Laboratory Manual for Introductory Geology 4e

EXERCISE 11.4
Locating an Earthquake’s Epicenter and Determining When It Occurred
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Park the cars, and let’s tackle an earthquake. Use the following series of steps to get all the information you need to
identify the location of the earthquake and pinpoint when it occurred:
1. Identify the four different seismic wave types on seismograms from three seismic recording stations.
2. Determine the arrival times and measure the delays between different wave types.
3. Use these data and the travel-time diagram to estimate each station’s distance from the epicenter.
4. Use triangulation to locate the epicenter.
5. Determine the time of faulting with the travel-time diagram.
Step 1: Reading a Seismogram
You will need to know what P-, S-, L-, and R-waves look like on a seismogram in order to identify them correctly.
Each seismic wave causes the ground to shake differently (see Fig. 11.2), and this produces a different appearance on a
seismogram. The differences are in wave amplitude (height) and frequency (the time between adjacent wave peaks).
These differences are summarized in the figure below and will help you interpret the seismograms in the next part of this
exercise.
This close-up of a seismogram shows the signals generated by different kinds of seismic waves.
1st seismic
wave arrives
2nd seismic
wave arrives
3rd seismic
wave arrives
4th seismic
wave arrives
Normal background seismicity
1 minute
P-wave
Lowest amplitude
Peaks 2–10 seconds apart
S-wave
Amplitude > P-wave
Peaks 2–10 seconds apart
L-wave
Amplitude > S-wave
Peaks > 40 seconds apart
R-wave
Highest amplitude
Peaks ~20 seconds apart
Now that you know what different types of waves look like on a seismogram, let’s see how to measure their arrival
times. The figure on the next page shows how waves look on a typical seismogram printout. The dashed vertical lines
are time markers and are 1 minute apart. The waves reflect time moving forward from left to right on each row, and
as a row ends, the time starts again on the left side of the next row below it (just like lines in a book). Seismic waves
rarely arrive precisely on a minute marker, so you have to estimate the number of seconds before or after each minute.
[Note: To avoid confusion involving time zones and changes to and from Daylight Saving Time, seismogram times are
recorded in Greenwich Mean Time (GMT)].
(continued)
11.3 LOCATING EARTHQUAKES
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