Laboratory Manual for Introductory Geology 4e

EXERCISE 11.1
When the Ground Shakes
Name:
Course:
Section:
Date:
When P- and S-waves reach the surface, the ground vibrates, and anything built on it is shaken in directions that depend
on which wave type is involved and the distance from the epicenter. The figure below shows the arrival of P-, S-, L-, and
R-waves at a skyscraper.
Ground shaking caused by seismic waves
P-wave at
epicenter
P-wave far from
epicenter
S-wave at
epicenter
S-wave far from
epicenter
L-wave
R-wave
(a) Draw arrows to indicate every direction in which each of the buildings will move in response to the different types of waves.
(b) Why do the P- and S-waves at an earthquake epicenter make the ground shake differently from the ground in an area
far from the epicenter?
11.3 Locating Earthquakes
Locating earthquakes helps us to understand what causes them and to predict
whether an area will experience more in the future. Most earthquakes occur in
linear belts caused by faulting at the three kinds of plate boundaries (FIG. 11.4)
and are used to define those boundaries. But intraplate earthquakes also occur, and
their causes are less well understood. How can we locate earthquake epicenters,
especially when they are in remote areas? Seismologists triangulate epicenter locations
by using sophisticated mathematical analysis of the arrival times of the different
seismic wave types at many seismic recording stations.
In this manual, we use a much simpler method to locate an epicenter, and determine
precisely when the earthquake occurred, with data from only three seismic
stations. The basis for this method is the fact that the four types of seismic waves
travel at different velocities. The same reasoning permits us to locate epicenters and
to determine the precise time of the faulting that causes individual earthquakes.
Let’s look at location first, then timing.
11.3 LOCATING EARTHQUAKES
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