EENS 3050

Natural Disasters

Tulane University

Prof. Stephen A. Nelson

Homework Assignment II. Seismological Exercises
Fall 2012


Answer the following questions. Typewritten answers should be turned in along with any maps required to answer the questions. Note that it is recommended that you use the PDF version of this file and print the figures on a high quality ink jet printer using the black and white only setting, to see the highest resolution images.  This will enable you to be much more precise in locating the earthquake in problem 1.  
  1. Shown below in Figure 1 are three seismograms that show the records of a small earthquake somewhere in California. Time is marked on each seismogram by offsets in the records. Each offset corresponds to the passage of 1 minute. The records are read from left to right. Both P and S wave arrivals are recorded.

 

 

 

    Shown below in Figure 2 are travel time curves for P and S waves determined from local earthquakes in California. Also shown is a curve (line) labeled S-P, which is travel time of the S wave minus the travel time of the P wave.

 

    1. From Figure 1, first carefully determine the time interval between the arrival of the first S wave and the arrival of the first P wave at each seismographic station. It is important that you determine these S-P intervals as precisely as possible. (2 points)

    2. From Figure 2, determine the distance of each seismographic station from the earthquake, again, it is important that you be as precise as possible. (2 points)

    3. Then, using the map below in Figure 3, and a drawing compass, locate the epicenter of this earthquake. (Note that you need to turn in the map showing how you located the epicenter) (2 points)

    4. On which fault did the earthquake most likely occur? (1 point)

 

 

  1. On October 17, 1989, just prior to game 3 of the World Series between the San Francisco Giants and the Oakland Athletics, a magnitude 7.1 Earthquake struck northern California (This earthquake is known as the Loma Prieta Earthquake). The earthquake occurred in the mountains west of San Jose California on the San Andreas Fault. As a result of the earthquake, 41 people were killed when a double-decked section of the Nimitz Freeway in Oakland collapsed, crushing people in the cars on the lower deck. A geologic map showing the various rock types present in the Oakland area and the location of the collapsed portion of the freeway (shown as a short dashed line) is depicted below in Figure 4. Between the thick bars is the area where the freeway was double-decked.

 

    On the map, the areas marked as Holocene mud are areas that were formerly occupied by San Francisco Bay, but have been filled with loose sediment in the last 100 years, the mud contains lots of water in the pore spaces between the grains. The areas marked Quaternary Alluvium are areas underlain by unconsolidated sediment deposited by streams over the last 2 million years. The area marked Franciscan Formation is underlain by solid sedimentary, igneous, and metamorphic rocks with a thin cover of soil.

    Several days after the magnitude 7.1 earthquake, small aftershocks shook the area. All had epicenters near the main shock of October 17. Seismologists placed several portable seismometers at stations S1, S3, and S4 (as shown on the map) and recorded these aftershocks. Seismographic recordings for one of these aftershocks, a magnitude 4.1 earthquake, for each of the three stations are shown in Figure 5, below. Note that the epicenter of the aftershock was far enough away that all of the recording stations could be considered to be about the same distance from the earthquake.

  1. What observations can you make about the seismic response (degree of shaking) on the three types of materials underlying the area? (2 points)

  2. What conditions were likely responsible for the double-decked Nimitz Freeway freeway to collapse where it did? (3 points)

  3. Considering that New Orleans is built on water-saturated river muds, how do you think New Orleans would fare if there were a major earthquake nearby? (2 points)

  1. Three of the factors that are largely responsible for building damage during an earthquake are the degree of ground shaking, type of building construction, and liquefaction.  This exercise is designed to explore these relationships.  Imagine that you are given the opportunity to become the San Francisco Bay Area regional manager for Denyallclaims Insurance Company.  Because this company has such a high profit margin, they can afford to provide housing for their regional managers. Thus, one of the perks of the job is that they will give you title to a house in one of the prestigious suburbs of San Francisco, Mill Valley, located on the Marin Peninsula, north of San Francisco.  The company is offering you a choice between three different homes that they have somehow acquired.  Because you know that the San Francisco Bay Area is prone to earthquakes and you also know that you will have to pay the insurance premiums for both home owner's and earthquake insurance, you want to make the best choice of which home to take based on safety concerns and the ability of the house to withstand damage from possible future earthquakes.  Since Denyallclaims requires that you purchase insurance from them, you know that the premiums that you will pay will depend on the home's vulnerability and susceptibility to earthquake damage.  While surfing the web, you have found a web site produced by the Association of Bay Area Governments (ABAG) that provides information that will help you in making a decision -  http://quake.abag.ca.gov/

    This site provides maps and information on such things as intensity of ground shaking, liquefaction, and the response of types of building construction to earthquakes, among lots of other useful information.

