Earthquakes, tides, tsunami triggering

Tides: the trigger for earthquakes

Source: December 2004 issue of Science et Avenir

**By increasing the pressure on faults, tides can trigger earthquakes. **

When a fault is under stress, an additional strain can trigger a slip, therefore an earthquake. A Japanese-American team, led by Elisabeth Cochran from the University of Berkeley, reached this conclusion. What is interesting is that she had published her work before the event of December 26, 2004, in Indonesia.

Her team analyzed more than two thousand earthquakes with a magnitude of 5.5 or higher on the Richter scale that occurred between 1977 and 2000 in subduction zones, where one plate slides under another, as is the case, for example, in Alaska, Japan, New Zealand, or on the western coast of South America, which are also the regions of the globe where the strongest tides are measured.

By tide, we usually refer to a phenomenon of rising and falling of the liquid mass. But we forget that the Earth's mantle also "breathes" under the combined gravitational effects of the Moon and the Sun. Every day, the Earth's solid surface rises and falls by fifty centimeters.

The team was able to highlight a correlation between the frequency of seismic activity and the amplitude of the tides. It is interesting to note that moderate suppressions (the correlation appears as soon as the tides exceed two meters in amplitude) can have a significant effect on the triggering of an earthquake. Two meters of water is a pressure of one fifth of an atmosphere. This shows that earthquakes, which involve very large energies, can be triggered by very modest causes, which Elisabeth Cochran compares to a matchstick compared to a forest fire.

Our comment:

If tides (marine and "terrestrial", combined) can play a triggering role in earthquakes, there is no reason why these phenomena could not also play a similar role in triggering a tsunami, which initially originates from an underwater earthquake, and no reason why the pressure related to a powerful underwater nuclear explosion could not in turn trigger the phenomenon artificially.

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