## Lab 3.3 – Earthquake and Tsunami Hazards

Fundamental Concept: Identify the the relationship of plate boundaries, depth of earthquakes and natural hazards such as tsunamis.  Hypothesize the risk of tsunamis in an area based on plate tectonics.
Estimated time to complete: 25 minutes
Data skills preparation: Lab 1.2 – Geography, Lab 1.3 – Latitude and longitude, Lab 2.2 – Bathymetric charts, Lab 2.3 – Bubble charts
Materials needed: None

We can learn something about the behavior of tectonic plates by examining the size and location of earthquakes. For example, the deepest earthquakes occur in convergent settings, where one plate subducts beneath another (Figure 3.3.1).

Because earthquakes occur within the brittle lithosphere, the depth and location of an earthquake tell us something about the geometry of a subduction zone (Image copyright: Benjamin R. Jordan, used with permission, left figure USGS).

When the seafloor is disturbed during an earthquake, either by direct movement of the seafloor or by the generation of an underwater landslide, the energy from this disturbance can be transferred to the ocean itself, forming a tsunami (Figure 3.3.2).  Thus, areas of active plate interactions within the ocean may present a significant natural disaster hazard.

Figure 3.3.2 If the surface of the seafloor is displaced, its motion will transfer energy into the overlying water, forming a tsunami wave. (Image copyright: Benjamin R. Jordan, Used with permission).

As an example of the many earthquakes that occur within the seafloor, the following video is a recording of the sound of an undersea earthquake that was detected in the North Atlantic in 2018 (1 min.):

Use this map of bathymetry and earthquakes in the northeastern region of the Pacific Ocean to make observations and answer the questions below. You can toggle on the volcano locations, zoom in and out of the map and examine the relationship between earthquake magnitude and depth relative to the seafloor features within the study area, indicated by the rectangle. For comparison, scroll the map over to Japan (Scroll out then slide over to Japan and scroll in) and see where Japan’s volcanoes are located relative to the coast and the deep-sea trench.

### Interpretation Questions:

1. Relative to their depth of occurrence, what is the relationship of the earthquake locations to the seafloor features? Do these make sense (explain why or why not)?
2. Where do the shallowest earthquakes occur?
3. Where do the deepest earthquakes occur?
4. Look closely at the deepest earthquakes. Do they have a pattern and if so, what is it?
5. What type of boundary is likely indicated by the deepest earthquakes and the locations of mountain ranges on the nearby land in the Northwest U.S.?
6. How do the land-based volcanoes support your answer to the last question?
7. What is the number of earthquakes at this boundary relative to the mid-ocean ridge segments and transform zones? Is it more or less?

### Application Questions:

1. What might the number of earthquakes in the subduction zone indicate? Why might it be different than at the mid-ocean ridge?
2. What might be indicated by the small number and size of earthquakes in the subduction zone, both presently and the future?
3. What does the reduced number of earthquakes in the subduction zone suggest about future natural hazards?
4. If a large earthquake were to occur near the continental slope, how might it affect the slope? What other hazards might be associated with such an earthquake?

### Reflection Questions:

1. Using your knowledge from Labs 3.1-3.3, explain how earthquakes, plate tectonics, and seafloor features relate.
2. In thinking about your answer to the last question and the 2011 earthquake in Japan, what do you hypothesize about the potential for tsunamis in the Pacific Northwest region?
3. List the coast closest to where you live (or a place you’d like to visit; i.e. near the east coast of the U.S.). What is similar or different about the plate boundaries? How might that change the natural hazard risk for tsunamis in that region?