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Seismology Lab SWHS Geology Name: Part I: Finding epicenters with seismic wave travel time curves 1. Refer to the following seismograms of an earthquake recorded at sites in Alaska, North Carolina, and Hawaii. Estimate, to the nearest tenth of a minute, the times of first arrival of the P-waves and S-waves at each station. Then subtract S from P to get S-minus-P. 1st P arrival 1st S arrival S-minus-p Sitka, AK Charlotte, NC Honolulu, HI 2. Using the S-minus-P times and the travel time curves (on the next page), determine the focus-to-seismograph distance for each station (in miles). Focus-to-seismograph distance (mi) Sitka, AK Charlotte, NC Honolulu, HI 3. Next, find the earthquake’s epicenter using the distances just obtained. a. First use the geographic coordinates below to locate and mark the three seismic stations on the map. Sitka, AK Charlotte, NC Honolulu, HI Lat/Long 57N, 135W 35N, 81W 21N, 158W b. Use a drafting compass to draw a circle around station whose radius is equal to the focus-to-seismograph distance from #2 (use the scale to set the radius). The circles should intersect at one point – the epicenter (if the do not quite intersect at a point, then find a point that is equidistant from the three edges). Record the location of the epicenter below: Latitude Longitude 4. What is the origin time of the earthquake (at what time did it occur?) 5. At what time did the L-waves from this earthquake begin to arrive at the Sitka Station? Part II: Finding epicenters without using travel time curves 1. For this part, you will determine the epicenter of the 2001 Nisqually Earthquake. Use the three seismograms from seismic stations OSD, SEP and MBW to find the arrival time of the P-wave for each station. Then use the P-wave arrival time and the Nisqually Earthquake origin time to find the P-wave travel time for each station. The Nisqually Earthquake occurred 2/28/01 at 18:54:32 GMT P-wave arrival time1 OSD SEP MBW 1 2 Time (GMT) which P-waves arrive (P-wave arrival time) – (Earthquake origin time) P-wave travel time2 2. P=wave travel times can be converted into a distance if we know how fast P-waves travel. For continental crust, the average P-wave velocity is 6.5 km/s. Using this information, calculate the distance from each seismograph to the epicenter using the following equation: Distance = P-wave travel time * P-wave velocity Station OSD SEP MBW Distance to Epicenter 3. On the map below, use a drafting compass to draw a circle around each seismographic station, using the distances you calculated above as the radius for each circle. 4. How far from Seattle (in km) is the epicenter, and in what direction? 5. For many earthquakes, the arcs do not intersect at a point. Can you think of two possible reasons why? Part III: Earthquake Hazard and Rick Assessment 1. Figure 15.7 shows the kind of materials which buildings have been constructed on in San Francisco. They include hard Fransican Sandstone, uncompacted beach and sand dunes, and artificial fill. Imagine you have been hired to assess the risk of buying new construction on points X, Y and Z. a. What is the risk at location X? Why? b. What is the risk at Y? Why? c. What is the risk at Z? Why? 2. On October 17, 1989, just as game 3 of the World Series was about to start, a strong earthquake occurred at Loma Prieta, CA and shook the entire San Francisco area. Seismographs at locations X, Y and Z are shown in Figure 15.8. Suggest a reason why more ground shaking occurred at sites Y and Z than at X.