The Moon's Gravitational Pull Can Trigger Major Earthquakes, Says Study (nature.com)

schwit1 writes: A careful statistical analysis of when major earthquakes occur has suggested they are more likely to be more powerful if they occur around the full and new moons when tidal forces are at their peak. Nature.com reports: "Satoshi Ide, a seismologist at the University of Tokyo, and his colleagues investigated three separate earthquake records covering Japan, California and the entire globe. For the 15 days leading up to each quake, the scientists assigned a number representing the relative tidal stress on that day, with 15 representing the highest. They found that large quakes such as those that hit Chile and Tohoku-Oki occurred near the time of maximum tidal strain -- or during new and full moons when the Sun, Moon and Earth align. For more than 10,000 earthquakes of around magnitude 5.5, the researchers found, an earthquake that began during a time of high tidal stress was more likely to grow to magnitude 8 or above." As these results are based entirely on statistical evidence, not on any direct link between tidal forces and actual quakes, they are quite uncertain and unproven.

Comments

It's the Sun, actually

[czert]

Well, it's actually the added stress from the Sun's alignment with Earth and Moon that's likely to play the major role here. The Moon itself doesn't do anything special during the full and new moons.

Re: It's the Sun, actually

[therealkevinkretz]

During full and new moons, Sun - Earth - moon ( or Sun - moon - Earth ) are aligned in a line. That's when their combined tidal forces are at a maximum

Re: It's the Sun, actually

[michelcolman]

It has nothing to do with the total amount of gravitational pull.

Tidal stress is the difference between the amounts of gravitational pull at different distances. If you are floating in space near a massive object (planet, star, moon), with your feet pointing towards it, your feet will be closer to it and will therefore experience more attraction, while your head will be further away from it and therefore experience less attraction. This will tend to stretch your body a little bit (or a whole lot if you happen to fall into a black hole). If the gravitational gradient is strong enough, it will make your hair stand up. Your entire body is actually accelerating towards the object, but your feet are being pulled harder while your hair gets left behind. Relative to the pull on your gravitational center, it feels like there are two forces pulling at you from opposite sides trying to tear you apart.

On earth, that means there's a high tide on the side of the moon (closer to the moon, therefore attracted more to it) but also on the other side (further from the moon, therefore attracted less and bulging the other way). The effect is the same on both sides because it's not the amount of attraction that matters, but only the difference with the attraction experienced by the earth's center.

The sun also creates a similar effect, though smaller.

When the sun and the moon are either on the same side (new moon) or on opposite sides (full moon), the two effects are added together and you get spring tide. And, apparently, possibly more or stronger earthquakes.

Unnecessary cushioning

[The Evil Atheist]

As these results are based entirely on statistical evidence, not on any direct link between tidal forces and actual quakes, they are quite uncertain and unproven.

Given that the article does not say tidal forces CAUSE quakes, this cushioning is completely unnecessary. They're only noting a suggestion of a link between tidal forces and the magnitude of the quake - not the occurrence/non-occurrence of a quake.

And something being "based entirely on statistical evidence" does not invalidate or weaken anything. It is the quality of the statistical evidence, not the mere use of statistical evidence, that would invalidate or weaken a claim.

Re:Unnecessary cushioning

[jheath314]

Earthquakes cause full moons!

Re:Unnecessary cushioning

[Alomex]

Correlation does not imply causation.

This has become a /. meme. What is your suggestion here, that there is a third agent responsible both for earthquakes and full/new moons?

In real life, science correlation is often converted into causation using Occam's razor or logical physical principles. E.g. while there is nothing in theoretical physics forbidding the interpretation that the light about to start shining made you turn the switch, we choose for a variety of other reasons, the interpretation that the causation is the other way around, we turn on the switch, then the light goes on.

Sellings eyeballs to the ad company

[DrYak]

Yeah, but
"Study shows statistical correlation that might suggest that tidal stress could be among the dozens of other factors that each contribute slightly in favor of stronger earthquakes", though much more accurate, doesn't have the same "oomph" and thus doesn't manage to sell the same amount of eyeballs to the ad-company paying for this fucking article.
"The moon will kill with its earthquake power! Cower in terror!! Panic!!! Buy products to feel better!!!!" works so much better...

Re:Is the reverse true?

[Black.Shuck]

And yet.

Tidal forces and moonquakes

[sjbe]

Are you serious? The moon is in gravitational lock, so experiences no tidal stress.

Not actually true. Because Earth is not tidal locked to the moon, the rotation of the Earth will cause tidal stresses on the Moon because the Earth is not a uniform body, nor is the moon. Tidal locking does not equal no tidal stresses.

Have you not noticed that the same side of the moon always faces the earth?

Again not completely true. See lunar libration. The orbit of the moon is not circular, the Earth itself rotates and their respective axis of rotation are not identical. So we don't always see precisely the same face of the moon. We actually see about 59% of the moon's surface though not all at the same time.

OK, there is some tidal force from the sun, but the moon is also kind of small, and solid. No molten core, no ocean, no thin crust, no plate tectonics.

There also are some tidal forces from the Earth on the moon. The effect appears to be quite minor. Moonquakes are apparently a thing and apparently ARE caused at least in part by tidal interactions between the Earth and the Moon.