EXTRA: Rapid Earthquake Viewer Lessons
What's THAT Inside our Earth?
The following lesson on inferring the interior structure of the Earth might be best suited in conjunction with (or before) a unit in plate tectonics, or prior to a unit including earthquakes.
This lesson also provides a good example of how models change over time as our understanding of nature improves with more detailed observations.
Students work in teams to investigate two possible models of the Earth, a simple, homogeneous interior and a layered Earth. They will use calculations and create a graph from data gathered from a model of Earth’s interior and using data from the Rapid Earthquake Viewer. Students will then use the graph to generate a model of the interior structure of the Earth. Students gather data directly from seismograms, organize the data into tables, create and interpret a complex graph of the data, and apply that interpretation to inferring a model of the interior structure of the Earth.
Note: This activity is adapted from Earth’s Interior Structure - Seismic Travel Times in a Constant Velocity Sphere. Copyright 2000. L. Braile. Permission granted for reproduction for non-commercial uses.
Materials / Preparation
Computers for half the students (or pre-printed seismographs as described below).
Materials for Theoreticians:
Large piece of paper: 11” x 17” to draw or print out the semicircle diagram a 1:20 million scale model of a cross section through the Earth. Print onto 11 x 17 inch paper to make a half circle with radius of 20 cm. Be sure that the tic mark identifying the center of the half circle is visible or print out each half and tape together left semicircle, right semicircle.
Materials for Seismologists:
If you will be working offline, you will need to print out seismograms from REV for the seismologist team and choose the earthquakes for them. To print a seismogram in REV, you’ll need to capture an image of the “record section” (the blue and green seismograms). If using a PC right click and choose “Print picture”; on a Mac click on the image, drag and drop it on the desktop and print that. Choose data from earthquakes of magnitude 6 or greater and select 15-20 stations that are distributed between 0 and 180 degrees from the epicenter, with an emphasis on the 80-120 degree interval. Use the default orientation (vertical or up/down).
Student Instructions for Part B (PDF)
The lesson can be taught without the poster; however, the poster contains diagrams and explanations that help immeasurably.
Additional resources that might be useful to show the class:
Examples of P-Wave and S-Wave travel time graphs:
Students will work in teams of 4 with 2 assigned to play the role of 18th century theoretician and two assigned to play the role of modern-day seismologist.
It is not necessary to use data from just one earthquake. You may use distance/arrival time data from any earthquake and from multiple earthquakes.
To generate a complete data set, students will need to use data across a range of distances, from stations close to the epicenter up to 180 degrees. For both teams it will be helpful to plot additional data for location in the 80 to 120 degree range to create a smoother graph.
If students are not familiar with geocentric angles you might reference the REV glossary entry for “Distance” to provide a visual explanation of this concept.
Caution - graphs constructed using graphing software, such as MS Excel, may show a misrepresentation of the data. You must choose the “XY scatter” otherwise MS Excel will connect the data points together. Blank graph template
Have students draw a hemisphere on 11 x 17” paper, or print the template onto 11 x 17” paper. Be sure to print at 100% scale and check that the semi-circle radius measures 20 cm.
Begin a discussion with students that reveals how the models of the interior structure of the Earth have changed over time.
Step 1: Begin with the question, “What did people think the shape of the Earth was 1,000 years ago?” Ask questions that direct student responses toward, “The Earth was flat.” Emphasize that this was the generally accepted scientific model and much evidence supported this view. Begin at the far left end of the white/chalk board with a diagram of what people thought the Earth looked like 1,000 years ago. Draw a diagram showing the Earth as flat, surrounded by water. Plan ahead on the usage of white/chalk board space – there will be five diagrams in all. Ask questions such as, “How did they know the Earth was flat and had an edge?” (Sailing ships would disappear over the horizon and often did not return; the horizon appeared flat). Point out to students that in all these cases the observations and evidence/data supported the model.
Step 2: Ask students what the model of the Earth was in the early 1500's. The flat Earth model persisted until the 1500's when Magellan sailed around the world. This led to a revision of the flat Earth model to a model of the Earth being round. As Magellan continued sailing to the west (more or less), his ship eventually returned to its starting point… though Magellan himself did not (he was killed in a conflict with natives in the Philippines). Discuss with students how the new model developed from the old model. Various religions or world views explained what the interior of the Earth contained, or what existed beyond the Earth, i.e., outer space.
Step 3: A new model emerged in the 1700’s with the notion that the Earth was made of rock, and outer space surrounds the Earth. Ask “How did the scientists know the Earth was filled with rock?” (Mines at the time had reached several hundred meters in depth, and in all cases, the mines penetrated into rock, therefore, the Earth must be filled with rock). This was also a time in history when supernatural explanations were set aside in favor of natural, or scientific, explanations. Draw a picture showing the Earth filled with solid rock.
Step 4: A new model emerged in 1906 when seismic waves were analyzed by Richard Oldham, an Irish geologist, and were found to have different characteristics when traveling through the Earth’s interior. What did he discover?
Note: This progression of models of Earth’s structure is an excellent example of how science is a dynamic venture with new information leading to new levels of understanding of the natural world. At each step, the model of the day DID match observations of the time.
Segue now to a role-playing activity:
A time warp has occurred during the annual seismology conference, transporting 18th century scientists (theoretical seismologists) to modern times. A debate breaks out between the early seismologists (theoretical seismologists or theoreticians) and the modern day seismologists about the structure of the interior of Earth. Students will take on the role of one of these groups and work in teams to explore this debate. How can each group model their assumptions and compare them to observed data?
Frame the activity to the students by asking a few of these questions:
What do you know about the interior of the Earth?
How do you know this? What tools do we have to explore and support this understanding?
Given what you know about seismic waves and earthquakes, how might these phenomena help us answer these questions?
How can we use this to create a model that will predict the structure of the interior of Earth?
Explain the concept of geocentric angle: Draw a cut-away diagram of the Earth (that does not show the core and mantle) to demonstrate what is meant by “distance in degrees from the epicenter”. The REV glossary entry for “Distance” provides an example of this. Show students examples of recording stations at various degree distances from an epicenter, for example, 45 degrees distant, 90 degrees distant, etc. This introduces the concept of geocentric angles and measuring distance on a sphere using degrees of arc.
Working in teams of 4, two students will act as theoretical seismologists from the 18th century and two students will play the role of current seismologists. Each pair will be making a prediction about the interior of the Earth using different assumptions and will then compare and analyze the data jointly.
Explain to the students:
1A. The theoreticians will develop a simple model of planet Earth that assumes that Earth is made of a single type of material all the way through (a “homogeneous Earth model”). With a homogeneous Earth, all seismic waves will travel at the same speed through this model Earth (11 km/sec). You will construct a geometric diagram of the Earth’s interior and predict earthquake travel times to different seismic stations around the world, based on this model.
Note: The worksheets (seismologist) (theoretician) include tables for students to fill in and have the necessary headings but not the angles or earthquake stations. This is done deliberately to encourage students to be ‘minds on’ by making them determine which geocentric angles and/or earthquake stations to use; this simulates real science. By not providing the exact angles, students need to think about why they are choosing each geocentric angle and earthquake station.
Note about graphing: Ideally students will create the graphs starting with blank graph paper, choosing the scale for the x (geocentric angle) and y (time) axis as a team. If this is too difficult to do or if time is running short, graph paper that has been set up with the axes is provided.