigzag horizontal motion.

Putting Them Together
Since these four types of waves shake a location on 
Earths surface in various ways and directions, a 
seismic station needs at least three seismographs to 
glean a reasonably good image of ground shaking at 
that location. One seismograph is built to measure 
vertical motions, and two others, aligned perpendicular 
to each other, measure horizontal motions. Although 
P, S, and Rayleigh waves may be recorded on all three 
seismographs, P waves are best recorded on the 
vertical component and S waves on the horizontal. 
Love waves are only recorded on the horizontal 
components, but Rayleigh waves are recorded on all 
three.

3.2 Pioneering Ideas

VOCABULARY
Continental drift: the theory, first advanced by Alfred Wegener, that 
Earths continents were originally one land mass, split off and 
gradually form the continents we know.

Epitaph: an inscription on a tombstone, often intended to sum up 
the achievements of a persons life.

Meteorology: the study of Earths atmosphere.

Pioneer: a person who moves into new and uncharted territory.

Plate tectonics: the theory that Earths crust and upper mantle (the 
lithosphere) are broken into a number a more or less rigid, but 
constantly moving, segments, or plates.

Seismology: the scientific study of earthquakes.

Topography (adj. topographic): the shape of the land; the contours and 
the arrangement of surface features that characterize a region.

RATIONALE
The people who have shaped our idea of the Earth are pioneers, just as 
truly as those who struck out in new directions across its surface. This 
idea may be new to students.

FOCUS QUESTIONS
Could a person be a pioneer without leaving home?

OBJECTIVES
Students will:
1. Read a biographical sketch about a pioneer of earth science.
2. Identify the characteristics of a pioneer.
3. Be able to tell why Wegener, Mohorovicic, and Lehman were 
pioneers.

MATERIALS
 Student copies of Master 3.2a, Three Pioneers (3 pages)
 Master 3.2b, Chronology: The Beginnings of the Seismological Age
 Overhead projector and transparencies (optional)
 Reference books for research (See Unit Resources.)
 Paper and pens

PROCEDURE
Teacher Preparation
Assemble a classroom reference shelf of biographical encyclopedias, 
studies on continental drift and plate tectonics, and books on earth- 
quakes. (See Unit Resources.)

A. Introduction
Tell students that you would like them to explore the notion of 
pioneer. Write the word on the chalkboard or overhead projector and 
ask the class to brainstorm about the meaning of the word and its 
implications. Be prepared to accept any reasonable suggestion. Such 
ideas as risk-taker, adventurer, initiator, innovator, frontier person, 
and explorer are likely to come forward. You may want to use these 
terms to build a concept map.

During the course of the brainstorming, remind the class that in a 
historical sense, we tend to think of pioneers as men and women who 
have moved beyond the edge of settlement. Daniel Boone was such a 
person, and so were Lewis and Clark and Matthew Henson, the polar 
explorer. There are other types of pioneers, howeverthose who are 
willing to advance new ideas and suggest new theories to explain 
physical or cultural phenomena. Albert Einstein, with his theory of 
relativity, is a good example of this kind of pioneer. So is Marie 
Curie, who worked to develop radium therapy and conducted some of 
the earliest experiments with radiation.

B. Lesson Development
1. When the brainstorming session has ended, ask each student to 
write a one-sentence definition of the term pioneer, using the list 
generated by the class as a reference. Then have students share their 
responses to learn if a consensus has developed about the meaning of 
the term. From the collection of definitions presented by class 
members, write what seems to be a representative definition on either 
the board or the overhead. Here are some likely definitions:

A pioneer is a person who is on the cutting edge, someone with the 
courage and the vision to try something new.

A pioneer is the first person to suggest a new idea or to try something 
that has never been tried before.

A pioneer is a person who prepares the way for others because of 
his/her courage and foresight.

2. Divide the class into groups of three students each. Provide each 
group with one copy of Master 3.2a, Three Pioneers. Each of the 
students may read one essay. When students have finished reading, 
ask them to give the essay a title and to write a two- or three-sentence 
summary of the essay in the space provided.

3. With this as context, remind students that when each of these 
discoveries was first published, it created discussion and even 
controversy. To understand the kind of excitement each advance in 
science causes, have half of the students who read about each scientist 
research the evidence that person offered to support the new theory 
and the other half research the views of his or her critics or the 
reasons why it may not have been accepted immediately. Later in the 
same period or the next day, invite the groups to present and discuss 
opposing points of view culled from their reading.

