GG101L Earthquakes and Seismology Supplemental Reading
“First the earth swayed to and fro north and south, then east and west, round and round, then up and down and
in every imaginable direction, for several minutes; everything crushing around us; the trees thrashing about as
if torn by a mighty rushing wind. It was impossible to stand, we had to sit on the ground, bracing with hands
and feet to keep from rolling over…”
-March 28, 1868, Mr. F.S. Lyman account of Big Island earthquake
Earthquakes are vibrations of the Earth caused by large releases of energy that accompany movements of the
Earth’s crust and upper mantle usually near tectonic plate boundaries, and volcanic eruptions. Seismologists
study earthquakes in order to learn about these subsurface Earth processes with the hope of reducing damage
and loss of life. Seismologists also use human-induced seismic energy to prospect below the Earth’s surface for
natural resources and buried geological structures. Today we will perform an experimental seismology
experiment to determine how fast seismic waves travel through soil, and then using this seismic velocity,
attempt to locate some home-made earthquakes.
I. Determining the Seismic Velocity of Soil – setup and data collection
Using the SmartSeis Exploration Seismograph we will make and measure earthquakes outside either POST or
Sakamaki. Our setup consists of an array of 12 geophones (simple seismometers) connected by a cable to a
SmartSeis recording device (a seismograph). The SmartSeis produces an image of seismic vibrations (a
seismogram). We will simulate earthquakes by whacking a metal strike plate with a sledgehammer.
Figure 1 below shows the paths of seismic rays as they emanate from the strike plate. They initially start
downward because the initial vibration is downward (the way the strike plate was hit), but then curve more
horizontally and eventually up to the surface. In reality they radiate outward in all directions, but the diagram
only shows those that go in the direction of the line of geophones. And of course there are also seismic waves
that come to the surface at places where there are no geophones, but they aren’t measured so they’re not shown.
In Figure 1, the little
tic marks along the
seismic rays represent
equal intervals of time, 10
ms, for example. So in
this example it would
take ~65 ms for the
seismic energy to get to
geophone #1, ~120 ms to
get to geophone #2, and
so on. As long as seismic
noise at the area is low,
geophone #1 wouldn’t
show any vibrations until
65 ms after the strike
plate was hit, geophone
#2 wouldn’t show any
until 120 ms after the
strike plate was hit, and
Figure 1: seismic ray paths
so on. If you know the
spacing between
geophones and the time difference between arrival of the seismic energy from the hammer blow, you could
figure out the seismic velocity of the subsurface material. This is because velocity = distance/time. This is
precisely what you will be doing in the first part of today’s lab.
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Notice that in order to reach geophones that are farther from the strike plate, the seismic rays have to pass
deeper and deeper under the surface. Notice also that the rays that travel to geophones #5 and #6 pass deep
enough so that they get into an underlying layer that has a faster seismic velocity. This is represented by the 10
ms tic marks being farther apart (if the same time represents a longer distance, the velocity must be higher).
This difference between the behavior of seismic waves traveling to close geophones vs. those traveling to far
geophones will be important to remember later.
You might say “Now wait a minute, the distance that the seismic energy is traveling isn’t just that measured
across the ground from one geophone to another, it includes the part heading into the ground and then up to the
surface!” You would be right. However, the curviness of the ray paths in Figure 1 is exaggerated so that you
can see what is going on. The real situation is more like that in Figure 2. The ray paths still aren’t exactly
horizontal from the seismic source to the geophones, but they are not too far from horizontal, and we are going
to ignore this slight complication.
Figure 2: seismic ray paths in a slightly more realistic orientation
Your TA will be busy setting up the SmartSeis seismograph. Meanwhile, this is what you will do
1. Stretch out the 50 meter tape in a straight line
2. Use the Brunton compass to determine the azimuth of this line, and record it in Worksheet 1
3. Put the receivers in the ground every 2 m along the tape (but not at zero)
4. Place the metal plate (which will be the earthquake location) at the near end of the line.
5. Record the geophone spacing in Worksheet 1
6. Once your TA has everything set up correctly…
7. Generate an earthquake by hitting the metal plate with the sledgehammer. Make sure you hit with the
green side up, and that you don’t hit the hammer trigger wire! By hitting multiple times (separated by
~5 seconds), the SmartSeis will “stack” the data. The result is that the seismic signals that are the same each
time (i.e., the waves generated by the hammer) add up, whereas those that are different (random noise) tend
to cancel – you get a cleaner trace.
