LAB 9: Earthquakes & Seismic Activity Objectives Identify P, S, and surface waves on a simple seismogram Locate the epicenter of an earthquake using seismograms and travel times curves Describe how the geology of a region can affect the intensity of an earthquake and observe the effects of earthquakes on structures Critically asses the relationship between plate tectonics and earthquake distribution Background Earthquakes are shaking motions and vibrations of the Earth caused by large releases of energy. Energy is released in several ways., through volcanic eruptions, explosions, and plate movement along plate boundaries. The epicenter is the point on Earth s surface directly above the focus, which is the origin of an earthquake. Strain is produced in bedrock by the periodic release of energy. This energy builds up until it reaches its limit and snaps producing elastic waves, called seismic waves. Seismic waves cause the ground to vibrate and shake, sometimes violently. Seismic waves are recorded on seismographs at seismic stations located throughout the world. The movement of Earth s crust along plate boundaries, or fault motion, is the most common source of earthquakes felt by people. However, intraplate earthquakes are also a source. One such example is the New Madrid and Wabash fault zones located in the central US. Earthquake damage varies from region to region. Most earthquake damage is caused by shaking. The intensity of shaking depends on 3 main factors: 1. Distance from an earthquake. The closer an area is to the source of earthquake, the more shaking occurs. 2. Magnitude. The larger the magnitude, the stronger the shaking and the larger the affected area. 3. The underlying geology of an area. Soils are affected differently than bedrock. Soft thick sediments cause greater amplification and therefore more shaking. (Fig 1) a. Thin sedimentary deposits cause less wave amplification. b. In thick sedimentary deposits, low frequency waves are amplified, which damages tall buildings and bridges. c. In thin stiff soil over bedrock, high frequency waves are amplified and cause damage to short buildings like houses
Low high Hard igneous rock sedimentary silt-mud alluvium Fig. 1- Amplification of shaking (surface waves) Using a Seismograph There are 3 main types of seismic waves generated by an earthquake at it s focus.; P-waves, S- Waves, and Surface Waves. P and S waves are body waves and can travel through the interior of the earth. When we are determining the epicenter of an earthquake we are concerned with the P and S waves. The P-wave travels the fastest and reached the seismic station first. S-waves arrive after P waves. The interval of time between the arrival of the P-wave and S-wave is what seismologists use to determine the location of an epicenter. Using the travel-time graph (Fig. 3) to determine the vertical separation between the P and S curves, which is equal to the difference in the arrival times between the P wave and S wave, scientists can determine the location of an epicenter. Figure 2: Typical Seismogram
Travel-time graphs Travel-time graphs depict the vertical separation between the P and S curves, which is equal to the difference in the arrival times between the P wave and S wave, found on a seismograph. To accurately locate an earthquake epicenter, records from three seismic stations are necessary. PART 1: Using seismographs and travel-time graphs 1. Use Figure 2 and determine the time difference between the arrival of the P-wave and the S- wave. 2. Refer to the travel time graph. What is the distance from the epicenter to the station that recorded the earthquake in Figure 2? miles 3. Use Figure 3 to determine the difference in arrival times (in minutes) between the first P wave and first S wave for stations that are the following distances from an epicenter. 700 miles: minutes difference 1200 miles: minutes difference 1800 miles: minutes difference 2500 miles: minutes difference
FIG. 3: Travel-time graph
PART 2: Finding and epicenter Finding the epicenter of an earthquake: 1. Using the seismograph stations provided in Figure 4, find the S-P time interval for each station, record your data in the table provided. 2. Use the travel-time graph (figure 4) to determine the distance of the epicenter to the seismograph station. Record your data in the table provided 3. Use a compass and draw a circle around each of the seismograph stations plotted on the map in Figure 5. Use the scale at the bottom of the map provided and the distance you plotted in the data table. 4. The epicenter is located where the three circles overlap Earthquake Data Table
Figure 4
Figure 5
PART 3: Finding Magnitude The magnitude of earthquakes is its size. It is measured on the Richter Scale, which was invented by Charles Richter in 1934. Magnitude is measured using the maximum amplitude of the highest wave on the seismograph, it doesn t matter which wave it is (see Fig. 1 surface waves) against the distance to the focus (S-P time interval). Using the information below determine the magnitude of the earthquake.
PART 4: Earthquake Hazards & Risks Earthquakes Hazards and Risks: The geology of an area can affect the hazards of an earthquake. Different sediments react differently to the ground shaking. In this activity we are going to simulate earthquake activity and infer risks. 1. Obtain a small cup 2. Fill the cup with dry sand about ¾ full 3. Place several coins in the sediment vertically 4. Observe and record what happens when you simulate and earthquake by shaking Experiment Observations when shaking occurs Dry Sand Wet Sand
1. What happened to the vertically positioned coins in the dry sand when you simulated and earthquake? 2. Add water to your cup, just enough to wet the sand. Then push down on the moist sand to ensure it is compacted. Place the coins in the sediment vertically. Simulate an earthquake and record your findings below. 2. Which type of material is more hazardous to build on in an earthquake prone region? Why do you think this is? 2. Consider the moist sand, do you think this material would become more hazardous or less hazardous if more water was added? Try this with your model by adding more water to your sand and inducing an earthquake, and compare with what you predicted.
