Laboratory Manual for Introductory Geology 4e

EXERCISE 11.5
Name:
Course:
Determining the Magnitude of an Earthquake (continued)
Section:
Date:
(a) What is m b
for an earthquake with exactly the same P-wave amplitude as the example in the chart below (red line)
but with an S-P delay of 30 seconds? 10 seconds?
(b) What is the relationship between wave amplitude, magnitude, and distance from the epicenter?
(c) What is m b
for an earthquake with exactly the same S-P delay time as the example in the chart below, but with a
P-wave amplitude of 2 mm? 100 mm?
(d) What is the magnitude of the earthquake you measured in Exercise 11.4, based on the following data from three
stations close to the epicenter? Station A: S-P delay 5 20 s, P-wave amplitude 5 100 mm; Station B: S-P delay 5 40 s;
P-wave amplitude 5 12 mm; Station C: S-P delay 5 50 s, P-wave amplitude 5 7 mm.
Note: The seismograms in Exercise 11.4 are artificial. If they were from a real earthquake, the values for m b
calculated from
each should be nearly identical.
Determining the magnitude of an earthquake by body wave amplitude.
Distance
S-P delay
(s)
Magnitude
Maximum
wave
amplitude
400
50
40
6
100
50
300
30
5
20
200
20
4
10
5.0
100
60
40
10
8
6
4
3
2
2.0
1.0
0.5
20
5
1
0.2
0.1
5
0
290 CHAPTER 11 EARTHQUAKES AND SEISMOLOGY