Lab 3 Earthquake Frequency
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Dec 6, 2023
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Geol 116, Spring 2023
Lab 3: 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/Yr
Minimum
Magnitude
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
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