1. 1 What is the Richter Scale? How large is a large earthquake? How is earthquake size measured? Earthquake Magnitude Module LRW-1 Prepared for SSAC by Laura Wetzel – Eckerd College, St. Petersburg Florida © The Washington Center for Improving the Quality of Undergraduate Education. All rights reserved. 2005 Quantitative concepts and skills Scientific notation Ratios Logarithms Graphs, linear Graphs, semilogarithmic

2. 2 Earthquakes occur when large bodies of rock shift suddenly along a fault. Seismographs record the waves produced during an earthquake. The magnitude of the earthquake can be calculated from the size of these waves. This presentation asks you to create spreadsheets and graphs to explore earthquake magnitude, wave amplitude, and energy release. Slides 3-4 briefly describe intensity and magnitude scales. Slides 5-7 ask you to create spreadsheets to calculate seismogram amplitudes corresponding to a variety of magnitudes. Slides 8-9 illustrate and further explain the concept of earthquake magnitude. Slides 10-13 ask you to create more spreadsheets, this time to compare the energy released by earthquakes of various magnitudes. Slides 14-15 ask you to create linear and logarithmic graphs to visualize how amplitude and energy increase with magnitude. Slides 16-18 give the assignment to hand in. Preview

3. 3 In 1935 Charles Richter developed a method to compare the sizes of earthquakes based on waves recorded by seismographs. A single magnitude is assigned based on maximum wave amplitudes. The intensity of shaking is one way to assess the size of an earthquake. A value is assigned based on damage reports and personal interviews of people who experienced the quake. The intensity depends on location; in general, the closer the observer to the earthquake, the higher the intensity. Intensity values assist in seismic hazard and historical earthquake analysis. Modern seismologists have modified his method and now analyze all waves recorded on a seismograph to produce a seismic moment. The seismic moment is then converted to moment magnitude, which is the standard size reported by the U.S. Geological Survey. Source: http://neic.usgs.gov/neis /seismology/people/ Measuring Earthquake Size

4. 4 This is a seismogram of the magnitude 9.3 Sumatra-Andaman Islands earthquake that occurred on December 26, 2004. The recording seismograph is located on the Cocos Islands in the Indian Ocean. Go to the Incorporated Research Institutions for Seismology website if you would like to see more seismograms (www.iris.edu). A seismogram is a graph of wave amplitude vs. time. In old seismographs, a pen drew the recording on a piece of paper. In new seismographs, the signal is recorded digitally. Seismograms

5. 5 One way for us to explore earthquake magnitudes is to use a spreadsheet. Recreate this spreadsheet. = Cell with an equation in it. = Cell with a number in it. We will consider earthquake magnitudes ranging from -1 to 9.5. Magnitudes above 2.5 can be felt by people. The magnitude scale is open-ended and is based on seismogram amplitudes; it is not a scale from 1 to 10. The maximum earthquake magnitude recorded is thought to be ~9.5, recorded from Chile in 1960. Earthquake Magnitude

6. 6 How are earthquake magnitude and seismogram amplitude related? We will use scientific notation for seismogram amplitudes. The quantity that is normally expressed as 1 x 10 -7 is written as 1.E-07 in Excel. Recreate this spreadsheet. Each step in the magnitude scale represents a 10-fold increase in wave amplitude. This means the scale is logarithmic. Understanding Magnitude

7. 7 How large is the amplitude of a magnitude-9 earthquake compared to the amplitude of smaller events? A M9 earthquake produces waves 1000 times the size of those of a M6 earthquake. (Three steps in magnitude correspond to a 10 3 -fold increase in amplitude.) Add 3 columns to your spreadsheet to calculate amplitude ratios. Understanding Magnitude

8. 8 How does the amplitude of a magnitude-9 earthquake compare to the amplitude of smaller events? If we likened earthquakes to hills and mountain peaks... Mag. 9 = 10x larger than Mag. 8 = 100x larger than Mag. 7 Mag. 8 = 10x larger than Mag 7 Mag. 7 Each peak is (approximately) 10 times the height of the previous one. Understanding Magnitude

9. 9 This is like comparing the elevation of Mt. Everest to the relief of a 300-ft hill! Understanding Magnitude How does the amplitude of a magnitude-9 earthquake compare to the amplitude of a magnitude-7 event? If we likened earthquakes to hills and mountain peaks...

