seismic waves assignment

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Why is an earthquake dangerous?

What are earthquake waves, how is earthquake magnitude measured, where do earthquakes occur.

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• USGS - The Science of Earthquakes
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Over the centuries, earthquakes have been responsible for millions of deaths and an incalculable amount of damage to property. Depending on their intensity, earthquakes (specifically, the degree to which they cause the ground’s surface to shake) can topple buildings and bridges , rupture gas pipelines and other infrastructure, and trigger landslides , tsunamis , and volcanoes .  These phenomena are primarily responsible for deaths and injuries. Very great earthquakes occur on average about once per year.

Earthquake waves, more commonly known as seismic waves , are vibrations generated by an earthquake and propagated within Earth or along its surface. There are four principal types of elastic waves: two, primary and secondary waves, travel within Earth, whereas the other two, Rayleigh and Love waves, called surface waves, travel along its surface. In addition, seismic waves can be produced artificially by explosions.

Magnitude is a measure of the amplitude (height) of the seismic waves an earthquake’s source produces as recorded by seismographs . Seismologist Charles F. Richter created an earthquake magnitude scale using the logarithm of the largest seismic wave’s amplitude to base 10. Richter’s scale was originally for measuring the magnitude of earthquakes from magnitudes 3 to 7, limiting its usefulness. Today the moment magnitude scale, a closer measure of an earthquake’s total energy release, is preferred.

Earthquakes can occur anywhere, but they occur mainly along fault lines (planar or curved fractures in the rocks of Earth’s crust ), where compressional or tensional forces move rocks on opposite sides of a fracture. Faults extend from a few centimetres to many hundreds of kilometres. In addition, most of the world’s earthquakes occur within the Ring of Fire , a long horseshoe-shaped belt of earthquake epicentres , volcanoes , and tectonic plate boundaries fringing the Pacific basin .

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earthquake , any sudden shaking of the ground caused by the passage of seismic waves through Earth ’s rocks. Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.” Earthquakes occur most often along geologic faults , narrow zones where rock masses move in relation to one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up Earth’s crust. ( See the table of major earthquakes.)

Little was understood about earthquakes until the emergence of seismology at the beginning of the 20th century. Seismology , which involves the scientific study of all aspects of earthquakes, has yielded answers to such long-standing questions as why and how earthquakes occur.

About 50,000 earthquakes large enough to be noticed without the aid of instruments occur annually over the entire Earth. Of these, approximately 100 are of sufficient size to produce substantial damage if their centres are near areas of habitation. Very great earthquakes occur on average about once per year. Over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property.

The nature of earthquakes

Causes of earthquakes.

Earth’s major earthquakes occur mainly in belts coinciding with the margins of tectonic plates. This has long been apparent from early catalogs of felt earthquakes and is even more readily discernible in modern seismicity maps, which show instrumentally determined epicentres. The most important earthquake belt is the Circum-Pacific Belt , which affects many populated coastal regions around the Pacific Ocean —for example, those of New Zealand , New Guinea , Japan , the Aleutian Islands , Alaska , and the western coasts of North and South America . It is estimated that 80 percent of the energy presently released in earthquakes comes from those whose epicentres are in this belt. The seismic activity is by no means uniform throughout the belt, and there are a number of branches at various points. Because at many places the Circum-Pacific Belt is associated with volcanic activity , it has been popularly dubbed the “Pacific Ring of Fire .”

A second belt, known as the Alpide Belt , passes through the Mediterranean region eastward through Asia and joins the Circum-Pacific Belt in the East Indies . The energy released in earthquakes from this belt is about 15 percent of the world total. There also are striking connected belts of seismic activity, mainly along oceanic ridges —including those in the Arctic Ocean , the Atlantic Ocean , and the western Indian Ocean —and along the rift valleys of East Africa . This global seismicity distribution is best understood in terms of its plate tectonic setting .

Natural forces

Earthquakes are caused by the sudden release of energy within some limited region of the rocks of the Earth . The energy can be released by elastic strain , gravity, chemical reactions, or even the motion of massive bodies. Of all these the release of elastic strain is the most important cause, because this form of energy is the only kind that can be stored in sufficient quantity in the Earth to produce major disturbances. Earthquakes associated with this type of energy release are called tectonic earthquakes.

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Seismic waves.

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When an earthquake occurs, the shockwaves of released energy that shake the Earth and temporarily turn soft deposits, such as clay, into jelly ( liquefaction ) are called seismic waves, from the Greek ‘seismos’ meaning ‘earthquake’. Seismic waves are usually generated by movements of the Earth’s tectonic plates but may also be caused by explosions, volcanoes and landslides.

Seismologists use seismographs to record the amount of time it takes seismic waves to travel through different layers of the Earth. As the waves travel through different densities and stiffness, the waves can be refracted and reflected. Because of the different behaviour of waves in different materials, seismologists can deduce the type of material the waves are travelling through.

The results can provide a snapshot of the Earth’s internal structure and help us to locate and understand fault planes and the stresses and strains acting on them.

This wave behaviour can also be used on a smaller scale by recording waves generated by explosions or ground vibrators in the search for oil and gas.

