LESSON FIVE

How do we measure global earthquakes? (two class periods)

For the Teacher:
Distribute the journals. Richter nomograms are fine for local earthquakes, but what about earthquakes from places far away? Break students into groups of three. Distribute South American seismograms from Bolivia, Brazil, and Argentina. Have students measure earthquake travel time on the seismograms, and the amplitude of each. If students have difficulty locating the start of the P and S waves, tell them to look for differences in patterns, and for patterns of peaks and valleys. To calculate an earthquake's magnitude, I have used a modified version of Richter's nomogram formula with some success. The formula is:



The formula is not as difficult as it looks. Walk them through it once or twice. When you get the magnitude of the three seismograms, average them to get the final magnitude.
Show students how to obtain an outline map of South America from the Outline Map site (http://www.eduplace.com/ss/ssmaps/). Have it ready to distribute. They will use this map to triangulate for the earthquake's epicenter. Distribute P and S wave travel-time graphs. Students will use the seismogram time measurement to locate the epicenter's distance in kilometers from the recording station using the chart. Convert kilometers to centimeters using the scale provided on the outline map. If the map gives miles instead of kilometers, students will have to first convert kilometers to miles using the proportion: 55 miles:88 kilometers = x miles:y kilometers (from the chart), then convert miles to centimeters using the scale on the outline map. Locate the station on the outline map by using the station map of the USGS Albuquerque Seismological Laboratory (http://aslwww.cr.usgs.gov/Stations/station_info/mainmap.htm). Adjust a compass to the desired centimeters and draw a circle with the station as its center. Do the same procedure for the other two seismograms drawing their circles on the same map. The three circles should overlap at the epicenter's location. If they don't, have students recheck their calculations. If intersection points are close, then average the distances of the separate intersections. The South American earthquake happened on April 30, 1999, at 19:23:46 GMT, 31.65S and 71.45W near the coast of central Chile measuring 5.0 on the Richter scale. You can obtain seismic information for the last thirty days from Current seismic event list
(http://www.iris.washington.edu/cgi-bin/
seismon_events.pl?date=99/04/23&lat=15.27N&lon=91.74W), and going back to 1977 from IRIS: Welcome to WILBER (http://www.iris.edu/cgi-bin/wilberII_page2.pl). Distribute a set of Asian seismograms and have students or groups work on finding the epicenter. Also distribute the appropriate outline map for triangulation. Students will obtain this for themselves in the future.

How do we measure global earthquakes?

For the Students:
Your teacher will give you three seismograms of a recent South American earthquake recorded by USGS stations. Write the names of the USGS stations in your journal.
Measure the S-P distance and use a P and S wave travel-time graph to find the epicenter distance for each seismogram. Record the distance next to the station in your journal. You will need this to locate the epicenter. Obtain an outline map of South America from your teacher.
Locate the USGS stations on the USGS station map provided by your teacher. Mark these locations on your outline map. Label each location with the USGS station name.
Locate the scale that is on the outline map. If it has kilometers, calculate the number of kilometers represented by 2 centimeters. Set up a proportion to find the number of centimeters represented by the epicenter distance (in kilometers) for each of the three USGS stations. If the scale is in miles, convert your epicenter distances to miles using the proportion 55 miles:88 kilometers = x miles:y kilometers (y is the epicenter distance for each station), then use another proportion to find the number of centimeters represented by the epicenter distance (in miles) of each USGS station. Record this information in your journal.
Take a compass and fix the radius the number of centimeters calculated for the first station.
Go to the outline map and, using the station as the center, draw a circle. Do the same for the other two stations using their centimeter values as the radius.
Where the three circles intersect is the epicenter of the earthquake. If the circles do not intersect but are close, take the average difference and approximate the center. If the circles are far apart, remeasure the seismograms and recalculate the centimeters. With practice, you will get better.
Your teacher will give you a formula to calculate the earthquake's magnitude. You will use it for each of the three seismograms. Here's the order of operations:

• Find the natural logarithm of the amplitude measured on the seismogram.
• Divide the S-P distance by 30 and get rid of the decimal (this is what is meant by integer).
• Subtract the above quotient from 8.
• Multiply the S-P distance by the above difference.
• Find the natural logarithm of the above product.
• Multiply the above logarithm by 3.
• Add the above logarithm and the logarithm of the amplitude.
• Subtract 2.92 from the above sum.

Ongoing Activity (one class period, then ongoing)

For the Teachers:
Break students into groups and have them access the Live Internet Seismic Server site (http://aslwww.cr.usgs.gov/Seismic_Data/heli.htm). Show them the stations the Albuquerque Seismological Laboratory has in its real-time seismogram network. Every day, one group will access the site and monitor the network for earthquake activity. Not all stations need to be contacted. For South America, contact Brazil or Bolivia or Argentina only. Activity at one station will surely be picked up at other South American sites that can then be contacted. The same will be true for China, Russia, Africa, and the other continents. If activity is found, seismograms from three separate stations must be printed. An outline map of the area must also be printed. The group then measures the seismograms, calculates the magnitude, and then triangulates on the map for the epicenter. When finished, the group reports back to the class. All research should be entered in students' journal folders.

For the Students:
Groups will take turns monitoring USGS stations worldwide for major earthquakes. Go to the Live Internet Seismic Server (http://aslwww.cr.usgs.gov/Seismic_Data/heli.htm) and click on stations to obtain their seismograms. Note that one country per continent is sufficient to observe seismic activity. Click on the small versions because they take less time to load. If you find an earthquake, then click on the twenty-four-hour version and print it. Use the map of the USGS stations provided by your teacher to find two other stations near the earthquake. Print their seismograms too. Don't forget to record the information in your journal. Go to the Outline Map Web site (http://www.eduplace.com/ss/maps/index.html) and print a copy of the geographic area of the earthquake. Locate the epicenter of the earthquake and calculate its magnitude as you did in Lesson Five. Make a report of your finding to the class.

Curriculum Extensions
Have a representative from your town's city hall speak to your class about the city's plans for an earthquake. Have a representative from the building industry or an architect speak to your class on how buildings can be built to withstand earthquakes. Create a Web page listing earthquake data and perhaps other data. Build your own seismometer (http://cse.ssl.berkeley.edu/lessons/indiv/davis/hs/Seismograph.html).