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Breakdown: Why do some earthquakes cause tsunamis, but others don’t?

Published: Nov. 12, 2021 at 1:02 PM CST
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MEMPHIS, Tenn. (WMC) - The Earth’s crust and the outer mantle layer beneath it are made up of seven massive plates and many smaller ones that fit together like puzzle pieces and are constantly moving above the molten core.

When these tectonic plates slip over, under, or past each other at the fault lines where they meet, energy builds up and is released as an earthquake. Undersea earthquakes sometimes cause ocean waves called tsunamis.

But why do some earthquakes spawn tsunamis that kill thousands while others hardly stir a ripple on the ocean surface?

Tectonic plates are pieces of the lithosphere and crust, which float on the asthenosphere....
Tectonic plates are pieces of the lithosphere and crust, which float on the asthenosphere. There are currently 7 plates that make up most of the continents and the Pacific Ocean.(letstalkscience.ca)

There are four conditions necessary for an earthquake to cause a tsunami:

  1. The earthquake must occur beneath the ocean or cause material to slide in the ocean.
  2. The earthquake must be strong, at least magnitude 6.5, but usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.
  3. The earthquake must rupture the Earth’s surface and it must occur at shallow depth – less than 70km (43.496 miles) below the surface of the Earth.
  4. The earthquake must cause vertical movement of the sea floor (up to several meters).
When a great earthquake ruptures, the faulting can cause vertical slip that is large enough to...
When a great earthquake ruptures, the faulting can cause vertical slip that is large enough to disturb the overlying ocean, thus generating a tsunami that will travel outwards in all directions.(International Tsunami Information Center)

Most tsunamis are generated by shallow, great earthquakes at subduction zones. More than 80% of the world’s tsunamis occur in the Pacific along its Ring of Fire subduction zones.

When these plates move past each other, they cause large earthquakes, which tilt, offset, or displace large areas of the ocean floor from a few kilometers to as much as a 1,000 km (621.371 miles) or more.

A subduction zone is a collision between two of Earth's tectonic plates, where one plate sinks...
A subduction zone is a collision between two of Earth's tectonic plates, where one plate sinks into the mantle underneath the other plate.(columbia.edu)

The sudden vertical displacements over such large areas, disturb the ocean’s surface, displace water, and generate destructive tsunami waves.

The height of a tsunami wave is influenced by the ground’s vertical movement, so changes in the seafloor’s topography can either amplify or dampen a wave as it travels along.

A tsunami wave typically travels at up to 500 or 600 miles an hour – roughly the speed of a jet. The waves can travel great distances from the source region, too.

For example, the Great 1960 Chilean tsunami was destructive not only in Chile, but also as far away as Hawaii, Japan and elsewhere in the Pacific. The tsunami was a result of the largest earthquake ever measured (magnitude 9.5).

Map showing the extent of the tsunami generated by the Chile earthquake of 1960.
Map showing the extent of the tsunami generated by the Chile earthquake of 1960.(Map showing the extent of the tsunami generated by the Chile earthquake of 1960.)

To detect tsunamis, scientists rely on a complex network of sea-level gauges and underwater pressure recorders.

To determine whether an earthquake will generate a tsunami, and to predict how severe it will be, researchers measure the height and energy of the ensuing wave by using ocean-pressure sensors and tide gauges, according to the USGS.

Tsunamis are detected and measured by coastal tide gages and by tsunami buoys in the deep ocean. The tide gages measure the tsunami wave directly. In the deep ocean, sensors on the ocean floor detect the pressure signature of tsunami waves as they pass by.

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