8. The seismogram is kind of like a graph, with geophone number on one axis and time (measured in
milliseconds – ms) on the other. One ms is 1/1000 of one second. Figure 3 on the next page is an example
of a seismogram from the SmartSeis.
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Figure 3. Example seismogram from outside POST: geophone spacing = 2 m. First arrivals have been circled.
9. The numbers up the left-hand y-axis (1, 2, 3 …) are geophone number, not the distance. To get distance,
you would have to multiply these numbers by the geophone spacing, which in the example of Figure 3 was
2 m.
10. The x-axis is time in ms, increasing to the right, and each dashed line is 5 ms. The left-hand y-axis
represents time = zero, and the timer started the instant that the hammer hit the strike plate (that’s why
there’s an electronic trigger attached to the hammer). Notice that the only labeled line happens to be 50 ms,
so when you get your hardcopy printout, it will probably be useful to write in values every 5 or 10 ms.
11. Extending to the right from each geophone’s number is a line that starts out straight (ideally, anyway),
and then starts to wiggle. This is the seismic signal recorded at each geophone. If you look at the signal at
geophone #1 above, you notice that there is a nice straight (no noise) line for about 5.5 ms. Then there is a
slight dip downward, and the maximum of this downward wiggle is reached just after 7 ms. Right where
the first deflection occurred, at 5.5 ms is the first arrival, and this is what you want to look for.
Unfortunately, it is usually very subtle.
12. That initial body wave pulse generates surface waves which have a much larger amplitude, and as you
follow the trace for geophone #1 to the right (increasing time), you can see that the ground at geophone #1
vibrated pretty significantly and continued to do so until the recording ended.
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13. Notice that the first arrival at geophone #2 is at time = 10 ms, later than when it arrived at geophone #1.
This, of course, makes sense because geophone #2 is farther from the strike plate than geophone #1 is.
14. As you go from geophone #1 to geophone #6 there is a pretty regular pattern of the first arrival and
subsequent surface waves occurring farther to the right (although geophone #6 has noisy data). This
progression to the right makes sense too, because it takes longer for the seismic energy to get out to these
farther geophones. Beyond geophone #6, the signals are unfortunately quite noisy.
15. It is important to notice that if you were going to draw a line connecting the first arrival positions, there
would be a kink at geophone #3, with one line connecting 1, 2, and 3 (and 0), and another line connecting 3,
4, 5, and 6. This kink is an indication that there is a boundary beneath the surface that is separating slower
material above (the soil) and faster material below (probably the ~70,000 year-old Tantalus lava flow).
Seismic rays that travel beyond the distance of geophone #3 (which is 6 m because they’re 2 m apart) has to
pass through this lower layer. Because sound travels faster in this deeper layer, the arrival at geophone #4 is
earlier than you would expect from just looking at the arrivals at geophones #1, 2, and 3.
16. OK, where were we? Once you get a good, clean-looking seismogram from stacking 4-5 hammer
whacks, have your TA print it out, and write on it something to the effect of “Distance Determination
Seismogram” S/he will xerox this so that each of you will have a copy. Instructions for actually
determining the velocity are presented in section III of this lab handout.
II. Determining the Location of an Unknown Earthquake – setup and data collection
17. Select a starting point somewhere in front of POST or Sakamaki, and extend the two 50 m tape
measures outward from this point at 90º from each other. Use the Brunton to make sure that it is a 90º angle
and to record what the direction of each tape is in Worksheet 2. These two tapes will form the coordinate
system for your seismic area, and will be analogous to latitude and longitude or Easting and Northing in the
UTM system. Record in Worksheet 2 and on the graph/map on p. 11 the azimuth of each axis with respect
to the place where they intersect.
18. Next, pull up the geophones (carefully) from the straight line and re-insert them roughly in a circular
loop within the angle produced by the two tape measures. Plant flags next to each geophone so that they
can be located easily. It does not have to be a perfect circle, and in fact it can actually be any shape you
want. Make sure each geophone is still connected to the geophone wire.
