Lesson 13: Earthquakes, Faults and Seismic Hazards
Ask anyone from the East Coast or Midwest what comes to mind when they think of California, and earthquakes are almost certain to rank right up there with Hollywood and Southern California beaches. This week's lesson on earthquakes is our second to explore a topic of statewide importance and, as with our discussion of water a few weeks ago, earthquakes are something that will impact almost all Californians at some time in their lives. Whether the state is struck by an earthquake that occurs here (like the 1906 San Francisco earthquake at right) or by a tsunami that comes ashore from elsewhere in the Pacific Ocean, Californians need to be prepared in order to minimize the loss of life and damage to property.
In chapter 13 Harden briefly describes how earthquakes are generated, how the various types of seismic waves they produce differ, and how we can determine the locations and strengths of earthquakes by measuring the properties of these waves. She also outlines what factors, in addition to magnitude, can influence an earthquake's destructiveness and describes the unique hazards that earthquakes are likely to pose in different parts of the state. Finally, this week's exercise will give you an opportunity to try your hand at measuring seismic waves and using your data to determine the location and magnitude of an earthquake.
As you read about earthquakes and the hazards they pose in California be sure to take careful notes on the topics covered by the learning objectives below. As during previous weeks, you'll find that writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and spot any gaps in your knowledge. This week it may also be helpful to do a few simple "thought experiments" about how ground motion and energy released differ for earthquakes of different magnitudes so that you really understand the logarithmic nature of the magnitude scale. Don't hesitate to post any questions you have to the Discussion Board so that your classmates or I can help you figure them out. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refer back to it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Explain why longer periods of strain accumulation along a fault might be expected to produce larger earthquakes in light of the elastic rebound model in which bent rocks "snap back" to their original shapes when a fault ruptures (Figure 13-3).
- Cite at least two criteria that you could use to distinguish an active fault from an inactive fault.
- Contrast an earthquake's magnitude with its intensity, briefly describe what each of these properties measures, and explain how factors such as distance and type of surface material can influence an earthquake's intensity.
- Use measurements of P-wave and S-wave arrival times from at least three seismograms to locate the epicenter of an earthquake, and use a distance-amplitude nomogram to estimate the quake's magnitude.
- Estimate how much (1) the amplitude of ground motion at a given distance from the epicenter, and (2) the amount of energy released will differ for two earthquakes with different magnitudes. (Assume that ground motion increases by a factor of 10 and energy output by a factor of 30 for each step up on the magnitude scale.)
- List three seismic hazards that may accompany an earthquake and indicate which areas of the state would be at particular risk from each. (For example, tsunamis are threats to coastal areas.)
- Briefly describe how geologists study the record of past movements on a fault and assess the frequency with which earthquakes recur on a given fault.
Reading and Browsing Assignment
- Read Chapter 13, focusing on the topics outlined in the learning objectives above.
- Take a few minutes to browse through the Southern California Earthquake Center's education site Investigating Earthquakes through Regional Seismicity. You'll find lots of in-depth information as well as some interestng activities that you can try.
- California is one of the most seismically active places in the world. You can see a near-real time map of the the past week's earthquake activity in the state by visiting the Recent Earthquakes Map. (Click on a quake and see what happens!) Also, check out the California Earthquake Forecast map based on recent aftershock activity. Finally, to see what's up at College of the Siskiyous. check out the online record of the campus seismometer.
Exercise 13: Earthquake! (Due by 9:00 AM on 18-Apr-2011)
After you have read the chapter and are comfortable with the concepts outlined in the learning objectives above, point your browser to Virtual Courseware's Earthquake exercise. Under "Main Activities" on the first screen, click "Travel Time" and work through the activity by following the instructions and using each tool in sequence to construct a travel time diagram. (Hint: It's a good idea to place your seismometers at a variety of distances from the explosion (E) so that you get a range of P-S delays on your seismograms.) Like the earlier radiometric dating exercise you worked on this one uses Java applets that may take a little while to load, so be patient. At the end of the first part of the exercise quit and return to the main screen (you do not need to save your results from the first part.) Next, work through the "Epicenter and Magnitude" activity (third button under "Main Activities".) At the end of this part of the exercise you will be asked if you want to take a quiz. Click yes, fill in your name, and enter 1778535 as your class code. (Write this number down now so that you have it when you go to the Earthquake site.) Complete the 10-question quiz. When you are done you will be able to review your score, and the program will automatically post a copy of it to a grade book that I will access next Monday (18-Apr-11). When you've finished both parts of the activity ("Travel Time" and "Epicenter and Magnitude") and taken the quiz, you'll be finished with Exercise 13; there is nothing to submit in Etudes. Have funthis is a very cool way to learn about how seismologists determine the locations and magnitudes of earthquakes.
Quiz 13: Earthquakes, Faults and Seismic Hazards (Due by 9:00 AM on 18-Apr-2011.)
After you feel you have met the learning outcomes outlined above please complete Quiz 13 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about earthquakes and seismic hazards in California and each is worth one point. If you can answer all of them correctly it means that you know your way around the California seismicity pretty well and are ready to start learning about the San Andreas fault next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.