Overview
Students become seismologists trying to triangulate the location of an earthquake by using data (seismograms) retrieved from at least three different seismic stations. The students get to make use of their knowledge about P and S waves. The options and help menus provide a great deal of valuable information about the program Epicenter and seismology.
We went through the introductory pages as a group and demonstrated how the activity worked with one station on the first exercise. It is important to make sure that the students understand the to read a seismogram. They need to be able to properly identify the arrival of the P and S waves. Another place where there can be some confusion is the time travel curves. You may have to explain about background noise that is shown in a seismogram when there is no earthquake. Another interesting phenomenon is the mercator projection and great circles.
We discussed the answers on the activity sheets as a group. The final part of this activity brought up an interesting conversation about timing and location of eathquakes, and the number of fatalities. Each time we implemented this activity there had recently been a large earthquake in a third world country that had cause a large number of fatalities. One time there had been a large earthquake in Japan that had caused no fatalities.

Goals:
To understand why earthquakes occur where they do and how the location of an earthquake is determined. Students make discoveries about plate boundaries.

Central Question:
How do scientists determine the exact location where an earthquake has occurred?

Materials for each team:
Access to a Macintosh computer
Epicenter program
Other resources:
Activity sheets

Setting the stage:
Quickly review P and S waves. Students will be interested about how great circles are portrayed on Mercator map projections. Also the P and S wave shadow zones makes a great place for an following discussion on how we know the Earth's outer core is liquid. You may want to go through the instructions as a class before splitting into groups of three or four students per computer.

Exploration/Investigation
Once the students have read throught the introductory information, there are seven "suggestions" to complete. It is prudent to make certain, early on, that the students understand the proper placement of the measure for the travel time curve.

Bringing it together:
Class discussion
You can have the students discuss their experiences with Epicenter and what they learned using the program.

Assessment:
Have each group of students create an earthquake and document the time, location, magnitude, etc... Then, have the groups switch computers and see how closely they can come in determining the parameters set by the first group. Study Questions:
Choose three locations on a map which reflect the three different types of active margins.
1) What tectonic setting may have caused this earthquake? Choose Show Map Legend from the Options menu to help you interpret the map symbols.
2)This program does not give you the means to determine the depth of the focus of the earthquake. In this case, however, would you expect this earthquake to have a deep or shallow focus?

Background:
The Determination of Earthquake Epicenters
When an earthquake occurs, rocks break underground and release strong vibration waves that spread out in all directions. Near the earthquake, these waves can cause destruction and loss of life. Even at very great distances, sensitive seismographs can detect and record the spreading waves.
The point underground at which the earthquake releases its energy is called the focus of the earthquake. The point on the surface of the Earth directly above the focus is called the epicenter.
In this program, you will interpret seismographs from seismic observatories around the world in order to find the location and time of a number of earthquakes. In addition, you will be able to set off your own earthquake and see how it records on seismographs.
The key to interpretation of a seismographs is the ability to distinguish the different incoming waves. Here, we shall deal only with P (primary) and S (secondary) waves. Usually there are many others, but in our synthetic seismographs only P and S waves are recorded
In the following seismographs, time moves from left to right along the top trace. The arrivals of the P and S waves are marked P or S.
Note that the P wave is always the first to arrive, and the S wave follows at a later time. Each wave starts out strong and then declines in amplitude until it dies out.
We measure the time elapsed between the arrivals of the P and S waves and match this against a graph showing the travel times for the two waves.
Once we have the distance to the earthquake, the travel time curve for the P wave may be used to determine how long the wave was traveling before we recorded it. This allows us to determine the time of the earthquake.
The program will guide you through these several steps until you have found the distance to the earthquake from a particular seismic station and its time. On a world map, a circle is drawn around the station with the distance as a radius. The earthquake must lie on this circle. By repeating the process for at least two more stations, it is possible to find the epicenter at the point where all three circles intersect.
On a globe, these circles would appear truly circular. On a flat computer screen, however, we must use a map projection (Mercator, in this case), and this distorts the circles into complex shapes. In all cases, however, the circles are plotted correctly.
For more information, choose Getting Started or Concepts of Epicenter Location from the Help menu. Watch for helpful messages at the bottom of the screen.
For each earthquake, it is up to you to determine the location and the time of its occurrence. Use three or more different seismic stations to determine the epicenter, then choose Show EQ Times under the Options menu to find the time.
Changing Things:
The Options menu contains two items that can help you to determine the time and epicenter of the earthquake. Choose Show Times after you have interpreted seismographs from one or more stations. You will see a summary of the times determined from each station, along with an average of the times. You may choose this item whenever you wish.
The second item in the Options menu is Show Coordinates. Choose this to display the latitude and longitude of the cursor as you move it around the map. This may be used to record the actual location of your epicenter. Once you choose Show Coordinates, the menu item changes to Hide Coordinates, which you may choose whenever you no longer wish to see the coordinate display.
The fourth item under the Options menu is Set EQ, which allows you to set the time, size, and epicenter of an earthquake. To enter data in a EQ entry Box, double click on the Box with the mouse, or press the Tab key repeatedly until the Box you wish to work with is highlighted. Once the Box is highlighted, anything you enter from the keyboard will replace the current contents of the Box.
You may use either of two methods to set the location of the earthquake. If you check the small Box labeled Locate by clicking on map, once you click the OK button, you will be instructed to click the mouse on the point where you wish the earthquake to occur. It does give you a bit of a feeling of power.
Or you may enter the latitude and longitude of the epicenter in the data entry spaces provided. Don't forget to click on the N or S, and the E or W buttons to place the epicenter in the correct hemispheres.

Activity Sheets:
Student Activity sheet can be downloaded and printed from HERE..