Lab 3 – Plate Tectonics and the Seafloor

On March 11, 2011 a 9.1 magnitude earthquake occurred 130 km (81 mi) east of Sendai, Japan. The earthquake was generated at a depth of 29 km (18 mi) (Figure 3.11). Sendai is the largest city in the Tohoku region of northern Japan. The earthquake was the largest quake recorded in Japanese history and the 4th largest in the world since records began in 1900. The earthquake caused widespread damage throughout northern Japan and shook buildings in Tokyo. In some locations, the land literally moved as much as 2.5 m (8 ft) eastward.

3-D map of Japan with epicenter of the earthquake. Includes topographic and bathymetric relief.

Figure 3.0.1 Epicenter location for the 2011 Tohuku Earthquake (image copyright: Benjamin R. Jordan, used with permission; underlying map image credit: UNAVCO).

Shortly after the earthquake (about 8 minutes later) a large tsunami wave inundated the Tohoku region. Along much of the coast, the tsunami was 10 m (33 ft) in height, but in some locations, it reached a height of 29.6 m (97 ft).

Over 15,000 people were confirmed to be killed, with nearly 5,000 still missing. Thousands of others were injured. In addition, 450,000 people were left homeless as the tsunami destroyed thousands of boats, homes and businesses. The tsunami travelled throughout the Pacific, reaching as far as North America, Peru, Chile, New Zealand, Papua New Guinea, and even caused an iceberg to calve from the Sulzberger Ice Shelf in Antarctica.

Similar to the 2011 Tohoku Earthquake, on 27 March 1964, a >8 magnitude earthquake occurred in Alaska.  Despite its distance, a tsunami generated by the earthquake devastated coastal communities in British Columbia, Washington, Oregon, and California.  A total of 121 people were killed, including 11 in the town of Crescent City, California.  This tsunami is the largest to hit North America in recorded history (The Tohuku tsunami also reached and caused some damage to Crescent City).  Both of these tsunami-generating earthquakes occurred as the result of plate tectonic interactions that may, or may not, be similar to those found in the northeast Pacific Ocean region near the U.S. states of Washington and Oregon.  This lab is designed to help you understand the potential for similar hazards in the northeast Pacific.

Learning outcomes

  • LO1. Identify seafloor features and their relationships using earthquake data. (Activity 3.1 and 3.2)
  • LO2. Determine plate tectonic settings by connecting real-world relationships to background knowledge. (Activity 3.2)
  • LO3. Articulate some of the challenges that are faced when data does not fit simple models. (Activity 3.3)
  • LO4. Develop a reasonable hypothesis for the potential natural hazards in the studied area. (Activity 3.3)

Background information

  • Key terms: earthquake, epicenter, seamount, mid-ocean ridge, transform zone, subduction, tsunami
  • Data collection sources:

    Figure 3.0.2 Locations of the OOI data stations and cabled arrays (left).  The red box indicates the location of the cabled array that collected the data in this lab.  Map of the OOI cabled array near Oregon and Washington (right).

    Geologic Background

    The Theory of Plate Tectonics is the grand, unifying theory of earth science. The basic idea is that the outermost, physical layer of the earth, the lithosphere, is brittle and thus broken into sections called tectonic plates, much like the thin shell of an egg is brittle and can be broken (Figure 3.0.3). These plates float on top of a more fluid, but still solid, layer called the asthenosphere. The plates of the lithosphere ride on top of the asthenosphere and move due to convection within the earth

    Image of egg shell broken and 2 images of earth broken into tectonic plates.

    Figure 3.0.3 Much like the shell of the egg in this image, the outer layer of the earth, or lithosphere, is a brittle layer that is broken into section. These sections are called tectonic plates (image copyright: Benjamin R. Jordan, used with permission; Earth images from Google Earth 2021.

    These plates move relative to each other, sometimes colliding (forming deep-sea trenches, mountains, and explosive volcanoes), sometimes pulling apart (forming rift valleys and mid-ocean ridges), and sometimes sliding past each other (forming strike/slip faults and transform zones between mid-ocean ridge segments) (Figure 3.0.4).

    Tectonic plate interact in three main ways: divergence (away from each other), convergence (colliding into each other), and transform motion (sliding past one another). the black arrows indicate the direction of movement (image copyright: Benjamin R. Jordan, used with permission; right image USGS.

    Figure 3.0.4 Tectonic plates interact in three main ways: divergent, convergent, and transform motion. The black arrows indicate the direction of movement (image copyright: Benjamin R. Jordan, used with permission; right image USGS.

    These interactions between tectonic plates lead to earthquakes as tremendous amounts of energy are built-up and then released, much like energy is built up within a pencil as it is bent. Eventually, the strength of the wood will be overcome by the energy and the pencil will break.

    Image of no stress as pencil, bending pencil as stress, elastic strain and then the pencil broken as no more stress.Graph indicating increasing stress as pencil is bent maximizing stress just before the pencil is broken, and then as the stress is released, the energy is released with an earthquake.

    Figure 3.0.5 Much like bending a pencil will eventually lead to breaking it, Tectonic forces within the lithosphere will lead to the breaking of rock, forming an earthquake (image copyright: Benjamin R. Jordan, used with permission).

    • Image of 6 ocean bottom seismometers (OBS) ( look like metal pan with very high rounded lid with sensos and cables attached) getting ready for deployment on the seafloor.

      Figure 3.0.6 Image of ocean bottom seismometers (OBS) getting ready for deployment on the seafloor.

      Sensors: The OOI cabled array uses many instruments to monitor the Axial Seamount Volcano. These instruments collect data on such things as ocean salinity and ocean chemistry, seawater pressure, as well as changes in hydrothermal vents. For this activity, data was collected by ocean bottom seismometers (OBSs), which detect and monitor earthquake events (see sidebar).

    • Ocean Bottom Seismometers: Seismometers are instruments that detect shaking and vibrations at the surface of, or within, the earth. The farther an earthquake wave travels, the weaker and harder to detect it becomes. OBSs are designed to be placed on the seafloor, close to the areas that are being studied, in order to collect reliable data. They are used throughout the world’s oceans by oceanographers and marine geologists and geophysicists to answer questions about plate tectonic movement, underwater volcanic processes, and the behavior of the deep earth.
    • Other need-to-know scientific background: Students should be familiar with general plate tectonic processes, the types of plate boundaries and the specific geologic features associated with those boundaries (i.e. mid-ocean ridge = divergent boundary, explosive volcanoes and/or deep sea trench = convergent boundary, etc.), the basic cause and process of earthquake formation, and the relationship between tsunami formation and earthquakes.


    Activities in this Lab