Natural Hazards - Geology 209
Homework assignment #4 - Earthquakes and Seismology
Due Date: Feb. 17th
Name____________
Seismic Waves
1. Which type of seismic waves can propagate in an unbounded solid medium? Which type
of seismic waves can be detected by a vertical seismometer? Which type of seismic wave
does not pass through a liquid? For each answer explain why?
2. An Earthquake was recorded in Oakland, California (Dr. Weil's home town) by three
seismometers (see attached figure). Calculate the time and location for this earthquake by
using the following information and the seismic travel time graph on the next page.
Hours
Minutes
Seconds
Seismometer 1
P-wave arrival time
13
19
58.9
S-wave arrival time
13
20
04.7
Seismometer 2
P-wave arrival time
S-wave arrival time
13
13
20
20
02.6
10.8
Seismometer 3
P-wave arrival time
S-wave arrival time
13
13
19
19
54.7
57.4
Earthquake Location and Magnitude
3. In this problem, you will locate an earthquake using S-P travel times and determine its
magnitude using an interactive program on the web at:
http://www.sciencecourseware.com/eec/Earthquake/
Work through the entire interactive example of earthquake location and magnitude
determination and do the project. This will entail going through each of the activities listed
under the heading Main Activities. Be sure to do each of the activities in order. Also, be
sure to read the text at each step so that you understand what you are seeing and doing. The
final result, which you will turn in, is an "electronic seismologist" certificate showing that
you have completed the exercise successfully.
4. Using the table listed below, calculate the total amount of energy released each year by
earthquakes. Remember that the amount of energy released by an earthquake is proportional
to its magnitude by (use an average of the magnitude ranges):
log E = 11.8 + 1.5 M
Compare the total amount of energy lost each year by earthquakes with the energy lost each
year by heat flow which radiates up from the Earth’s crust, which equals 4 x 1013 Watts. 1.0
Watt is equal to 1.0 Joule per second. So you will have to convert your calculated total
energy for earthquakes, which is in Joules, to Joules per second.
Magnitude Range
 8.6
7.9-8.5
7.0-7.8
# of Shallow focus
earthquakes
(0-70 km)
9
66
570
# of Intermediate
focus earthquakes
(70-300 km)
1
8
214
# of Deep Focus
earthquakes
(>300 km)
0
4
66
5. a) The daily electrical consumption of the United States in 1985 was about 7 x 109 kilowatt
hours. If an earthquake released this amount of energy what would its magnitude be?
1 kilowatt hour is equal to 3,600,000 Joules. Remember that the energy released by an
Earthquake is proportional to its magnitude – see equation for problem number 4.
b) What type of mathematical relationship exists between earthquake energy and magnitude?
Make a plot of energy versus magnitude to support your answer.
c) Finish the following sentence:
The seismic energy of a M=6 earthquake is about ______________ times as large as that of a
M=5 earthquake, and is about _______________ times that of an M=4 earthquake.
6. A 100 kilometer long and 25 kilometer deep fault is located at a plate boundary where the
absolute plate motion between the two plates is 50 mm/yr. Assuming that half of the plate
motion is related to earthquake slip, how long will it take for this fault to produce an
earthquake with a moment magnitude (Mw) of 7.5? Remember that the Moment magnitude
Mw is:
Mw 
2
log 10 ( M o )  16
3
where (Mo) is the seismic moment
Mo  A  d  
(Dyne-cm)
where A is the area of the fault, d is the total amount of slip released by the earthquake and 
is the shear modulus of rock rigidity, which for typical crustal material is 3x1011 Dyne/cm2.