TEACHING CLUES AND CUES
Use Master 3.2b, Chronology: The Beginnings of the 
Seismological Age, for your own reference and to help 
students place the work of these three scientists in the context of 
other disciplines.

C. Conclusion
Return to the consensus definition of pioneer that the class developed 
and ask students to apply that definition to these three individuals. To 
do this, they should write epitaphs for the tombstones of the three 
scientists. Remind students that the purpose of an epitaph is to 
summarize a persons life in a brief and pithy fashion. Post the 
epitaphs on the bulletin board to present the variety of impressions 
class members have about Wegener, Mohorovicic, and Lehmann.

ADAPTATIONS AND EXTENSIONS
1. Make a time line from the information on Master 3.2b, Chronology: 
The Beginnings of the Seismological Age. You could do this either 
before class or during class with student participation.

2. As a class, brainstorm suitable epitaphs for some of the other 
pioneers mentioned in this lesson and those students know about from 
other areas of study.

3. Encourage students to read biographies of intellectual explorers and 
learn about some of the challenges pioneers have faced. ?

3.2a Three Pioneers

Name
Date

1. In 1912, when Alfred Wegener proposed in print that Earths 
continents floated on denser and more stable material below, he 
was openly ridiculed and even scorned by his colleagues. Not until 
several decades later did his ideas receive any acceptance. Today 
he stands as the forefather of modern plate tectonics because of his 
theory of continental drift. His widely accepted theory of land 
displacement holds that Earths continents have been in motion 
throughout geological time.

Wegener believed that there was once a single supercontinent, 
which he called Pangea (or Pangaea). He said that Pangea broke 
apart millions of years ago to form two large continents. He called 
the one in the northern hemisphere Laurasia and the one in the 
southern hemisphere Gondwanaland. After a very long span of 
centuries, Wegener said, Laurasia split to form North America, 
most of Asia, Greenland, and a large section of Europe. 
Gondwanaland became Africa, South America, Australia, India, 
and Antarctica. Wegener believed that the land masses drifted for 
millions of years before assuming their present shapes and arriving 
at their present locations. He was led to this notion by the 
congruity he observed in the shorelines of the lands bordering the 
Atlantic Ocean and several other kinds of evidence. Further, he 
said, the process of continental drift is still going onthe 
continents are still on the move.

Alfred Wegener, who was educated to be a meteorologist and an 
Arctic climatologist, insisted that his theory was correct because of 
the evidence he saw. To support his ideas about continental drift, 
Wegener pointed to the similarities in the fossils of the southern 
continents. Fossils of the same sort from ferns and freshwater 
reptiles had been found in all of the southern continents. He saw 
this as evidence that all the lands south of the equator has once 
been part of a single land mass. He argued that such land-based 
life forms could never have crossed the thousands of miles of open 
ocean that now separate these land masses. His critics scoffed 
because the physical model that Wegener proposed to explain the 
movement of continents did not fit what was then known about the 
physics of the Earth.

For the next 30 years or so, scientists paid little attention to 
Wegeners theory. In the 1960s, however, geologists discovered 
that the ocean floors had been spreading, thus influencing the 
shapes and sizes of the continents. This new theory, called plate 
tectonics, provided a mechanism that made sense in physical terms 
to account for Wegeners idea of continental drift.

Although the continents themselves do not drift, as Wegener 
proposed, he was correct in his thesis that Earths surface is not 
fixed. He was a man well ahead of his time whose insight went 
beyond safe and conventional thinking. So important is Wegener 
to our current understanding of plate tectonics that in the 1970s a 
crater on the dark side of the moon was named for him, to honor 
his courage and vision.

Tragically, Alfred Wegener never lived to see his ideas accepted 
by the scientific community. He perished while attempting to cross 
Greenland from a camp on the ice cap in the winter of 1930. His 
purpose was to learn more about atmospheric conditions in the 
Arctic in order to better predict world weather patterns.

What title would you give this essay?

Write a two- or three-sentence summary of the essay, then add a one-sentence 
comment.

