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Earth rocks are frequently folded at surface.

Rocks folded at Crete, Greece

Folds in alternating layers of limestone and chert in Crete, Greece.

Source: commons.wikimedia

Are earthquakes the main mechanism producing the folds?

If so, have ever geologists observed the development of a fold after an earthquake?

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No, earthquakes are sudden, often very energetic events which produce faults and discontinuities. There is a sharp break in the strata when subjected to a powerful earthquake. The folding of strata is a much more gradual process, where rock deep underground is subjected to enormous lateral pressure over millions of years. This pressure is caused by plate movements (continental drift). Earthquakes have a variety of causes, but the most violent ones are also caused by continental drift as one plate slides past another along a fault line. The San Andreas Fault in California is an example of this. One of the plates is moving, but friction holds it in place for a while, sometimes for many years. Pressure builds up until suddenly the friction force holding it in place gives way, and there is an earthquake as the moving plate slips forward for a few metres. For these reasons, geologists have never seen a newly created fold after an earthquake. It is impossible for one to form in so short a time.

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  • $\begingroup$ Even most of folds are not formed as the result of an earthquake, couldn't those faults and discontinuities generate isolated folds? $\endgroup$ – user12525 Jul 9 at 11:18
  • $\begingroup$ @Universal_learner if you apply enough force to try to bend rock on a timescale of minutes, it'll just break. Hence the discontinuities that Michael mentioned. To bend rocks like this takes slightly more modest forces over long time periods. Think of it like bending toffee... $\endgroup$ – Semidiurnal Simon Jul 9 at 16:47
  • $\begingroup$ @Semidiurnal Simon Being a bit fussy at plastic/non consolidated rocks earthquakes for sure produce folds, at least at microscale. But for consolidated rocks as those ones of the picture I post I understand those folds cannot be produced by an instantaneous mouvement of rocks $\endgroup$ – user12525 Jul 9 at 17:51

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