Page 431 - Laboratory Manual in Physical Geology

A CTIVIT Y 16.2 How Seismic Waves Travel through Earth

Name: ______________________________________ Course/Section: ______________________ Date: ___________

Notice the seismic data provided with the graph in FIGURE 16.5 . There are data for 11 recording stations where seismograms were
recorded after the same New Guinea earthquake (at 3° North latitude and 140° East longitude). Th e distance from epicenter
(surface distance between the recording station and the epicenter) and travel time of main seismic waves are provided for each

recording station. Notice that there are 3 lines of data from most of the recording stations. They show the travel times for all three
main kinds of seismic waves (P-waves, S-waves, and L-waves). However, instruments at some locations recorded only one or two
kinds of waves (P-waves, or P- and S-waves). Location 1 is the Australian recording station where the seismogram in FIGURE 16.4
was obtained.

A. On the graph in FIGURE 16.5 , plot points from the data table in pencil to show the travel time of each main seismic wave

in relation to its distance from the epicenter (when recorded on the seismogram at the recording station). For example, the
data for location 1 have already been plotted as red points on the graph. Recording station 1 was located 1800 km from the
earthquake epicenter and the main waves had travel times of 3.8 minutes, 7.0 minutes, and 7.9 minutes. Plot points in pencil
for data from all of the remaining recording stations in the data table, and then examine the graph.
Notice that your points do not produce a random pattern . They fall in discrete paths close to the three narrow black lines
(or curves) already drawn on the graph. These black lines (or curves) were formed by plotting many thousands of points from
hundreds of earthquakes, exactly as you just plotted your points. Explain why you think that your points, and all of the points
from other earthquakes, occur along three discrete lines (or curves).

B. Study the three discrete, narrow black lines (or curves) of points in FIGURE 16.5 . Label the line (curve) of points that

represents travel times of the P-waves. Label the line or curve that connects the points representing travel times of the
S-waves. Label the line or curve that connects the points representing travel times of the L-waves. Why is the S-wave curve
steeper than the P-wave curve?

C. Why do the L-wave data points that you plotted on FIGURE 16.5 form a straight line whereas data points for P-waves and

S-waves form curves? ( Hint: The curved lines are evidence of how the physical environments and rocks deep inside Earth are
different from the physical environments and rocks just beneath Earth’s surface.)

D. Notice that the origin on your graph (travel time of zero and distance of zero) represents the location of the earthquake

epicenter and the start of the seismic waves. The time interval between first arrival of P-waves and first arrival of S-waves at
the same recording station is called the S-minus-P time interval. How does the S-minus-P time interval change with distance
from the epicenter?

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