[w.h.rogers]

w.h.rogers asked the Naked Scientists:
   
Is there any connection between high tides and earthquakes?

What do you think?

[LeeE]

I would imagine that the extra weight upon the crust might have some effect, but our resident rock-botherers will know more.

[geo driver]

i wouldn't of thought so. high tide is what twice a day?  and the shift in weight would be regular so you could say the reduced weight at low tide would cause the same thing.  but again the real geologists would know

[Bass]

Several years ago, I analyzed correlations of over 80,000 earthquakes of various magnitudes and depths with position of the moon (tides), position of the moon and sun (full moon, etc), apogee and perigee of the moon and earth, and alignment of the planets.  (This was to win a small wager).

There is no correlation of earthquakes to any of these factors, with one exception- there was a very slight negative correlation of deep, moderate earthquakes with full-new moon cycles.

Ocean tides add/subtract insignificant strain to the crust.  The moon also causes tidal movements in rocks- one would guess this might have the greatest effect on small, shallow earthquakes (less than 2M and 2km)- I didn't test these very small, shallow earthquakes because of the overwhelming number of the sample.

Short answer- no.

[thedoc]

Chris -  It sounds a bit bizarre, doesn’t it? To think, could the sea be causing earthquakes? But actually, the answer is, yes, it possibly could. Now it’s a slightly indirect answer to this but there was a paper that came out. It was in the journal Nature and it was last June, and it was by a US Geologist who’s called Selwyn Sacks and a researcher in Taiwan. Taiwan is interesting because it’s got a very, very rapid rate of tectonic plate movement. Plates there are moving and colliding at the rate of about 15 centimetres a year which is a huge amount of movement. This means that where you have faults, you have enormous amounts of energy being stored up.
So, Selwyn Sacks and his colleagues were measuring strain energy. They were putting strain gauges into the ground there to measure how these faults are moving and storing energy over time. What they were really surprised to see were some rather weird recordings on their strain gauges at certain points, and what they found is that they were seeing the arrival of typhoons – these big tropical storms associated with very low pressures.
Normally, when a low pressure moves in over land, what happens is that the low pressure makes the land swell up a bit. So their strain gauges were recording that the land was swelling up. But sometimes, instead of the land swelling up, they actually found the land shrinking and the only way they can explain this is if there’s been an earthquake – a so-called slow earthquake.
What’s going on, it turns out, is that when you have a very low pressure system, the air pressure drops, so the land swells, but where there’s sea, which is adjacent to the land of course, because the water doesn’t get out of the way, water doesn't swell, instead, more water comes in to fill the area with low pressure. So this means the pressure on the sea floor is roughly the same, but the pressure over land is lower, and therefore, any faults open up for this reason because you've now got a pressure differential between the land and the sea, and this is more likely to unload faults and trigger earthquakes.
What they found is possibly happening in Taiwan is that you've got these so-called slow earthquakes which are earthquakes that happen over hours to days; they don't go all of a sudden. They gently let go of the energy and this slowly dissipates the stored energy in the fault, but the thing that was triggering it, they found on their recordings was the arrival of these typhoons, and the typhoons are basically making the tide come in metaphorically.
So, I guess you could say that the movement of large bodies of water can potentially trigger an earthquake. So the answer is, yes.
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[Eric A. Taylor]

Earthquakes happen when the rock of the crust fails due to pressure that has built up over time. Earthquakes in tectonic zones are inevitable to the the movements of the plates. Think of blowing up a balloon. As you blow more and more air into the balloon the latex will stretch farther and farther. Eventually the air pressure will exceed what the balloon will hold and POP!

  The tides do pull on the rock of the Earth and could possibly pull enough cause it to fail. However because the stress builds steadily, and the pull is very small, tides could ONLY cause an earthquake if the rock is very close to failing anyway. Even without this pull the rock would fail very soon. Perhaps within just a few hours.