EARTHQUAKES. Compressional Tensional Slip-strike

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1 Earthquakes-page 1 EARTHQUAKES Earthquakes occur along faults, planes of weakness in the crustal rocks. Although earthquakes can occur anywhere, they are most likely along crustal plate boundaries, such as the area called the Ring of Fire around the Pacific Rim. At plate boundaries the pressures can be compressional, tensional, or slip-strike faults where plates move past each other. Compressional Tensional Slip-strike For instance, the recent earthquake in the Los Angeles area was along a compressional thrust fault deep in the earth, a side effect of the main San Andreas slip-strike fault movements. The San Andreas Fault marks movement of the North American plate against the Pacific plate, which southern California and Baja California are attached to. These areas are essentially on their way to Alaska, while the rest of the U.S. is moving westerly. Earthquakes start at a focus, a point in the earth where the break starts. We calculate this point reflected on the earth s surface as the epicenter, by calculating the distance to the earthquake epicenter from three seismographs, the epicenter being where the distance circles cross each other. Earthquake waves (seismic waves) come as surface or body waves, that is they travel on the earth s surface or through the earth. Primary (P) waves, a body wave, arrive first, followed by the Secondary (S) wave, and then the surface waves on a seismograph. Damage from earthquake waves is caused by the effect at your location of the combination of the waves and the time the earthquake lasts. The longer the quake, the more damage. The closer to the epicenter usually implies more damage. The immediate damage is from shaking side to side, although tall buildings will have a vertical acceleration component also. The safest building is a single story frame house that does not have a tile roof.

2 Earthquakes-page 2 Other dangers from earthquakes are fires, ground failure, and the resultant landslides and tsunamis near the ocean. In fact, fire and the inability to put them out often causes the most loss of property from and earthquake. Earthquake power is measured by the Richter scale, which measures the size of the wave on a seismograph. On this scale, each whole number means a 10X greater amplitude and 30X greater release of energy. This is why a 5.0 is a minor shaking, and a 7.0 can cause major damage. (The 7.0 has a 100X greater amplitude and a 900X greater energy release than the 5.0.) We will now do a lab exercise to locate the epicenter of an earthquake and time of occurrence by working with seismographic records and a time travel graph.

3 Earthquakes-page 3 EARTHQUAKE RECORDS The earth is continually undergoing change due to stresses that exist within. At least a million times each year the earth suddenly fractures and the seismic waves of an earthquake radiate in all directions from the focus. Seismograph stations located throughout the world record these waves on seismograms. In this exercise we will examine how seismograms are used to determine the time of occurrence and location of an earthquake. Examining Seismograms The three basic types of waves produced by earthquakes are: P waves, S waves, and Surface waves. P waves travel most rapidly, therefore, they reach the seismograph station first. Examine Diagram 1, which is a typical seismogram. The dots on this seismogram mark time intervals of one minute. Answer the following two questions by referring to Diagram How many minutes elapsed between the arrival of the first p-wave and the arrival of the first S-wave? 2. How many minutes elapsed between the arrival of he first S-wave and the first surface wave? _

4 Earthquakes-page 4 Locating An Earthquake The difference in the velocities of the P and S waves provides a method for determining the distance to the location of the earthquake or epicenter. The principle used is analogous to a race between two autos, one faster than the other. The greater the distance of the race, the greater will be the difference in the arrival times at the finish line. 1. From the seismogram in Diagram 1, you should have determined that the arrival times of the first P and S waves differed by 4 minutes. Using the travel-time graph, determine the distance to the epicenter in this instance. This is done by finding the place on the graph where the vertical separation between the P and S lines is equal to 4 minutes (one square on the graph is one minute). Draw a vertical line at this location extending to the bottom of the graph. Label the line Bloomington. The distance to the earthquake in degrees can be read directly below your line. Answer = degrees (Have your answer checked before you continue.) 2. Using your answer from number 1 above, calculate how many kilometers to the earthquake epicenter. (One degree = 111 km or 69 Miles) km miles 3. The time travel graph can also be used to determine the time required for seismic waves to reach a seismograph station. Note on the time-travel graph that time is given on the vertical axis. Thus, for the example you did above, the first P-wave required 5 minutes and 15 seconds to reach the seismograph. This is determined by finding the place where the Bloomington line crossed the P-wave curve, and reading this value from the vertical axis. How many minutes were required for the first S-wave to reach the seismic station? _ minutes 4. In question #2 you determined the distance from an earthquake to a seismograph station. However, all we know is that the earthquake

5 Earthquakes-page 5 occurred somewhere along a circle, a given number of kilometers away. Records from three different seismograph stations are needed to more accurately locate an earthquake. In the following exercises you will interpret three seismograms in order to determine the location of an earthquake. a. On Diagram 3, label the arrival of the first P-wave and the first S- wave on each seismogram. b. In each case, how much time elapsed between the arrival of the first P-wave and the first S-wave? (Determine to the nearest ½ minute.) Nagpur Darwin Paris minutes minutes minutes c. Using the time-travel graph determine the distance to the epicenter from each seismograph station. Draw a line on the graph representing each situation and label each line with the city s name. Nagpur Darwin Paris DISTANCE degrees km miles degrees km miles degrees km miles d. Using the world map provided, determine the location of the earthquake epicenter. Do this by drawing three circles (one from each seismic station) using the distances from part c. Use the scale on the side of the map and a drawing compass to make your circles. In what country was the earthquake located? What is the approximate latitude and longitude of the earthquake epicenter?

6 Earthquakes-page 6 5. Next we will determine the time of origin of this earthquake. Each seismogram has a time designated on it. (9:43 GMT means 9 hours, 43 minutes, Greenwich Mean Time.) a. For Nagpur, India, note the time of the arrival of the first P-wave. b. Using the time-travel graph, determine the time required for the first P-wave to travel from the epicenter to the seismograph. c. Determine the time of the occurrence of the earthquake by taking the arrival time of the first P-wave minus the time required for the wave to reach the recording instrument. 6. Using the data for Darwin, determine the time of origin of the earthquake. a. For Darwin, note the time of arrival of the first P-wave. b. Using the time-travel graph, determine the time required for the first P-wave to travel from the epicenter to the seismograph. c. Determine the time of the occurrence of the earthquake by taking the arrival time of the first P-wave minus the time required fort he wave to reach the recording instrument.

7 Earthquakes-page 7 d. Does the time of the occurrence of the earthquake determined for Darwin agree with that determined for Nagpur? YES NO (circle one) Should they agree? YES NO (circle one) EXTRA CREDIT Determine the velocity of the wave from Darwin in feet per second and meters per second. We already have a value in miles per minute derived from how far the wave traveled and how long it took. Velocity of wave m/sec Velocity of wave ft/sec

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