Exercise 7
Physical Geology

Semester Project - Part 2 : Earthquakes


For this exercise you will need a map showing plate boundaries which is a small map or this one which is larger [if you print from the browser, turn the image on its side and reduce the size to about 75% for the best effect), or you can copy the larger figure on the inside cover of the text.

You should start your observations at the beginning of the semester. The questions can be deferred until the appropriate chapters are covered in the course.

The velocity of a P or S wave is a function of the physical properties of the rock the wave is traveling through. Different rocks have different physical characteristics (such as how compressible they are and how the respond to shear stresses) and different P and S wave velocities. In fact, if the velocity of the wave can be measured, it may be possible to predict the type of rock the wave traveled through - indirect detection of rock type!

The velocity of a P wave can be expressed as:

  1. Velocity P wave = ((B + 1.3G)/Density)1/2, where:

    • B = the bulk modulus - the resistance to change in volume
    • G = the Shear modulus - the resistance to change in shape
    • Density = mass/volume

    As the pressure increases (increasing depth of travel) the quantity (B+G) increases faster than Density. Therefore, with increasing depth we expect the velocity of the P wave to increase.

    HOWEVER, if the wave passes from one material to another, the wave may speed up, slow down or not change its velocity, depending on the contrast in properties of the two types of material.

  2. Velocity S wave = (G/Density)1/2

    Questions

    1. If you put a cube of rock in a fish bowl will the rock change shape? Therefore, G for solids is a big number. (Remember, however, that some rocks may flow at high temperatures). If you take milk which is in a rectangular container and pour it into the fish bowl will the milk offer resistance to a change in shape? Therefore, G for a liquid is 0.0.

    2. If a P wave were to go from a solid to a liquid - what will happen to its velocity (use the equation)?

    3. If an S wave were to go from a solid to a liquid - what will happen to its velocity (use the equation)?

    4. Describe a boundary where a P and S wave would go from solid to liquid.

    Each day new Earthquakes are plotted at the Earthquake Locator . Select the most recent earthquakes of magnitude 4.5 and higher. Prepare a table with columns for:

    Location of EpicenterFocal DepthMagnitudeRemarks

    On Shaking Ground is a similar exercise that may give you some ideas.

    Prepare a copy of the map of plate boundaries and locate the epicenters in red:

    1. Locate each epicenter and record the magnitude and depth to the focus on the map for each earthquake.

    2. In the remarks, indicate whether the epicenter was near a plate boundary; if it was, briefly describe the nature of the plate boundary and the sense of motion. If the epicenter was within a plate, give the name of the plate and its general direction of motion. Based on this very limited sample, comment on the relationship between earthquake epicenters and plate margins.

    3. Between now and the date assigned by your instructor, record new earthquakes that occur (magnitude 4.5 and higher) in your table in blue.

    Plate Tectonics - The Cause of Earthquakes is a very good introduction to earthquakes and plate tectonics. Read through this section for a general review. Incorporate your answers to the following questions into the article you are producing. Read the following section on Seismic Waves. Finish you reading by quickly going through the section on Richter Magnitude. Don't get bogged down in the mathematics. Focus on differences between Richter's scale and the older Mercalli scale.

    Questions

    1. In your own words, describe the Richter scale and the Mercalli scale for describing earthquakes. Include the distinction between Intensity and Magnitude. Why is there a concern about continued use of the Richter scale?

    2. What are some of the problems with using the Mercalli scale? How do the users of this scale counter these criticisms?

    3. Use your rapidly improving ability to search the Internet for a list of the ten largest magnitude earthquakes on Earth. Add these earthquakes to your table and plot their epicenters in orange

      The Table and Map that you complete for this exercise will be handed in with the written report as Appendix II.