In the theory of plate tectonic where a plaque sliding over another causes an earthquake?

Question

does moisture content on the plaques rim make a difference in the sense that the more moisture content would make the plates to slide easier? If so and if no moisture or very little were present then the build up of stresses would be greater and the earthquakes would be more severe?

Answer 1

Earthquakes come down to stress and strain. It is not 'the plates rubbing together' as many people say; or at least that's a very simplistic view. And although water plays some role in speeding up the process, it is mainly the stress that does it.

Plate tectonics causes stress on the continents and oceans, all over the surface of the earth. In some places, the stress is very small (usually within the plates). Elsewhere, the stress is high, usually where the plates meet each other. Since each plate moves, when two come in contact, the stress each other. They can push on each other and cause compressional stress, they can pull with extensional stress, and they can slide past or shear each other with tensional stress.

Faulting, causing earthquakes, comes from the fact that this stress is building up all the time, but rocks and continents are strong materials. Just like hitting a rock with a small hammer, you do put stress on it, but a small amount. It would take a sledgehammer to put enough strain (effects and accumulation of stress) to build up and cause breakage. In the earth, the area around an active fault builds up strain from the stress of plate tectonics. Most faults become locked, because of this strength, and thus can not release their strain. Away from the fault, the stress produces very small and slow movement of the rock masses as a whole. Eventually, the strain is too much and the rest of the plate has moved too far and the fault releases the strain build-up all at once in a big stress release called an earthquake. This is called the elastic rebound theory, and it explains most (but not all) movements.

This is why an earthquake's size is relative to the fault size. The bigger the fault, the bigger the strain build up, and the bigger the release in an earthquake. Subduction zones and collision zones, where large portions of plates actually can rub together as a whole, have the really big earthquakes, like the 2004 Sumatra earthquake. In other plate boundaries, the entire plate boundary does not act together, so the faults become spread out and many faults take up the strain from the tectonic stress, like with the San Andreas Fault in California (it only takes up ~3/4 of the stress between the plates).

Earthquakes are really tricky things; there is still so much we need to learn. As of now, there is no way to predict them, but we can say where the danger is highest and about how long between events.

Answer 2

A lubricant, like water in this case, reduces the friction on moving objects, by building up a water film between the two surfaces. When the movement stops the film will vanish and the two surfaces will come together.
What I am trying to say is that once the tectonic plates start moving, the water will make them slide a little further before the stop, but before they start moving, the water doesn't make much difference.

PS. I'm an engineer, not a geologist, I don't know how well this applies to earthquakes.

Answer 3

Sounds reasonable. The co-efficient of friction would have something to do with it.

But the great forces would generally overcome that and make differences very minute.

Also, there is great heat built up. Water would evaporate, even at great pressure. If it vents, there would be no moisture to speak of. If it doesn't vent, the great pressure of steam could further cause plate slippage.

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Answer 4

It might make some difference but given the size of the tectonic plates (i.e., their thickness) it makes no practical difference.

Answer 5

It's also the great weight of the plates and that geology moves slowly