How can we use seismic waves to determine the epicenter of an earthquake?

09 The Geosphere: 09.05 Geologic Events

Geologic Events

Geologic Events Lab Report


Your task will be to locate the epicenter for the earthquake using the arrival time of P-waves and S-waves from several seismic stations. Upon completion of the virtual lab, you will write a laboratory report of your results.


How can we use seismic waves to determine the epicenter of an earthquake?


Review the Introduction tab of the pre-lab within the lesson. Hypothesize how seismographs will be used to pinpoint the epicenter of the Earthquake.


Use the reference image in the Reading Seismographstab of the pre-lab to identify the S-P Time interval of a seismograph reading. Use the Determining Distance to an Epicentertab and the Locating an Epicentertab of the pre-lab as practice before completing the procedures below.


Examine the reference image under Materials to learn to identify the S-P time interval of a seismograph reading.
UnderData and Observations, determine the S-P time intervals of all three stations using the time scale under the seismograph readings. For example, if the P wave arrives at 2:02 pm and the S wave arrives at 2:27 pm, the total S-P time interval is 25 minutes.
Record the time interval for each station in Table 1under Data and Observations.
To determine the distance from each seismograph to the epicenter, you multiply the S-P time interval by 10. For example, if the time interval is 25 minutes, 25 x 10 = 250 kilometers.
Record the calculated distance to each station from the epicenter in Table 1under Data and Observations.
Next, use the distance scale under Data and Observations to measure the three circles provided, each with a radius that equals the distance of each station. To measure, place the center of each circle at zero. An example measurement is shown below for a 75 km radius.

Place each of the measured circles around the station on the map that they represent. All three circles should have one intersecting point that matches the epicenter.
Complete the Questions and Conclusion section of the lab report.

For this investigation, list the independent, dependent, and controlled variables.

Data and Observations:

S-P Time Intervals

Table 1: Seismograph Data

Based on your results, were you able to locate the epicenter of this earthquake?


S-P Interval Time (min)

Distance (km)

Distance Scale:
Place each station on the map below to find the epicenter.

Station 1

Station 2

Station 3

Questions and Conclusion

Why are earthquakes monitored worldwide instead of in earthquake-prone areas only?
How many seismographs are needed to find the epicenter of an earthquake? Why is this number significant?
What is the relationship between S- and P-waves?
How did the study of earthquake waves lead to a greater understanding of the interior structure of Earth?
Not all earthquakes are felt by people; however, such earthquakes are often detected on seismographs. Why?
How did your hypotheses about the use of seismographs compare to your experimental results? Were you able to find the epicenter of the earthquake using this method?
If you were unsuccessful in locating the epicenter, describe how you could modify seismograph resources to find the epicenter successfully.

Tectonics and Geologic Events When the ground beneath our feet moves, it is sometimes with great violence. Geologic events, such as earthquakes and volcanic eruptions, can be deadly. Several infamous geologic events are shown in the activity below: Sh

You’re with a team of geologists studying the latest earthquake in China. While digging through the rubble, you find an old, tattered journal. Use the activity below to investigate the contents of the journal:

Map shows the author lives about 80 kilometers northeast of Xi’an, in the Shaanxi region of central China.
Show Text Version

The Chinese journal indicates that people have long been aware of ground-shaking disturbances in Earth’s crust. Advances in earthquake-detection technology have led to a better understanding of these disturbances. Scientists no longer use a dragon-and-frog pot to detect earthquakes. But to avoid extensive damage and death, scientists want to predict earthquakes before they happen.

Does this lead to the question: Are earthquakes and other geologic events, such as volcanic eruptions, predictable?

In this lesson, you will learn how earthquakes happen and why volcanoes erupt. You will see the dangers associated with each, as well as examples of several famous earthquakes and volcanic eruptions. Finally, you will learn how people make predictions about geologic events.


What can earthquake and volcanic activity tell us about Earth?
At the end of this lesson, you will be able to:
define earthquakes and identify causes
identify how earthquakes are predicted and measured and how they are located
discuss how waves affect a variety of mediums
identify the effects of volcanoes
discuss how we can use models to predict earthquakes or volcanic eruptions
Tectonics and Geologic Events
When the ground beneath our feet moves, it is sometimes with great violence. Geologic events, such as earthquakes and volcanic eruptions, can be deadly. Several infamous geologic events are shown in the activity below:

Show Text Version

Earth’s lithospheric plates are in constant motion. This movement makes earthquakes and volcanic eruptions possible. These naturally occurring events are potentially dangerous. The destruction that results from earthquakes and volcanic eruptions can cost billions of dollars, in addition to being life-threatening.

