lab 9
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Purdue University, Fort Wayne *
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Course
GEOG-237
Subject
Geography
Date
Dec 6, 2023
Type
docx
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8
Uploaded by LieutenantButterfly3655
The Benioff zone and Geographic Information Systems
NOTE: If you do not already have Google Earth Pro installed on your computer, you will need it for this lab. Download the app from https://www.google.com/earth/versions/
and follow the instructions.
One of the key discoveries that led to the invention of the theory of plate tectonics was that of the Benioff zone in 1949 (Also called the Benioff-Wadati zone. Hugo Benioff and Kiyoo Wadati independently discovered it at about the same time). This is a zone of earthquakes that is found adjacent to oceanic trenches, on the same side as the volcanic arc. The zone extends at an angle from the surface to within the upper mantle. The significance of the zone was quickly realized, but was not widely appreciated until the 1960s, when Dr. Mauwia Baranzangi (bear uh ZAN gee) created detailed maps in the early 1960s, of the epicenters of earthquakes, color coded by their depth. (As an aside, although both Benioff and Barazangi worked in the US, both were immigrants. Wadati was Japanese.)
The meaning of Benioff zones was not fully appreciated at the time of its discovery. The invention of the theory of plate tectonics in the mid-1960s explains these zones at the outline of an oceanic plate that is in the process of being subducted. (The overlying plate may be either oceanic or continental.)
In this lab, you will make a regional map of the Benioff zone, in the style of Barazangi. From that, you will measure the angle at which the plate is being subducted into the mantle. To do so, you will use real-life data retrieved from the United States Geological Survey, and view it Google Earth Pro.
1.
If you have not already done so, Download and install Google Earth Pro.
See the note at the start of this document for instructions.
2.
Download data from USGS as a KML file, for two regions
Go to USGS website, at https://earthquake.usgs.gov/earthquakes/search/
This will take you to a form from which you will download the data you’ll need. The form is long, and here’s what you need to enter:
Magnitude
4.5+
Date and Time
Custom Start
set the year to 1900
End
today’s date
Advanced options
Geographic Region
Location 1: Andes
North: -20
West: -74
East: -60
South: -28
Location 2: Japan
North: 41
West: 135
East: 145
South: 35
Depth (km) Minimum
35
Output Options
Format
KML
KML – specific options
Color by depth
Magnitude – Largest first
Limit results:
Number of Events
500
Press SEARCH
If the file does not download automatically, check your browser’s help to see how to make that happen.
3.
Open the KML file in Google Earth Pro
Usually, just double-clicking the icon will do it.
You should now see a globe of the Earth. The earthquakes may not be visible in your starting view. (For some reason, mine always open looking at the US.) The locations are
in either South America or the Western Pacific. You should also see a legend showing you what the different colored circles represent.
Below is a similar map, from an area that we are not using in this lab.
ASSIGNMENT
Include your maps of both locations in a Word document (or PDF), as I have done here (two maps, then). To make an image of your map, select in the File menu: Save > Save Image…
You can hide the legend and title by unchecking them in the “More Options…” pulldown just above the map.
Answer these questions for each map:
1.
Describe the pattern between map location and depth of the earthquakes. Are they scattered randomly, or are the grouped? If grouped, are they grouped in bands, ovals, or some other pattern?
2.
Where in your image is the trench, relative to the earthquakes? (direction and distance)
If there are any volcanoes or volcanic arcs in your map, where are they, relative to the earthquakes? (direction and distance)
The trench is the dark blue line in the ocean, offshore of land. Volcanoes may be marked with small triangles on the land, or appear as islands, or as circular objects in the images.
4.
Collect data from your map.
Each circle on your map represents an earthquake. If you click on one, a popup window
will tell you that earthquake’s time, location, and depth. Select 25 earthquakes with several of each color. For each, record its depth and distance to the trench (in kilometers, of course).
You can measure the distance between two points using Google Earth Pro’s ruler
tool. You can get to it either through the Tools menu, Tools > Ruler; or by clicking on the ruler icon in the Google Earth Pro toolbar.
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Location 1
Japan
Location 2
Andes
Depth
Distance
Depth
Distance
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
5.
Find the angle of subduction
Graph the data, and calculate or measure the angle that the subducted plate is descending into the mantle. Draw a best-fit line through your data, and use the angle between your best fit line and the axis representing Earth’s surface. You can either do this by using a protractor from the graph (assuming you were careful in your choice of axes), or by using trigonometry (tangent of the angle = rise/run).
Angle of subduction for Japan
Angle of subduction for Andes
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