Tectonic Earthquake Basics and How Tsunamis Form

The term tsunami has been adopted by the scientific community in recent years to define the powerful and destructive waves that are caused by earthquakes, landslides, and other similar events that have the ability to suddenly displace large amounts of water. The term tidal wave was used to mean the same thing prior to the introduction of tsunami, but fell out of favor due to tides being – by definition – caused by a gravitational effect.

Having said that, tsunami isn’t exactly a spot-on term for what needs to be described either. Tsunami is a Japanese term literally meaning harbor wave – but it’s not like harbors are the only places on the coast that have need for worry when a large submarine earthquake hits in close proximity. In any case, these are The internal structure of the Earth illustratedthe generally accepted definitions and I’m not one to question or dispute its place.

In order to understand how tsunami are formed, you have to first understand how earthquakes are triggered – as earthquakes are responsible for most devastating tsunami. An earthquake is the result of the sudden release of large to enormous amounts of built-up energy in the Earth’s surface due to plate tectonic friction. Tectonic plates are fragments or sections of the Earth’s lithosphere, and can be thought of to float on the bed of magma comprising the lower asthenosphere.

Tectonic Plate Boundaries

Illustration of tectonic plate boundariesThese tectonic plates are about 50 miles thick and are continually moving, exhibiting a combination of three movement-relationships with their neighboring plates. The locations at which these movements occur are known as tectonic plate boundaries:

  • Convergent plate boundary. This is where tectonic plates will collide with each other, resulting in either a subduction zone or a continental collision. A subduction zone is where one plate will move beneath the other, and a continental collision is when two plates collide, resulting in a formation of a mountain range.
  • Transform plate boundary. This is where tectonic plates will slide past each other, at times resulting in the build-up of potentially earthquake-triggering friction due to “getting caught” on irregular geometry.
  • Divergent plate boundary. This is where tectonic plates will move in opposite directions creating a gap where magma rises, gets cooled by the seawater, and solidifies into rock, replacing the lithosphere that is lost in convergent plate movement. (See image above for visual aid.)

All three forms of plate boundaries have the ability to trigger earthquakes, as though divergent plate boundaries move apart on one end, transform and convergent plate boundaries must also occur simultaneously on the remaining ends. This built-up energy caused by friction at the tectonic plate boundaries is what triggers an earthquake when the stress is suddenly released.

Tsunami illustration 1 Tsunami Illustration 2
Tsunami Illustration 3 Tsunami Illustration 4

And when the released stress is in a convergent plate boundary, where one tectonic plate is violently and suddenly pushed upward, it can displace a monumental volume of water. Where one tectonic plate is pushed upwards, it is called a reverse or thrust fault, and when it’s pushed downwards, it’s called a normal fault. Although thrust faults are one of the chief causes of tsunami, normal faults can’t usually generate enough energy.

  • For those who would like an illustrated explanation, I found the following site quite simple and to the point – How Earthquakes Happen.

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