LAB 6: Earthquakes & Faults

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1 Name School LAB 6: Earthquakes & Faults An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter. Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Earthquakes are measured using the Richter Scale. What causes earthquakes and where do they happen? The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake. Why does the earth shake when there is an earthquake? While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth s surface, they shake the ground. Patrich Physical Geography Lab 1

2 How to Locate an Epicenter: When an earthquake occurs, it releases seismic waves that can be detected at stations all around the globe. Remember that there are three types of waves released by an earthquake: primary (P), secondary (S), and surface waves (L). Primary waves (P-waves) travel via compression and are the first to arrive at seismic stations. They generally travel at a rate between 5.95 and 6.75 kilometers per second (km/sec), depending on various factors in the crust including density, compressibility, and rigidity. Secondary waves (S-waves) have a shearing motion and are the second type of wave to be detected, traveling between 2.9 and 4.0 km/sec. The last waves to arrive are the surface waves (L-waves), which have velocities around 2.7 and 3.7 km/sec. Geologists can use these known travel times to approximate the distance from the reporting station to the epicenter. Three or more stations can compare their distances to the epicenter in order to determine its exact location. Let s assume that you work at the seismic reporting station in Golden, Colorado. At 1:45pm you receive the first P-waves from a quake at an unknown location. Surface waves (L-waves) follow at 1:59pm. To determine your distance from the quake you must first establish the difference in arrival times: 1:59pm - 1:45pm = 14 minutes. Using a ruler or a scrap piece of paper, figure out the distance between 14 minutes on the Y axis and move your ruler along the graph until you find a spot where the two lines (P- and L-waves) are exactly 14 minutes apart. Project that location down to the X axis to determine the distance to the epicenter. You determine that your epicenter is 2125 km away from your location; however, you do not know the direction to the epicenter! In order to determine the exact location, you must call two colleagues from different reporting stations around the globe. Patrich Physical Geography Lab 2

3 Epicenter Location Continued: Let s do it again with data from three reporting stations. Pay careful attention to the type of wave arriving at each time. Record your answers on your answer sheet. Station A: The first S-wave arrives at 6:32:45 pm. L-waves begin to arrive at 6:39:45 pm. Station B: P-waves first appear at 6:30:45 pm, L-waves at 6:51:45 pm. Station C: P-waves arrive at 6:34:27 pm, S-waves follow at 6:45:27 pm. 14. Plot the location of the epicenter by drawing a circle around each station. The radius of each circle should be equivalent to the distance from that station to the epicenter. Use the same scale as the graph above to determine your distances. Show your circles and a dot to indicate the location of the epicenter. Distance (kilometers) Patrich Physical Geography Lab 3

4 Epicenter Location Continued: Station C Station B Station A Patrich Physical Geography Lab 4

5 Modeling the Faults A fault is a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture. Strike-slip, normal, and reverse faults. Using the model found on the last page you will create each of these three faults to observe and describe what it is you see. Please color then assemble the model before continuing. Normal fault: Often associated with extensional forces as a result of a divergent boundary 1. Locate points A and B on your model. 2. Move point B so that it is next to Point A. 3. Observe your model from the side (its cross-section). 4. Draw the normal fault as represented by the model you have just constructed. Normal Fault Diagram Questions: 1. Which way did point B move relative to point A? 2. What happened to rock layers X, Y and Z? 3. Are the rock layers still continuous? 4. What likely happened to the river? The road? The railroad tracks? Explain your answer. Patrich Physical Geography Lab 5

6 Reverse Fault: Associated with compressional forces as a result of a convergent boundary 1. Locate points C and D on your model. 2. Move point C next to point D. 3. Observe the cross-section of your model. 4. Draw the reverse fault as represented by the model you have just constructed. Reverse Fault Diagram Questions: 5. Which way did point C move relative to point D? 6. What happened to rock layers X, Y and Z? 7. Are the rock layers still continuous? 8. What likely happened to the river? The road? The railroad tracks? Explain your answer. Patrich Physical Geography Lab 6

7 Transform fault: associated with strike-slip forces as a result of a transform boundary 1. Locate points F and G on your model. 2. Move the pieces of the model so that point F is next to point G. 3. Draw an overhead view of the surface as it looks after movement along the fault. Transform Fault Diagram Questions: 9. If you were standing at point F and looking across the fault, which way did the block on the opposite side move? 10. What happened to rock layers X, Y and Z? 11. Are the rock layers still continuous? 12. What likely happened to the river? The road? The railroad tracks? Explain your answer. Patrich Physical Geography Lab 7

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