EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Homework Assignment II. Seismological Exercises
 Answer the following questions. Typewritten answers should be turned in along with any maps required to answer the questions. 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.

 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) 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) Then, using the map below in Figure 3, and a drawing compass, locate the epicenter of this earthquake. (2 points) On which fault did the earthquake most likely occur? (1 point)

 On October 17, 1989, just prior to game I 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.
 What observations can you make about the seismic response (degree of shaking) on the three types of materials underlying the area? (2 points) What conditions were likely responsible for the double-decked Nimitz Freeway freeway to collapse where it did? (3 points) 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)