19. Now you need to use the tape measure to determine the location of each geophone with respect to your
coordinate system. Probably the easiest way to do this is to use the piece of rope. Have one person hold
one end of the rope at a geophone and another person pull taut it so that it is perpendicular to one tape
measure. You can use the Brunton to make sure that it is actually perpendicular if you like. Record the
distance along that particular axis for that particular geophone, and record it in Table 2. It will probably be
quicker to measure the position of all the geophones with respect to one axis first, and then measure their
positions with respect to the second axis, but it will require care not to get the numbers mixed up. Having a
team of 3-4 people working on this will help.
20. Next, the TA will send half the class around the corner so that they can’t see the seismic area. The folks
left behind will then select an “earthquake” location somewhere in the area defined by the tape measures.
Place the strike plate here, and determine its location relative to the two tape measures the same way you
determined the positions of the geophones (use the rope). Report this position to your TA, and record it in
Worksheet 2 where it says “Unknown Earthquake Location #1”.
21. With half the class still out of sight, make sure your TA has the SmartSeis all set and ready to record,
and then whack the strike plate 4-5 times as before. Remember that you hit with the green side up, and that
you need to take care not to get tangled up in the hammer trigger wire.
22. After your TA makes sure that the seismogram is good, pick up the strike plate, and bring it and the
hammer back to where your TA is. This is so that the other students don’t get any hints about where the
unknown earthquake has taken place. It might even be a good idea to put a few leaves on the spot where the
strike plate was, especially if it has made a tell-tale square indentation into the ground.
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23. Have your TA print out the seismogram, and label it “Unknown Location #1”
24. The two groups next trade places, and those who were out of sight will now repeat steps 21-23, filling in
“Unknown Earthquake Location #2” in Worksheet 2. This time have your TA label the seismogram
“Unknown Location #2”.
25. At this point you are pau with data collecting. Carefully pull up all the geophones and disconnect them
from the geophone cable. Pull up the flags also. Put all the geophones and other equipment back in the
buckets and on the cart, and return them to POST 704. Please scan the experiment area to make sure that
nothing has been left behind.
III. Determining the Seismic Velocity of Soil – data reduction and interpretation
26. While your TA is making xeroxes of
the seismograms, you can plot geophone
positions from Part II on the graph/map
provided on p. 11 of this lab handout.
Make sure to label the azimuths of the
axes with the same numbers you recorded
with the Brunton compass.
27. For the velocity determination, you
will be looking at the seismogram that was
produced when the geophones were all in
a straight line. It will be labeled “Distance
Determination Seismogram” or something
similar. Try to find the first arrival for
each geophone’s trace, and circle it. These
are pretty subtle, and even trained
seismologists sometimes argue about
them. Figure 4 shows some first arrivals
selected for the sample seismogram. Use
the time scale to determine the time of
each first arrival, and fill in the last
column of Table 1. If the first arrivals are
Figure 4: example seismogram showing first arrival picks.
too hard to see or are all messed up, you
can instead use the first full trough or peak in the seismogram trace. But you have to be consistent.
28. For the data from the example seismogram, the first arrival times for the first 6 seismograms would look
like the following:
Geophone #
0
1
2
3
4
5
6
Distance from strike plate (m)
0
2
4
6
8
10
12
Time of first arrival (ms)
0
5.5
10
14.9
15
16.5
17.5
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29. You don’t actually have a geophone #0, but if you did, its travel time and distance would both be zero.
30. Next, create a travel-time plot from the second and third columns of Table 1. Graph paper for this can
be found on page 11 of this handout. You want to plot travel time on the x-axis and distance on the y-axis.
Join your points with one or more
straight, best-fit lines, and if you have
noisy data for your farther geophones,
extrapolate the line out to those distances.