Types of seismic waves

There are three basic types of seismic waves – P-waves, S-waves and surface waves. P-waves and S-waves are sometimes collectively called body waves.

P-waves, also known as primary waves or pressure waves, travel at the greatest velocity through the Earth. When they travel through air, they take the form of sound waves – they travel at the speed of sound (330 ms -1 ) through air but may travel at 5000 ms -1 in granite. Because of their speed, they are the first waves to be recorded by a seismograph during an earthquake.

They differ from S-waves in that they propagate through a material by alternately compressing and expanding the medium, where particle motion is parallel to the direction of wave propagation – this is rather like a slinky that is partially stretched and laid flat and its coils are compressed at one end and then released.

S-waves, also known as secondary waves, shear waves or shaking waves, are transverse waves that travel slower than P-waves. In this case, particle motion is perpendicular to the direction of wave propagation. Again, imagine a slinky partially stretched, except this time, lift a section and then release it, a transverse wave will travel along the length of the slinky.

S-waves cannot travel through air or water but are more destructive than P-waves because of their larger amplitudes

Surface waves

Surface waves are similar in nature to water waves and travel just under the Earth’s surface. They are typically generated when the source of the earthquake is close to the Earth’s surface. Although surface waves travel more slowly than S-waves, they can be much larger in amplitude and can be the most destructive type of seismic wave. There are two basic kinds of surface waves:

• Rayleigh waves, also called ground roll, travel as ripples similar to those on the surface of water. People have claimed to have observed Rayleigh waves during an earthquake in open spaces, such as parking lots where the cars move up and down with the waves.
• Love waves cause horizontal shearing of the ground. They usually travel slightly faster than Rayleigh waves

What can seismic waves tell us?

Studies of the different types of seismic waves can tell us much about the nature of the Earth’s structure.

For example, seismologists can use the direction and the difference in the arrival times between P-waves and S-waves to determine the distance to the source of an earthquake. If the seismographs are too far away from the event to record S-waves, several recordings of P-waves can be crunched in a computer program to give an approximate location of the source.

Activity idea

In the activity Earthquake location , students are introduced to some of the methods scientists use to record earthquakes. They extract data from seismograms to locate the epicentre of an earthquake, which they plot on a map of New Zealand. Students then consider the location and predict possible damage.

Useful links

Watch these videos on YouTube, from GNS scientists:

• Yoshihiro Kaneko models the propagation of seismic waves across New Zealand
• John Ristau explains of how seismic waves are used to locate an earthquake .

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The Science of Earthquakes

Originally written by Lisa Wald (U.S. Geological Survey) for “The Green Frog News”

What is an earthquake? 

An  earthquake  is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the  fault  or   fault plane . The location below the earth’s surface where the earthquake starts is called the  hypocenter , and the location directly above it on the surface of the earth is called the  epicenter .

Sometimes an earthquake has  foreshocks . These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the  mainshock . Mainshocks always have  aftershocks  that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

What causes earthquakes and where do they happen?

The earth has four major layers: the inner core, outer core, mantle and crust . The crust and the top of the mantle make up a thin skin on the surface of our planet.

But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces  tectonic plates , and the edges of the plates are called the  plate boundaries . The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

Why does the earth shake when there is an earthquake?

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the  friction  of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of  seismic waves  like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!

How are earthquakes recorded?

Earthquakes are recorded by instruments called  seismographs . The recording they make is called a  seismogram . The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

How do scientists measure the size of earthquakes?

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the  seismogram  recordings made on the  seismographs  at the surface of the earth to determine how large the earthquake was (figure 5). A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

The size of the earthquake is called its  magnitude . There is one magnitude for each earthquake. Scientists also talk about the intensity  of shaking from an earthquake, and this varies depending on where you are during the earthquake.

How can scientists tell where the earthquake happened?

Seismograms come in handy for locating earthquakes too, and being able to see the  P wave  and the  S wave  is important. You learned how P & S waves each shake the ground in different ways as they travel through it. P waves are also faster than S waves, and this fact is what allows us to tell where an earthquake was. To understand how this works, let’s compare P and S waves to lightning and thunder. Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right after the lightning, but if you are far away from the lightning, you can count several seconds before you hear the thunder. The further you are from the storm, the longer it will take between the lightning and the thunder.

P waves are like the lightning, and S waves are like the thunder. The P waves travel faster and shake the ground where you are first. Then the S waves follow and shake the ground also. If you are close to the earthquake, the P and S wave will come one right after the other, but if you are far away, there will be more time between the two.

By looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location. However, they can’t tell in what direction from the seismograph the earthquake was, only how far away it was. If they draw a circle on a map around the station where the  radius  of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle. But where?

Scientists then use a method called  triangulation  to determine exactly where the earthquake was (see image below). It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the  radius  of each is the distance from that station to the earthquake, the intersection of those three circles is the  epicenter !

Can scientists predict earthquakes?

No, and it is unlikely they will ever be able to predict them. Scientists have tried many different ways of predicting earthquakes, but none have been successful. On any particular fault, scientists know there will be another earthquake sometime in the future, but they have no way of telling when it will happen.