Name
Date

2. In the earths interior, where seismic waves travel invisibly and inaudibly, 
they can be followed only by mathematical equations.
Dragutin Skoko, Mohorovicics biographer

The boundary separating Earths crust from its upper mantle is 
called the Mohorovicic discontinuity, or the Moho, for short, in 
honor of the Croatian seismologist Andrija Mohorovicic. In 1909, 
he used data on the travel tame of earthquake waves to 
demonstrate that their velocity changes at about 50 km beneath the 
surface. Others later refined the study of crust and upper mantle 
and applied new, methods, but Mohorovicic paved their way.
Mohorovicics father, also named Andrija (or Andrew), was a 
maker of anchors. The young Mohorovicic loved the sea, and 
married a sea captains daughter. He taught for nine years at the 
Royal Nautical School in Baka. After becoming director of the 
Meteorological Observatory in Zagreb, in 1891, he studied and 
wrote primarily about clouds, rainstorms, and high winds. After a 
severe earthquake in 1901, however, Mohorovicic and his 
colleagues petitioned their government to establish a seismic 
station in Zagreb. In 1910, Mohorovicic published his account of 
the earthquake of November 9, 1880. In it, he plotted a now- 
standard transit time graph-arrival time versus epicenter distance to 
recording station-using the data for 29 stations that ranged to a 
distance of 2,400 km from the epicenter.

After plotting data for a large number of earthquakes over a wide 
area, he had begun to notice that the P wave arrivals required two 
curves on his graph. Because it was not possible to have P waves 
traveling in the same medium at different velocities, and the earlier 
P arrivals were only seen at some distance from the epicenter, he 
reasoned that the two (different arrival times represented two 
different phases of P waves traveling different paths. After 
working out the refraction equations and tests to determine optimal 
values for the depth of the focus, the ray paths of the two P waves, 
the corresponding two S phases, and their reflection paths, he 
concluded that at approximately 50 km there must be an abrupt 
change in the material that composes the interior of the Earth, 
because he observed an abrupt change in the velocity of the 
earthquake waves. Although this conclusion was not accepted 
immediately, Beno Gutenberg was able to confirm it with his own 
research as early as 1915.

Even after earthquakes became one of his primary interests, as 
chief of the observatory, Mohorovicic was responsible for 
recording all the meteorological data for Croatia and Slovenia
precipitation, tornadoes, whirlwinds, thunderstorms, and more
with only an occasional assistant. He was responsible for all the 
mathematics involved in keeping records and for answering 
hundreds of letters and requests for assistance, as well as teaching 
classes at the University. He was patient and precise in his 
collection and analysis of data, but he loved good scientific 
instruments, and was frequently frustrated at the inadequacy of the 
instruments available and the difficulty of obtaining new ones. An 
accurate clock was particularly important to his research, because 
in studying earthquakes, an error of one second in the time of 
arrival means an error of 5.6 km in estimating the length of its 
travel making it impossible to accurately locate the focus of an 
earthquake. By 1913 he had finally obtained a crystal clock with a 
radio receiver that allowed him to synchronize with the Paris 
Observatory, but in 1914, during World War I, the army 
commandeered it for military use. When the dock was returned to 
him after the war, he also received a new radio receiver that took 
two railroad cars to transport.

Mohorovicic published a paper in 1909 on the effect of 
earthquakes on buildings that described periods of oscillation (see 
lesson 4.3). In this he was at least 50 years ahead of the times both 
in his own country and elsewhere. Croatias first national 
Provisional Engineering Standards for Construction in Seismic 
Areas were published in 1964.

During his lifetime, Mohorovicic maintained contacts with 
seismologists all over the world. He retired in 1922, but remained 
active until shortly before his death in 1936. His only grandchild, 
Andre, remembers that he was always good natureda kind and 
peaceful man. Mohorovicic, like Alfred Wegener, received the 
honor of having his name given to a crater on the dark side of the 
moon.

What title would you give this essay?

Write a two- or three-sentence summary of the essay, then add a one-sentence 
comment.

Name
Date

3. By 1936, scientists had learned from the study of earthquake 
waves that the Earth has three layers, crust, mantle, and core. 
Denmarks Inge Lehmann was the first to demonstrate the 
existence of a change in composition midway through the core, 
dividing it into an inner core and an outer core. This division is 
now known as the Lehmann discontinuity.

As a girl, at the turn of the century, Lehmann attended the first 
coeducational school in Denmark, which was founded and run by 
Hanna Adler. (Adlers nephew, Neils Bohr, was the first to 
describe the physical makeup of the atom.) At that school, 
Lehmann wrote many years later, No difference between the 
intellect of boys and girls was recognizeda fact that brought me 
disappointments later in life when I found that this was not the 
general attitude.

Lehmann studied at Oxford, earned a masters degree in 
mathematics from the University of Copenhagen, and went to work 
as an actuary, calculating life expectancies and statistical risks for 
insurance companies. Beginning in 1925, however, she also served 
as a staff member of the Danish Geodetic Institute, helping to 
establish seismological stations in Greenland and in 
Copenhagena part of the world not noted for its seismicity. 
Seismology soon became her life work, and for 25 years, until just 
before her retirement, she was the only Danish seismologist.