The popular conception of a volcano is a high mountain with a hole at the top where the volcano erupts. In fact, a volcano is any location where molten rock and other materials make their way to Earth’s crust and onto the surface. Usually, but not always, volcanoes build mountains, as the once-molten rock hardens on Earth’s surface.

Volcanoes allow scientists to study processes inside Earth’s interior. Volcanoes are found worldwide, often where two tectonic plates meet. Many of the world’s volcanoes are along the Ring of Fire surrounding the Pacific Ocean. The map below shows the locations of Earth’s tectonic plates and the location of volcanoes.

Volcanic activity. See text version.
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The movement of tectonic plates creates volcanism. Use the following activity to investigate how tectonic plates create volcanoes:


Volcanoes differ in the types of igneous rocks and the frequency and strength of eruptions. Active volcanoes erupt often. During an eruption, molten rock from Earth’s interior reaches the surface and solidifies to form igneous rocks. Eruptions can be deadly for people who are caught in them, but most volcanoes are not active. Most volcanoes are dormant or extinct. A dormant volcano is not presently erupting or is unlikely to do so in the near future. An extinct volcano is one in which all activity has ceased so that the volcano will not erupt again.

Volcanoes are an essential part of the geosphere. The rock cycle would not be possible if volcanic eruptions did not form igneous rocks from magma. Magma is molten rock that forms under Earth’s surface. Lava is molten rock that reaches the surface. Look at the two photographs below to learn about different kinds of volcanic eruptions. Select the image button to move to the next photograph.

The various types and locations of eruptions lead to different classifications for volcanoes. Use the activity below to investigate how volcanoes are classified:

Show Text Version

Through careful monitoring, scientists are learning more about how to predict volcanic eruptions. One indication an eruption may be about to take place is an increased frequency of earthquakes. When conditions are right, magma moves into a central location in a volcano known as the magma chamber. As the material expands and squeezes into the space, it can trigger earthquakes. In this way, earthquake data can help scientists predict volcanic eruptions.

Earthquakes occur anywhere rock structures are under stress from movement. The upper portion of Earth’s crust is brittle. Tectonic plate motion can cause the crust to break, resulting in an earthquake.

Use the following activity to understand how tectonic movements cause earthquakes, and how earthquakes cause damage:

Visit this site to see the earthquakes that happened in the previous seven days:

Earthquakes occur most often at transform plate boundaries. The best-known transform plate boundary runs through California. Here, the eastern side of the Pacific Plate meets the west side of the North America Plate. The two plates slide past each other, building stress within the rock structures. The stress causes the rocks to bend. The rocks deform as more stress builds. Deformation is the process in which any material changes shape.

The deformation transfers energy from the movement of tectonic plates to rock structures. Therefore, deformed rocks along fault lines contain a high amount of potential energy. At some point, the rocks can no longer bear the stress and suddenly give way. All the stored energy is released, and an earthquake occurs.

© 2011 Brand X Pictures/Thinkstock
Scientists model the buildup and release of energy in rocks as elastic rebound. In this process, Earth’s rocks are equated to a rubber band. If you pull back a rubber band, it gains potential energy. If you pull far enough, the rubber band snaps, with a sudden release of that energy. In other words, the excessive deformation of the rubber band results in elastic rebound, which we experience as the release of potential energy suddenly transformed to kinetic energy. A similar process takes place during an earthquake when the potential energy of stressed rock is transformed to the kinetic energy of moving rock.

The stressed point in the Earth’s crust where the rock ruptures is the focus. The point on Earth’s surface directly above the focus is the epicenter. The sudden release of energy during the earthquake causes seismic waves to move outward in all directions. There are two types of seismic waves: body waves and surface waves. In the next section, you will examine body waves and surface waves in more detail.

Earthquakes cause intense shaking of the ground. The shaking is caused by waves moving through the ground. Scientists study the seismic waves using a seismograph (also called a seismometer). These machines record ground movement by making lines on paper. The recorded lines on paper are called a seismogram. Both a seismograph and a seismogram are shown in the images below:

Seismograph and Seismogram. See text version.
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There are two main types of waves, as follows:

Stress in rock structures can be measured. Earthquakes are usually sudden, but some rock structures can give clues. For example, there is obvious stress on rocks in the following image. At some point, the rocks will be unable to withstand the stress and will suddenly rupture, causing an earthquake.

Hayward fault. See text version.
The Hayward Fault in California puts stress on rock structures that can be seen at the surface. This curb is slowly shifting, proving there is stress building deeper within Earth.
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To ensure help is quickly sent to the right place, it is important to locate the earthquake’s epicenter and focus. Scientists use triangulation to find the epicenter of an earthquake. Seismograph stations worldwide continually monitor and record ground motion. An earthquake’s waves travel through Earth at predictable speeds. Therefore, a station closer to the epicenter of an earthquake will record the arrival of P-waves first.