Is there such a thing as earthquake weather? Can some animals or people tell when an earthquake is about to hit?

These are two questions that do not yet have definite answers. If weather does affect earthquake occurrence, or if some animals or people can tell when an earthquake is coming, we do not yet understand how it works.

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Introduction to seismology, course description.

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Animation 3: Seismic Waves

You now have some basic knowledge about the nature and characteristics of mechanical waves : what they are, how they are measured, and the different ways in which  longitudinal and transverse waves move through a medium as they transfer energy from one place to another.

Watch this animation to learn about seismic waves , which are the literally ground-shaking mechanical waves that occur during earthquakes.

Stop the animation at any time, watch it again, or read the transcript.

A great place to see longitudinal and transverse waves in action is during an earthquake. When energy builds up in the earth’s crust, it is released during an earthquake by waves of energy traveling through the solid ground. These waves, called seismic waves, are examples of longitudinal and transverse waves and transfer energy in predictable ways.

The first type of wave that occurs and travels faster are called primary or P-waves. P-waves are longitudinal, meaning they compress and stretch the earth to move the energy outward.

Then, slower moving secondary, or S-waves, follow. These are transverse waves and transfer energy by moving the earth’s layers up and down, while moving the energy outward. 

Both of these longitudinal and transverse waves transfer energy over long distances and can be measured (wavelength, frequency, amplitude) to determine the location of the source of the earthquake.

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• Earthquakes Living Lab: Finding Epicenters & Measuring Magnitudes

Hands-on Activity Earthquakes Living Lab: Finding Epicenters & Measuring Magnitudes

Grade Level: 10 (9-11)

Time Required: 1 hour

Expendable Cost/Group: US $0.00

Group Size: 2

Activity Dependency: None

Subject Areas: Earth and Space, Physical Science

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Every year, earthquakes cause death and destruction worldwide. These natural disasters may be mitigated, however, by insightful and creative engineering. Engineers first determine where earthquakes are likely to occur, and how severe they are likely to be. They use three seismographs in a process called triangulation to determine earthquake epicenters. Using historical seismographs, engineers forecast the strength or magnitude of earthquakes and make predictions and determine building codes and safety protocols.

Scientists and engineers around the globe gather data through observation and experimentation and use it to describe and understand how the world works. The Earthquakes Living Lab gives students the chance to track earthquakes across the planet and examine where, why and how they are occurring. Using the real-world data in the living lab enables students and teachers to practice analyzing data to solve problems and answer questions, in much the same way that scientists and engineers do every day.

After this activity, students should be able to:

• Use the process of triangulation to locate an earthquake's epicenter.
• Explain the difference between S and P waves, and how their time interval is used to determine the epicenter location.
• Describe the logarithmic nature of the earthquake magnitude scale.

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Each group needs:

• computer or other device with Internet access
• journal or writing paper for each student
• pen or pencil, one per student
• Finding Epicenters and Measuring Magnitudes Worksheet , one per group

Seismographs are measuring devices designed by engineers and used by researchers to determine the locations and magnitudes of earthquakes. Several thousand seismographs exist at locations around the planet, continuously measuring abnormalities in the Earth's movement. Specifically, seismographs make recordings (seismograms) of the seismic waves generated from earthquakes, providing engineers and other researchers with data that they use to make predictions about future earthquakes.

What types of engineers might use this data the most? How might they use this information? (Listen to student ideas.) Civil engineers, who design houses, apartment buildings, schools, skyscrapers, bridges, highways, tunnels, water treatment facilities, factories and other structures, may use this data to help them create safer structures that are less likely to sustain damage during earthquakes. While no one can predict earthquakes, knowing the intensities, frequencies and locations of past earthquakes and fault planes helps us to better anticipate the locations and forces to expect, so we can do our best to prepare our communities and infrastructure to withstand them safely.

Teacher Background

In this activity, students use an online simulation—Virtual Earthquake—that is accessible through the Earthquakes Living Lab interface, to locate the epicenter of an earthquake by making simple measurement on three seismograms, recordings of an earthquake's seismic waves detected by instruments (seismographs) far away from the earthquake. The process is called triangulation. Then from the same recordings they determine the earthquake's magnitude, an estimate of the amount of energy released during the earthquake. The point of origin of an earthquake is called its focus and the point on the Earth's surface directly above the focus is the epicenter.

Since the 1970s, the use of the Richter magnitude scale has largely been replaced in the scientific community by the moment magnitude scale (MMS). In the U.S., earthquake measurements under the MMS (those of magnitude 3.5 or higher) are still commonly erroneously referred to by the general public and media as being on the Richter scale, due to more familiarity with the Richter scale.

The Richter scale was created in the 1930s to assign a single number to quantify the energy released during earthquakes. It's a logarithmic scale from 1 to 10 with each succeeding level representing 10 times as much energy as the last. Today, most seismologists no longer follow Richter's original methodology because it does not give reliable results when applied to stronger earthquakes and it was not designed to use data from earthquakes recorded at epicentral distances greater than ~600 km.

