Obtaining evidence for subduction
Step 1
First, use Google images to source an image of subduction.
To do this, log onto Google, select images and type in subduction zone diagram
Step 2
Select the image you wish you use
Step 3
Log onto the Earthquake data base at
http://neic.usgs.gov/neis/epic/
Then Select the Circular Area option
Step 4
Once you have selected circular area, the top part of the screen will appear as below, select output file
type 4. Spreadsheet format (comma delimited)
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Earthquake Hazards Program: Circular Area Earthquake Search
http://neic.usgs.gov/neis/epic/epic_circ.html
Step 5
Scroll down the screen until you come to the area that allows you to input circular area search
parameters.
As we want to search for earth quakes in and around the Marianas Trench, we will need to input the
latitude (11 N) and longitude (142 E) figures for this area with a search radius of 500 Km's.
There will be a large amount of data available so we will want to limit the amount of information that we
focus our analysis on. Select a minimum magnitude of 4 and a maximum magnitude of 10, with a date
range between 1/1/2008 till end of July 2009 as seen below.
You can choose different dates if you like and select different depth parameters. That way a number of
groups could independently collect different datasets from which different graphs can be drawn.
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Earthquake Hazards Program: Circular Area Earthquake Search
http://neic.usgs.gov/neis/epic/epic_circ.html
Hint: Leave the intensity fields blank
Then press the submit search button immediately below minimum intensity.
Step 6
The data base will generate a list of events (part of which is seen below). Highlight the entire list (including
headings). With the cursor on the highlighted list, right click the mouse button and select copy. One of the
first things you might notice is just how many earthquakes occur in this relatively small area. This suggests
that the area is a "tectonically" active zone.
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NEIC: Earthquake Search Results
Step 7
Start Microsoft Excel, use alt-tab to switch screens back to the search results, select and copy the
data from the Earthquake Search Results and paste these into the top left hand cell (A,1)of your
blank Excel spreadsheet.
It should look like the data set shown below:
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Results in Excel
Step 8
Highlight Column A then, from the Data toolbar select text to columns
This will place the data selected into separate columns and enable you to manipulate the data
more easily in order to analyse trends and identify patterns.
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Data converted to an expanded table in Excel
Step 9
Highlight the longitude column and the depth column. If they are side by side you may simply drag the
mouse over both column headers to select both. If they are in columns separated by other data select the
longitude column and while holding down the control key, highlight the depth column.
Screen clipping showing scatter graph tool selected
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Step 10
With both columns highlighted, select Insert and select Scatter, then select Scatter only with markers.
This will then produce a plot titled Depth. (sample shown below)
Scatter graph drawn in Excel
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Step 11
To enable us to build a model from the data that enables closer analysis of the characteristics of
earthquake activity around a subduction zone we can manipulate the data in the following way.
Right click on the axis of the chart to
format the chart. Click in the box to
reverse the axis so depth goes down.
Select the Layout menu and Axis Titles.
Change the axis titles to longitude on the
horizontal axis and depth on the vertical
axis.
Select Chart title to add a label to identify your graph if you want.
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This will then produce a plot with depth (in Km's) on the Y axis and longitude on the X axis.
Assessment of learning
Describe how this graph supports the concept of subduction.
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