Benioff Box

Original Source: Dr. Leslie Sautter, Department of Geology, College of Charleston

COASTeam Program, Project Oceanica, College of Charleston

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Focus Question

  • How are locations of high seismic activity related to patterns of the location and depth of earthquake origins (foci)?

Objectives

Students will:

  • plot on a map the distribution of earthquake epicenters for shallow, intermediate and deep-focus earthquakes.
  • demonstrate how the geophysicist, Hugo Benioff, identified locations on the earth where seismic activity (earthquakes) showed distinct patterns with respect to location and depth of earthquake origins.

Key Words

  • bathymetry
  • earthquake epicenter
  • earthquake focus and foci

Materials

Per student group:

  • shoe box (at least 9” X 6”) with lid
  • photocopy of map (from attached sheet)
  • 25 wooden (bamboo) skewers
  • markers: red, green, blue
  • Play-Doh: red, green, blue (optional)
  • tape
  • push pin (type with plastic spool on it)
  • exacto knife or good scissors (optional)
  • thin piece of cardboard measuring 8” x 8”

Teacher Preparation

Review bathymetry of the seafloor and plate tectonics, including key terms so that you are familiar with these before the activity.

Time Frame

50 to 75 minutes

Suggested Learning Environment

The teacher may wish to construct one large model for the class to observe, or students should work in small collaborative groups of 3 or 4.

Relevant pages in Of Sand and Sea

Chapter I, The Ocean Planet, pp. 10-13.

COASTeam Program, Project Oceanica, College of Charleston

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Benioff Box

TEACHER PAGES (draft)

Procedure

The text below is identical to the text on the Student Pages. Additional information for the teacher is forthcoming!

  1. Tape a copy of the attached map (Figure 1) onto the top of the shoe box. Make certain that the latitude and longitude marks are displayed.
  1. Your teacher will discuss the bathymetry (the water depth variations) of the ocean floor depicted by the map.
  2. What seafloor features can be found? Describe these features.
  3. Describe any land masses present.
  1. Use Table 1 to plot the 25 earthquake epicenters on the map. Label each point using the “epicenter i.d. number” from column 1 of the table. Labeling the epicenters is important, as you’ll need to return to each point later.
    Use a blue marker to plot the epicenters for “shallow” focus earthquakes. With a green marker, plot the epicenters for “intermediate” focus earthquakes. Use a red marker for deep focus earthquake epicenters.
  1. Do you observe any pattern to the distribution of color dots?
  1. With a dashed blue line, mark the approximate boundary between the shallow and intermediate focus earthquakes. Use a green marker to mark the boundary between intermediate and deep focus earthquakes.
  1. Compare the distribution of epicenters for the shallow, intermediate and deep focus earthquakes to the volcanic islands and seafloor features. Write down your observations. You are now doing what Hugo Benioff did in the early 1950s.
  1. Use the push pin to poke a hole at each of the 25 epicenter locations on the map, poking through the shoe-box cardboard.
  1. Use the scale bar provided to make a scale model of the foci depths. For each epicenter, measure and mark on a wooden skewer the focus depth, using a blue (shallow), green (intermediate), or red (deep) pen. Mark all the way around the girth of the skewer to make a color ring.

0 100 200 300 400

Scale

bar:

(km)

  1. Then, use the same marker to color the pointed tip of the skewer. This colored tip is where the earthquake focus is located – at the bottom of the skewer.
  1. Carefully break the skewer approximately ½” above the color ring. Then, push the skewer through the appropriate numbered push pin hole on the shoebox map. Push the skewer into the box to the level of the measured depth (the color ring around the skewer).
  1. Repeat the procedure in steps 7-9 for each of the 25 skewers. Do NOT lift the top of the shoe box until instructed.
  1. When all of the skewers are in place (and before you lift the box top off!!), answer the following question:
  2. What do you predict will be the arrangement of earthquake foci (i.e., the colored tips of the skewers)below the earth’s surface (inside the box) when you lift off the shoebox top?
  1. Carefully lift the shoebox top off to reveal the third dimension! Compare what you observe to your prediction from step 11. Record these observations.
  1. With the exacto knife or scissors, carefully cut along the line marked A – A’ on the map. Slide the cardboard along the skewer tips. This cardboard represents the “plane of earthquake focus depths.”
  1. Answer the following thought questions:
  2. What might the motion of the cardboard represent?
  3. Does this motion generate earthquakes?
  4. At what seafloor feature does the cardboard plane (the plane of foci) intersect the seafloor?
  5. What role do you think the cardboard plane has in causing the earthquake foci?