The moment magnitude scale (MMS) was developed in the 1970s as a modification of the Richter scale and is better for measuring big earthquakes but less good for small ones. Even though the scale formulae are different, MMS retains the familiar continuum of magnitude values defined by the older scale. Thus, the Richter scale is used for measuring small earthquakes (3.5 M or less), while the moment magnitude scale is used for measuring stronger earthquakes (3.5 M or higher). The USGS now uses the MMS to estimate magnitudes for all modern large earthquakes.

For purposes of this activity, the least complicated and probably most accurate approach is to just use the term "magnitude," without needing to say "on the Richter scale" or "on the moment magnitude scale." To abbreviate, use the symbol M (a capital M, plain text, no sub/superscripts) expressed to the nearest 0.1. If less precision is desired, precede M with a tilde, such as "M ~7" or "magnitude ~6.8."

Before the Activity

• Make copies of the Finding Epicenters and Measuring Magnitudes Worksheet , one per group. The worksheet serves as a student guide for the activity.
• Make arrangements so that each student group has a computer with Internet access.

With the Students

• Divide the class into student pairs, and have them assemble at their computers with journals/paper and writing instruments.
• Hand out the worksheets to the groups and direct them to read through the instructions. Encourage them to explore all of the Earthquakes Living Lab as they complete the worksheet.
• Before looking at the Earthquakes Living Lab, have pairs complete the Engage section of the worksheet: What is the Richter scale? What is an epicenter?

• Of the four Earthquakes Living Lab seismic areas, choose the "Chile" box, as shown in Figure 3.

Take a few minutes to read the information on the left side of this page for the 2010 earthquake off the coast of central Chile. Then locate and click the link in the center of the page under the question: "How is an earthquake epicenter located and how is magnitude determined"? 

This opens a new window to Michigan Tech’s UPSeis informational site about earthquakes and seismology. Read through the sections “What Is Seismology and What Are Seismic Waves?,” “Where Do Earthquakes Happen?,” and “Why Do Earthquakes Happen?” to answer the following questions:

What is an earthquake?

What is a seismic wave?

What is the difference between S and P waves?

• After reading about earthquakes, open a new window to an Earthquake Simulator at https://www.newpathonline.com/api_player/enus_54_6304/LXX/index.html . Select the location of Chile (the middle location).
• Follow the simulation instructions. As you read and complete the activity, take notes so you can complete the Explain questions next.
• Direct student pairs to independently complete the tutorial/simulation to find an epicenter location via the triangulation method and compute the earthquake magnitude:
• The simulation directs students to look at three simplified seismograms from seismic stations in Chile (Talca, Santiago, Osorno) and select the correct measurements of the S-P intervals.
• Doing this generates an S-P interval graph (time vs. distance, called the travel-time curve graph) from which they determine and select three epicentral distances.
• The simulation renders three circles on a map and directs students to find the epicenter. Success is figuring out that the epicenter is just off the coast of Chile, where the three circles intersect.
• To make a magnitude determination, two measurements are needed: the S-P interval (already determined earlier in the tutorial) and the maximum amplitude of the seismic waves. So next, students compute the magnitude of this same earthquake by looking at the three simplified seismograms again, but this time selecting each's maximum S wave amplitude (height). Tips: Make sure students are reading the S waves and not the P waves.
• Entering the three maximum amplitudes generates a nomogram, a graphical device that simplifies the process of estimating magnitude from distance (determined earlier in the tutorial from the S-P interval process) and amplitude. Looking at the nomogram, students click on each location data point to see where the three lines intersect to read the estimated magnitude. Success is figuring out that the estimated magnitude is 5.9.
• Next, have student groups answer the eight questions in the Explain section of the worksheet (also listed below). Allow students to return to Michigan Tech’s UPSeis website and read through the other informational sections. Allow students to return to Michigan Tech’s UPSeis website and read through the other informational sections.
• How is an earthquake located?
• What is an epicenter?
• How are S and P waves used to determine how far away epicenters are?
• How does distance from the epicenter affect the S-P time interval?
• Describe the process of triangulation to locate an epicenter.
• How is the magnitude of an earthquake determined?
• Describe what the "magnitude" of an earthquake is.
• What data is used to determine magnitude?
• Then have student pairs complete the Elaborate section of the worksheet.
• Why might the triangulation method not always produce an exact point (other than your measurement errors)?
• How does distance from the epicenter affect the magnitude (height) of the seismograph reading?
• Direct students to finish the activity by completing the Evaluate section. To answer the two questions, have students each write paragraphs to explain their opinions about the reliability of the science of seismology and ways that engineers use seismic data.
• Conclude the activity with a class discussion (and perhaps homework questions) to share ideas and answers, as described in the Assessment section.

epicenter: A point on the Earth's surface that is directly above the place where the underground forces of an earthquake originate.

moment magnitude scale: Similar to the Richter scale, but replacing its use starting in the 1970s for more accuracy in measuring big earthquakes (magnitudes > 3.5) from greater distances. (Source: USGS, Wikipedia)

P wave: The first seismic wave of an earthquake. Short for "primary wave" or "pressure wave." It travels faster than the same earthquake's S wave (almost double the speed) and is similar to sound waves.

