Ocean STEMulation: Tsunamis and Plate Tectonics
Focusing on science, technology, engineering, and math, STEM, as they pertain to the ocean.
On April 11, the Indian Ocean experienced an 8.6 magnitude earthquake about 300 miles north of Sumatra, bringing back terrible memories of 2004 – when an earthquake triggered a tsunami that devastated coastlines from Indonesia to Madagascar, killing over 230,000 people. Thanks to improved monitoring and communications technology, alerts were issued across the region. Coastal residents, remembering what happened eight years earlier, executed a rapid evacuation…
However, this time, no tsunami came.
Tsunamis happen when a large amount of ocean water is displaced. This usually happens when a large piece of the seafloor moves because of an earthquake, but it can also come from underwater landslides. Not all earthquakes are the same, however.
The outermost layer of the earth, the crust, sits on top of a thick liquid layer called the mantle. Plate tectonic theory shows that the crust, anywhere from 3 to 30 miles (4 to 40 km) thick, is made up of several big plates that move on the mantle, causing earthquakes.
The earth’s crust is very thin and floats upon the thick, liquid mantle. Image by Jeremy Kemp via Wikimedia Commons, Creative Commons License.
The crust is made up of multiple moving pieces, called plates. Image by USGS via Wikimedia Commons, Creative Commons License.
A fault is the border where two plates meet. There are different types of faults. If two plates slide past each other, like two huge boats going opposite directions on a river, then it is called a transform fault. If they are moving away from each other, it is a divergent fault. And if they are moving toward each other and colliding, it is called a convergent fault.
This month’s Indian Ocean earthquake happened due to a transform fault earthquake. The plates slid past one another, but they didn’t change how much space was taken up by ocean bottom versus how much space was taken up by water. That is why no tsunami occurred.
Earthquakes often happen along faults, which occur where two tectonic plates meet. There are three main types of faults - transform, convergent, and divergent - classified by which way the plates are moving. Image by Jose F. Vigil, USGS via Wikimedia Commons, Creative Commons License.
The 2004 tsunami in Indonesia, and the devastating 2011 tsunami in Japan, were both caused by convergent fault earthquakes. Two plates collided, one slid underneath the other, and a ton of friction built up between the two plates. This is called a subduction zone. This can cause massive earthquakes that move a big piece of the earth’s crust up or down. If the fault is in the ocean, immense amounts of water can be displaced. This triggers big tsunamis.
In the open ocean, a tsunami can be almost impossible to detect. They are low in height, or amplitude, and long in width, or wavelength. The wave rapidly travels extremely long distances. (Click here for an animation of how far the 2004 tsunami traveled). When it hits shallow water you see a wave above water level because the energy is pushed upward by the ocean bottom.
The wave is usually low in height until it reaches shallow seafloor, which pushes it upwards. Image by Lauchame via Wikimedia Commons, Creative Commons License.
Tsunamis are not necessarily very large in height. Much of the footage of the Japanese tsunami of 2011 showed waves only a few meters high. The reason they are damaging is the enormous volume of water they carry. While normal waves typically break and then come to a stop when they hit an obstruction like a reef or beach, tsunamis have enough energy to continue over obstructions and can travel far inland.
Waves draw the water in front of them into their energy cycle, which causes a dip in the water in front of a wave. A tsunami’s great amplitude will cause water to recede far in front of it. When the wave nears shore, this can look like a fast and sudden low tide. If you ever see the ocean suddenly retreat like an extremely low tide, this is a sign of serious danger and you should evacuate.
To see an interactive diagram of how tsunamis happen, click here.