    The homes from which you get to choose are the following, all within the 94941 zip code:
  • 50 Plymouth Ave. (Single Family Wood Frame 2 story house built after 1940)*  
            
  • 75 Avon Ave. (Unreinforced Masonry 2 story house). 
  • 5 Catalpa Ave. (Single Family Wood Frame 2 story house built before 1939). 

*Note that this exercise is hypothetical.  The actual existence of houses at these addresses is not known with certainty, nor are the construction characteristics at any of these houses known.  Any resemblance to actual houses or addresses is purely coincidental and is unintended.

You should use the interactive liquefaction maps and shaking maps to first determine the susceptibility of each of these homes to liquefaction and shaking during a worst case scenario earthquake. Proceed as follows:

First, click on the Liquefaction link under "Earthquake-Related Hazards" on the ABAG Earthquake and Hazards Program web page. Then click on the USGS Interactive Liquefaction Susceptibility Map (the third map down from the top). This will open the interactive liquefaction susceptibility map.  The map is color coded for Liquefaction Susceptibility as shown in the Legend. To find the locations and thus the Liquefaction Susceptibility for each house, move the mouse cursor over the blue cube at the top of the map until "Find Location in a small blue box is displayed, then click on the blue cube and select Address from the drop down menu.  A box then opens into which you can type the street address and zip code (94941) for a particular house.  Once you have entered that information, click on "Locate" and the map will show you where the house is located.  You can then use the color in which the house is located to determine the Liquefaction Susceptibility for that house.  You can repeat the processes by clicking on the first icon at the top of the address box then clicking the "Clear" button so you can enter another address.  For each of the three properties, determine the Liquefaction Susceptibility and record the information for later use.  

Next, go back to the main ABAG Earthquake Hazards page and click on Shaking under Earthquake-Related Hazards. Then click on the Earthquake Shaking Potential map (second one down). The map that opens works just like the one you just looked at, except the color code now refers to Shaking Potential. The colors represent Modified Mercalli Intensity (MMI) values, with the top (dark brown color representing a MMI value of X, the lighter brown IX, red VIII, dark orange VII, etc.  For each of the homes, determine the MMI values. 

Note that you don't need to print these maps for this exercise. 

Answer the following questions -

  1. For each property, what is the Liquefaction Susceptibility and the Modified Mercalli Intensity (MMI) value for an earthquake a large earthquake that could affect this area? (3 points)
     
  2. From your acquired knowledge of the relationships between shaking intensity, liquefaction, and the type of geologic material underlying an area, make an educated guess about the topography and materials that underlie each of the three properties (3 points).

Before you use the answers to the above questions to decide which property you will accept from Denyallclaims, you should consider the type of construction involved in each of the houses. 

To determine the effects of construction type during the shaking of the worst case scenario earthquake,  go to the following ABAG web page - http://quake.abag.ca.gov/wp-content/documents/2010-On-Shaky-Ground.pdf   Once this PDF file opens, go to Page 6 and read about SHAKING INTENSITY AND BUILDING DAMAGE. As you scroll down, you will see pictures of various types of construction damaged in other earthquakes., then look at Table 1 on page 8.  From the information you obtained above on MMI values knowledge of the construction type for each address, answer the following question:

  1. Which property would you accept from Denyallclaims based on all of the information available and your desire to have the safest and least earthquake susceptible house available, keeping in mind that Table I does not say anything about possible damage that may result from liquefaction (recall that liquefaction may cause damage to the foundation of the home)?  Discuss the reasoning used to obtain your answer. (3 points)
  1. On the internet, use the search engine of your choice (I recommend www.google.com) to find information about a major earthquake that occurred in the U.S. or Canada  in the year 1700. Then answer the following questions about the earthquake.

    1. Where did this earthquake occur and how big was it? (2 points)

    2. Considering that humans capable of recording information about the earthquake did not live in the area at the time of the earthquake, how do scientists know the exact date, time and size of this earthquake? (2 points)

    3. What is the plate tectonic setting of the area where the earthquake occurred? (1 point)

    4. What implications does this have for future earthquake potential in this region?  Specifically, what major cities could be affected by future earthquakes that could occur in this area? (2 points)

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