# Real Evidence of a Subducting Plate

```SAUNDERS
EARTH SCIENCE
LAB: Exploring Focus Depth
NAME _______________________________
HOUR ________________
CHAPTER 6: Earthquakes
EQs and Subduction Boundaries: Real Evidence of a Subducting Plate
Introduction
As you learned in Chapter 4- Plate Tectonics, the crust of the Earth is believed to be made up of
many plates. The rocks that make up the continental plates are less dense than the rocks that
make up the oceanic plates. Earthquakes and volcanoes are the result of the interaction of the
rocks that make up these plates, and the kinds of earthquakes and volcanoes produced depends on
whether the plates are coming together, spreading apart, or sliding past each other.
In the following lab, you will be using authentic earthquake data—location and focus depth—to
determine if plate tectonics can explain what you find.
EARTHQUAKES AND FAULTS
We learned in the 6.1 Notes that earthquakes and faults are caused by the movement of the
earth’s crust as it is pushed together or pulled apart. As the movement occurs, the rocks will
often break and grind together, releasing vibrations of energy into the ground (seismic waves).
The plane along which the rocks break and move is called a fault (see figure below). This fault can
occur near the ground surface where humans can actually see it, or deep below the ground. The
technology we use to detect earthquakes can actually tell us exactly where the faulting has
occurred below the surface. The figure shows two blocks of earth that are sliding against each
other. The location at which the rocks actually break during a single event is called the focus of
the earthquake. The point on the earth’s surface that is directly above the focus is called the
epicenter. This is the area where the most violent shaking will occur because it is directly above
the actual rock movement.
Procedure:
On your data table you will find information on earthquakes near the west coast of South
America. Provided are the latitude, longitude, depth and magnitude (how strong the earthquake
was) of each earthquake. From this data, you will be constructing a 3-D model of these events,
and determining what type of pattern (if any) that the earthquakes create in the region.
Step 1. Locate the epicenter of each earthquake and plot it on the map.
- Find where the latitude and longitude lines
meet. That is the location of the EQ epicenter.
- Latitude is the measure of the distance
north and south of the equator.
- Longitude is the measure of the distance
east and west of the prime meridian .
- We will plot the first point together in class.
Step 2. Lay the box on its side and tape the map
on what is now the top surface of the box. Orient
the map so that the boundary between the ocean
and the continent is perpendicular to the open end of the box
(35&deg;S is above the opening).
The map represents the surface of the land, while the open
side of the box represents your view of the crust beneath the
surface. This view is called a cross-section view of the earth.
Step 3. At each epicenter point, punch a hole with the pushpin
through the map and box. The hole needs to be big enough to
Step 4. Hang a colored bead so that it represents the focus of
each earthquake.
- For each of your epicenters, you have information about the depth of the EQ.
- To make a 3-D representation of the EQ in this area, you are going to use a colored bead to
represent where the FOCUS of the EQ is.
- From the epicenter, you need to hang a colored bead on a length of string that represents the
depth of the earthquake. Be careful of your scale!!
Tip for choosing a scale for string length: Consider the deepest earthquake and use a scale
so that it will fit into the box!
-Determine what EQs are shallow (red; 0-70), intermediate (green; 70-300), and deep (blue; 300650). Set aside beads in those colors.
-To hang the beads, tie one end of the string to the bead. Then, determine the length of the string
that will represent the depth of the earthquake. Mark the string at that point.
- Pull the string up through the hole you made at the epicenter until the ball is at the correct depth.
Repeat for each of the earthquakes.
To be done AT HOME:
Using the data table provided, plot the points to represent your 3D cross-section model as a 2D
cross-section of the focus depth. If you have a graphing program such as Excel, please use that.
Otherwise, talk to Miss Saunders if you need graphing paper in order to do it by hand—you should
still be neat and use a ruler!!!
You will be graphing the Longitude &deg;W (negative for plotting purposes) vs. Focus Depth (km).
On the y-axis should be Depth (km) and on the x-axis should be the Longitude—which makes
sense if you think about it…
Below are some Tips for Graphing—if you forgot how!!
When graphing, start by drawing two base lines—one is a
horizontal line and the other is a vertical line, which are called
the x-axis and y-axis respectively. You will need to label these
axes according to the numbers in your data table. I would suggest
labeling the x-axis from -72 to -62 and the y-axis from 0 to -600
(to show going down into the earth). A suggested example is
shown below:
The position of any point on the graph is described by using two numbers, (x, y), that are called
coordinates. The first number, x, is the horizontal position of the point from the origin. In this
lab, it will be the Longitude. It is called the x-coordinate. The second number, y, is the vertical
position of the point from the origin. In this lab, it will be the focus depth. It is called the ycoordinate.
For example, on the graph above, there is a point in the lower right corner. The coordinates of
the point are (-63, -500), which means the Longitude is 63&deg;W and the focus is 500km deep. Use
this method for plotting the other points in the data table.
Looking at the Results:
1. What information does an epicenter provide about an earthquake?
2. What is the difference between the focus and the epicenter of an earthquake?
3. What is happening to the rocks at the focus of an earthquake, and what do we call this?
4. What do you notice about the depth of the focus of the earthquakes as you go further inland
from the coast of South America?
5. What appears to be happening to the two plates that meet along the west coast of South
6. Draw and label a diagram showing what is happening to the plates along the west coast of SA as
you described in question 5:
Making Conclusions:
1. Describe the type of plate boundary which you think is present along the west coast of South
America.
3. How can our model explain the deep trench that lies just off the coast of South America?
4. Looking at the map of the world below, can you find some other areas where this same type of
plate boundary is occurring? Highlight or color those areas.
HINT: Use the key to identify where there are trenches, and mid-ocean ridges.
Ridge and trench
systems of the world.
- Ridges are where new
ocean crust is being
formed (plates are
moving apart).
- Trenches are formed
where an oceanic plate
is being subducted
(plates are coming
together).
TABLE 1. Table with data from actual earthquakes in South America from 1993-1994.
Station
Latitude (S)
Longitude (W)
Depth (Km)
Magnitude
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
19.8
27.8
26.2
31.2
23.2
23.2
24.5
21.3
23.6
23.5
22.9
34.1
22.3
23.2
22.5
19.5
21.4
27.0
27.2
20.4
25.6
22.2
66.6
63.2
63.3
71.5
66.4
71.0
70.8
68.2
70.0
70.5
68.3
69.8
66.1
69.3
67.4
65.8
68.1
63.0
67.1
66.0
66.0
64.5
259
513
550
33
200
25
33
122
42
50
115
45
274
67
168
305
123
500
155
300
385
440
4.6
5.1
4.8
5.0
4.8
5.0
5.0
4.7
5.0
na
4.8
Na
5.0
4.9
4.5
4.5
5.1
4.9
4.7
4.5
5.0
4.8
```