Part 3—Explore Earthquakes and Seismic Waves
Step 1 Add Earthquakes and Shear Wave Velocities Layers
Earthquakes generate seismic waves that move through Earth. Changes in the speed of some of these wave help scientists learn about Earth's structure. In this step, you'll compare the types of plate boundaries with seismic shear wave velocities near them. You'll observe different types of plate boundaries around the world and the pattern of shear wave velocities near them.
- If necessary, launch My World and open the Seismic Wave.m3vz project file.
- Select the Zoom to Full Extent
button to ensure that you are at the full world view. Turn off the Tectonic Plate names layer. Make sure only the Plate Boundary Types and Countries layers are turned on.
- Observe the distribution of tectonic plate boundaries around the world. What do the different colored plate boundaries represent?
- Turn on the Seismic S-wave Velocity at 100 km layer. Observe the relationship between color and velocity for this layer. The values are in kilometers per second (km/sec). What color represents the slowest shear wave velocity? What color represents the fastest?
- Turn on the Earthquake layers. What do you notice about the distribution of earthquakes and the plate boundaries? The distribution of earthquakes coincides with the boundaries between tectonic plates.
- In the next few steps, you'll examine the relationship among shear wave velocities at a depth of 100 km, types of plate boundaries, and earthquake locations. You'll examine these data for Japan, India, and the middle of the Atlantic Ocean.
Look at the Legend to the right of the map in My World or use the Get Information tool.
Step 2 Examine Data near Japan
- Use the Zoom In tool to Zoom In on the area around Japan.
- Save your work as a new 'Map View'. Click the Map View pull-down menu located above the map. In the save the current as... dialog box, type the word "Japan."
- Examine the plate boundary near Japan.
- Is this a convergent or divergent plate boundary?
- Turn on and activate the Tectonic Plate Names layer. Use the Get Information tool to determine which plates are east and west of the boundary.
- East: Pacific
- West: Okhotsk and Amur Plates
The velocities on the western side of the boundary are slower than those on the eastern side. The darker orange-red color on the western side represent slower wave velocities and the lighter orange and blue on the eastern side are faster.
The majority of earthquakes are on the western side of the boundary.
Think about how the plates here are moving at this convergent boundary. Consider the three dimensional shape of the boundary and what you would find deep below the surface on each side of it. Which side of a subduction zone would you expect to have lower density (and therefore, slower shear wave velocities) overall? On which side of the boundary is a plate being subducted?
The slower shear wave velocities on the western side of the boundary are the result of the Pacific plate plunging below the Eurasian plate (and others). Seafloor sediments and water that are subducted along with the Pacific plate have a lower density than the undisturbed mantle material below the surface on the eastern side of the boundary. Earthquakes on the western side provide further evidence that the Pacific plate is grinding its way under the Eurasian plate. The figure below shows the relationship of those plates from a viewpoint looking south.
You may want to examine shear wave velocities near other locations along this plate boundary to see if the relationship you discovered holds true.
Step 3 Examine Data near India
- Zoom In to the area around India.
- Save your work as a new 'Map View'. Click the Map View pull-down menu located above the map. In the save the current as... dialog box, type the word "India."
- Examine the plate boundaries near northern India.
- Is this a convergent or divergent plate boundary?
- Identify the tectonic plates north and south of the boundary.
- North: Eurasian
- South: Indo-Australian
- In terms of geographic features, how is India different from Japan?
- What major mountain range is located along India's northern border?
- Are the earthquakes evenly distributed on each side of the plate boundary or only on one side?
India is on a continent rather than an island in the ocean.
The Himalayas, the largest mountain chain in the world.
No. The majority of earthquakes are on the northern side, under the Himalayas.
The velocities on the northern side are slower than those on the southern side. The darker blue color on the southern side represent faster wave velocities and the lighter blue on the northern side are slower.
On which side are you still in the crust and which side are you in the mantle?
Because this is a convergent boundary between two continents, mountain building is occurring. The reason that the earthquakes are to the north of the actual plate boundary is because the subducted slab is contacting the base of the crust north of the plate boundary. This is where the earthquakes are generated. The reason that the velocities are faster in India than below the mountains is because at this level (100 km), under India we are well into the mantle while under the mountains we are just crossing into the mantle.
Step 4 Examine Data in the Mid-Atlantic
- Zoom in on the Mid-Atlantic Ridge.
- Save your work as a new 'Map View'. Click the Map View pull-down menu located above the map. In the save the current as... dialog box, type the word "Mid-Atlantic."
- Examine the plate boundary.
- What is the primary type of plate boundary here?
- Which plates are on the east and west side of the boundary?
- East: African
- West: South American
- What other type of plate boundary is seen along the Mid-Atlantic Ridge?
- Look at the shear wave velocities on each side of the plate boundary. Are they similar on each side?
- How are the earthquakes distributed in regard to the plate boundary - evenly or or only on one side?
- Can you think of a reason for this pattern of velocities and earthquakes?
- Examine more plate boundaries around the world. See if the patterns you discovered in shear wave velocities at the three plate boundaries above can be generalized for the whole world. If you find locations that exhibit a different pattern, hypothesize about the factors that might be affecting that situation.
- Zoom back out to the full world view by clicking on the Zoom to Full Extent button.
- Save your edited project with a new name such as: Seismic Wave Part 3.m3vz. Save the file to a location where you can locate it easily, such as the desktop or your documents folder, if you are using a shared computer.
Yes, the velocities are similar on both side of the boundary.
The earthquakes are evenly distributed on each side of the plate boundary.
Think about how the plates are moving in relation to each other at this divergent boundary. Would you expect to find any differences below the surface of a spreading center?