10. 10 Create this spreadsheet to determine the energy released for a range of magnitudes. How is magnitude related to energy? One advantage of moment magnitude (M W ) is its direct relationship to the amount of energy released during an earthquake. The energy released is the seismic moment (M o ) and is expressed in dyne-cm. Rearrange this equation to solve for M o. Understanding Magnitude

11. 11 How is magnitude related to energy? E = M o. 2 x 10 4 The energy released (E) is related to the seismic moment (M o ) and is also expressed in dyne-cm. The following equation is a good approximation for the relationship between energy and seismic moment. Add one column to your spreadsheet to calculate energy release.

12. 12 How large is a magnitude-9 earthquake compared to smaller events in terms of released energy? A M9 earthquake releases ~32,000 times the energy of a M6 earthquake. This means that ~32,000 M6 earthquakes must occur to produce the energy released in one M9 earthquake. Add 3 columns to your spreadsheet to calculate energy ratios. Working with Magnitude

13. 13 How large is a magnitude-9 earthquake compared to smaller events? To plot seismogram amplitude and moment on the same graph, multiply all amplitudes by 1 x 10 20. Note: At the end of this module, you will be asked to hand in copies of this spreadsheet and the three graphs that you will create on the following slides. Combine your magnitude, amplitude, and moment columns into one spreadsheet for magnitudes 5 through 9. Working with Magnitude

14. 14 How large is a magnitude-9 earthquake compared to smaller events? Create two graphs showing amplitude vs. magnitude. Draw one with a linear and one with a logarithmic scale. Add trendlines. Logarithmic Scale Hint: After creating the linear scatter graph, click on the Y-axis. Select Scale and click the box on the bottom labeled Logarithmic scale. Working with Magnitude Linear Scale Amplitude

15. 15 How large is a magnitude-9 earthquake compared to smaller events? Create a graph showing both amplitude and energy vs. magnitude. Note that energy increases at a much faster rate than amplitude. Working with Magnitude Amplitude (mm x 1E+20); Energy (dyne-cm)

16. 16 Hand in copies of the master spreadsheet in Slide 13 and the three graphs you created in Slides 14-15. Thus far, you have explored general relationships between earthquakes. Next, apply your knowledge to some real events. Hand in answers to the following questions, including copies of all new spreadsheets and graphs. 1.Create a spreadsheet to calculate the seismic moments for these earthquakes. 2.Create linear and logarithmic scatter graphs illustrating magnitude on the X-axis and seismic moment on the Y-axis for these earthquakes. End of Module Assignments

17. 17 3. Compare the energies of the Loma Prieta, San Francisco, and Sumatra earthquakes. a. _______ Loma Prieta earthquakes = 1 San Francisco earthquake b. _______ Loma Prieta earthquakes = 1 Sumatra earthquake c. _______ San Francisco earthquakes = 1 Sumatra earthquake End of Module Assignment

18. 18 4. Create a scatter graph with magnitude on the X-axis and fatalities on the Y-axis. 5. Create another scatter graph of magnitude vs. fatalities, but omit the Sumatra earthquake. (To do this, you can copy your original graph and reset the Y-axis maximum to 10000.) 6. Briefly describe two reasons why earthquake magnitude and the number of fatalities are not directly correlated. For example, the Alaskan earthquake has a high magnitude, but relatively few fatalities. Details of Interest: The 2004 Sumatra earthquake produced a devastating tsunami. A fire caused by the 1906 San Francisco earthquake destroyed most of the city. The 1989 Loma Prieta earthquake rattled the San Francisco Bay area during the World Series. The 1964 Alaska earthquake struck Anchorage on Good Friday. End of Module Assignment