Name: _________________________________________GOL 105: Physical Geology
Earthquake Laboratory Assignment
Objective:
This assignment is meant to give you practical experience on locating the epicenter of an earthquake and
determining its magnitude from seismogram data.
Procedures:
● Download Google Earth Pro onto your computer or laptop. You will need the download version
to draw the great circles required to complete this exercise; this option is not available on the
phone app. It is also not available in the Google Earth web app in Google Chrome. You need to
use the download version of Google Earth Pro.
● It may be easiest if you print this lab assignment out onto paper and work from that, but it’s also
possible to complete the entire lab online, save your final completed lab as a PDF and upload
your final work.
● Here is a great video to help you with a step-by-step tutorial:
● Also, another great video on determining the Richter Magnitude:
Part 1: Determining Earthquake Epicenter
Directions:
We have provided you with three seismograms from a single earthquake. Each seismogram is from a
different location in the vicinity of the earthquake.
1) Label the P-wave and S-wave on each of the three seismograms (by hand or annotate with simple
online tools available in Word or Google Docs).
2) Determine the time difference between the first arrivals of the P and S waves for each location
and enter the times in the Table below.
3) Determine the distance to the epicenter of the earthquake from each of the stations using the time
difference between the P and S-wave arrivals determined in the previous step and the provided
graph of arrival times vs. distance.
a. Determine the length along the vertical axis representing the time interval between the P
and S-wave arrivals for each station (P-S arrival). Use the red line in Figure 1 as an
example.
b. Slide the P-S arrival line into the wedge between the two curves in Figure 1 to where it
best fits. See the second red line fit between the curves in the example.
c. Find the distance to the epicenter by dropping straight down line fit between the curve in
step b. above. In the example, the P – S arrival is 20 seconds; this would correspond
to a horizontal distance of approximately 200 km (follow the blue arrows in Figure
1).
d. Complete as above for your 3 seismic stations and finish Table 1.
Open Google Earth Pro. All of the remaining steps take place using functionality in Google Earth
Pro.
4) Locate the first station (Los Angeles) by searching on the name in the Search box (upper left).
See screen capture below.
5) Open up the Ruler Tool by selecting it from the Tool Icon (labeled in the screen capture).
6) Select “Circle” from the pop-up Ruler Tool window. Assure your “Radius” is in units of
Kilometers.
7) Move your cursor over the location placemark for Los Angeles.
8) Left click on the Los Angeles placemark and begin to move your cursor away from Los Angeles
as your Great Circle begins to appear.
9) Left click again when you reach the Distance to Epicenter (km) that you calculated for Los
Angeles in Table 1 below. This is the Radius of your circle. If you make a mistake, simply select
“clear” on the Ruler Tool.
**Note: This is just an example. Your great circle around Los Angeles will be much larger.**
10) Once you have the circle drawn to your liking, select “Save” and name your Circle Measure, “Los
Angeles.” (First screen capture below). Select OK You will see “Los Angeles” show up in your
Temporary Places Panel on the left side of the screen. (Second screen capture below).
11)
You may need to Zoom Out to see the full circle.
The Zoom tool is on the right hand side of the screen
(labeled in the screen capture at left – NOTE: you may
need to hover over the Zoom control for it to appear on
your screen).
12)
Search on the next station (San Jose) in Google
Earth Pro from Table 1. Follow the same steps as above
(6-11) to draw your Great Circle Distance to Epicenter
around San Jose.
13)
Search on the final station (Bakersfield) in
Google Earth Pro and follow the same steps as above
(6-11) to draw your final Great Circle Distance to
Epicenter around Bakersfield.
14)
If you have measured and drawn everything
correctly and carefully, the three circles around the
stations should intersect at only one point (or make a
small triangle), which will represent the epicenter of the
earthquake. Note: there are many places to make small
errors in this exercise and each will add cumulatively. If
the circles do not intersect in one point (or make a small rectangle), you should go back and make
the measurements and calculations again, but more carefully.
15) Select the Yellow Push Pin in Google Earth Pro from the same ribbon of tool icons where the
Ruler Tool is located. From the pop-up menu, name your Placemark “Epicenter.” Move the Push
Pin and drop it on the location where the Great Circles intersect. Select OK and this location will
now show up in your Temporary Places panel.
16) Zoom out to an altitude that captures all three of your Great Circles and select the “Save Image”
from the tool icons (labeled on the screen capture below).
17) A box will pop-up that says “Untitled Map.” Select this box to enter your Name and include the
date. Write a short description about what is displayed on your map.
18) Select the “Save Image” button (labeled in screen capture below) to save this image to your
computer documents file.
19) Finally, open this image on your computer, copy and paste it into the final page of this lab
exercise. No image, or an image without your Name included will cause you to earn zero
points on this exercise. Save your document as a PDF and upload it as your final submission.
You can use the Table provided below to organize your data. Show all work to get full credit!
Table 1
Location
Time between P and S arrivals (s)
Distance to Epicenter (km)
Los Angeles
San Jose
Bakersfield
Part 2: Determining Earthquake Magnitude
Using the seismogram from the Bakersfield location and the Richter scale diagram (Figure 2; the last two
pages of the lab handout) provided at the end of this exercise, determine the magnitude of the earthquake.
As with the example Richter magnitude calculation (Fig. 2a), you will use the Bakersfield data (in Fig.
2b) to calculate the magnitude of the earthquake:
1) Mark the Distance to the earthquake epicenter from the Bakersfield station on the vertical axis
on the left. You can also use the S-P time, also marked on that axis.
2) Measure the amplitude (in millimeters) of the highest seismic wave on the Bakersfield
seismogram from the horizontal equilibrium line (do not measure the distance from the lowest
point to the highest point of a wave, that is the wave height, not the amplitude, which should be
approximately half that distance) and plot it on the vertical axis on the right (marked Amplitude).
Enter your measured amplitude in the space provided below.
3) Use a straight line to connect the two points that you plotted. This line will intersect the center
vertical axis, and the spot where it crosses is the Richter magnitude of the earthquake. Record the
Richter magnitude of the earthquake in the space below.
Amplitude of largest seismic wave:
Richter magnitude of earthquake:
(mm)
Figure 1
Figure 2a
Just an example, to show you how it’s done.
Figure 2b
This is where you use the Bakersfield data to calculate the magnitude of the
earthquake in this lab exercise. Follow the same three steps (also described on
page 6.