The downward motion of the cardboard is similar to the motion of a thin slab of oceanic plate that slides, or subducts, beneath another plate. This region of subduction is where earthquakes are generated. We now refer to the plane of earthquake foci as a Benioff Zone. The top of the zone is defined on the seafloor by a trench. Earthquakes occur in much greater concentrations on one side of the trench – which tells us the location of the subducting slab of oceanic lithosphere (or plate).

Notice the volcanoes. They are commonly found parallel to deep ocean trenches. Volcanoes typically form on the same side of trenches as earthquakes. Why do volcanoes form? As the subducting slab of oceanic lithosphere descends it heats up, causing many minerals in its composition to melt. The melted, or molten, rock is less dense than surrounding rock, so it rises. Magma may reach the surface through cracks and fissures in the lithosphere above, resulting in the formation of a volcano. Volcanoes are commonly found parallel to deep ocean trenches. Compare your model to the two figures of convergent plate boundaries (Figures 2 and 3). Determine where the “Benioff Zone” is located on EACH figure.

  1. Write a paragraph summarizing the relationships among the Benioff Zone, the trench, and the volcanoes.

COASTeam Program, Project Oceanica, College of Charleston

1

Benioff Box

TEACHER PAGES (draft)

COASTeam Program, Project Oceanica, College of Charleston

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Benioff Box

TEACHER PAGES (draft)

Assessment

Use the student’s ability to determine the “Benioff Zone” on the figure of a convergent plate boundary to assess their understanding of seismic activity and locations of earthquakes.

COASTeam Program, Project Oceanica, College of Charleston

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Benioff Box

TEACHER PAGES (draft)

Figure 1. Contour map of the earthquake zone being studied. The land masses are volcanic islands.

COASTeam Program, Project Oceanica, College of Charleston

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Benioff Box

TEACHER PAGES (draft)

Table 1. Earthquake focus depths for epicenters associated with Figure 1.

Epicenter I.D. Number / Latitude (degrees North) / Longitude (degrees east) / Focus depth
(km) / Shallow (S)
Intermediate (I)
Deep (D)
1 / 28 30’ / 5140’ / 310 / D
2 / 27 50’ / 52 20’ / 250 / I
3 / 28 00’ / 54 10’ / 50 / S
4 / 27 10’ / 51 20’ / 390 / D
5 / 27 00’ / 54 00’ / 110 / I
6 / 27 20’ / 55 00’ / 45 / S
7 / 26 30’ / 52 40’ / 210 / I
8 / 25 40’ / 51 00’ / 400 / D
9 / 25 30’ / 52 20’ / 260 / I
10 / 25 30’ / 54 20’ / 100 / S
11 / 25 20’ / 53 30’ / 160 / I
12 / 24 50’ / 53 00’ / 200 / I
13 / 24 10’ / 51 50’ / 320 / D
14 / 23 50’ / 52 50’ / 250 / I
15 / 24 30’ / 54 40’ / 50 / S
16 / 22 30’ / 51 10’ / 380 / D
17 / 22 50’ / 52 10’ / 290 / I
18 / 22 40’ / 53 20’ / 180 / I
19 / 23 00’ / 54 00’ / 40 / S
20 / 22 00’ / 54 50’ / 50 / S
21 / 21 20’ / 51 50’ / 350 / D
22 / 21 30’ / 52 30’ / 220 / I
23 / 20 30’ / 51 00’ / 370 / D
24 / 20 50’ / 53 20’ / 190 / I
25 / 20 40’ / 54 10’ / 100 / S

COASTeam Program, Project Oceanica, College of Charleston

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Benioff Box

TEACHER PAGES (draft)

Figure 2. (a) Cross-section diagram of an oceanic-oceanic convergent plate boundary. (b) The Benioff zone is the plane of shallow focus (blue solid dots), intermediate focus (green solid dots) and deep focus (red solid dots) earthquakes that follows the surface of the subducting plate. The epicenters of these earthquakes (depicted as open circles in corresponding colors) are plotted on the map of the earth’s surface, directly above the earthquake foci. (Figure fromOf Sand and Sea, by P. Keener-Chavis and L. Sautter)

Figure 3. (a) Cross-section diagram of an oceanic-continental convergent plate boundary, and (b) the corresponding distribution of earthquake foci and epicenters. See Figure 2 caption for key to symbols.

(Figure fromOf Sand and Sea, by P. Keener-Chavis and L. Sautter)

COASTeam Program, Project Oceanica, College of Charleston

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