Richter magnitude scale: An earthquake measurement scale created in the 1930s to assign a single number to quantify the energy released during earthquakes. In this 1-to-10 logarithmic scale, each succeeding level representing 10 times as much energy as the last. The magnitude is the logarithm of the amplitude of the ground wave. Considered best for measuring small earthquakes (3.5 M or less). (Source: USGS, Wikipedia)

S wave: The second seismic wave of an earthquake. Short for "secondary wave" or "shear wave." It is slower than the same earthquake's P wave and cannot travel through liquids.

seismograph: An instrument that measures motions of the ground, including those of seismic waves generated by earthquakes. Also called seismometer. The instrument detects and documents the intensity, direction and duration of ground vibrations, which are used to determine the epicenters and strength/magnitudes of earthquakes or other seismic events.

S-P interval: The time interval between the arrivals of P and S waves.

triangulation: A method to determine exactly where an earthquake originates. It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter. (Source: USGS)

Pre-Activity Assessment

Introduction: Before student pairs look at the Earthquakes Living Lab, direct them to complete the Engage section of the Finding Epicenters and Measuring Magnitudes Worksheet , which asks them to apply any prior knowledge and/or speculate as to what the Richter magnitude scale is and what an epicenter is. Review their answers to assess their base knowledge of the subject matter.

Activity Embedded Assessment

Triangulation and Magnitude: Student pairs complete the worksheet , which includes following a tutorial/simulation accessed through the Earthquakes Living Lab. Students first triangulate the location of an earthquake's epicenter, then calculate its magnitude. Have students turn in their answers for the Explain portion of the worksheet. Assess their understanding based on the thoroughness of their answers.

Post-Activity Assessment

Sharing Information/Thinking Ahead: In the Evaluate section of the worksheet, student pairs are asked to compose answers to two questions—whether they think seismology is a reliable science and ways seismic data is useful for engineers. In a concluding class discussion, have groups share their ideas about engineering and one new thing they learned about earthquakes. Continue the discussion with the following questions (or assign these questions as homework):

• Do limits exist on what science can predict? What are those limits?
• Must engineers be content with mitigating disasters, instead of preventing them?
• Do you think we will someday be able to prevent earthquakes?

Have student groups explore one or more of the other two regions (Southern California and Japan) provided in the Virtual Earthquake simulation.

• For lower grades, just introduce the concepts of triangulation and the magnitude scale; a thorough understanding of P and S waves is not vital.
• For upper grades, have students work individually, do two of the three seismic area tutorials, and look up historical earthquakes to learn their magnitudes, and make data tables or graphs with this information.

Show students some of the numerous online animations comparing the movements of P and S waves.

Students learn what causes earthquakes, how we measure and locate them, and their effects and consequences. Through the online Earthquakes Living Lab, student pairs explore various types of seismic waves and the differences between shear waves and compressional waves.

Students learn about the types of seismic waves produced by earthquakes and how they move through the Earth. Students learn how engineers build shake tables that simulate the ground motions of the Earth caused by seismic waves in order to test the seismic performance of buildings.

They make a model of a seismograph—a measuring device that records an earthquake on a seismogram. Students also investigate which structural designs are most likely to survive an earthquake.

Students learn about factors that engineers take into consideration when designing buildings for earthquake-prone regions. Using online resources and simulations available through the Earthquakes Living Lab, students explore the consequences of subsurface ground type and building height on seismic d...

Moment magnitude scale. Last updated November 26, 2013. Wikipedia, The Free Encyclopedia. Accessed December 11, 2013. http://en.wikipedia.org/wiki/Moment_magnitude_scale

Novak, Gary. Virtual Earthquake (tutorial/simulation) 1996. Geology Labs On-Line, Department of Geological Sciences, California State University, Los Angeles, CA. Accessed December 11, 2013. http://www.sciencecourseware.com/virtualearthquake/

USGS Earthquake Magnitude Policy (implemented on January 18, 2002). Last modified July 18, 2012. U.S. Geological Survey, U.S. Department of the Interior. Accessed December 11, 2013. http://earthquake.usgs.gov/aboutus/docs/020204mag_policy.php

Other Related Information

This activity is designed around the Earthquakes Living Lab, a resource and online interface that uses real-time U.S. Geological Survey seismic data from around the world. The living lab presents earthquake information through a focus on four active seismic areas and historic earthquakes in those areas. The real-world earthquake data is viewable via a graphical interface using a scaling map.

Contributors

Supporting program.

Last modified: May 8, 2020

Module 8: Earthquakes

Assignment: earthquakes.

In this assessment, you will read and interpret various seismograms to determine the location of an earthquake. You will also determine the magnitude of the earthquake. When you are finished you will be presented with a Certificate of Completion making you a Virtual Seismologist. Remember to access the tips and hits at the bottom of the page.

