Lab 4 Earthquakes doc (1)
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San Diego State University *
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Dec 6, 2023
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GEOL101 Dynamics of the Earth – Fall 2023 Name:
Emily Thomson
Laboratory 4: Earthquakes Section:
Introduction
Earthquakes generate shaking and vibrating of the land surface. Such a phenomenon
commonly is produced when Earth material (rocks) ruptures during brittle failure (breaking)
along an old or new fault releasing stored up energy as ground displacement seismic waves.
Think back to the Plate Tectonics lab, all three of the plate boundaries are capable of
producing earthquakes. The Earth’s plates are not in constant motion, instead they move in
sudden bursts and each burst results in an earthquake. It is important to note that not all
earthquakes are generated by movement along brittle faults. In fact, earthquakes can be
generated during volcanic eruptions and nuclear explosions. Here, for the sake of simplicity,
we only consider earthquakes generated during rupture along a new or old fault.
Earthquakes can occur at a variety of depths in the Earth’s crust. The depth where
they generate from is called the
focus
or
hypocenter
(figure 1). Located directly above the
focus on the earth’s surface is the
epicenter
(figure 1)
.
When enough energy is stored along
a fault to overcome the strength of the rock, it will break releasing energy as seismic waves
that travel away from the focus in all directions as spheres (figure 1). A common analogy for
this is dropping a pebble into water and watching the ripples (waves) travel away from where
the pebble was dropped. Seismic waves are generally strongest at the focus and gradually
grow weaker further away from the rupture site.
Figure 1: Block diagram illustrating the locations of focus and epicenter along a fault.
Seismic waves propagate away from the focus as the earthquake occurs.
Question 1. What type of fault is shown in Figure 1 (strike slip, normal, or reverse)? How
do you know? How does one side of the fault move relative to the other?
It is a strike slip and I can tell by the direction of the arrows. One side of fault moves forwards
and one backwards, in opposite directions
Seismic Waves
Seismic waves are disturbances that elastically distort the material they travel
1
through. Meaning after a seismic wave has passed through a portion of the Earth it returns to
its original form. Seismic waves include body and surface waves. The former type of wave
emanates spherically from the focus traveling
entirely within
the interior of the Earth while the
latter travels along
the surface
of the Earth. Body waves are compressional or P-waves and
shear or S-waves. Surface waves are Love and Rayleigh waves. For the purposes of this lab
we are going to focus on body waves.
P-waves
or primary waves are compressional, meaning that the p-wave energy moves
outward from the focus it produces a series of contractions and expansions (Figure 2b) in the
direction of wave movement. You could think about this like a slinky being pushed and pulled.
Typical P-waves speeds range from 5 to 8 km/s, but they can be much lower near the ground
surface. The speed at which they travel depends on part of the Earth’s interior it’s traveling
through. Generally, the speed of the P-waves increases with depth.
S-waves
or secondary or shear waves have an up/down and/or side-to-side motion (Figure
2c) as the seismic wave energy moves outward from the focus. The motion of S-waves is
similar to shaking one end of a rope. S-waves are not able to travel through liquid like
P-waves. They have average speeds of around 3.5 km/s in crustal material like granite, but
S-wave speed can be much lower near the ground surface.
Figure 2: a. A section of
undisturbed material within the
Earth. b. The same section of Earth
material but with a P-wave passing
through. Notice that in areas of
contraction the squares are smaller
and in areas of expansion they are
bigger as the wave moves through
the material. Wave movement or
propagation is from left to right. c.
S-wave moving through the same
section of Earth material. As the
wave propagates from left to right,
notice that the size of the squares
does not change, rather they move
up and down.
Question 2. Based on the descriptions of the motion associated with P and S waves
above and in Figure 2, hypothesize which wave type causes the least intense ground
shaking and therefore the least damage to buildings and why.
I predicts that P waves cause the least intense ground shaking because they only
travel in a push pull method, while S Waves travel in all directions therefore having a
greater amplitude.
