Gravitational pull 'has role in quakes' | Hacker News Reader

Gravitational pull 'has role in quakes'

bbc.com

47 pointsadanto684026 comments

Quote: "When the tug of gravity is strongest, they added, "the probability of a tiny rock failure expanding to a gigantic rupture increases"."

The article's author clearly didn't understand the original paper. It's not gravitational force that was being examined, but its first derivative -- changes in gravitational force from one location to another and/or one time to another, i.e. tidal stresses.

In fact, any change in gravitational force -- either an increase or a decrease -- could produce the effects being examined. So it's not the strength of the gravitational force, but the fact that it changes over time, and from one location to another.

Consider Jupiter's moon Io as a classic example. If Io were in a circular orbit, it would be a cold, dead world. But Io's orbit is elliptical, which causes perpetual changes in tidal force (the first derivative of gravitational force) across Io's diameter. The result is a very highly seismically and volcanically active body.

Without gravity, there would not be very many earthquakes.

Or planets lol.

Plus no stars and any large scale formation, totally diffuse dark matter, and only the heavy elements from very early nucleosynthesis, assuming you could have a BB without gravity. Practically though, it's hard to imagine spacetime that has no curvature at all beyond a local IF.

This is kind of old news, I found references in late 1990's to tides influencing earthquakes around volcanos while doing a high school science project. The problem is the effect size is tiny. I thought it was a cool project, but someone else at that science fair did the same thing. I suspect a lot of people have looked into it over the years.

PS: USGS has a great database if earthquake data going back decades, it was downloadable as a ~100 meg txt file. Just remember there is a lot of different kinds of faults, and volcanos cause a lot of earthquakes. Modern version looks rate limited to 20,000 events though.

There's some research out there that correlates certain types of marine quakes with tidal levels: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2009....

For those reading the article (it was free to read for me at home), it looks like their central point is shown in Figure 2a. Mind the vertical axis as you look across Figure 2.

The statistics are thankfully low for strong earthquakes, but another century or so will provide an ample test of their hypothesis.

I looked up the San Francisco 1989 earthquake and it was 2 days past a full moon. I tried to look up tide data, which is out there, but I'm on my phone and didn't want to bother with ftp or CSV stuff, heh. Probably just a coincidence but was interesting to me at least.

Roughly half of all earthquakes are within one week of a full moon.

For those surprised by this (like me), consider that roughly half of your life is within one week of a full moon. A lunar cycle is 29-30 days, and two weeks (one before and one after) is roughly half of that cycle.

I dont believe the phase of the moon as anything to do with the earthquakes, more the orbital position...

Psst. The moon takes about four weeks to go from full moon to full moon, so "within a week of full moon" is roughly 50% of all days - earthquakes or otherwise.

I'm pretty sure OP was joking. Except what if the quakes are not evenly distributed? Then the joke is wrong :-)

It is a joke - but intuitively you'd expect some correlation between tides and earthquakes. It would be somewhat surprising if such correlation did not exist, and then I'd wonder what does that mean for the mechanisms that produce EQs.

But the actual phase has nothing to do with it, the moons position around the earth does. It's possible to have a full moon over head or a new moon over head, the gravitational pull will be the same regardless of the phase. It's purely coincidental about it being a full moon.

The phase is important because tidal forces are the sum of forces from both the Sun and the Moon. When the Moon is full, that is because the Moon is directly opposite the Sun. When the Moon is new, that is because it is directly in front of the Sun.

This causes minimums and maximums in the tidal forces on the Earth.

> The phase is important because tidal forces are the sum of forces from both the Sun and the Moon. When the Moon is full, that is because the Moon is directly opposite the Sun. When the Moon is new, that is because it is directly in front of the Sun. This causes minimums and maximums in the tidal forces on the Earth.

More pedantically, both those phases cause a spring tide (max tidal range). A neap tide (min tidal range) is when the sun and the moon are at right angles to the earth.

Good point.

Tidal forces cause a lot of stress on celestial bodies.

The Moon is tidally locked to Earth. Its rotational energy was spent by rotating it while being tidally pulled.

A good analogy is: get a rubber ball, squeeze it on a flat surface and roll it (while squeezing it). That's what happened to the Moon.

That can't be strictly true, otherwise we wouldn't be able to extract energy from the tides. What am I missing?

> That can't be strictly true,

That The Moon is tidally locked to Earth? It's a one way relationship - The Moon is tidally locked to Earth, but Earth isn't tidally locked to The Moon.

I assume you'd have trouble extracting as much energy from any tides that might exist on The Moon, if it had liquid oceans.

No. I was referring to

  "Its rotational energy was spent"

or rephrased

  "Tidal forces cause a lot of stress on celestial bodies",
  but not with the Earth-Moon system because the Moon's
  "rotational energy was spent".

raverbashing's point as I read it was that while tidal forces matter on celestial bodies (i.e. not in the Earth-Moon system) they no longer matter in the Earth-Moon system because the Moon is tidally locked.

I'm not disputing that the Moon is tidally locked, just the consequences in terms of tidal forces. But perhaps the issue (and maybe err) is in confusing rotational energy with orbital energy and the relationship to tidal forces.

You misunderstood me

> "Tidal forces cause a lot of stress on celestial bodies",

Correct

> but not with the Earth-Moon system because the Moon's "rotational energy was spent".

Not now. But the Moon is still stretched tidally (and permanently in that position)

It causes a lot of stress when the tidal pull happens while the body rotates (w.r.t their main "puller" - the Moon rotates, but not relative to the Earth)

And this pulling/stretching is what caused the Moon to stop rotating

Maybe Wikipedia explains it better https://en.wikipedia.org/wiki/Tidal_locking

Ahh. I interpreted more along the lines of the rephrasing: "... but not on the Moon because the Moon's 'rotational energy was spent'" - that is, no claim about Earth not still being under significant stress.

Taken as a whole, I read the comment as possibly along the lines of "This seems reasonable (or obvious) - you end up with dead lifeless geologically inert celestial bodies like the moon if you take away the stress of tidal forces / (varying) gravitational pull", as an example to contrast/compare against.

I can confirm, we didn't have gravity here in Oklahoma until the 2000s.

Queue the fracking-causes-quakes deniers! I can hear the floor speeches already.

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