This lesson provides an overview of destructive earthquakes and their connection to tectonic movements of the Earth's crust. It includes a discussion of some especially destructive historic earthquakes, and a brief introduction to contintental drift and the theory of plate tectonics. There is also discussion of basic seismology (types of waves) and measures of the magnitude of an earthquake (the Richter Scale). The lesson inlcudes an activity in which students use an online simulator to locate the epicenter of an earthquake using readings from three different seismograph stations. After they have completed the simulation, they attempt to locate the epicenter of a real earthquake using data from the United States Geological Survey (USGS) earthquake website. Educational levels: Undergraduate lower division, High school

John Pratte

English

Digital Library for Earth System Education

Plate Tectonics: Earthquake Epicenter 
Introduction 
History 
Nothing can strike fear in the hearts of people quite like a natural disaster.  The awesome power 
unleashed by a hurricane, tornado, volcano, or earthquake can cause large losses in life and property.  
Even the Earth, which we often think of as solid and unchanging, can be irrevocably scarred by one of 
these events.  In ancient days, the occurrence of a natural disaster was often attributed to the anger of the 
gods who were displeased with some action of mankind.  Today, most people do not hold with such ideas, 
relying on our knowledge of science to explain their appearance.  In each case, we are able to describe 
the physical situations that produce these disasters.  For instance, earthquakes occur when stresses that 
have built up in crust of the Earth are very quickly relieved by large blocks of rock moving very rapidly.  
However, for all of our knowledge of these disasters, we have yet to be able to accurately predict them.   
 
Of these four natural disasters, the one that has the 
greatest reputation for loss of life and societal damage is 
an earthquake.  History is replete with stories of massive 
damage from these events.  Until very recently, the 
recording of earthquakes required that a civilization be 
near the epicenter of the earthquake and that this society 
have some way of recording it.  It is, therefore, not 
surprising that the earliest recordings of earthquakes that 
we have today come from China in 1177 B.C.  China, 
which has had a literate, recording society for many 
millennia, is situated in a location that has many large 
and lethal earthquakes.  Throughout time, they have 
recorded a large amount of death and destruction, from 
the 1556 Shensi earthquake that killed 830,000 to the 
1976 Tangshan earthquake that killed over 250,000 
people.  Other countries have also recorded vast amounts of death and damage, as seen in Table 1. 
Year Location Dead 
1290 Hopeh Province, China 100,000
1556 Shensi Province, China 830,000
1667 Shemaka, Russia 80,000
1737 Calcutta, India 300,000
1908 Messina, Italy 75,000
1920 Kansu Province, China 180,000
1923 Tokyo, Japan 142,807
1932 Kansu Province, China 70,000
1935 Quetta, India 60,000
1976 Tangshan, China 255,000
Table 1: Historical earthquakes2
 
The history of recorded earthquakes in the U.S. is filled 
with many large earthquakes, but few that rival the loss 
of life as those already mentioned.  In 1811-1812, a 
series of large earthquakes that have been estimated 
above an 8 on the Richter scale hit near the area of 
New Madrid, Missouri.  The vibrations from these 
earthquakes were felt as far away as the New York 
and Boston.  In 1886, a devastating earthquake hit 
Charleston, South Carolina that killed over 60 people 
and destroyed almost all buildings in the city.  Possibly 
the most destructive earthquake in the U.S. occurred 
in San Francisco during 1906.  This earthquake 
caused a series of large fires that burned large 
sections of the city and killed over 700 people. 
 
Most of these large earthquakes occurred before we 
were able to accurately measure their magnitudes.  One of the largest that has occurred since we have 
been able to measure magnitudes happened in Alaska on March 27, 1964.  It had a magnitude of 9.2 and 
created vertical displacement of over 11 meters high.  The focus was situated under the Prince William 
 
Figure 1: 1964 Alaskan earthquake 
Sound, and resulted in a tidal wave that killed 110 people in Alaska, Canada, Hawaii, and the continental 
U.S.  It was so large that it caused the water in pools in Texas and Louisiana to slosh.3
 
Structure of the Crust 
In order to understand in better detail the reasons for earthquakes, we must come to a greater 
understanding of how our crust operates.  In 1912, Alfred Wegener, a German meteorologist, proposed 
what seemed to be a rather preposterous scientific theory.  He posited that the crust of the Earth was split 
up into giant plates of rock that were free to move about on the top of the mantle.  This theory, known as 
continental drift, had several key pieces of evidence to support it.  The first of these was that easternmost 
margins of the continental shelves of North and South America seemed to fit into the westernmost margins 
of those of Europe and Africa.  Furthermore, the geographic features, such as rock layers, mountain 
ranges, and glacial scoring, lined up if one were to place the continents together as such.  The fossil 
record also showed some anomalies that were easily explained if these continents had been placed 
together some time in the past.  Although there was much to support this theory, it suffered from one great 
problem: there was no known mechanism that could have driven the rock layers to their locations. 
 
