SIMULATED SEISMOGRAMS

Transcription

1 SIMULATED SEISMOGRAMS Great Falls, Montana I I I I I I I I I I I I I I I I I I I I I I I I I 8:03 8:04 8:05 8:06 8:07 P-wave arrival S-wave arrival Little Rock, Arkansas I I I I I I I I I I I I I I I I I I I I I I I I I 8:00 8:01 8:02 8:03 8:04 Augusta, Maine I I I I I I I I I I I I I I I I I I I I I I I I I 8:01 8:02 8:03 8:04 8:05 Long Beach, CA I I I I I I I I I I I I I I I I I I I I I I I I I 8:05 8:06 8:07 8:08 8:09 I I I I I I I 8:10 8:11

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3 Seismology Lab A seismological station is usually a quiet place. The delicate seismographs at the station are housed in deep cellar rooms where they are not disturbed by vibrations from passing trucks, trains, or other manmade disturbances. A few scientists and technicians do routine work at the station, but for the most part there is not much activity. Sometimes, however, the pen of the seismograph suddenly begins to swing in wide arcs. An automatic alarm sounds and the station comes to life. An earthquake has occurred! The seismologist on duty is alerted and immediately they rush to the station to examine the seismogram. The seismologist notes the arrival of the earthquake s P-waves and S-waves on the seismogram. By calculating the lag time between the arrival of the P and S waves, the seismologist gets a rough idea of how far away the earthquake center (epicenter) is located. They also study the size of the swings made by the pen. When the swings are large, the seismologist knows the earthquake is severe. After doing this preliminary analysis, the seismologist contacts colleagues at other seismological stations around the world to share data that allows the position of the earthquake epicenter to be determined. With data from at least two other stations, the location of an earthquake s epicenter can be determined. If the earthquake occurred near a large city a great disaster may have taken place. Many buildings would be destroyed and hundreds, even thousands of people could be injured or killed. If the epicenter of the earthquake is located beneath the ocean, energy from the shaking earth is transferred into the water creating a huge wave called a tsunami. This wave, commonly misnamed as a tidal wave, can cause great destruction to coastal areas. Today, seismology makes the rapid location of most large earthquakes possible. Aid can be rushed to a specific location on the surface and tsunami warnings can be issued for coastal regions. In the past, tsunami warning systems did not exist and people had no way of knowing about the huge wall of water approaching their community. Seismology plays a key roll in saving lives and bringing rapid assistance to earthquake stricken areas. Locating an Earthquake When an earthquake occurs, several different kind of seismic waves are produced. The waves move through the earth in all directions. Seismologists are mainly interested in two types of seismic waves: primary waves (P-waves) and secondary waves (S-waves). The P-wave travels faster than the S-wave. Therefore, the farther away a seismological station is from the earthquake epicenter, the greater the time-delay between the arrival of the P-wave and the S- wave. Your Investigation Use the seismograms recorded at four different seismological stations and the epicenter distance formula to find the epicenter of an earthquake.

4 Procedure Observe the simulated (made-up) seismograms from Great Falls, Montana, Little Rock, Arkansas, Augusta, Maine, and Long Beach, California. A. Find Distance of Epicenter From Seismological Station 1. The points on each of the four seismograms where the P-waves and the S-waves arrived have already been located with arrows. Determine as closely as possible the arrival times of the P-wave and S-wave for each station. Record the arrival times in the data table provided. 2. Determine how much time passed between the arrival of the P-wave and S-wave. Record this time in the data table. 3. We know that P-waves travel 7.75 km/s and S-waves travel 4.35 km/s. Therefore, the P- wave arrives at a seismograph station before the S-wave. Using P-wave and S-wave data, scientists have determined a formula to calculate the distance from the seismograph station to the earthquake s epicenter- the point on Earth's surface directly above the focus of the earthquake. Where: d = 9.91 (ts - tp) d = distance in km between epicenter and seismograph t s - t p = time lag in seconds between the arrival of the P-wave and S-wave 9.91 km/s = constant 4. Use the epicenter distance formula to calculate the distance of the epicenter from each seismograph station and record the epicenter distance into the data table. B. Determine the Location of the Epicenter 1. Use a drawing compass to draw a circle representing the distance of the epicenter from each seismological station. 2. Mark the epicenter with a colored star. Identify where all four circles come together on your map. In theory you would find an exact point where all four circles intersect. This is the epicenter. In reality, because of measurement errors interpreting the seismograms and measuring distances, the four arcs will probably not intersect at a specific point. Instead, the intersecting arcs will form a small triangle. In this case, the center of the smallest triangle on your map is probably the epicenter.

5 Earthquake Data Name Hour Data Great Falls Little Rock Augusta Long Beach P-wave arrival time in minutes and seconds. S-wave arrival time in minutes and seconds. Time Lag Time between P- wave and S-wave in minutes and seconds Time Lag in Seconds (60 seconds/minute) Distance to Epicenter in Km: d = 9.91Km/s(time lag)

6 Seismology Lab Completion Questions Name Hour 1. Could you have found the earthquake epicenter with less than four seismograms? Explain. 2. Describe the situation where information from only two seismographic stations would be enough data to determine the location of an earthquake's epicenter. 3. In which state was the epicenter located? 4. How did you identify the epicenter? 5. What is the difference between the epicenter and the focus of an earthquake? 6. What is the difference between the P-wave and the S-wave? 7. Why should we continue to spend money and time studying earthquakes? 8. One of the seismograph stations probably made an error. Which station shows an error and how did you recognize the error?

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8 Great Falls Augusta Long Beach Little Rock Scale: 1 cm = 200 km