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Everywhere I've looked so far that talks about the possibility the LIGO detection was an earthquake, involves being ruled out due to the large distance between the two LIGO sites. Two identical earthquakes would be necessary is what I'm reading.

I don't think this is correct, because earthquakes are recorded as deep as 800 km below the surface. There's a region in the mantle, I should think, from which earthquakes may register identically at the two locations. Particularly as we're talking very tiny vibrations.

What I would like to ask is, firstly, were deep-mantle quakes controlled for? As a follow-up question, was there a consulting academic seismologist with deep-mantle expertise on the LIGO team?

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marked as duplicate by Kyle Kanos, ACuriousMind, CuriousOne, Daniel Griscom, John Rennie Mar 23 at 6:38

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The gravitational wave travels at the speed of light, and they had a measurable difference in detection time. Imagine an earthquake wave travelling at the speed of sound to arrive so simultaneously! You have to place it with the precision of some hundreds of meters, what are the odds of this! :) – Ilja Mar 22 at 11:19
Hi - what I would answer to that, is it's plausible what you say is correct in terms of the relevant sciences of the interior. But you are speculating about a complex scientific field with a large accumulated body of knowledge, skills, specialisms and publications. Intuitively, at least, I don't think it's implausible that a deep mantle quake occurred in a region of mantle in a suitable location to reach the surface at both locations at the same time. – Lucy Meadow Mar 22 at 11:30
But I'm not a specialist any more than you are, but I mean, we don't have to do things this way because it is a science in its own right. With it's own community of scientists. From amongst them there will be several high calibre scientists who have and/or are dedicating their entire career to this sort of thing. Ask them! – Lucy Meadow Mar 22 at 11:42
It's probably safe to assume that the LIGO people had seismometers and that they did suitable analysis on what the seismometers recorded at the time of the detection. Now, perhaps there are special earthquakes which don't show up on seismometers but do shake the mirrors suitably. Perhaps. – tfb Mar 22 at 12:41

From the FAQ of LIGO:

LIGO uses two basic strategies to shield the detectors from vibrations of the Earth. They are referred to as “passive” and “active” vibration isolation systems.


Basic "quad" pendulum to demonstrate passive damping through suspension.

LIGO’s passive vibration isolation system absorbs vibrations before they reach the all-important mirrors. One way these vibrations are absorbed is through a pendulum suspension system: LIGO’s main mirrors are suspended at the bottom of 4 pendulums--each node in the pendulum being a hefty mass (LIGO's mirrors alone each weigh 40 kg, or 88 lbs.). Since heavy things don’t like to move (that’s Newton’s Law of Inertia), by their sheer weight, each mass at the top of each step of the 4-step pendulum absorbs vibrations from the mass above it until nothing reaches the lowest, all-important hanging mass: LIGO’s laser-reflecting mirror. To illustrate the effect for yourself, tie four heavy washers or bolts together in a line, each one separated by an equal length of string (see illustration at left). Hold the string at the top and rapidly shake the top back and forth by a small amount (you're simulating vibrations from the environment around LIGO). You'll see that the lowest mass just won’t move much at all compared to how much you're moving the top mass because each mass in the chain absorbs or ‘damps’ the vibration, isolating the last one from a lot of noise above it.

As it suggests, “active” isolation is an active process whereby a set of sensors feels vibrations and sends signals to “force actuators” that generate counter-forces to cancel out vibrations. This is the same basic principle by which noise-cancelling headphones operate. In LIGO's case, the active isolation process actually occurs outside of the suspension system, removing the largest vibrations before they reach the first step in the pendulums described above (as the name implies, LIGO's suspensions are also 'suspended' below the active seismic isolation platforms). The suspensions then remove the smaller vibrations, leaving LIGO’s mirrors perfectly still and primed to detect gravitational waves.

So it is not " ruled out due to the large distance between the two LIGO sites" but because the mirrors are constrained to be at rest, no matter what ground vibrations come up.

Edit after finding the paper Reducing the effect of seismic noise in LIGO searches by targeted veto generation: it seems that seismology noise experts are among the ones signing the LIGO publication, so due respect to seismological noise has been paid.

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That's right....that's the normal way in modern times tremors are handled. I'm sure it's very precise. But it would be necessarily to factor-in deep mantle effects at the time of design. Because all dampeners have real or possible vulnerabilities to resonance scenarios. The sensitivity is at extremes enough to warrant, I should imagine. But I'm speculating beyond my knowledge. The question is whether I'm the only one! – Lucy Meadow Mar 22 at 12:04
@LucyMeadow why would a deep mantle quake be worse that a truck passing outside? Note that they are not using concidence or not in the two experiments. They just exclude all vibrations. – anna v Mar 22 at 12:58
I don't think this really answers the question. The mirrors have very good seismic isolation but it is still finitely good, and there is still some magnitude of noise getting in. However good the isolation is, the signal is still sensitive to e.g. trucks rumbling nearby. These local disturbances are mainly ruled out by the comparison between distant sites but in principle a strong enough distant earthquake, as posited in the OP, could cause the signal if not specifically controlled for. – Emilio Pisanty Mar 22 at 13:28
@EmilioPisanty But such an earthquake would have shown up on the seismometers. Which they have. – tfb Mar 22 at 14:34
If the effect of seismic noise had been entirely eliminated, the title of the linked paper would not be "Reducing the Effect of Seismic Noise..." – sdenham Mar 22 at 15:41

It isn't ruled out. The LIGO team have carefully looked at, modelled and empirically measured all sorts of possible noise effects that could give the observed signature in both detectors within a few ms of each other. This includes a battery of other instruments which monitor all sorts of things, including seismic activity, that are used to veto sections of data so that they aren't used in any analysis.

The confidence with which they are suggesting they believe this event was not caused by any noise source that they have considered or measured, is quantified by saying that noise would only produce a signal that looked like the merger of two black holes in both detectors once every 200,000 years of LIGO observation.

Note that this "false alarm probability" does not rule out some other cause besides the merging of black holes for the event. As far as I know, nobody has suggested a credible alternative that can mimic the chirp signature of merging black holes.

However, if anybody has any ideas, then now is the time to publish and make predictions for the distribution of such events and their strengths, since I imagine it is quite likely that LIGO will publish results from several months of observing shortly.

As far as your specific suggestion goes, would a deep mantle earthquake produce almost simultaneous signatures in detectors thousands of km apart that lasted for a fraction of a second and swept across an order of magnitude range in frequencies? Given p-wave velocities of 5 km/s or so, there would have to be an incredible lack of any wave dispersion/refraction/diffraction for a wavepacket to arrive in such a short space of time after travelling thousands of km?

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Do you know how much they could mitigate that by looking at the traditional seismic sensors? That would rule out all large earthquakes. – Lacklub Mar 22 at 12:17
@Lacklub the LIGO detection was many orders of magnitude smaller than what a conventional seismometer could measure. The previous generation of LIGO wasn't able to collect good data at the Louisiana site during the day due to noise from logging trucks in the surrounding forest. I suspect anything large enough to register on conventional seismometers would end up knocking the LIGO interferometer out until it passed. – Dan Neely Mar 22 at 17:34
@DanNeely I'm not so sure about this. If you have a complex attenuation system that knocks many orders of magnitude off the effect as seen by LIGO it might be quite feasible to detect said effect using an "unshielded" instrument. They certainly do have veto periods when data is rejected for seismic reasons. – Rob Jeffries Mar 22 at 17:38
@DanNeely Ah, ok. I thought that the system was so incredibly isolated that only a large earthquake would even register. What you said makes much more sense. – Lacklub Mar 22 at 17:38

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