As early as 1910 scientists had noticed a shadow zone in the 
Earths interior, but seismographs had not been refined enough to 
explain this observation. In the course of the 1930s more and more 
sensitive seismographs were being developed. At the Copenhagen 
Seismological Observatory, Lehmann studied waves reflected 
through the core from earthquakes in Japan. In 1936, after 10 
years of studying seismograms, she interpreted the newly revealed 
data to confirm the existence of a relatively small inner core in 
the center of the Earth. The paper in which she reported her 
findings has one of the shortest titles in the history of seismology, 
if not of all science: It was called P.

Lehmann was among the founders of the Danish Geophysical 
Society in 1936, and served as its president from 1941 until 1944. 
She helped to formulate the constitution of the European 
Seismological Federation and was elected its first president in 
1950. She found time to attend most of the meetings of the 
International Union of Geodesy and Geophysics, and served on 
the executive committee of the International Seismological 
Association from 1936 to 1944, from 1951 to 1954, and from 
1957 to 1960. International cooperation in the sciences was one 
of her passions. She was active in national and international 
scientific organizations, and traveled in France, the Netherlands, 
Belgium, and Germany, where she worked with some of the 
leading seismologists of the day. In Canada, she worked at 
Ottawas Dominion Observatory, and in the United States she 
conducted research at the Seismological Laboratory, California 
Institute of Technology; the University of California at Berkeley; 
and the Lamont-Doherty Geological Observatory, Columbia 
University, New York.

She loved hiking, skiing, and mountain climbing. Her favorite 
place indoors, aside from her own cottage in Denmark, was an art 
gallery. She loved to visit galleries and look at paintings 
whenever she traveled, and she traveled widely, especially after 
her retirement in 1953. She also loved music and gardening. Inge 
Lehmann died in February, 1993, at the age of 105, leaving a 
worldwide network of friends.

What title would you give this essay?

Write a two- or three-sentence summary of the essay, then add a one-sentence 
comment.

3.2b Chronology: The Beginnings of the Seismological Age

1883: The English seismologist John Milne hypothesized that with the proper 
equipment, it should be possible to detect seismic waves from a large earthquake 
occurring anywhere on Earth.

1889: Milnes 1883 hypothesis was proven correct when E. von Rebeur 
Paschwitz used delicate pendulum seismographs to record the April 18, 1889, 
Tokyo earthquake in Potsdam and Wilhemshaven, Germany.

1897: Richard Dixon Oldham noticed that seismograms from earthquakes 
consistently showed three different disturbances, the first and second 
preliminary tremors (now known as P waves and S waves, respectively) and 
the large waves that followed the preliminary tremors, and that the difference 
in arrival time between the large waves and the preliminary tremors 
increased in a predictable fashion with increasing distance from the earthquake.

1900: Oldham established that the preliminary tremors (P and S waves) have 
travel paths that take them through the body of Earth (we now call them body 
waves), and that the large waves (now called surface waves) travel along 
Earths surface.

1906: Oldham used evidence from earthquake waves to demonstrate the 
existence of a large central core at a depth of about 3,821 km beneath the 
surface.

1909: Andrija Mehorovicic, a Croatian seismologist, used seismic waves to 
discover a discontinuity at a depth of about 50 km beneath the surface. This 
marks the boundary between what we now call the Earths crust and the 
underlying mantle. In his honor, we call the boundary separating the crust from 
the mantle the Mohorovicic discontinuity, or the Moho for short.

1914: Beno Gutenberg used an extensive data set of earthquake wave travel 
times to compute the average distance to the top of the core at about 2,900 km.

1926: Harold Jeffreys measurements of tides in the solid Earth suggested that 
the Earth was less rigid than had been previously assumed. This led to the 
assumption that the core is fluid.

1936: Inge Lehmann, Danish seismologist, demonstrated the presence of an 
inner core.

3.3 Sizing Up Earthquakes

VOCABULARY
Epicenter: the point on Earths surface directly above the focus of an 
earthquake.

Focus (pl. foci): the point within the Earth that is the origin of an 
earthquake where stored energy is first released as wave energy.

Intensity: a subjective measure of the amount of ground shaking an 
earthquake produces at a particular site, based on geologic effects, 
impact on human structures, and other human observations. Mercalli 
intensity is expressed in Roman numerals.

Isoseismal line: a line on a map that encloses areas of equal earthquake 
intensity.