After World War II, more evidence was discovered that helped support the idea that the crust was made of 
moving plates.  The discoveries of the Mid-Atlantic Ridge and the mirrored striping of magnetic fields 
across the Ridge led to a re-thinking of the continental drift theory and efforts to actually measure relative 
movements of the plates.  The later discoveries of this movement and evidence for mantle convection 
brought the idea to the forefront of geological thinking.  The theory of plate tectonics is now widely 
accepted and used as a basis for all work in geology and paleontology. 
 
It is this fact that the plates of the crust are moving that accounts for almost all earthquakes.  The plates 
are not greasy pucks sliding around on frictionless ice.  They are giant slabs of solid material that are 
moving past other solid pieces of rock with great deals of friction, stress, and strain.  As they move, the 
plates deform, warp, and stretch, sometimes buckling in two or cracking in place.  This means that 
earthquakes can occur in any location on the Earth.  However, they are much more likely to occur at the 
boundaries between plates, as this is where the most amount of stress and buckling occur.  To understand 
this, let us look at the types of boundaries that there are. 
 
 Since the Earth is not changing in size, the fact that 
there are moving plates means that there will be 
three types of boundaries between the plates.  
There will be boundaries where plates are moving 
apart, where plates are moving together, and where 
plates are moving past each other.  Divergent zones 
(Figure 2A) are characterized by large-scale 
volcanoes and shallow, small magnitude 
earthquakes.  As the plates come apart, molten 
magma from near the crust-mantle interface is 
allowed to seep to the surface and fill in the cracks. 
 This magma pushing up causes some stress and 
cracking in the surrounding rock, thus creating small 
earthquakes.  The Mid-Atlantic Ridge is a perfect 
example of this type of zone.  Figure 2: Plate margins 
 
Convergent zones are separated into two categories depending upon the types of rocks on either side of 
the boundary.  When continental crust (thicker and less dense) crashes into oceanic crust (thinner and 
denser), the oceanic crust goes under the continental crust to form a subduction zone (Figure 2C).  The 
oceanic crust that is driven down toward the mantle melts as it encounters higher temperatures and 
pressure.  As this magma is driven up through cracks created in the continental crust, dissolved gases 
within it begin to expand out of solution due to the reduced pressure.  This forces the magma up through 
the cracks faster, which often results in explosive vulcanism.  These zones are characterized by deep-
seated earthquakes and stratovolcanoes that form island arc near the continental shelf. 
 
The other type of convergent zone occurs when continental crust crashes into continental crust (Figure 
2B).  When this occurs, the two plates fold and crumple at the boundary, uplifting material into mountain 
chains.  This results in few, if any, volcanoes and numerous shallow earthquakes.  The best example of 
this is the Himalaya Mountains that form the boundary between the Indian Plate and Asia. 
 
The last type of boundary is a transform fault (Figure 2D) that will form between two plates that are sliding 
past one another.  This type of boundary has never resulted in volcanoes in recorded history.  The most 
likely explanation of this is that there is no crustal plate melting, nor any exposure to the mantle, with this 
type of boundary.  However, earthquakes are very prevalent in this type of zone, as there is a great deal of 
friction between the plates that will allow cause no movement for long periods of time while stress builds 
up.  Earthquakes in this zone are usually shallow.  In the U.S., our favorite example of this type of 
boundary is the San Andreas fault that runs through California. 
 
