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 use another figure that you locate.

This exercise is worth a total of 30 points and will be handed in on April 28, 2001 when you take exam 3.

Background

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 velocity or direction, depending on the contrast in properties of the two types of material.

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

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 within the Earth where a P and S wave would go from solid to liquid.

Each day new Earthquakes are plotted at the Earthquake Locator . Another good resource for recent earthquakes is provided by IRIS

Starting on March 27, 2001, select the 20 most recent earthquakes of magnitude 5.0 and higher. Prepare a table with columns for:

Location of Epicenter	Focal Depth	Magnitude	Remarks

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 of these 20 most recent magnitude 5 or higher events in red. Record the magntude and depth to the focus (if it is given) for each event in your table.

1. 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.

2. Between now and April 21, record new earthquakes that occur (magnitude 5.0 and higher) in your table in blue.

3. Use your rapidly improving abilities and 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

4. Based on this very limited sample, comment on the relationship between earthquake epicenters and plate margins.

Plate Tectonics - The Cause of Earthquakes is a very good introduction to earthquakes and plate tectonics. Read through this section for a general review. 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.

a Letter to an Editor

You live in the Cascades area. In recent editorials the regional planners have written articles about how much the new Cascades Convention Center will add to the regional economy. You believe that these writers have confused the public about the frequency and magnitude of events in the Cascades. Write a 300 word (or so) letter to the editor in which you state your opinion as to the safety of such a facility. How likely are earthquakes in this area? Use the information you have gathered to formulate your letter.

In your own words, describe the Richter scale and the Mercalli scale for describing earthquakes. Include the distinction between Intensity and Magnitude.

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

The Table and Map and editorial that you complete for this exercise will be handed in on the day of exam 3.

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Copyright by John C. Butler, July 29, 1995