Transcribed Text
1. On each of the seismograms on page 3, analyze the recorded ground motion (the
squiggly lines on each seismogram) to determine the signals of the first P and S waves
(surface waves are not shown on those seismograms). Pages 337 and 338 in your text
might help you make the picks. Keep in mind that:
the P waves arrive first, but creates the lesser amount of ground motion. Don't
be fooled by very small background motions that occur prior to the P waves'
arrival.
the S waves arrive second, they show an increase in ground motion relative to
the P wave
no surface waves are shown on these seismograms
Mark the first arrival of each P and S wave on the seismogram using a vertical line.
Extend the line upward to the "Time since earthquake" scale, and determine the time
when that wave type first arrived (in seconds  note that the values appear on the
seismogram in the form 00:00:00, which stands for hours:minutes:seconds) Record
the time of the P and S wave in the chart below.
2. Determine the time interval (Atsp, in seconds) between the arrival of S and P waves.
Record this value (in seconds) on the chart below.
3. Use the plot of travel time interval (Atsp, in seconds) versus distance shown on page 3
(top of next page) to determine the distance (in kilometers) to the earthquake epicenter
from each seismograph station. Record this value on the chart below. An example is
shown in red arrows for a Atsp of 90 seconds and explained on the top of page 4.
4. Repeat steps 13 for each seismogram.
Seismograph
Pwave
Swave
Atsp (difference in
Distance to the
Station ID
arrival time
arrival time
arrive times, in
epicenter (km)
(seconds)
(seconds)
seconds)
BBR
TIN
W12A
2
Part 1: Seismograms
Time since the earthquake
1
00:00 00:00:20 00:00:40 00:01:00 00:01:20 00:01:40 00:02:00 00:02:20
00:02:4
80
60
Station BBR
40
20
20
40
60
80
Time since the earthquake
2
00:00 00:00:20 00:00:40 00:01:00 00:01:20 00:01:40 00:02:00 00:02:20 00:02:4
I
I
100
80
Station TIN
60
40
X
20
SIL.
0
20
40
60
80
100
Time since the earthquake
3
00:00 00:00:20 00:00:40 00:01:00 00:01:20 00:01:40 00:02:00 00:02:20 00:02:4
I
21.00
18.00
15.00
Station W12A
12.00
EVO
9.00
6.00
X
3.00
$
0.00
3.00
6.00
9.00
12.00
15.00
18.00
Red lines show an example of how to use this graph. If you know Atsp is 45 seconds for
seismogram XXX, find that value on the xaxis, project upward along the red arrow to
the diagonal line, then horizontally to the yaxis to estimate distance between
seismograph stationXXX and the epicenter. In red line example, that distance is  360
kilometers.
Now repeat that process for the Atsp values you put into the chart on the bottom of the
prior page.

I
400
300
200
100
10
20
30
40
50
Atsp Interval Time (sec)
Now you will go to Part 2, BUT If you did not pick the correct first arrivals for the P and
S waves in part 1 (for any or all seismographs), you will not get a solution to the
earthquakes epicenter in Part 2 and will have to go back to part 1 and reexamine the
seismographs. You may have misidentified one or more of the first P and S waves.
And if so, then you have an erroneous Atsp, which also means an erroneous distance
determined from the above graph. Analyzing the solution you initially get in Part 2, and
deciding what might be wrong with it is a key part of this exercise (i.e., monitor & reflect
on you own work as a selfregulated learner).
And yes, the above paragraph says the same thing as what is on page 1, but Dr. Budd
has learned over the years that not all students take the time to not only read the
information provided, but also process it.
4
Part 2: Map of
Seismograph
Stations
Locate that
Epicenter!
1. Using information from only one seismograph station, you can determined the distance
from the station to the earthquake epicenter  but you do not know the direction from the
seismograph station to the epicenter (you know the distance, but not the direction).
2. To locate the earthquake epicenter, you will use the information from all 3 seismograph
stations. To do this, for each seismograph station, draw a circle that is centered on the station
and that has a radius equal to the estimated distance to the epicenter (in km) from your
chart. Use S compass to draw the circle. Use the map scale below to determine how much
to open your compass. Do this for each of the 3 seismograph stations.
3. Put an "X" on the spot where the three circles tell you is the location of the epicenter.
Nevada
Utah
Califomia
an Jose
TINO
W12A
Los Angeles
Arizona
BBR
47
San Diego
Mexico
100
200
300
400
500
kilometers
5
PART 3: Determine the earthquake magnitude.
The magnitude of an earthquake can be determined from the maximum amplitude of the
Swave (your text, p. 339).
1. Determine the maximum amplitude (height) of the Swaves on each of the
seimograhs. Draw a horizontal line from that maximum to the yaxis and determine the
amplitude in. Record the maximum amplitudes for each seismograph in the chart below
2. Convert maximum amplitude to millimeters by dividing by 20. Record that in the
chart.
Seismograph Station ID
Max amplitude
Column 2 divided by 20 = Max
amplitude in millimeters
BBR
TIN
W12A
3. Using the nomograph below, determine the magnitude of the earthquake. Draw a line
connecting the distance of each seismograph from the epicenter (left vertical line) to the
amplitude you calculated for that seismograph (right vertical line). The three lines you
draw (one for each seismograph) should intersect the middle vertical line at one point,
which is the earthquake's magnitude. Because we are doing this by estimating values
off of graphs, don't be surprised if one of the lines does not intersect the others at the
same point. If two intersect, we will consider the job down. Record that magnitude in
large writing in the blank line (make it easy for the grader to see)
500
800
8.0
700
200
Magnitude of the
600
7.0
100
quake is
500
50
6.0
400
20
300
5.0
10
8
6
4
200
4.0
2
100
3.0
1
60
0.5
2.0
30
0.2
1.0
0.1
20
Distance (km)
Magnitude
Amplitude (mm)
6
PART 4: Questions.
1. What were some of the difficulties you had in deciding what was the first P wave at
each seismograph. Use sketches to illustrate those difficulties.
2. What were some of the difficulties you had in deciding what was the first S wave at
each seismograph. Use sketches to illustrate those difficulties.
3. Did the circles exactly intersect on the map in one point? If not, what are some
ideas that you have to explain why they did not do so exactly?
7
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