Seismic Stations
Seismic stations house the equipment seismologists use to record ground motion from
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earthquakes. Globally, there are thousands of stations on land and on water recording in real
time. Seismic stations consist of a
seismometer
(records the ground movement), a computer,
communication equipment (antenna and gps), and often a solar panel to power everything
(Figure 3).
Figure 3: Schematic illustrating how a seismic station receives, records, and transmits data from
an earthquake. usarray.org/about/how
Figure 4 demonstrates how a Seismometer can record up and down motion (not used for
practical purposes anymore). A weight hanging from a spring is attached to the seismometer
frame and when an earthquake occurs the relative motion between the weight and the moving
Earth provides a measure of the ground motion. The movement is recorded onto a
seismogram
with a pen attached to the weight. More modern seismometers record and store the data
digitally, by inducing electrical currents from a magnet moving along with the ground.
Seismograms allow us to visualize the arrival times of
the different seismic waves, P and S. Figure 5 is an
example of what seismograms look like. The y-axis or
vertical is acceleration of the seismic waves and the
x-axis is time in seconds. Because P-waves travel
faster than S-waves they arrive at the station first and
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are recorded first (blue arrow, Figure 5). S-waves travel
a bit slower and arrive at the seismic station after
P-waves, but because of the way S-waves travel (up
and down/side-to-side) they produce greater
accelerations (y-axis) or higher peaks on the
seismogram (red arrow, Figure 5). If you have ever
experienced an earthquake you have likely felt the
S-wave but probably not felt the P-wave.
Figure 4: A classic seismometer based
on weight suspended on a spring.
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Figure 5: Example of a seismogram with the P-wave arrival in blue and the S-wave arrival in red.
P-waves travel faster and arrive at the station first, so the first peak on the seismogram is the
P-wave. The second major peak is the S-wave arrival.
Seismologists and scientists use the difference between the P and S-wave arrival times (called
the
S-P time interval
) to determine the distance the station is to the epicenter of the earthquake.
The closer the station is to the epicenter the closer the P and S-wave arrivals will be and vice
versa. In Figure 5 the
S-P time interval is 9 seconds
(34 s – 25 s = 9 seconds). We use the
S-P interval to determine the distance the seismic station is away from the epicenter with the
help of graph 1.
To determine the
distance between
a seismic
station and the
epicenter of an
earthquake, find
the S-P separation, in this example 9 sec on the
y-axis and follow that time over until it intersects
the S-P line on the graph. Follow this
intersection down to the x-axis and read the
distance, in this example the distance is ~ 90
km. Meaning the earthquake occurred 90 km
away from the seismic station in any direction.
Unfortunately, we do not yet know in what
direction. For this we need data from at least
three stations.
Graph 1: Distance is on the x-axis in km and
time in seconds is on the y-axis. For this lab we
are only going to use the S-P line that plots
distance vs. time.
4
Figure 6: Three seismograms from a 2019 earthquake, time in seconds is on the x-axis. Blue
arrows indicate the P-wave arrival and red arrows the S-wave arrival.
Figure 6 is showing seismograms from three seismic stations, LUG, CAA, and ALP, the x-axis is
showing time in seconds. For each station the P-wave arrival is indicated with a red arrow and
the S-wave arrival with a red arrow.
5
Question 3. Based on the spacing of the P and S-wave arrivals, hypothesize which station
is furthest to the epicenter? Why?
S wave is furthest from the epicenter because they are slower than the P waves, and the larger
the arrival time difference, the further the location is
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Question 4. Use Figure 6 to determine the arrival times for the P and S waves for all three
stations. Record your results in Table 2.
Question 5. Calculate the S-P separation (S-wave arrival – P-wave arrival) and record your
results in Table 2.
Question 6. Based on your S-P separations, rank the three stations in order from closest
to furthest to the epicenter.
CAA, LUG, ALP
Question 7. Plot the S-P separations on Graph 1 to determine the distance each station is
from the epicenter of the earthquake. Record your results in Table 2.