Magnitude: a number that characterizes the size of an 
earthquake by measuring the motions on a seismograph and 
correcting for the distance to the epicenter of the earthquake. 
Magnitude is expressed in Arabic numbers.
 
ACTIVITY ONE
THE MERCALLI SCALE: CALLING STATION KWAT

RATIONALE
Students need to know how seismologists establish earthquake 
intensity in order to understand how much damage earthquakes can 
cause and how building codes are developed.

FOCUS QUESTIONS
How do seismologists determine the intensity of an earthquake?

OBJECTIVES
Students will:
1. Interpret the Modified Mercalli scale and assign values on the basis 
of descriptions by citizen observers during and after a quake.
2. Use the assigned values to construct an isoseismal map.

MATERIALS
 Overhead projector
 Student copies of Master 3.3a, Modified Mercalli Intensity Scale
 Transparency made from Master 3.3a, Modified Mercalli Intensity Scale
 Back of Master 3.3a, Teacher Background Reading: Richter vs. Mercalli
 Student copies of Master 3.3b, Wattsville Map
 Transparency made from Back of Master 3.3b, Wattsville Map Key
 One copy of Master 3.3c, KWAT Television Script, cut into strips
 Student copies of Master 3.3c, KWAT Television Script
 Colored pencils and lead pencils

VOCABULARY
Modified Mercalli scale of 1931: a qualitative scale of earthquake 
effects that assigns an intensity number to the ground 
shaking for any specific location on the basis of observed effects. 
Mercalli intensity is expressed in Roman numerals.

Qualitative: having to do with perceived qualities; subjective. 
Examples: large, cold.

Quantitative: having to do with measurable quantities; objective. 
Examples: 10 m long, 5 C.

PROCEDURE

A. Introduction
Ask students on what basis scientists classify earthquakes. Many 
students may be aware of the Richter Scale and some may be aware of 
the Mercalli Scale. Explain that the Richter Scale is a quantitative 
measure of the energy released by an earthquake, while the Mercalli 
Scale is a qualitative measure of the amount of damage it does. The 
Richter rating is referred to as a quakes magnitude, and the Mercalli 
rating is referred to as its intensity. The original Mercalli Scale was 
developed in 1902 by the Italian geologist Giuseppi Mercalli. Wood 
and Neumann adapted it to U.S. conditions and introduced the 
Modified Mercalli Scale in 1931. Both the Richter and the Mercalli 
scales have their uses. Have students give other examples of 
quantitative vs. qualitative measurements.

B. Lesson Development
1. Project the transparency of the Modified Mercalli Scale and 
distribute student copies. Share the background information about the 
two earthquake scales (back of Master 3.3a) with the class and discuss 
the importance of intensity in establishing building codes. Briefly 
discuss each of the values assigned on the Mercalli scale. If some 
students in the class have experienced an earthquake, ask them to 
estimate its intensity from the scale.

2. Have the students compare and contrast the differences between the 
two types of measurements. Ask: Why do you think magnitude is 
more often reported than intensity? (Most earthquake-prone areas 
have equipment already in place to determine magnitudes, so this 
measurement can be quickly established. Mercalli ratings are 
sometimes not arrived at until several days later, when a full estimate 
of the damage can be made.) Point out to students that the lack of 
news accounts reporting intensity does not diminish its value to city 
planners and engineers.

TEACHING CLUES AND CUES
Have students listen carefully for significant phrases in the callers 
reports. They will be found in the Mercalli scale (Master 
3.3a).

It may take class discussion to reach agreement 
on the values for some of the reports. Have 
students read the descriptions on the Mercalli scale carefully, and reread 
them as necessary, checking with the script as they proceed.

3. Tell students that in this activity they will be using data adapted 
from reports of an earthquake that struck California in 1971.
Distribute copies of Master 3.3b, Wattsville Map. Appoint one 
student to be Jake Wilde, the television news anchor, and tell the 
other students that they are citizens of Wattsville and the surrounding 
area. The town has just been struck by an earthquake.

4. Distribute the strips cut from Master 3.3c and have students take 
turns reading them in order, starting with the news anchors report. 
Distribute student copies of the script as well.

5. As each student reads a part, have the other students locate the site 
of the report on their maps, scan the Modified Mercalli Intensity 
Scale, and mark a Mercalli intensity in pencil next to the location.

6. After the last student has read, have students draw blue lines 
enclosing areas with equal intensity ratings to develop an isoseismal 
map. (They will be drawing a series of concentric lines.)

