Part 4—Select and Analyze Data
Step 1 Visually Compare S-Waves at 100 km and 28 km depths
How do Seismic S-wave velocities compare at different depths? Examine 2 depths of waves, at 28 km and 100km, to get a sense of their velocities.
- If necessary, relaunch My World, and open the Seismic Wave Part3.m3vz that you saved at the end of Part 3.
- Click the Zoom to Full Extent button to ensure that you are at the full world view. Turn on Plate Boundaries, Countries, and the Seismic S-wave Velocity at 28 km layers. Turn all other layers off.
- The Seismic S-wave velocity at 28 km layer, pictured above, shows the seismic shear wave velocities measured at a depth of 28 km below Earth's surface. What do the dark red areas of this layer represent? The dark red areas represent continental crust, which transitions to the denser upper mantle beneath the oceans. Reminder: Shear waves move more slowly through continental crust and faster in the denser upper mantle.
- At 100 km depth, has the velocity of seismic waves under the continents increased or decreased compared to measurements at 28 km depth?
Step 2 Select S-Wave Data for Graphing and Export the Data
In this step, you'll select the data necessary to create line graphs of shear wave velocities across North America. Each line graph will represent a vertical profile that shows how shear wave velocities change across the continent.
- Turn on the Seismic S-wave velocity at 28 km layer and make it active (or return to your Map View) and turn off the Plate Boundary types layer.
- Then zoom into North America so that you can see the individual grid cells of velocity drawn on the map.
- Click the Lasso Selection tool to select it. You'll use this tool to select a line across one row of cells representing Seismic S-wave Velocities under North America. On the Map View, place your cursor in the Pacific Ocean off the coast of western North America. Click and drag a line across the continent and beyond into the mid-Atlantic, release your cursor to end the line. A yellow line of selected cells will appear on your map. You will be prompted to name your selection. You can name it "Transect 1 at 28 km." Depending on how steady your hand is it may take several tries to get a single line of cells. If you make a mistake you can click the Delete Selection button, located in the toolbar above the Layer List.
- With the Seismic S-wave Velocity at 28 km layer active, click on the Show Table of Selection button. The layer table will open. Drag it by to a position so you can view both the table and the map at once.
- Each record in these layers represents a measurement of shear wave velocity at the indicated depth. Out of the total 15,454 records in the layer, how many did you select with your line?
Step 3 Import Data into a Spreadsheet and Create a Line Graph
- Launch Excel (or another spreadsheet program).
- Choose File > Open..., navigate to the CSV file that you created (and saved) in the previous step, select it, and click Open. Repeat with the other file. You will have two open workbooks.
- In the columns titled "Velocity", add 28 km or 100 km so that you can tell the two apart in your final graph.
- Copy and paste the cells from the 100 km workbook into the 28 km workbook so that the two columns are side-by-side.
- Click and drag to select the two Velocity columns. Select only the rows for which you have both sets of data.
- Choose Insert > Chart....
- Select the Line type of graph.
- Adjust the scale, title on the axes, and chart title to finish your graph.
- Save your work.
Step 4 Compare Graphs to Maps
- Compare the two graphs to the data on the maps. How does the velocity of the Seismic S-wave at 28 km and 100 km change as you go from oceanic to continental crust?
- After examining your line graphs, answer the following questions:
- What general statement can you make about shear wave velocities in the crust and in the mantle? (Do seismic waves travel at the same speed through both?)
- Based on your observations, what can you say about the depth of the transition from crust to upper mantle under ocean basins and under continents?
- Optional: Save the graph as a linked file in your My World Project. This way you can use the link tool to show the graph from within the project. Choose Project > Files..., select your saved graph, and click add.
Seismic waves travel more slowly through Earth's crust than through the mantle. The relative low density of crustal material cannot pass the shear wave energy along as efficiently as the higher density material of the mantle can, so the shear waves travel more quickly whenever they move through mantle material.
Under ocean basins, denser upper mantle material is present at a depth of 28 km or less. Under continents, the transition from crustal material to mantle material appears to occur at a depth between 28 km and 100 km.