Potential dangers from earthquakes and volcanoes are not limited to ground shaking or eruptions of lava. Many hazards are associated with each kind of event.

Landslides are often a problem with both earthquakes and volcanoes. The largest landslide in recent history occurred when Mount St. Helen’s erupted in 1980. The entire side of the mountain was blown away, and then the hot ash cloud melted snow, causing a massive mudslide down the side of the mountain.

A tsunami is a high-speed, long-wavelength wave that happens when large volumes of ocean water are displaced by earthquakes, volcanoes, or underwater landslides. Underwater earthquakes move the seafloor, driving the water around it upwards and outwards. An underwater volcano or landslide can displace large amounts of water, creating a tsunami that travels a great distance from the original event.

© 2011 Jupiterimages/
The main impacts of earthquakes and volcanic eruptions on humans are destructions of property and loss of life. Volcanic ash or pyroclastic flows can bury entire towns or villages. But earthquakes are more dangerous in terms of loss of life. Earthquakes cause the collapse of buildings, pollution to water supplies, gas leaks, fires, damage to roads, and power outages.

Earlier, you learned about the Shaanxi earthquake of 1556. This earthquake holds the record for the greatest number of deaths in a single earthquake event. Today’s increased safety in building codes and better communication lead to fewer deaths. However, thousands can still lose their lives. In 1976, an earthquake in China caused more than 250,000 deaths. A tsunami in Sumatra in 2004 caused by an undersea earthquake resulted in nearly 230,000 deaths. An earthquake in Haiti in 2010 caused over 220,000 deaths. [Source:]

Earthquake intensity is measured on the Richter scale. This scale gives a rating of 1 to 10 for the intensity of an earthquake. The explosiveness of volcanoes is rated using a volcano intensity scale known as the Volcanic Explosivity Index. This scale has an intensity rating from 1 to 8.

Honors Lesson: Geologic Events
How can heat energy within Earth be used?
At the end of this lesson, you will be able to:
describe how processes within Earth create surface features such as geysers and hot water springs
describe how geothermal energy can be tapped for use
determine the reliability and cost-effectiveness of using geothermal energy
examine the environmental impacts of using geothermal energy
IntroductionEarth’s Internal HeatFinding Heat 1Finding Heat 2Using Geothermal Energy

Your friend has just returned from a trip to Yellowstone National Park. While at the park, she took a series of photos. The photos are of fountains of erupting hot water. View her slide show of photos below.

Anemone Geyser
Show Text Version
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What do you think causes these eruptions of hot water?

You will find the answer to this question in the lesson. You will then learn how heat energy within Earth may be used as a source of power. You will also learn the benefits and drawbacks of using this type of heat energy.

Geothermal energy exists as heat within Earth. Geo- means “Earth,” and thermo- means “temperature” or “heat.” Earth has a crust, mantle, and core. The molten mantle and outer core are sources of heat. You can see this heat in surface features such as geysers.

A geyser forms when underground water is heated by geothermal energy. The water heats to very high temperatures under high pressure. The superheated water rises toward the surface. The pressure shoots streams of hot water into the sky. These are the geyser and hot spring images you viewed in the introduction to the lesson.

Use the following activity to understand how geysers form:

Show Text Version

Scientists already know that hydrothermal features on Earth have energy. Hydrothermal energy can be used to generate electricity. In fact, hydrothermal energy can power an entire electrical gener

Most geothermal energy is unusable. It exists too deep within Earth. Typically, the boundaries of tectonic plates are good locations for finding usable geothermal energy. For instance, erupting volcanoes bring Earth’s heat to the surface.

Geysers are another source of heat energy that can be tapped at the surface. More than 300 geysers are found in Yellowstone National Park. Other hydrothermal features are also found worldwide, as shown in the image below:

These locations indicate where geothermal energy is near the surface. Scientists believe an active hotspot volcano system exists under the surface in Yellowstone National Park. They have traced the type of activity at Yellowstone to past locations as far away as Nevada and Oregon.

Use the following activity to investigate these thermal features.

Another location where geothermal heat is easily accessible is the divergent plate boundary in Iceland. The Mid-Atlantic Ridge runs through Iceland. The country has many locations from which to access geothermal heat. Nearly all the buildings in Iceland’s capital, Reykjavik, use geothermal energy as a power source. An image of Iceland showing the divergent boundary and Reykjavik’s location is shown on the map below:

Midatlantic ridge. See text version.

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There are several ways we can tap into geothermal heat to generate power. A geothermal power station requires access to the heat. As you have learned, this can be deep inside Earth. In most situations, drilling is necessary to access the heat. A diagram of a geothermal energy plant is shown in the image below:

Geothermal energy plant. See text version.
Public Domain

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How can we use seismic waves to determine the epicenter of an earthquake


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