Basic Requirements (assignment criteria):

• Go to the Virtual Earthquake (Links to an external site.)   site from Geology Labs online.
• Choose a location to “experience” your earthquake.
• Read over the next page on how to determine the P-S wave time interval from a seismogram.
• On the next page you will view three different seismograms. Determine the time difference from the P wave to the arrival of the S wave on each seismogram and enter it into the box.
• Click on the convert button to go to the next page.
• Use the bottom graph to get the epicenter distance. For example, if you have stated that the S-P interval is 35 seconds, you will find 35 seconds on the vertical (y-axis) and follow it across until it intersects the diagonal line. Then draw your line straight down to the horizontal line (x-axis) to get the distance. In this case, 35 seconds translates to 340 Km. Another example, 51 seconds is 500 Km. You will enter these values in the box on the right.
• Click on find epicenter. Depending on well you did, you can either re-enter your data to try again or click View True Epicenter if you are close.
• When you get close, go click on the View True Epicenter, look at how close you came to the true epicenter. Take a screenshot of the two pictures and save them for later.
• Click on Compute Richter Magnitude. Read the explanation of magnitude, then go to the next page. Read about the nomogram and how to use it.
• Go to the next page and complete the questions on magnitude. Click submit and proceed to the nomogram of your data.
• Answer the magnitude estimate and then click on confirm magnitude. If you were successful you will have a screen congratulating you.
• Fill in the necessary information to get your certificate, make sure you DO NOT email a copy to me. Click get certificate and you will see a green certificate and chart.
• Take a screen shot and save this for later.
• Open up a word document and insert both the picture of your earthquake location (the two side-by-side pictures) and the picture of your completion certificate and chart. Adjust them to fit on one page.

Answer the following questions on the same page:

• Based on the location and magnitude of your earthquake, speculate on the type of damages your earthquake might have caused.
• Now compare this to the Mercalli Intensity scale. What classification is your earthquake based on this scale?
• How did the different waves (P, S & Surface Waves) assist you in determining the epicenter and the amount of damage caused?
• Was your location in an area prone to earthquakes? Speculate on what might have caused this earthquake (be specific).
• Why is it so difficult for geologists to predict when and where an earthquake will occur?
• What connections can you make between the behavior of the seismic waves and the Earth’s interior?
• Make sure your name is ON your document before submitting.

Tips and hints can be found here  (Links to an external site.) .

This assessment is adapted from “Virtual Earthquake” by Gary Novack, originally found here  (Links to an external site.) .

Contribute!

Improve this page Learn More

• Earthquakes Assessment. Authored by : Kimberly Schulte. Provided by : SBCTC. Located at : http://www.columbiabasin.edu . License : CC BY: Attribution

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Part of Hall of Planet Earth .

Seismologists study shock, or seismic, waves as they travel through the Earth’s interior. These waves originate from natural sources like earthquakes, and from artificial sources like man-made explosions. Knowing how the waves behave as they move through different materials enables us to learn about the layers that make up the Earth. Seismic waves tell us that the Earth’s interior consists of a series of concentric shells, with a thin outer crust, a mantle, a liquid outer core, and a solid inner core.

P waves, meaning primary waves, travel fastest and thus arrive first at seismic stations. The S, or secondary, waves arrive after the P waves.

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• Published: 16 January 2008

Using seismic waves to image Earth's internal structure

• Barbara Romanowicz 1  

Nature volume  451 ,  pages 266–268 ( 2008 ) Cite this article

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Seismic waves generated in Earth's interior provide images that help us to better understand the pattern of mantle convection that drives plate motions.

Forty years after the discovery of seafloor spreading and the acceptance of the theory of plate tectonics, important gaps remain in our understanding of the pattern of convection that drives the motions of the plates, leading to earthquakes, tsunamis and volcanic eruptions. There are still many heated debates. Does oceanic lithosphere pushed down into the interior at converging plate boundaries reach the bottom of Earth's mantle? Do deep-rooted, thin hot plumes rise through the mantle under mid-plate 'hot spot' volcanoes? What is the relative importance of compositional versus thermal heterogeneity in mantle convection? And what role does Earth's solid inner core have in the 'geodynamo', which keeps Earth's magnetic field alive, and in the thermal evolution of our planet (see page 261 )? To address these controversies, seismology has been brought to bear to image Earth's deep interior. From the construction of accurate models of Earth's one-dimensional radial structure ( Fig. 1 ) to the current models of its three-dimensional structure ( Fig. 2 ), progress in seismic imaging has gone hand in hand with improvements in the design of seismic sensors, the capacity to record digitally increasingly massive quantities of data, theoretical progress in handling seismic-wave propagation through complex three-dimensional media and the development of powerful computers for simulating seismic waves and for the inversion of large matrices.

The first-order structural units of Earth — its suite of concentric shells and their approximate composition — were established over the first half of the twentieth century from measurements of the travel times of seismic waves refracted and reflected inside Earth, whereas proof of the solidity of the inner core had to await the capability to record and digitize long time series and measure the frequencies of free oscillations. The '660 km' discontinuity is a phase change, and possibly a compositional change, in the silicate mantle. This illustration is of the preliminary reference Earth model 14 .