7. Project the transparency of Master 3.3b, Wattsville Map, and have 
students take turns giving you the values to draw on the transparency. 
Then draw in the isoseismals according to their interpretation of the 
data.

C. Conclusion
Ask students to find the area where the damage was most intense and 
color this area red. Then build a class discussion around these 
questions:

 Where do you think the epicenter of the earthquake is located? 
(Most students will think that the area with the most damage is the 
epicenter. In certain cases this is a false assumption. Damage may 
be related to siting or construction practices. An example of this is 
the 1985 Mexico City earthquake in which the fault was over 350 
km away.)

 Why did some buildings near the epicenter withstand the shocks 
better than others? (perhaps because their structures were sounder 
or the soil under them was more firm)

 What are some advantages of the Mercalli rating procedure? (In 
ideal conditions, it can be done quickly, and it doesnt require any 
instruments. It describes the impact in human terms: on human 
beings and their structures.)

 What are some disadvantages to this procedure? (It requires human 
observers, so it could not assign an intensity to an earthquake in the 
middle of an ocean or in a deserted place. It also requires the 
exchange of information over a fairly wide area. To arrive at 
Mercalli ratings today, an official survey is conducted through the 
U.S. mail. It is not as precise or objective as instrument readings; 
different people might describe the same situation differently.)

Now place the answer key transparency on the overhead so it overlays 
student data. If there is a discrepancy between the isoseismals shown 
on the master and those students produced, they will see one of the 
drawbacks to this system of measurement. Ask students for 
suggestions to solve this problem.


ADAPTATIONS AND EXTENSIONS
1. Write a modern-day version of the 1931 Mercalli Intensity Scale.
2. Go to the library and search newspapers for qualitative information 
on an earthquake in your area. Construct an isoseismal map for the 
earthquake.

ACTIVITY TWO
RICHTER MAGNITUDE

VOCABULARY
Amplitude: a measurement of the energy of a wave. Amplitude is the 
displacement of the medium from zero or the height of a wave crest or trough 
from 
a zero point.

Duration: the length of time that ground motion at a given site shows 
certain characteristics. Most earthquakes have a duration of less than 
one minute in terms of human perceptions, but waves from a large 
earthquake can travel around the world for hours.

Lag time: the difference between the arrival time of P waves (Tp) and S 
waves (Ts).

Magnitude: a number that characterizes the size of an earthquake by 
recording ground shaking on a seismograph and correcting for the 
distance to the epicenter of the earthquake. Magnitude is expressed 
in Arabic numbers.

Richter magnitude: the number that expresses the amount of energy 
released during an earthquake, as measured on a seismograph or a 
network of seismographs, using the scale developed by Charles Richter 
in 1935.

Seismogram: the record of earthquake ground motion recorded by a 
seismograph.

RATIONALE
To determine Richter magnitude, we need to know both the maxi- 
mum wave amplitude as recorded on the seismogram and the distance 
between the seismograph station and the earthquake, which can be 
calculated by finding the lag time.

FOCUS QUESTIONS
How do scientists determine the size of an earthquake?

OBJECTIVES
Students will:
1. Interpret seismograms to calculate Richter magnitude.
2. Compare Richter magnitudes from several seismograph stations for 
the same earthquake.

MATERIALS
 Transparency made from the first page of Master 3.3d, Five Seismograms
 Overhead projector
 Transparency marker
 Copies of Master 3.3d, Five Seismograms (simplified) (3 pages), 
one set for every two students
 Copies of Master 3.3e, Distance, Magnitude, Amplitude, one for 
every two students
 Paper and pencils or pens
 Rulers or straightedges
 Transparency made from Master 3.3e, Distance, Magnitude, 
 Amplitude
 Transparency made from back of Master 3.3d (page 2), Richter 
 Data Table, Answer Key


PROCEDURE
A. Introduction
Tell students that Richter magnitude is a quantitative measure that is 
related to the amount of energy released during the earthquake and is 
not attracted by factors such as population, building materials, or 
building design. Ask them to name some other kinds of quantitative 
measures (minutes, hours, centimeters, dollars). Be sure they under-
stand the distinction between quantitative and qualitative description.

B. Lesson Development
1. Divide students into pairs. Give each pair a ruler and copies of 
Master 3.3d, Five Seismograms, and Master 3.3e, Distance, 
Magnitude, Amplitude. Tell students that they are going to be working 
with five seismograms, all recorded for one earthquake on September 
2, 1992, with its epicenter at St. George, Utah. Explain that the 
seismographs have been enlarged and simplified for their use but that 
this has also changed the scale, so they must use the vertical scale for 
all measurements. You will be demonstrating the steps to determine 
magnitude by using the first seismogram on Master 3.3d. They will 
record their data on the bottom of the third page of seismograms.

