Laboratory Manual for Introductory Geology 4e

EXERCISE 11.4
Name:
Course:
Locating an Earthquake’s Epicenter and Determining When It Occurred (continued)
Section:
Date:
Step 5: Determining the Time When an Earthquake Occurred
When, exactly, did the earthquake occur at the epicenter? Align one of your station worksheets from Appendix 11.1 with the travel-time
curves in Figure 11.5. Draw a horizontal line from the P-wave intercept on your worksheet to the time scale on the vertical
axis, as shown by the green dashed arrow in the travel-time diagram on page 287. Read directly how many minutes the P-wave
took to travel to the station, using a millimeter ruler to estimate the number of seconds—in this case, 9 minutes and 35 seconds.
Subtract that amount of time from the P-wave arrival time in the data table of this exercise to get the time when the
earthquake occurred. Repeat for the other two stations—the answer should be the same for each.
Time of the earthquake based on Seattle : : Boston : :
Los Angeles : :
11.4 Measuring the Strength of an Earthquake
The last piece of information we need is the strength of the earthquake. There
are two ways to describe the strength of an earthquake. The Mercalli Intensity Scale
is based on the amount of damage sustained by buildings. But because damage to
buildings can depend on factors unrelated to the energy released by an earthquake—
such as the quality and nature of construction and the type of ground beneath the
buildings—this method is not very useful in our study of the Earth. In 1935, Charles
Richter designed the first widely accepted method for estimating the energy released
in an earthquake: the Richter magnitude scale, which is based on the amount of energy
released during faulting and calculated from the amount of actual bedrock motion.
Each level of magnitude indicates an earthquake with ground motion 10 times greater
than the next lower level. Thus, a magnitude 4 earthquake has 10 times more ground
motion than a magnitude 3 and one-tenth that of a magnitude 5.
We now know that Richter’s method is accurate only for local, shallow-focus
earthquakes. Modern estimates of earthquake strength use different methods
involving body waves, surface waves, and a moment magnitude scale to calculate accurately
the magnitude of shallow- and deep-focus, local and distant, and large and
small earthquakes. They also depend on complex analyses of the rock type that
broke in the fault, how much offset took place at the fault, and other factors that
can’t be determined from seismic records alone. In Exercise 11.5, we will use a
simple graphical method to determine the magnitude of an earthquake.
EXERCISE 11.5
Determining the Magnitude of an Earthquake
Name:
Course:
Section:
Date:
In this simplified exercise, you will estimate m b
, the magnitude of an earthquake based on the amplitude of a body wave
(P-wave). Because ground motion decreases with distance from a seismic station, distance from the epicenter must also
be taken into account. To determine m b
of the earthquake you measured in Exercise 11.4, mark the left-hand scale of the
chart on the next page at the appropriate S-P delay for one of your stations to account for distance from the epicenter.
Measure the maximum P-wave amplitude and mark it on the right-hand scale. Now draw a line connecting these two
points. The value for m b
is found where the line intersects the center magnitude scale.
(continued)
11.4 MEASURING THE STRENGTH OF AN EARTHQUAKE
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