PART 5: Analyzing Earthquake Data (TO BE DONE INDIVIDUALLY) Using the chart on the following page, plot the earthquakes on the map provided below. Mark the earthquakes with the highest fatalities with a red star. 1. Do you notice a pattern in the location of earthquakes? If so, what is the pattern? 2. Many earthquakes occur every day on the San Andreas fault. Why are there not many dots along the San Andrea Fault on the map (refer to Fig. 6 for reference)? 3. The Earthquakes on this map represent large magnitude earthquakes over the past few years. Describe the relationship between large magnitude earthquakes and plate boundaries, refer to fig. 6 which shows the plate boundaries.
Plot the following earthquakes on the map. Date Latitude Longitude Geographic Location Magnitude Fatalities 3/11/2011 38.297 N 142.372 E Sendai, Honshu, Japan 9.0 20,352 2/21/2011 43.583 S 172.680 E Christchurch, New Zealand 9/3/2010 43.530 S 172.120 E Christchurch, New Zealand 6.1 181 7.0 0 7/23/2010 6.470 N 123.532 E Mindanao, Philippines 7.6 0 5/9/2010 3.747 N 96.013 E Sumatra, Indonesia 7.2 0 4/13/2010 33.224 N 96.666 E Gyegu, China 6.9 2,968 4/4/2010 32.259 N 115.287 W Maxicali, Mexico 7.2 2 2/27/2010 35.909 S 72.733 W Chillan, Chile 8.8 577 1/12/2010 18.443 N 72.571 W Port-Au-Prince, Haiti 7.0 316,000 1/10/2010 40.652 N 124.692 W Fortuna, CA, USA 6.5 0 12/19/2009 23.763 N 121.689 E Hua-Lien, Taiwan 6.4 0 11/13/2009 19.385 S 70.266 W Iquique, Chile 6.5 0 11/9/2009 17.212 S 178.413 E Suva, Fiji 7.3 0 10/7/2009 13.145 S 166.297 E Lata, Solomon Islands 7.4 0 9/30/2009 0.725 S 99.856 E Sumatra, Indonesia 7.6 1,117 9/29/2009 15.509 S 172.034 E Hihifo, Tonga 8.1 192 8/10/2009 14.013 N 92.923 E Port Blair, India 7.5 0 7/15/2009 45.750 S 166.577 E Invercargill, New Zealand 7.8 0 5/28/2009 16.733 N 86.22 W La Ceiba, Honduras 7.3 7 1/24/2009 59.414 N 152.836 W Nanwalek, AK, USA 5.8 0 10/28/2008 30.656 N 67.631 E Quetta, Pakistan 6.4 166 7/29/2008 33.953 N 117.761 W Los Angeles, CA, USA 5.5 0 7/19/2008 37.552 N 142.206 E Iwaki, Honshu, Japan 7.0 0 7/5/2008 53.888 N 152.869 E Petropavlovsk, Russia 7.7 0 6/30/2008 58.169 S 22.014 W Bristol Island 7.0 0 5/12/2008 30.986 N 103.364 E Chengdu, Sichuan, China 7.9 87,587 5/2/2008 51.935 N 177.595 W Tanaga, AK, USA 6.6 0 10/8/2005 34.493 N 73.629 E Islamabad, Pakistan 7.6 86,000 12/26/2004 3.316 N 95.854 E Sumatra, Indonesia 9.1 227,898 12/26/2003 29.004 N 58.337 E Kerman, Iran 6.6 31,000
Figure 6 4. Compare your plotted map with figure 5 showing the types of plate boundaries. Is there a pattern associated with boundary type? If so, what is the pattern? 5. Is there a pattern associated with numbers of fatalities? If so, what is the pattern?
Critical Thought (To Be Completed Individually): Consider what you have learned about earthquakes. Using the maps, graphs, and charts in this lab to answer the following questions. 1. The 1812 New Madrid earthquake was only one of a series of powerful quakes to occur along the fault line in the winter of 1811-1812, including 3 that are among the 10 largest earthquakes in U.S. history. Fortunately, very few people were killed in these events. The next time the area experiences such an enormous quake, how do you think the death toll would compare? Refer to the image above and justify your answer. 2. What is the seismic risk here at San Jacinto College? At your home? How safe or unsafe is the building in which you are currently living? attending classes? your home? What are some precautions you would need to take if you lived in San Francisco instead of Houston? 3. Predict what would happen if a magnitude 7 earthquake were to hit Houston, Texas? Do you think the city would be prepared?