2. Project a transparency of Master 3.3d, page 1, and demonstrate how 
to measure amplitude. (See vocabulary definition.) Using the scale on 
the left side of the graph (the y axis) determine the greatest deflection 
in millimeters above or below zero of the largest seismic wave. 
Record this measurement in the amplitude column on the data table.

3. After all students have measured amplitude, start the next part of 
the activity by asking them which earthquake wave travels fastest and 
therefore should be the first wave recorded on the seismogram. Most 
of them should be aware from previous lessons that it is the P wave. 
On the transparency of Master 3.3d, page 1, point out the arrival of 
the P and S waves. Ask students what the difference between the 
arrival of the two waves is called (lag time (Ts-Tp). Ask students what 
should happen to the lag time recorded at stations farther from the 
earthquake. Offer a hint by comparing the progress of the waves to a 
race between the family car and a race car. The longer the race is, the 
more of an advantage the race car will have.

4. Using the projection of Master 3.3d, page 1, demonstrate how to 
determine distance to the earthquake recorded by the first 
seismogram. Using the scale at the top of the graph (the x axis), 
measure the difference between P wave and S wave arrival times in 
seconds. Use the formula
          Distance = (Ts-Tp) x (8 km/sec) or (4.96 mi/sec)
to convert time to distance. Have students record this in their data 
tables. Give students time to calculate the distances for the other four 
seismograms, following the procedure you have modeled.

5. Project the transparency of Master 3.3e and demonstrate how to 
find the magnitude of the earthquake. To do this, place the left end of 
the ruler on the left scale at the distance calculated in step 4. Holding 
the left edge of the ruler in place, move the right edge of the ruler to 
the correct point for the base-to-peak amplitude. Read the Richter 
magnitude on the center scale where the ruler crosses the graph and 
record it in the data table. Have students repeat this process on their 
own for the other stations. Average the magnitudes from the five 
locations to determine the final magnitude for this earthquake, and 
record it in the data table.

TEACHING CLUES AND CUES
The seismograms on Master 3.3d are seismograms that have 
been simplified and enlarged for classroom use. The 
printed millimeter scale has also been enlarged proportionately.
 
Do not use rulers for this step or the measurement will be 
inaccurate. (Be sure they make all their measurements on the same wave 
cycle, not the crest from one and the trough from another. The diagram on 
p.116 will be helpful.)
 
To measure the amplitude, have students lay a piece of paper across 
the wave and mark the high and low peaks, then use the 
scale on the y axis or the scale printed at the bottom of the seismogram to 
measure the amplitude or 
distance.

C. Conclusion
Project the back of Master 3.3d, Richter Data Table, Answer Key, and 
allow students to compare their calculations with the actual data. Ask 
students how a more accurate magnitude could be calculated. (by 
collecting more data points or by averaging the class averages of the 
calculated magnitude)

ADAPTATIONS AND EXTENSIONS
Seismologists today use a variety of magnitude scales and many types 
of instruments to record precise information on earthquakes. Interested 
students can read about P wave or body wave magnitude (called Mb), 
surface wave magnitude (Ms), moment magnitude (Mw), as well as the 
concept of seismic moment, drawing on materials in the unit resources 
list and others from local libraries.

ACTIVITY THREE
FIND THE EPICENTER: DECODING SEISMOGRAMS

VOCABULARY
Epicenter: the point on Earths surface directly above the focus of an 
earthquake.

Focus (pl. foci): the point within the Earth that is the origin of an 
earthquake, where stored energy is first released as wave energy.

Seismogram: the record of earthquake ground motion recorded 
by a seismograph.

Seismograph: an instrument that records vibrations of the Earth, 
especially earthquakes.

Triangulation: using data from three or more known points to locate an 
unknown point, in this case the epicenter of an earthquake.

RATIONALE
Students can find the location of an earthquake by triangulation if 
they know the distances from at least three seismograph stations.

FOCUS QUESTION
How do seismologists use seismograms to locate the epicenter of an 
earthquake?

OBJECTIVES
Students will:
1. Calculate the distance from an earthquake to a seismograph station.
2. Use five calculated distances to triangulate the location of the 
earthquakes epicenter.