Station
s-wave arrival (sec)
p-wave arrival (sec)
S-P separation
Distance (km)
CAA
25 sec
13 sec
12 sec
120 km
ALP
40 sec
25 sec
15 sec
140 km
LUG
43 sec
29 sec
14 sec
150 km
Table 2.
8. Does your ranking from Question 6 match the distances you determined in Question 7?
Why or why not?
Yes because I ordered them from closest to furthest from the epicenter and the one listed
closest has only a 12km separation distance and the numbers increase further from there.
To determine the location of the epicenter the location of the three seismic stations are placed
on a map and the distance to the epicenter (question 7) is used to draw a circle with the center
placed at each station and the radius equal to the distance to the epicenter. Where the three
circles overlap is the approximate location of the epicenter.
To visualize these locations and draw circles we are going to use Google Earth. Download and
open ‘Earthquake.kmz’ in Google Earth by double clicking on the file. When the file opens it
should look something like the screenshot below.
6
The three seismic stations are shown with their names as yellow push pins. Also shown are the
Quaternary aged faults as the United States Geological Survey (USGS) has them mapped. Turn
off all other layers besides ‘Earthquakes’.
To draw circles in Google Earth follow the steps below for station LUG:
1. Open the ‘ruler tool’ (screen shots available at the end of the lab for help with the ruler tool).
2. A dialog box will open, select the ‘circle’ tab along the top and change the units next to
‘Radius:’ to Kilometers.
3. With the ruler tool open, zoom into the LUG pin close enough that you can see where the pin
is sticking into the Earth and click once to place the center of the circle on this location.
4. Now, zoom out far enough that you can see all three seismic stations and move your mouse
away from the LUG station. You should see a yellow circle with a radius that increases as you
move your mouse away from the station. Move your mouse away from LUG until it reaches the
distance you determined from question 5. It can be hard to get the exact distance, but try your
best. Once you’ve drawn a circle with the correct radius click again.
5. Save the circle by clicking ‘save’ in the ruler dialog box, name the circle after the station
(LUG).
Repeat the five steps above for stations CAA and ALP, and be sure to save each circle.
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Question 9. All three circles ‘should’ overlap at the same point, the epicenter of the
earthquake, or close. If your circles do not overlap at the same point, hypothesize where
in the methods error was introduced (i.e., reading seismograms, plotting S-P separations,
etc.). There are a number of nonhuman errors that can affect the speed of seismic waves,
such as variation in rock type.
The circles did not overlap because of common human error and because we were estimating
the times and distances so they were not super exact measurements with a ruler. However they
are pretty close to overlapping so we were on the right track.
Question 10. Submit a screenshot of your plotted circles via Canvas. Make sure your
screenshot includes the date and time it was taken!
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Turn on the ‘X’ pin by clicking in the box to its left so a black check mark appears. This pin marks
the actual epicenter of this earthquake. Zoom in so you can see all the mapped faults in the
area.
Question 11. To find the name of a fault click on it (orange line). Did this earthquake occur
along a previously mapped fault? Based on the name of this fault do you think it is an
important fault? How do you know?
Garlock Fault Zone. it is very important and second largest in southern California after San
andreas. It occurred with the other one they both show on google earth.
Question 12. What is the name of the closest major fault? (Hint: Look around on google
earth to see where the nearest continuous fault line is).
The Closest major fault in the San Andreas fault line.
Question 13. Turn on ‘Borders and Labels’ in the ‘Layers’ panel. Zoom into the area
around the location of ‘X’. What is the closest developed city to ‘X’ based on the
development you can see in Google Earth?
Ridgecrest looks like the most developed city by the X because we can see roads and
highways and closer than that is just canyon and rock and no city
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Now that we’ve found the location of this earthquake, let’s evaluate how strong an earthquake
was. To do this, we will discuss two methods, the
Richter Magnitude
and
Moment Magnitude
.