Each column of images represents a different model of mantle shear-velocity structure (using various data sources) shown at three representative depths (140 km, 925 km and 2,770 km). Left, SAW24B16, developed at the University of California at Berkeley 15 ; centre, S362D1, developed at Harvard University 16 ; and right, S20RTS, developed as a collaboration between the University of Oxford and California Institute of Technology 17 . In the top ∼ 250 km of the mantle, the structure follows the surface tectonics: slow ridges and back-arcs (red), fast roots under stable continents (blue) and a progressively faster velocity away from mid-ocean ridges, consistent with expectations from a simple cooling plate model. Below the thickest lithospheric roots (250 km), the pattern changes, and in the transition zone a clear signature of fast anomalies associated with subducted slabs emerges. Recent models show a variety of behaviours for these slabs: some seem to be stagnating in the upper mantle; for others, the fast-velocity anomaly seems to continue at oblique or steep angles into the lower mantle. Two regions, in northwestern America and Southeast Asia, show fast-velocity anomalies that may be related to past subduction down to considerable depths ( ∼ 1,200–1,400 km). In the mid-mantle, the spectrum of heterogeneity becomes white, which indicates that it is dominated by smaller-scale features. In the bottom 1,000 km of the mantle, as we approach the core–mantle boundary, a new pattern of long-wavelength heterogeneity progressively emerges, with two very large antipodal low-velocity regions centred in the Pacific Ocean and under Africa and surrounded by faster than average material. The units of the key are relative shear-velocity changes (as percentages) with respect to the global mean at the given depth.

From seismic tomography, first introduced in the late 1970s (refs 1 , 2 ), we now have a good understanding of the first-order characteristics of the long-wavelength ( ∼ 1,000–2,000 km) three-dimensional elastic structure of Earth's mantle 3 . At shorter wavelengths ( ∼ 200 km), fast-velocity 'slabs' representing oceanic lithosphere plunging back into the mantle are, today, the best-resolved 'objects', because of the favourable geometry; many earthquake sources illuminate such slabs from both below and above, at least down to ∼ 600 km depth ( Fig. 3a ). It is tempting to interpret the large-scale features imaged throughout the mantle in terms of lateral variations in temperature, which can be as much as several hundred degrees Celsius. For example, the fast ring of high velocities at the bottom of the mantle (shown in blue in Fig. 2 ) might well represent the 'graveyard' of cold subducted lithosphere, and the slow regions, commonly referred to as 'superplumes', the hot rising return flow (shown in red in Fig. 2 ). It is increasingly clear, however, that compositional variations also have an important role in mantle convection.

a, Worldwide distribution of earthquakes of magnitude ( M w ) greater than 5.0 from 1 January 1991 to 31 December 1996. Earthquakes occur mainly along plate boundaries, delineating, in particular, the global mid-ocean ridge system. Earthquakes are generally shallow (yellow). In subduction zones around the Pacific Ocean and in the collision zones in southern Eurasia, intermediate-depth (orange) and deep (red) earthquakes indicate the presence of cold lithospheric slabs plunging into Earth's mantle. b, The current global broadband digital seismic network (shown as at October 2007) has been constructed through an international effort coordinated by the Federation of Digital Seismic Networks (FDSN), complemented by denser permanent regional arrays (not shown) and temporary regional deployments. GSN, Global Seismic Network (the US component of the international network). (Panel b courtesy of R. Butler, IRIS, Washington DC.)

With the deployment, starting in the early 1980s, of high-quality digital broadband seismic stations around the world ( Fig. 3b ), finer-scale imaging became possible. Particularly striking is the accumulating evidence for complexity in the lower 300–400 km of the mantle, the so-called D″ region, an important chemical and thermal boundary layer. Many intriguing seismic observations have been made in this region 4 , 5 , including the remarkable observation that the lateral transition from fast shear velocity regions in D″ into the superplumes occurs abruptly, over a much smaller range than would be possible if lateral variations in temperature were the only cause 6 . Perhaps less surprisingly, closer to Earth's surface such strong lateral contrasts are also found at lithospheric depths, especially at the edges of tectonic provinces of different origin and age.

Characterizing the sharpness or fuzziness of the boundaries of the heterogeneous structures deep inside the planet, and detecting and mapping small-scale heterogeneity, are the next steps. This will mean extracting more information from seismograms than has traditionally been done. Indeed, neither remnants of compositionally distinct lithosphere in the lower mantle nor narrow plume conduits (if they exist) can be accurately mapped by standard tomographic approaches that make use only of information carried by the most direct waves — those that travel along the shortest paths — according to the simple rules of ray theory. It will be necessary to take account of the energy bouncing off weak scatterers that can have a wide range of sizes. In practice, this means working in a wide frequency band, at short spatial wavelengths, using both the amplitude and the travel times of all possible seismic phases — that is, the entire seismogram — and applying signal-enhancing techniques.

