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Microsoft Word - In-Class_Earthquake_F13.docx 1 GEOL 1110 Assignment #4: Locate That Epicenter! (15 pts) Name_________________________________________________ This assignment has 3 parts: Part 1 (5 pts) In Part 1, you will observe seismic waves for an earthquake as recorded on seismograms from 3 different seismograph stations. For each seismograph station, you will use the seismograms to determine a) the arrival times of the first incoming P waves and S waves, b) the time difference (time delay) between the P wave and S wave arrival times, and c) the distance to the earthquake epicenter from the seismograph station. Part 2 (5 pts) In Part 2, using your 3 distance estimates from each of the 3 seismograph stations, you will locate the earthquake epicenter on a map. You will need a string, pin, and pencil, OR a drafting compass to draw some arcs (see figs): Part 3 (5 pts) In Part 3, you will answer two questions related to your observations, estimates, and mapping. 2 Part 1: Observations and Measurements 1) P and S waves travel at different velocities Modified from Mies, 2004 2) The difference in arrival times between P and S waves (∆tsp) has a known relationship to distance from the earthquake epicenter (thus, if you know ∆tsp, you can determine the distance to the epicenter). Modified from Mies, 2004 3) Seismographs from at least 3 different locations (seismograph stations) are necessary to accurately determine the epicenter location. 3 Part 1: Observations and Measurements Name: 1. Observe the 3 seismograms on Page 4. 2. On one of the seismograms, determine the time (in seconds) for the arrival of the first P waves and also for the arrival of the first S waves (use hints on Page 2 to help you). Mark the arrival times on the seismogram (e.g., use a vertical line to show when the first P waves arrive, then do the same for the S waves). Record the arrival times on the chart below (in seconds - Note that the values might appear on the seismogram in minutes). Note: scientists perform this and subsequent steps with a computer these days, but you can obtain excellent estimates using this paper-and-pencil method! 3. Determine the time interval (∆tsp, in seconds) between the arrival of S and P waves. Record this value (in seconds) on the chart below. (Note: this needs to be in seconds) 4. Next, use the "Travel-Time Scale" (this ruler-like scale is provided on page 6) and the graph of Travel Time versus Distance (Page 5) to estimate the distance (in km) to the earthquake epicenter from the seismograph station. To do this, orient the Travel-Time Scale vertically (increasing numbers upward) and align the "0" value on the Travel-Time Scale with the P- wave line on the graph. Move the scale along the P- wave line until your recorded value of ∆tsp lines up with the S-wave line (see example on Page 2). The scale will now line up with the estimated distance traveled on the x-axis (the distance from the seismograph station to the epicenter). Record this value on the chart below. 5. Repeat steps 1-4 for each seismogram. Seismograph Station ID P-Wave Arrival Time (seconds) S-Wave Arrival Time (seconds) ∆tsp S-Wave Arrival Time - P-Wave Arrival Time (seconds) Distance to Epicenter (km) W12A BBR TIN 5 5 Part 1: Travel-Time vs Distance Graph Name:________________________ Travel time versus distance traveled (0 to 400 km on Earth's surface) for P and S waves. 6 CUT OUT RULER SCALE (Cut this out or fold over to use with Part 1, #4, explained above) 6 Part 2: Map of Seismograph Stations Name: 1. Using information from only one seismograph station, you can determine the distance from the station to the earthquake epicenter - but you do not know the direction from the seismograph station to the epicenter (so, you know the distance, but not the direction). 2. To locate the earthquake epicenter, you will use the information from all 3 seismograph stations. To do this, for each seismograph station, draw a circle that is centered on the station and that has a radius equal to the estimated distance to the epicenter (in km) from your chart. Use your string, pin and pencil to draw the circle (or a compass). Use the map scale below to determine the circle radius. Do this for each of the 3 seismograph stations. 3. The earthquake epicenter is located where the 3 circles intersect! San Utah California San Jose Station TIN Station W12A Arizona Los Angeles Station BBR N San Diego Mexico Part 3: Questions Geology 1110- Earthquake Exercise Name: Question 1: List the types of data you needed to locate the earthquake epicenter Question 2: List the steps you took to locate the earthquake epicenter (in other words, tell us what you did to locate the earthquake epicenter). Question 3: Briefly discuss why you think it might be important to society for scientists to be able to locate earthquake epicenters. Question 4: Where in the world is this epicenter? Get on Google Maps and report what geographic features or places are nearby and what population centers (over 100,000 people) are nearest it: Question 5: Finally, go to the U.S. Geological Survey’s awesome interactive fault map for the country: https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf 88412fcf and try this state’s interactive fault map: http://maps.conservation.ca.gov/cgs/fam/ Can you identify a likely fault? How active is it? What is its name? What type of fault is it? https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf http://maps.conservation.ca.gov/cgs/fam/ GEOL 1110 Assignment #4: Locate That Epicenter! (15 pts) This assignment has 3 parts: Part 1 (5 pts) Part 2 (5 pts) You will need a string, pin, and pencil, OR a drafting compass to draw some arcs (see figs): Part 3 (5 pts)