The
Richter Magnitude Scale
(or ‘local magnitude’, M
L
) is the most well-known scale and was
introduced by the seismologist Dr. C. F. Richter of California Institute of Technology in Pasadena
in 1935. It is determined by the amplitude of the largest seismic wave in millimeters from the
zero line and the distance from the epicenter. The Richter magnitude of an earthquake is a
number: about 3 for earthquakes that are strong enough for people to feel and 8 or larger for the
strongest earthquakes (largest measured event was a M9.5). The scale is logarithmic, meaning
for every increase on the scale, the magnitude increases 10-fold. The Richter Magnitude is also
known as local magnitude and is easy and quick to calculate. [the energy actually increase by a
factor of 32 for a log unit, but you probably don’t want to get into that]
Moment Magnitude
(M
w
) is based on physical properties of the earthquake analysis of all the
waveforms recorded from the shaking. It considers several factors such as the rigidity or
strength of the rock, the area of the fault that slipped or moved, and the distance that the fault
moved. This information is used to calculate first the Moment then the Moment Magnitude that is
roughly equal to the Richter Scale for earthquakes smaller than about 6.
Because the Richter Magnitude is quick and easy to calculate, we will use this method to
determine the M
L
of the earthquake. To do this, we need two pieces of information: the
distance
from the S-P time and the
Maximum Amplitude of the seismic waves.
We already have the
distance so let’s determine the maximum amplitude.
Follow the steps below to determine the M
L:
Question 14. Measure the distance in mm from the zero line up to the maximum
amplitude from Figure 7 and record this value here (could be negative or positive):
145 mm
Question 15. Record the distance for station LUG from Table 2 here:
140 km
Question 16. Use Figure 8 to plot the distance and amplitude values. Draw a line to
connect them and record the magnitude here:
Magnitude = 5
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Figure 7: Seismogram for station LUG with millimeters on the y-axis.
Figure 8: Nomogram used to determine Richter Magnitude M
L
by plotting the max amplitude of a
seismogram and its distance from the epicenter. Connect these two points with a line and where
this line intersects the Magnitude scale read the magnitude.
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Question 17. The data shown in this lab are from the 2019 M
w
7.1 Ridgecrest earthquake.
What was the M
L
you determined? Based on what you know about Richter Magnitude (M
L
)
and Moment Magnitude (M
w
) hypothesize why the values are either the same or different.
We determined that the magnitude is 5. Moment Magnitude is favored over Richter
Magnitude because it can be more accurate over measuring events at a larger scale but
they overall have the same purpose.
Question 18. The Los Angeles Times wrote an article about the Ridgecrest earthquake
and included excellent close-ups of the surface rupture produced by this earthquake:
https://www.latimes.com/california/story/2019-07-22/ridgecrest-earthquake-images-broken-ground
What type of fault is shown? What is the sense of motion across this fault?
This is again a strike shift fault. The motions are faults moving in opposite
directions but stay leveled on the ground.
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Google Earth help:
When Google Earth (GE) first opens, all the ‘Layers’ are
turned on. This can be distracting and make finding what you
are looking for hard. Turn off everything in ‘Layers’ to make
your life easier. When a layer is on, a check mark will be in
the box to its left. To turn it off click the box and the check
mark will
disappear.
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To expand a folder to see its subfolders/layers, click the black
arrow head located to its left. An expanded folder has a black
arrow head pointing down. A collapsed folder has a black
arrow head pointing to the right.
The ruler tool is located in the menu bar at the top of Google Earth, to the right of the search bar
and it looks like a small blue ruler. Click the ruler tool once to get the ruler tool dialog box. Make
sure the circle tab is open. You can change the units by clicking the drop-down box located to
the right of ‘Radius’.
Once you have the units set, zoom in to one of the stations close enough that you can see the
pin sticking into the Earth’s surface and click once. You do not click and drag. Now move your
mouse away from the station and watch the radius distances in the ruler tool dialog box, until
you reach the desired distance and click again.
Be sure to save your circles by clicking ‘Save’ in the bottom right of the ruler tool dialog and
name them after each station. To measure another radius, open the ruler tool again. To close
the ruler tool, click the red x located in the top right corner of the ruler dialog box.
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