Then, do the following to address one of the sections in the lab packet (you’ll need google earth):
Click on the following link in order to open a USGS site with various KMZ fault files:
https://www.usgs.gov/natural-hazards/earthquake-hazards/google-earth-kml-files (Links to an external site.)
Click on and download the “Quaternary Faults & Folds in the U.S.” file. Once downloaded and opened, enter the information found below.
Below are the coordinates for each seismic station, along with the epicenter of the earthquake ‘X’:
LUG: 34.365597, -117.366819
CCA: 35.152523, -118.016477
ALP: 34.687092, -118.299471
X (epicenter): 35.799212, -117.598834
Type in these coordinates and drop pins where they land. Once you’ve pinned all these locations, you’ll be able to answer questions 9 & 10.
For questions 11 & 12, refer to the earthquake lines that should be enabled from the file you downloaded from the USGS site.
Name:
Earthquakes
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 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, seen as yellow circles, propagate away from the focus as the earthquake occurs.
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1. What type of fault is shown in figure 1 (strike slip, normal, or reverse)? How do
you know? How does the hanging wall move relative to the footwall?
Seismic Waves
Seismic waves are disturbances that elastically distort the material they travel
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
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 a 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
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.
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 most intense
ground shaking and therefore the most damage to buildings and why.
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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.
Seismic stations
Seismic stations house the equipment seismologists use to record ground motion
from 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
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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- Figure 4: Cartoon depiction of a
axis or vertical is acceleration of the seismic waves seismometer
and the x-axis is time in seconds. Because Pwaves travel faster than S-waves they arrive at the
station first and 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 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 to the station first, the first peak on the seismogram is the Pwave. The second major peak is the S-wave arrival.
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Seismologist and scientist 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.
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.
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.
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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.
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Figure 5 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.
3. Based on the spacing of the P and S-wave arrivals in Figure 6, hypothesize
which station is closest to the epicenter? Why?
Table 2
Station p-wave arrival (sec)
s-wave arrival (sec)
S-P separation
Distance (km)
LUG
CAA
ALP
4. Use figure 5 to determine the arrival times for the P and S waves for all three
stations in Figure 6. Record your results in table 2.
5. Calculate the S-P separation (S-wave arrival – P-wave arrival) and record your
results in table 2.
6. Based on your S-P separations, rank the three stations in order from farthest to
closest to the epicenter.
7. Plot the S-P separations on graph 1 (page 5) to determine the distance each
station is from the epicenter of the earthquake. Record your results in table 2.
8. Does your ranking from question 6 match the distances you determined in
question 7? If not, you need to go back and make sure your calculations are
correct.
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
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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 it the file opens is should look something like the screenshot below.
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).
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Repeat these steps for stations CCA and ALP, and be sure to save each circle.
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 SP separations, etc.). There are a number of nonhuman errors that can affect the
speed of seismic waves, such as variation in rock type. Don’t stress (too much) if
your circles don’t overlap perfectly.
10. Insert a screen shot of your plotted circles in Google Earth below.
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
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mapped faults in the area.
11. Did this earthquake occur along a previously mapped fault? If not, what is the
name of the fault closest to the epicenter. To find the name of a fault click on it
(orange line). Based on the name of this fault do you think it is an important fault?
How do you know?
12. What is the name of the closest major fault? (Hint: This fault is located to the
south of X and is roughly east-west trending).
13. Turn on ‘Borders and Labels’ in the ‘Layers’ panel. Zoom into the area
around the location of ‘X’. What is the closest large city to ‘X’ based on the
development you can see in Google Earth?
Now that we’ve found the location of this earthquake let’s determine its Richter
Magnitude. Those are the two basic questions we ask about earthquakes, where was
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the earthquake located and how strong was it.
There are many ways to evaluate how strong an earthquake was, we are going to
discuss two, the Richter Magnitude and Moment Magnitude.
The Richter Magnitude Scale (or ‘local magnitude’, ML) 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 (Mw) 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 ML of the earthquake. We need two pieces of information: 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 ML:
1. 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):
2. Record the distance for station CAA from table 2 here:
3. Use figure 8 to plot the distance and amplitude values. Draw a line to
connect them and record the magnitude here:
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Figure 7: Seismogram for station CAA with millimeters on the y-axis.
Figure 8: Nomogram used to determine Richter Magnitude ML 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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14. The data shown in this lab are from the 2019 Mw 7.1 Ridgecrest earthquake.
Was the ML you determined 7.1? Based on what you know about Richter
Magnitude (ML) and Moment Magnitude (Mw) hypothesize why the values are
different.
15. The Los Angeles Times wrote an article about the Ridgecrest earthquake and
created a GIF showing the land surface before and after the rupture or
earthquake. Navigate to the below url to see the GIF
https://www.latimes.com/california/story/2019-11-13/ridgecrest-earthquakeruptured-dozens-of-faults
What is the sense of motion across this fault? Is it a right-lateral strike slip fault
or a left-lateral strike slip fault?
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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.
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.
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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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