MATERIALS
 Student copies of Master 3.3f, Several Seismographs
 Student copies of Master 3.3g, Sample Seismograms
 Student copies of Master 3.3h, Map of Station locations
 Transparency made from one page of Master 3.3g, Sample Seismograms
 Overhead projector
 Transparency made from Master 3.3h, Map of Station Locations
 Student copies of Master 3.3i, Time/Distance Reference Table
 Drawing compasses
 Metric rulers with millimeter scales

PROCEDURE
Teacher Preparation
Make one copy of each of the masters (3.3f through 3.3i) for every 
two students in your class.

A. Introduction
Ask students chosen at random to explain the difference between an 
earthquakes focus and its epicenter and between a seismograph and a 
seismogram. Review these distinctions if necessary.
Distribute copies of Master 3.3f, Several Seismographs, then project a 
transparency of Master 3.3f and describe their operation.

B. Lesson Development
1. Divide the class into pairs of students. Distribute a set of 
seismograms (Master 3.3g, 3 pages), one map (Master 3.3h), and 
copies of the Time/Distance Reference Table (Master 3.3i) to each 
pair. Tell students that all the seismograms are from the same 
earthquake, a quake that occurred on January 14, 1993, with a 
magnitude of 3.3, but each was recorded by a different seismograph in 
the seismograph network.

2. Project transparencies of one seismogram and the map. Model the 
procedure for students as necessary.

3. Give these directions for finding the epicenter of the earthquake 
recorded on the five seismograms:
a. On the first seismogram, use the second scale to measure the time- 
distance from the nearest 10-second mark to the P wave arrival of the 
earthquake. Record the P wave arrival times in the table to the nearest 
second.
b. Repeat for the S wave, measuring from the same minute mark.
c. Find the Ts-Tp time by subtracting the arrival time of the P wave 
from the arrival time of the S wave. Record this time in the table.
d. Use the time/distance table on Master 3.3i to determine the distance 
to the epicenter.
e. Repeat this procedure for all of the stations.
f. For each seismogram, draw a circle on the map with the compass, 
using the distance you calculated as the radius of the circle. Place the 
point of the compass at zero on the map scale and adjust the compass 
width to the calculated distance. With the distance set, place the point 
of the compass on the station and draw a circle. Mark the outer edge 
of each circle with a letter to identify the station.
g. Repeat, setting the compass and drawing circles for 0 five stations.

4. Instruct students to circle the area where all the circles intersect. 
Ask: What is this area called? (It is the epicenter of the earthquake.)

TEACHING CLUES AND CUES
Emphasize the importance of accurate measurement, particularly 
the need to set the compass accurately for distance. 
Dont worry if the circles dont intersect perfectly, however. Remind 
students that an earthquake actually occurs along a fault, not at one single 
point.

C. Conclusion
Build a class discussion around these questions:

 What information can be obtained from one seismogram? (The 
distance from that seismograph in a 360 circle.)

 After the arcs for stations TRYN and FGTN were drawn, where 
was the epicenter of this earthquake? Explain. (We dont know yet. 
It could be at either place where the two arcs cross. They are the 
common points.)

 After all the stations were drawn, where was the actual epicenter of 
this earthquake? Where was its focus? (In the area where the arcs 
cross just south of station BHT. Directly under the epicenter.)

Why is it necessary to have measurements from at least three 
different stations to locate the epicenter of an earthquake? 
(Answers will vary but should relate to the above questions.)

 Why dont all of the arcs pass through the same point? (Answers 
will vary. Accuracy in measurement and drawing should be two 
most common. Also, an earthquake does not occur at one point but 
along a fault surface. Have students speculate as to the location and 
strike of the fault.

 Which station was closer to the earthquakes epicenter, BBG or 
FGTN? Cite two kinds of evidence from the seismograms to 
support your conclusion. (BBG. Evidence: amount of lag time and 
amplitude difference.)

 Would it be possible for an earthquake at this location to be felt 
where you live? Why or why not? (Answers will vary; will depend 
on distance from the focus and the magnitude of the quake.)

 P waves travel at an average velocity of 6 km/sec in the Earths 
crust. How long would it would take for the P waves from this 
quake to reach a seismic station in your city, if they continued to 
travel at a constant speed? (Answers will vary. Multiply 6 km/sec 
times the distance to your city.)

ADAPTATIONS AND EXTENSIONS
1. Challenge students to research these questions:

 Would a seismograph work on the moon?

 Have scientists placed seismographs on the moon and other planets?

 If so, which planets? Have quakes been detected there?

2. Interested students may research several types of seismographs and 
build their own models.