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

[bytesex]

The moon is heavier when it shinest brightest, didn't you know that? Those photons don't weigh nothing, you know...

No, actually, it has something to do with the position of the moon and the sun, and how that both creates the optimal conditions for gravitational pull, and gives it the most exposure to the earth. Think, young Anakin, think!

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.

Re: It's the Sun, actually

[tomhath]

They're usually not quite lined up due to the inclination of the Moon's orbital plane relative to the plane of the ecliptic, but they're close enough. An eclipse happens occasionally when the Moon lines up exactly with the Earth and the Sun.

Re: It's the Sun, actually

[Anonymous Coward]

Just to pick a minor nit. Photons really don't weigh nothing. They lack mass. But they do have weight. Weight is the force of gravity on something - gravity affects light (hence we get things like gravitational lensing) ergo - light has weight.

I can't tell if you're joking, so I'll assume that others may not be able to tell either. Light doesn't have mass, but it does have momentum. It does not "weigh" anything. Gravity bends spacetime, and the path light takes can also be bent.

If you were joking, well played. Carry on.

Re:It's the Sun, actually

[Applehu Akbar]

A 'land tide' of about one meter corresponding to the ocean tides has been noted for years, and so has been the effect on earthquakes.

Re: It's the Sun, actually

[tinkerton]

i think he means it the other way round. If you have a sphere filled with light, the light will add to the gravity of the sphere. For calculating the mass of the sphere you need the amount of energy inside and you don't need the details about what kind of energy it is.

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

[ilguido]

To say the truth, this is a very old theory. A guy spent his life searching for connections between tides and earthquakes.

Re:Unnecessary cushioning

[jheath314]

Earthquakes cause full moons!

Re:Unnecessary cushioning

[silentcoder]

>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.

Well, in this case, that evidence is purely of correlation. Correlation does not imply causation. Now as it happens - they aren't even claiming causation and there is no reason to suspect it. The suggestion is that tidal forces can make quakes worse - and there are conceivable mechanisms that might cause that effect, this does not however prove any of those mechanisms actually happen. They could all cancel out for all we know.
What this does say is -there is reason to study more, there's something here worth further study.

Some of that study may be statistical - in which case it's important to remember the golden rule of these things: rule out all the data so far. Basically, the statistical clump you noticed, is not a valid sample in determining whether that clump matters or represents a real thing. This is where the scientific method kicks in. Based on the clump we can now predict that, in future, quakes that that coincide with new or full moons will also be more severe than quakes which do not. If that prediction holds- then we learn that there must, indeed, be some causal mechanism at play. If it does not -it means we picked an interesting but meaningless data set either deliberately or unintentionally because human brains are programmed to look for interesting patterns whether or not they exist.

Re:Unnecessary cushioning

[jrumney]

And something being "based entirely on statistical evidence" does not invalidate or weaken anything.

It depends if you consider correlation to be weaker than causation. Statistical evidence is correlation. You now need to come up with a theory to explain the cause of that correlation, and test the theory to see if it can be proven. With only a correlation, there is still a possibility, however remote, that the relationship you have noticed has appeared in the data purely by chance.

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.

Is the reverse true?

[bazmail]

Do moonquakes occur during these periods?

Re:Is the reverse true?

[Black.Shuck]

And yet.

Re:Is the reverse true?

[Anonymous Coward]

"When New Horizons sent us the first close-up pictures of Jupiter recently - the single biggest shock was how varied the land is."

Actually, the biggest shock would have been that it was Jupiter, not Pluto, like it was supposed to be...

Re:Nuke the Moon?

[Calydor]

Video blocked in Germany for copyright reasons. Yay free world.

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...

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.

Re:Why does the phase of the moon matter?

[93 Escort Wagon]

It's about tidal forces - the relative angles at which the sun and the moon are acting on the earth as the phases of the moon change. The overall strength of the moon's gravitational pull does not change, since its mass is constant.

This just in ...

[Qbertino]

Forces of nature can trigger forces of nature.
Film at eleven.

Re:It's the Sun, actually. NOT!

[khallow]

If someone actually bothered to compute the gravitational force on the Earth by both the sun and moon, you would find the force of the moon is about 1E6 times larger than that of the sun.

The Sun is roughly 400 times further way than the Moon. The Sun masses roughly 30 million times more than the Moon, We have that gravitational force is proportional (using the pretty accurate point mass Newtonian model of gravity) to the mass of the object and inversely proportional to the square of its distance. Hence, the Sun's gravitational force is crudely 200 times stronger than that of the Moon.

By a similar calculation one would get that the tidal force of the Sun is roughly half that of the Moon since tidal force under the above model is proportional to mass and inversely proportional to the cube of the distance.

So maybe there was a reason your highly accurate motion platform wasn't.

I thought we knew this ...

[NoSalt]

Didn't we already know this? The moon can cause tides, earthquakes, and other Earth-bound changes.