Earthquakes Are Vibrations of the Earth's Crust

Name: Date: Per: Earthquake Recording Station Gizmo Background Information Earthquakes are vibrations of the earth's crust. Earthquakes occur when the rocks on either side of a fault break, or fracture. As they slip into new positions, the rocks release energy in the form of vibrations called seismic waves. The point on the earth's surface directly above the area where slippage first occurs is the earthquake's point of origin, or epicenter. Seismic waves can be detected and recorded by using an instrument called a seismograph.

During an earthquake, primary waves, or P waves, move the fastest and thus are the first waves to be recorded. Secondary waves, or S waves, are the second waves to be recorded.

Purpose In this activity, you will simulate an earthquake. During the earthquake, you will record time and distance data that will enable you to determine its location.

Procedure 1. In the Gizmo, look at the map on the right-hand side of your screen and at the legend on the lower left part of the screen. The “A “ in the triangle represents the seismograph station. The circle with the cross through it represents the epicenter of the earthquake. 2. Click on “Show grid” below the map. This will show gridlines on the map where each square represents 100 km. 3. Move the epicenter of the earthquake to the center of the grid. (x=500km, y=500km) 4. Move the seismograph station so that it is 100 km south of the epicenter. (x=500km, y=400km) 5. Look at the upper left-hand corner of the Gizmo. The recording station detector graph represents a seismograph. Time is displayed on the horizontal x-axis. The strength of the earthquake is displayed on the vertical y-axis. 6. Click “Play” and watch the earthquake animation. After the earthquake is done, select “Show time probe.” Click and drag the left-hand (green) time probe to align with the moment when the P wave first arrived at the recording station. Click and drag the right-hand (blue) time probe to align with the moment when the S wave first arrived at the station. 7. Record the amount of time between what two events in the data table. 8. Move the seismograph station to 200 km south of the epicenter. (x=500km, y=300km). Repeat steps 6 and 7. 9. Move the seismograph station to 300 km south of the epicenter. (x=500km, y=200km). Repeat steps 6 and 7. 10. Move the seismograph station to 400 km south of the epicenter. (x=500km, y=100km). Repeat steps 6 and 7. 11. Move the seismograph station to 500 km south of the epicenter. (x=500km, y=0km). Repeat steps 6 and 7. 12. Graph your data on a sheet of graph paper. Time difference between the P waves and the S waves will be on the x-axis. Distance will be on the y-axis. Data Table Time Difference (sec) Distance from Epicenter (km)

Questions 1. What do the first set of waves represent? What do the second set of waves represent?

2. What happens to the time difference between the two sets of waves when the seismograph station is closer to the epicenter of the earthquake?

3. What happens if the seismograph station were further from the epicenter?

4. Is the line connecting the points on your graph straight or curved?

5. What would the time difference be if the seismograph station were 1000km away from the epicenter? (Hint: Use the ratio of distance to the time difference for 500 km)