A significant challenge is the limited distribution of seismic-wave sources and receivers. Ideally, one would want to sample the volume of Earth uniformly. But unlike other disciplines that use imaging, such as medical tomography or petroleum exploration, earthquake seismologists cannot optimize their experimental geometry ( Fig. 3 ). To overcome these limitations, several promising approaches are being pursued.

New and exciting horizons have recently opened up with increasing capabilities in both computation and data collection. There are now powerful numerical schemes to compute synthetic seismograms in structures of arbitrary complexity, such as the spectral element method 7 , which are well adapted to the spherical global geometry of Earth. They can be used in a variety of ways, for forward modelling of observed seismic waveforms, as well as for inversion of the seismogram to retrieve the three-dimensional structure. They are still heavy on computation but hold much promise for the construction of the next generation of global tomographic models. Anisotropy and dissipation, which also influence seismic-wave propagation, can now be better characterized and provide additional information on flow directions, temperature variations and the presence of partial melting. At the higher end of the seismic spectrum, the deployments of dense permanent regional arrays, such as Hi-net in Japan, or temporary ones such as those of PASSCAL ( http://www.iris.edu/about/PASSCAL ), are stimulating the development of techniques that are beginning to erode the difference between global seismology and exploration geophysics.

Through the utilization of energy scattered both backward and forward, impressively detailed images of slabs are starting to be constructed. For the first time, it is possible to use the results of seismic imaging to trace the fate of water as it is entrained down into the mantle with the subducting slab 8 . The global seismic network, complemented by PASSCAL-type deployments 9 , 10 , and local dense arrays provide sufficient spatial sampling in some continental areas to investigate fine-scale layering in the deep mantle using newly developed sophisticated back-projection techniques. Much is expected from the data set now being assembled through the USArray programme of Earthscope ( http://www.iris.edu/USArray ). Seismologists can start to put precise constraints on velocity contrasts and the sizes and depths of heterogeneous bodies. These can be combined with experimental and theoretical data about mineral physics to determine lateral variations in composition and temperature. For example, in the case of the recently discovered post-perovskite transition, which is thought to occur in the temperature/pressure range of the D″ region (see page 269 ), mineral physicists and geodynamicists are working hand in hand with seismologists to search for its presence in the deep mantle and evaluate its consequences for mantle dynamics 4 , 5 .

The approaches mentioned above assume that appropriately distributed earthquake sources are available. Where this is not possible, a rapidly developing technique to eliminate the constraints associated with natural earthquakes is building on the data set of continuous broadband waveforms accumulated by many stations in the world. Background seismic noise continuously excited by the oceans and the atmosphere can be used to construct tomographic images through noise cross-correlation. The promise of this approach has been demonstrated in the investigation of the crust 11 , for which the presence of strong energy in the microseismic frequency band ( ∼ 1–15 s) can be exploited. A possible extension of the technique to longer-period seismic waves presents interesting prospects for imaging the upper mantle at high resolution down to at least the base of the lithosphere.

This still leaves the oceans, where recording is limited to sparsely distributed islands. Yet there are key geodynamic problems to be addressed: for instance, the deep structure and anisotropy of ocean basins are not well understood. Most volcanic hot spots are in the oceans. The recent controversy about the 'banana-doughnut kernel' technique 12 indicates the level of frustration: improvements in wave-propagation theory and inclusion of scattering effects cannot make up for the fact that stations on hot-spot islands are isolated, so that it is not possible to accurately constrain the depth and lateral extent of underlying slow anomalies. Many areas in the deep mantle and the core are currently not accessible because of a lack of stations in the oceans. Although efforts to instrument the ocean floor have been ongoing for more than 20 years, long-term ocean-floor broadband stations are still few. Local temporary deployments, such as those beneath mid-ocean ridges, have led to spectacular results 13 , and other ongoing projects, such as the Plume project in Hawaii, will help to address specific targets. A cabled observatory is planned in the northwest Pacific, combining Canadian and US efforts ( http://www.orionprogram.org/OOI/default.html ). But an internationally coordinated programme is needed to systematically deploy large-aperture (1,000 km × 1,000 km) broadband ocean-floor arrays that would be left in place for at least one or two years, to record a sufficient number and variety of earthquakes and progressively fill the gap in illuminating deep structure under the oceans.

Finally, as the images provided by seismologists become sharper, there is an increasing opportunity to work closely with other geoscientists — geochemists, geodynamicists and mineral physicists — to make the best of complementary constraints for the challenging 'inverse problem' that the interior of our planet represents — that is, to use observations at or near the surface of Earth to constrain ideas about its deep structure and dynamics. Better communication and cross-education among these disciplines is key to progress. This is why interdisciplinary programmes such as the Cooperative Institute for Deep Earth Research ( http://www.deep-earth.org ) are needed.

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Barbara Romanowicz is at the Berkeley Seismological Laboratory and the Department of Earth and Planetary Science, University of California at Berkeley, 215 McCone Hall, Berkeley, California 94720, USA.,

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Romanowicz, B. Using seismic waves to image Earth's internal structure. Nature 451 , 266–268 (2008). https://doi.org/10.1038/nature06583

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Published : 16 January 2008

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DOI : https://doi.org/10.1038/nature06583

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