Finding the Epicenter: Name____________________
Introduction: Most earthquakes occur in remote areas where people are not present. They sometimes occur underwater in oceans. They may leave a scar on the land where the fault slips, but sometimes they do not. Even if no one is at the epicenter (the location where the fault slips), scientists can still locate it. In this activity, you will learn how P and S waves are used to find the epicenter and the location of two epicenters. You will use a principle called “triangulation”. To triangulate, you find the distance a place is from 3 other locations. Using three points, it can only be in one place. Triangulation is the same technology used in your GPS on your phone. Add your answers in RED INK.
Materials: drawing
compass (you can replicate this technique virtually- computers are good at making perfect circles)
Support: Watch this video on how to complete this assignment. You can also reference the procedure directions listed below.
Video Link- How to Locate the Epicenter
Procedure:
1. Record the p- and s-wave arrival times on your data table. Use the charts below. Each start time is the very first infection in the wave (not the peak amplitude). Each little line is about 1.5 seconds.
2. Subtract the p-wave arrival time from the s-wave time to find the
lag time. Lag time equals s-wave start time minus p-wave start time.
3. Convert seconds to minutes to use the
Travel Time Chart to determine the distance from the epicenter. You are determining where, between the P-curve and S-curve (on the graph), the correct lag time is. See video example.
4. Use the scale in the middle of the maps to find the distance from the station. This distance obtained is the
radius from the seismic station to the epicenter. But only plotting one station means that the epicenter can occur ANYWHERE within that radius from that station, so plotting three distances from three stations is key. (EX: Chicago is 1600km from the epicenter, so you should draw a circle around Chicago with a radius of 1600km). Repeat with other station information.
5. Use the drawing compass/computer to draw a circle around the station at the correct distance/radius from the epicenter.
6. Complete all four stations (the fourth station is for accuracy). The epicenter is the place where at least three of the circles meet/cross/intersect.
Put a star on that spot.
7. I understand there will be some sway with your results, so place your star in the closest/most accurate position.
Notice: You may need to edit the map outside of the Word document. You can copy and paste the map and edit the image (to add circles to it) in a program like PowerPoint. Then you can “snip” (but using the snipping tool) or screenshot your final product and save it as an image. Replace your finished copy image with the original blank map provided below. Edit the document as needed, but you are submitting this edited document as a PDF to the assignment block.
Data: Here are the following recordings from multiple seismic stations across the US. Chicago is done for you.
Data Table: Complete the following data table using the seismograms above. You will need to use the graph on the next page to complete the “Distance from City” column.
Earthquake 1
|
Seismographic Station City |
p-wave arrival time |
s-wave arrival time |
Lag Time |
Distance from City |
|
Chicago |
18s |
152s |
134s |
1600km |
|
Kansas City |
|
|
|
|
|
Santa Barbara |
|
|
|
|
|
Seattle |
|
|
|
|
Earthquake 2
|
Seismographic Station City |
p-wave arrival time |
s-wave arrival time |
Lag Time |
Distance from City |
|
Newport |
|
|
|
|
|
Tucson |
|
|
|
|
|
McMinnville |
|
|
|
|
|
Rockville |
|
|
|
|
Analysis:
1. Between the S & P waves, which one arrives first? Why?
2. What instrument recorded the information from the P and S-wave arrival times?
3. Why didn’t the P-waves arrive in each city at the same time?
4. Does the lag time get shorter or longer the further you get from the epicenter?
5. How could you prove an epicenter was where you calculated it to be?
Distance graph: As shown in the video, take the note card and line it up along the Y-axis, put a small mark on your notecard at zero, and another mark at the lag time (data table). Once you have your marks, slide the notecard to the right until the bottom mark (zero) lies up with the P-Wave curve, and the upper mark (Lag time) lines up with the S-Wave curve. Where those lines meet, follow the line to the X-axis and determine the distance.
Plotting the distance: Once you have your distance from above, adjust your compass/circle to that distance (radius) using the scale at the bottom of the map. Draw your circles next to each city, and where they all intersect is the location of the epicenter.
