Lab 10 Earthquake Frequency
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Geol 116, Fall 2023
Lab 10: Earthquake Recurrence and Evaluating Seismic Hazards
One of the ways that seismologists can assess the likelihood of future earthquakes is by investigating
the frequency of historical earthquakes in a region of interest. Numerous studies have shown that the
number of earthquakes in an area decreases logarithmically with increasing earthquake magnitude. In
this lab exercise you will explore the rate of earthquake occurrence for areas of Earth that you choose.
Data is accessed through the simple interface of the IRIS Earthquake Browser. After compiling your data
for various sized earthquakes in the region, you will calculate recurrence intervals for each magnitude
and plot the data on a semi-log graph for interpretation.
Discussion of the collected data touches on
strengths and limitations of the data set, possible societal implications (e.g. exposure, vulnerability,
mitigation), as well as concepts related to earthquake prediction and forecasting.
The Exercise
We’ll be using the
IRIS Earthquake Browser
to compile earthquake data in some region. Your first step is
to familiarize yourself with the tool. Navigate to it on your computer. You might watch the tutorial, but
if you feel brave (or don’t like to be given instructions), you can navigate from this site directly to the
app. Then we’ll use the app to compile how many earthquakes have occurred in a region within distinct
magnitude ranges.
Example of using the app.
Using the tools on the left (select new region) I outline an area that to study.
When you do this, choose a region that is neither too small (not very many earthquakes) nor too large
(too many earthquakes to plot); you’re best choosing an area that includes a plate boundary. For this
example, I randomly chose a zone across central Africa. The information panel on the right indicates
that there are 3460 earthquakes recorded in this area and reported in the earthquake catalog; 1000 are
shown.
If you click on Magnitude Range, you can (and should) limit your investigation to a range of magnitudes
of 4 and above. When I did this, I still had 3323 earthquakes.
We want to examine how the number of earthquakes in this region changes with increasing earthquake
magnitude. The easiest approach is to figure out how many earthquakes of magnitude 4 or greater, 5 or
greater, 6 or greater, etc. have occurred in the region and divide by number of years of record (here,
1970-2022). I’ve this in the example extra half magnitude (4+, 4.5+, 5+, etc.). I have the app tell me how
many earthquakes of a particular magnitude or larger by changing the magnitude range (e.g. 4-10, then
4.5-10, 4-10, etc.) in the right-hand box. Inserting the data into the spreadsheet on Brightspace yields:
# of events of
Mag or greater
Years of
Record
Number/Y
r
Minimum
Magnitud
e
3323
53
62.70
4.0
2170
53
40.94
4.5
668
53
12.60
5.0
130
53
2.45
5.5
34
53
0.64
6.0
11
53
0.21
6.5
2
53
0.04
7.0
0
53
0
7.5
I plot the number/yr for a given magnitude or greater on a logarithmic scale against the magnitude to
obtain this graph, which is known as a Gutenberg-Richter diagram:
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
1.00
10.00
100.00
Magnitude
Number of Events/yr
Ideally, the decline in number of events per year (i.e. frequency) will be linear in this space (as it is
between 4 and at least 6.5 in this case) So we can estimate recurrence (1/frequency) of events either
directly from the graph or by extrapolating our best guess for a straight-line fit. Thus, to predict the
likelihood of earthquakes of M 7 or greater in this case, read off the graph (`0.04 events/yr, which
corresponds to a recurrence or repeat time of 25 years). That is, this area should experience 1 M 7 or
greater earthquake, on average, once in 25 years. By extrapolating the trend, I predict the number of
earthquakes of M 7.5 or greater to be about 0.008/yr, or a recurrence time of 125 years.
Your Mission
You will be assigned a partner.
Each of you chooses a different area of interest and performs the same kind of analysis to obtain the
Guternberg-Richter curve for your chosen region. You will want to pick a region that includes a plate
boundary in order to have enough earthquakes of M 4 or greater recorded in the period of record
(1970-2022). Look for a “Goldilocks” area size: not too big, not too small, just right. Limit your
magnitude range to 4 and greater.
Use the template spreadsheet, inserting the appropriate values for the region you chose. You may find
that the maximum recorded earthquake in your region is greater than M 8, and the frequency values
may be quite different than in my example, so you may have to adjust the scales on the axes, but
otherwise the template should work.
Estimate the recurrence time of a M 7.0, M 7.5, M 8.0 (if possible) earthquake for your area. Ideally you
would have the program calculate a best-fit linear line to the data, but it’s a difficult set of steps to
produce a trendline in Excel for a semi-log plot like this. So simply use a ruler to identify your best guess
of a trend line that is linear on this graph and read off or project and estimate the number of M 7.0,
7.5, and 8.0 events for your region. If you’ve chosen an area with a particularly large historical
earthquake, you may be able to estimate the recurrence time for M 8.5 or even M 9.0. Convert to
recurrence interval (1/the value) and record those recurrence estimates on your spreadsheet.
Once you have completed this, compare your results with your partner(s)’s. Are the trends of your data
similar? In other words, do they show a similarly steep decline in frequency of occurrence with
increasing magnitude?
How similar/different are the predicted recurrence intervals for the different
areas? Now that you know something about the earthquake hazard in at least two areas, discuss with
your partner(s) the exposure and vulnerability of the population in each area. How great is the
population? Is the area wealthy enough to be able to design buildings to be earthquake-resistant? Do
moderate-sized earthquakes cause unusually large damage or death tolls?
What to Hand In
You will provide a brief written report with the following elements:
Your name and that of your partner(s)
Area that you investigated; include a screen shot of the map from the app.
Copy of the spreadsheet with your analysis and output, including a Gutenberg-Richter graph.
Estimates of recurrence intervals for earthquakes of M 7.0, 7.5, and larger if appropriate for your area
and that of your partner(s).
Summary of the discussion of exposure and vulnerability for your area and how it compares to the area
your partner chose.
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Jake: U.S. Virgin Islands
Mag.
No./yr
7.
100
7.5.
200
8.
200
8.5.
500
9.
1000
Anya Donders and Jake
I investigated the southern California area which is along the San Andreas fault, and it is a transform
fault. The Virgin Islands are on a subduction fault due to volcanos from millions of years ago. There are
two plates sliding past each other on the Southern California fault, which often leads to an offset of the
land. Southern California has less vulnerability since it is usually wealthier in terms of economics, and
they can afford to pay for better infrastructure. On the other hand, there are more people who live in
Southern California therefore it puts more people at risk.
The Virgin Islands doesn’t have the same type of infrastructure and it’s more vulnerable to earthquakes
making a larger impact. The Virgin Islands have a greater recurrence value for all of the magnitudes, so
it puts them more at risk as well as the fact that it’s surrounded by water. Less developed infrastructure
doesn’t help.