31. Is there a break in slope for your
points? In the example data (Figure 5)
the answer is yes because there is an
obvious bend in the line connecting the
points. One straight line fits the data for
out to 6 m (geophones 1-3 and zero), and
another fits the points from 6 m outward
(sort of). This means that in order for
seismic waves to travel as far as
geophone #4 (8 m), they must pass
through a layer with a higher seismic
velocity that is beneath the soil. The
slopes of lines on travel-time plots are
equal to seismic velocity. Remember, the
slope of a line is the vertical distance
divided by the horizontal distance. For
the upper layer, which seismic waves
traveling outward as far as 6 m travel
Figure 5: example travel-time plot for seismogram shown in Figure 4.
through, the slope would be:
Note the kink at 6 m (geophone #3). The more gradual slope from 0 to 6 m
(6-0)m / (14.9-0) ms = 0.402 m/ms
is an indication of a slow seismic velocity whereas the steeper slope
beyond 6 m is an indication of a higher seismic velocity. Note that the
upper slope has been extrapolated out to 18 m.
There are 1000 ms in 1 second, so to get
the seismic velocity in m/s, you multiply by 1000, and get 402 m/s. Between geophones #3 and #6, the
slope would be:
(12 – 6)m / (17.5 – 14.9) ms = 2.307 m/ms = 2307 m/s. Thus, there is quite a difference between the
seismic velocities of the soil and whatever that underlying layer is.
32. Keep track of where the break in slope between the two best-fit lines is because you’ll need this
information.
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IV. Determining the location of the unknown earthquake
If we have a travel-time plot (or we know the velocity) in a region such as the POST yard, and we know how
long it took a seismic wave to travel from an earthquake to each geophone, then we can calculate the
earthquake-to-geophone distances. Because there was only one earthquake location for each unknownearthquake experiment, the earthquake-to-geophone distances should theoretically all place the earthquake in
the same location.
33. The idea is that each geophone records nothing until the seismic energy arrives. The time between when the
hammer hits the strike plate (and starts the recording) and when the geophone records the arrival of seismic
energy tells you how long it took for the seismic energy to travel from the strike plate to that particular
geophone. If you can measure how long that time is, then you can look up the corresponding distance on the
travel-time plot.
34. All this tells you is that the earthquake was a given distance from that particular geophone; what this does is
narrow down the possible earthquake location to a circle that is centered on that geophone and has a radius
equal to the geophone-earthquake distance. This is because from the point of view of that geophone, any
location on that circle satisfies the calculated earthquake-geophone distance.
35. If you have a signal from a second geophone, it will allow you to calculate a second earthquake distance,
and plot it as a circle centered on that second geophone. Every point on that second circle will satisfy the
calculated earthquake-geophone distance for the second geophone.
36. Two circles intersect in two places, so you have narrowed down the possible earthquake position
considerably. What you need is a signal from a third geophone, and a corresponding circle centered on that
third geophone. Three circles can only intersect at a single point, and that point will be the only location that
satisfies the data from all three geophones.
37. Of course, no data are perfect, and it may be that the three circles don’t quite intersect at an exact point.
Instead, they may define a small triangle. All you can say is that the earthquake was somewhere in that triangle.
Better yet, if you have data from >3
geophones (which you do), you can
make up for some of the noisiness of the
data, and get a better earthquake
location determination.
38. Figure 6 illustrates how this would
work for data from 6 geophones (the
filled squares). A distance to the
earthquake is calculated for each
geophone, and a circle with that
distance as its radius is drawn around
each corresponding geophone. The
point where all 6 circles intersect is the
only place where the data from all 6
geophones can be accommodated, and
that must therefore be the earthquake
location.
39. From the seismogram you have for
an unknown earthquake, determine the
first-arrival time for each geophone.
Remember that the first arrival is
indicated by the first, usually very
subtle, downward deflection of the
Figure 6: diagram illustrating how data from 6 different geophones allow
you to locate an earthquake.
seismic trace.
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40. The hardest thing to do is pick out the all-important first arrivals for each geophone. Figure 4 shows
examples, and admittedly they are often pretty subtle and/or ambiguous.
41. Once you have determined the earthquake-geophone distances for each geophone and entered them in Table
2, use a compass (the circle-drawing kind, not the which-way-is-north kind) to draw circles around each
geophone on the final map. Remember, the radius of each circle will be equal to the earthquake-geophone
distance you calculated for that geophone.
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