AuSIS Module: Earthquake Location
Aim:
The aim of this module is to introduce students to recordings of earthquake
waves, and to techniques used to locate an earthquake and calculate its
magnitude.
Introduction:
For this activity, we assume that students have already been introduced to the
different types of waves, at least P-waves and S-waves (see AuSIS Module:
Seismic Waves). This activity is an online java applet that goes through the steps
of earthquake location and computation of an earthquake’s magnitude. Students
can complete this exercise in groups or individually.
For this earthquake location technique it is important to keep in mind that we
assume the earthquake occurred at the surface and that the crust is uniform. The
technique takes advantage of the differences in the speed that P- and S-waves
travel and use the difference in the arrival times to work out how far away the
earthquake is from the recording station. After the waves are observed at 3 or
more stations we can use overlapping circles to determine the epicenter.
The magnitude of an earthquake is an estimate of the amount of energy released
by the event; therefore, the magnitude should be the same (or very similar) at
each recording station. The calculation of magnitude takes into account the
amplitude of the largest wave recorded at the station (often the S-wave for local
earthquakes) and the distance of the earthquake from the event. In this exercise
a special sliding scale is used to determine the magnitude.
Note: Magnitude should not be confused with earthquake intensity. Intensity is
the observation of the amount of shaking at a particular site and will decay with
distance from the earthquake. Often intensity is calculated from personal
accounts and is how we can estimate the size of historical earthquakes.
Activity:
(adapted from Virtual Courseware for Earth and Environmental Sciences, 2002-7.
http://www.sciencecourseware.com/eec/earthquake/)
Getting Started:
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Go to the website http://www.sciencecourseware.com/eec/earthquake/
Please note that this is activity/website is not designed or maintained by
AuSIS, however we have found it a great tool for demonstrating how to
locate earthquakes.
The activity works best in Internet Explorer and Firefox (v. 21.0 or
higher), make sure you allow for pop-ups and have java enabled.
Under ‘Main Activities’ Open Epicentre and Magnitude
Click on Start Activity
For instructions, click on the Background and Assignment
Walkthrough:
Trigger Earthquake
 Click on the earthquake button and select the location of the earthquake’s
epicenter to random location. Click “Trigger Earthquake” button.
Find the Epicenter
 Click on Journal tab
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Click on the MAP
, this will display a partial map of the west coast of
the U.S. (the epicenter of the earthquake is somewhere on the map).
There are nine seismic monitoring stations displayed on the map. Try to
select a number of stations positioned around the map (surrounding the
earthquake) to ensure a good azimuthal spread for triangulation.
Like this:
The surrounding stations allow for greater confidence in triangulation, and
errors in calculation will have less of an effect.
Not like this:
Any errors in calculation and measurement will have a far greater effect on the
calculated location of the epicenter.
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Select the S-P Tool
. This tool will allow you to measure the distance
of the station selected from the epicenter of the earthquake. Ensure you
measure from the beginning of the P-wave (‘First arrival’) to the
beginning of the S-wave (‘Second arrival’). P-waves travel faster than Swaves, measuring the difference is one way to find the distance to the
epicenter from the detector/observer. [NOTE: if the P-waves and S-waves
overlap, calculating the amount of lag can be difficult, perhaps select
another station.]
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Type the data generated for three different stations into the Journal
section of the tab.
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To find the distance from the epicenter, use the Distance Tool
and
drag the slider to find the distance from each station to the epicenter with
the amplitude. Enter the results into the Journal section.
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Type the data generated for each of the three stations into the Journal
section of the tab.
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Use the triangulation tool
to find the epicenter of the earthquake
with the distances generated with the Distance Tool.
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Use the Epicenter Tool
to mark the epicenter you have chosen. Place
the marker on the intersections of the circles.
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Use the Latitude and Longitude Tool
to bring up a grid to read the
location of the earthquake’s epicenter in terms of co-ordinates. (NOTE:
the grid provided is in degrees and minutes not decimal degrees.)
Calculate the Magnitude
 To determine the magnitude of the earthquake, click on the Amplitude
Tool
. Use this tool to measure the maximum amplitude of the wave.
Increase the scale on the seismogram to get a more accurate reading.
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Use the Magnitude Tool
to find the magnitude of the earthquake.
The magnitude of the earthquake should be similar for all the stations, as
the slide takes into account distance from the epicenter and the energy
released in the seismic event dissipates as it travels. The measured
amplitude of the earthquake is dependent on distance from the epicenter,
so the further the observer from the epicenter, the lower the experienced
intensity of the earthquake.
Final Thoughts
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After finishing the exercise students can engage in a conversation about
the activity. Some discussion topics might include:
o Where could errors be introduced during the activity?
o What would happen if the earthquake occurred at some depth
beneath the surface?
o What is the problem with using a uniform crust? If the crust had
layers with different properties (density and velocity) what would
happen to the arrival of the waves?