Geophysics 189 Natural Hazards Homework 2 Earthquake Locations and Hazards Due February 15, 2005

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1 Geophysics 189 Natural Hazards Homework 2 Earthquake Locations and Hazards Due February 15, 2005 Objective: The objective of this homework is to give you experience using real seismograms to locate an earthquake in New Mexico. Your tasks include identifying P and S wave arrivals on seismograms, using the arrival times to determine event location. In addition, you will use government websites to assess earthquake hazards in the San Francisco by area and synthesize knowledge from lecture to make judgments about regional hazards. To Hand In: For the earthquake location, turn in the seismograms with your picks for P and S arrivals as well as the map showing the earthquake location. All answers to questions in this assignment should be typewritten and turned in as well. 1. Earthquake Location As discussed in class, earthquakes do occur in New Mexico, although they are typically small magnitude events. There is a network of seismometers in the state that record data that seismologists use to locate earthquakes. Today you get to put on your seismologist hat to locate a small earthquake that occurred in May In order to locate the earthquake, you will need to measure arrival times of the P and S waves on seismograms. From these arrival times, you can calculate the distance between the station and the event. Each station will provide a distance that is the radius of a circle (i.e. the earthquake could have occurred anywhere along that circle). Thus, you will need three stations in order to estimate the location of the event, which should be the intersection of the 3 circles. Three seismograms recorded at seismic stations in the state are provided with this assignment. A map is also included showing the station locations. Step 1: Pick the P and S wave arrivals on the 3 seismograms at ANMO, SBY, and CPRX. Remember that the P wave is the first arrival, and the S wave is the secondary arrival (will be the largest arrivals on the seismograms after the P wave). A hint is given on station SBY for the S wave arrival it s a tough one. Note that picking these arrivals is not always easy this is real data, which can be messy sometimes. For each seismogram, write down the time (in seconds, found at the bottom of each graph) next to the seismograms. Step 2: Compute distance using arrival times. In class, we discussed using a travel time curve, a graph that plots travel time for each phase as a function of distance from the station. We could use a travel time curve for this dataset as well, however, because this earthquake is reasonably close to the stations, we can use a formula (which can be a little easier than extrapolating from the curve).

2 The equation we will use is t s t p = R( 1 V s 1 V p ) t s is the arrival time of the S wave t p is the arrival time of the P wave R is the distance between station and earthquake V s is the S wave velocity (use 3.43 km/s) V p is the P wave velocity (use 6 km/s) Use your arrival times to calculate R for each station. Step 3: Plot distances on map. Using the map provided, you will need to draw a circle around each station with a radius of R. Use the scale bar on the map to get an accurate distance. To plot the circle, you can use a compass, or more low-tech, a piece of string cut to the proper length. The earthquake location is at the intersection of the three circles. Please turn in your seismograms (15 points) and your map (20 points). Also answer the questions below. a). Based on your location, what town is the earthquake closest to? You can look at another map if you want to get a better sense of the location. (5 points) b). Describe some of the errors involved in your earthquake location. (5 points) 2. Earthquake Hazards in the San Francisco Bay Area During an earthquake, buildings can experience significant ground shaking and liquefaction, causing damage and even failure. Type of building can be very important as well in terms of withstanding major damage during the event. In this portion of the exercise, you will explore information available about hazards and building type for the San Francisco Bay area of California to make informed decisions about risk. We will work with a completely hypothetical scenario (note that addresses in this assignment are made up, I do not know what sort of structure exists at these locations) you have been offered a job at a startup computer company in San Francisco. You are not making quite enough money to live in a house in the city, but can afford to buy a house in the suburb of Mill Valley, located north of San Francisco on the Marin Peninsula. You are lucky there are 3 houses on the market in this town in your price range. Before you buy however, you want to do a little research. Because you have taken the Natural Hazards course, you know that the region is prone to earthquakes, and you know that building damage can depend on shaking, liquefaction, and construction. You want to choose the house that would have the best chance of withstanding a major earthquake, similar to the one that occurred in 1906.

3 Knowing that the internet is a wonderful resource, you surf around to find a website that contains information and a database of hazard information for this area This site contains searchable maps that you can use to find out the susceptibility of liquefaction and expected shaking at your possible house locations during a worst-case scenario earthquake (1906 scenario). The only caveat the site works best with Internet Explorer browser. Your possible houses are at the following addresses: 50 Plymouth Avenue, zip code (Unreinforced masonry 2 story house) 45 Escalon Drive, zip code (Unreinforced masonry 2 story house) 200 Sycamore Avenue, zip code (Wood frame 2 story house) Go to the ABAG website and start by looking at the Shaking Maps/Info link. Here follow the link for Interactive (GIS) Maps for Future Earthquake Scenarios. You will get a new window read about screen resolution (adjust if necessary) and click on the circle near the top of the screen. This will take you to a map of the Bay Area with color coded shaking hazards (this can be a little slow). On the right, a list of earthquake scenarios appear choose Entire San Andreas (1906 quake). On the bottom right, under the map a location button, click on search by address, then click on find address button to get a form where you can input an address, zipcode. Put in the address of one of the possible homes. You will get a zoomed-in map with that address noted. Click on the left-hand legend View Legend to see a color scale for expected intensity, and note this for each address (you do not have to print out the maps). Next go back to the main ABAG website and click on the Liquefaction Maps/Info link. From here, follow the link to Liquefaction Susceptibility. The process will be similar as described above, and continue to use the 1906 quake scenario for you maps. Note the liquefaction hazard for each address (but you do not have to print out the maps). a). What is the expected Liquefaction Susceptibility and Modified Mercalli Intensity for each address? (15 points) b). Based on these values and knowledge from class, describe what types of geologic materials might be underneath each site. (15 points) You also have information about the construction of each house. Go back to the main ABAG webpage to the Shaking Maps/Info link. From there go to How and Why does ABAG Make These Maps and to What Does Ground Shaking Intensity Really Mean?. Table 1 on this page provides information relating to intensity and damage. Use this information to answer the question below. c). Which property would you purchase, based on your knowledge of the regional hazards and construction? Describe why you made your choice. (25 points)

4 Farmington Raton Gallup Santa Fe Albuquerque 35 ANMO Socorro 34 SBY CPRX Las Cruces El Paso km

5 Hint S wave Time (s)