LAB 15
Speed of Seismic Waves and Earthquake Epicenter
Location
Purpose
The purpose of this lab is for you to identify the different velocities at which seismic waves generated
by an earthquake travel through the Earth. You will then use this information to calculate the approximate
distance to an earthquake’s epicenter.
Materials
Graph 1 & 2, Drawing compass & Colored pencils
Procedure- Do not use complete sentences for this LAB!
The first graph shows the arrival times of P waves and S waves based on the distance the location is
from the epicenter. The second graph shows the difference in the arrival time of the P & S waves based on the
distance. Use these two graphs to answer the following questions (1-8):
1. Using your graph on P and S wave travel times, approximately how long will it take an S wave to travel
4,000 kilometers?
2. Using your graph on P and S wave travel times, approximately how long will it take a P wave to travel
4,000 kilometers?
3. Using your graph on P and S wave travel times, approximately how far will a P wave travel in three
minutes?
4. Using your graph on P and S wave travel times, approximately how far will an S wave travel in three
minutes?
5. Based on the data you plotted on your graphs, which seismic wave travels at a greater velocity?
6. How far away from the epicenter of and earthquake are you if the difference between the arrival of P and S
waves is nine minutes?
7. If a P wave arrives at your location at 3:25 P.M. and the S wave arrives at
earthquake epicenter?
3:32 P.M. how far away is the
8. If a P wave arrives at your location at 11:15 A.M. and the S wave arrives at 11:17 A.M., how far away is the
earthquake epicenter?
Find the Epicenter!
Using your graph on the difference between P and S wave arrival times, the data below and help from your
instructor, locate the approximate location of an earthquake’s epicenter location on the map in Figure 15-1.
Use a drawing compass to draw the radius distance from each seismic station where the earthquake occurred.
Where the three circles converge is the approximate location of the earthquake epicenter. (SHOW YOUR
WORK!)
Example: Tucson, Arizona: P wave arrival time=11:52:30., S wave arrival time= 11:57:15
Step 1 Determine the difference in arrival time (11:57:15-11:52:30) = 4:45
Step 2 Determine the distance using the P/S wave graphs = 3,100 km
Step 3 Determine the distance in cm on the Graph : 3,100 km/250 = 12.4 cm
Step 4 Using a ruler and a compass- trace a circle with a 12.4 radius around Tucson.
Seattle, Washington: P wave arrival time = 9:41:45 P.M., S wave arrival time = 9:45:30 P.M.
Portland, Maine: P wave arrival time = 9:40:20 P.M., S wave arrival = 9:43:10 P.M.
Miami, Florida: P wave arrival time = 9:39:40 P.M., S wave arrival = 9:42:00 P.M.
9. What is the general location of the earthquake?
10. What time did the earthquake occur?
11. What time would the LA area detect the first seismic waves?