As previously stated, earthquakes do occur in other locations than plate boundaries.  They will occur as 
stress is relieved slowly over time from the entire plate moving in the direction it is going.  Oftentimes, 
these earthquakes occur at former plate boundaries between the “welded” sections of the plate.  For 
instance, the Appalachian Mountains are the site of a former boundary between North America and 
Europe over 300 million years ago.  They were formed when the two proto-continents crashed into one 
another and welded together.  Because of their history, they are a weak area of the plate.  Possibly the 
most famous intracontinental earthquake zone in the U.S. exists 
near New Madrid, Missouri, where the famous 1811-1812 
earthquakes occurred. 
Fig 3: Seismic waves (USGS) 
 
Earthquake Waves 
When a violent release of energy occurs because of rock 
movement, vibrations are generated as the rock moves past each 
other.  These vibrations create waves, which travel away from the 
focus of the earthquake.  Some of these waves only travel along 
the surface of the earth and are, therefore, called surface waves.  
Those that travel through the interior of the earth are called body 
waves, and fall into two different categories: compressional and 
shear waves.  The frequencies of some of these waves are high 
enough to be heard, while others fall below our hearing level of 
about 10 hertz.  As they travel, they obey all of the principles of 
other types of wave motion, meaning that they will reflect and 
refract at boundaries between parts of the Earth that have 
different wave speeds. 
 
The surface waves are designated as either Rayleigh or Love 
waves, depending upon whether there motion is vertical or 
horizontal (Figure 3).  These types of waves have the slowest 
wave speed, but are usually responsible for most of the damage 
that occurs during an earthquake, as their motion drives the 
ground up and down, and side to side.  Since these waves spread 
in all surface directions, their amplitudes decrease with distance 
away from the epicenter, making them truly dangerous only within 
a hundred miles or less of the epicenter. 
 
Of the two types of body waves, shear or S-waves have the slowest speed.  These waves travel in a 
transverse fashion, as the material through which it travels is moved at right angles to the direction that the 
wave is travelling.  The analog to this type of wave is the type of wave that travels on a plucked, taut 
string.  As the wave travels down the string, the string vibrates back and forth.  This is different from the 
compressional or P-waves, which travel by material movement in the same direction as the wave is 
moving.  To visualize how this wave travels, imagine a Slinky that is placed on a floor and pulled into a 
straight line.  If the Slinky is moved back and forth in the direction of the line that the Slinky makes, a 
compressional wave will travel down its length.   
 
This difference between P and S-waves in how they travel is very important.  In order for S-waves to 
propagate, they require a fairly stiff material matrix, i.e. they need for the molecules in the material through 
which they are travelling to be tightly bound to their nearest neighbors.  This is so that there will be internal 
forces that will pull material back to its original location as it is perturbed away from it.  Because of this, S-
waves cannot travel through a liquid or a gas.  P-waves, on the other hand, have no such restriction, and 
can travel through all types of media (sound waves in air are an example of a compressional wave).  This 
difference allows us to determine a very important fact about the interior of the Earth: the outer core is 
liquid.  S-waves are not able to penetrate this layer and are reflected from the core/mantle boundary. 
 
These waves can be detected through the use of a seismograph.  As the waves pass by the seismograph, 
they will jiggle the base of device, which causes the pens to mark out the amplitude of the wave on the 
paper as it moves underneath it.  The amplitude of the wave will depend upon the strength of the 
earthquake and the distance it is from the seismograph, as the wave energy is spread over a greater 
region as it travels and loses intensity due to reflections.  The time that it takes for the waves to reach the 
seismograph also depends on this distance, as well as the type of wave.  The differences in their wave 
speeds means that P-waves hit a spot first, followed by S-waves and then surface waves. 
 
Earthquake Magnitude 
The earliest attempts to quantify the magnitude of an earthquake relied upon assessing the damage near 
its epicenter.  Different numbers were assigned to an earthquake depending upon whether buildings were 
knocked down, trees toppled over, or things fell off of shelves.  The scale that was developed for doing 
this was known as the Mercalli scale, after its inventor Giuseppe Mercalli.  The problem with using this 
scale to quantify earthquakes was that it was too subjective.  Depending upon many factors such as the 
quality of the buildings, the population density of the region, and the investigator doing the survey, different 
numbers could be assigned to the same earthquake. 
 
In order to get a more objective measure of the magnitude of an earthquake, Dr. Charles Richter 
developed the Richter scale in 1935.  This system assigned each earthquake a number dependent upon 
the amplitude of the waves generated by it.  Since there was such a broad range of values of amplitudes 
for earthquakes measured, Richter made the scale logarithmic.  This means that each increase of one on 
the Richter scale corresponds to an increase of 10 in the amplitude of the waves.  Thus, on the Richter 
scale, a magnitude-8 earthquake is 10 times greater in amplitude than a magnitude-7 and 100 times 
greater than a magnitude-6.   
 
While the Richter scale does allow one to objectively measure an earthquake, it still does not give one a 
sense for the damage that an earthquake can render.  For instance, the 1964 Alaska earthquake that we 
mentioned before was 9.2 on the Richter scale, the second highest value every recorded.  While Figure 1 
attests to the damage to buildings on the fault line being severe, only 110 people died in this earthquake.  
This is far less than the numbers of people killed in the earthquakes listed in Table 1, even though each of 
these earthquakes was probably less than a magnitude 9 on the Richter scale.  In order to give one a 
sense of how much damage is done by an earthquake, scientists have developed the Modified Mercalli 
scale.  Earthquakes are given a letter from Roman numeral I up to Roman numeral XII based upon 
eyewitness accounts and observations of the epicenter after the earthquake.  While not as precise as the 
Richter scale, its narrowed classification schemes over the old Mercalli scale do allow for a reasonable 
estimation as to the damage to humans and their buildings. 
 
References 
 
1 http://pubs.usgs.gov/gip/earthq1/, September 13, 2003. 
2 The World Book Encyclopedia, Volume 6, p. 19, Field Enterprises Educational Corporation, 
Chicago, 1974. 
3 http://neic.usgs.gov/neis/eqlists/USA/1964_03_28.html, September 13, 2003. 
Updated link: http://earthquake.usgs.gov/regional/states/events/1964_03_28.php, August 15, 
2006. 
 
Activity 
 
In this week’s activity, we are going to try to locate the epicenter of an earthquake using readings from 
three different seismograph stations.  This will be done by measuring the difference in time between the 
arrival of the P and S-waves.  The difference it time of their arrivals is due to the difference in speeds for 
both waves. In particular, the difference in time is given by (distance to epicenter)/(Vp – Vs).  Thus, we can 
find out how far away a particular seismograph is from an earthquake by solving this equation for distance. 
 Since there are three stations, we can triangulate between all three to find the exact spot. 
 
Before we use actual data, we will first do a “test run” using a simulator from California State University – 
Los Angeles.  This simulator will walk you through the steps of epicenter triangulation that we will then 
apply to some real data recorded by seismographs.  To get to the simulator, click on  
 
http://vcourseware3.calstatela.edu/VirtualEarthquake/VQuakeExecute.html
 
When you get to the simulator site, select “San Francisco area” at the bottom of the page, and follow the 
instructions on the succeeding pages. 
 
Once you have completed this activity and understand how to use triangulation to find epicenters, you are 
ready to test out your skills on a real set of data.  The USGS Earthquake website maintains a running 
record from seismographs that are stationed around the world.  You will be able to pull up complete data 
sets for any day in the past several years.  Your instructor will designate which day or days from which you 
are to retrieve data.  Follow the steps below: 
 
1. Print out the seismographic records from three different stations on that day, and determine the 
arrivals of the P and S waves on them.   
2. Measure the differences in the arrival times for each station, and plot these on the Travel Time 
Curve plot on the Activity Sheet to determine the distance from the earthquake’s epicenter to the 
station.   
3. Using a scale of 1 inch = 2,500 miles, determine the radius of the circle to draw from each station 
on the attached world map 
4. For each station, set a compass to draw a circle of the corresponding radius in inches.  Placing 
the sharp end of the compass on the station, draw a circle to represent the distance that the 
epicenter is away from the station. 
5. Repeat this process for the other two stations. 
6. Determine the epicenter by the intersection of the three circles.  
7. Answer the questions on the activity sheet. 
 
Links 
 
USGS Seismograms - http://earthquake.usgs.gov/eqcenter/helicorders.php
  
Science 1102 Activity Sheet Earthquake Epicenters 
Name: 
 
1. For the “Virtual Earthquake” activity, what is the epicenter of the earthquake?   
 
 
 
 
 
2. What is the epicenter for the actual earthquake you studied?  Can you find any evidence of this  
 earthquake in the news or online to support your supposition? 
 
 
 
3. How well does this triangulation method work? 
 
 
 
 
 
 
Mark your travel-times on the above chart and attach your world map to show your triangulation work. 
 


Plate Tectonics: Earthquake Epicenter

http://esa21.kennesaw.edu/activities/epicenter/epicenter.pdf

Plate Tectonics: Earthquake Epicenter

Abstract

Similar works

Full text

Available Versions

Digital Library for Earth System Education