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Edit: Two new items may shed more light on this question:

The Nature Communications paper Prompt gravity signal induced by the 2011 Tohoku-Oki earthquake presents a "...report on the search for a prompt gravity signal during the rupture of the 2011 Mw 9.0 Tohoku-Oki earthquake in data recorded by a superconducting gravimeter in the underground Kamioka Observatory and five nearby broadband seismometers from the Japanese network F-net."

They conclude that with a certainty of about 99% that a prompt gravity signal was detected. The change in position of large masses of crust resulted in a tiny change in gravity even 500 km away and this was within detectable limits of this extremely sensitive instrument.

Since gravity's influence "travels" at the speed of light it is suggested this might be an avenue to be explored for a potential early warning system.

Considering that electrical and optical signals (e.g. the internet or radio or a more dedicated communications system) travel about 2/3 the speed of light in wires and fiber, and even conventional internet pings are of the order of 100 milliseconds or better, is there any suggestion or discussion anywhere else besides this paper that super-sensitive gravimeters would be in any way better and/or cheaper than a network of cheap sensors?

In either case the idea is an early warning for something "big" rather than accurate measurements. Although the paper and analysis is interesting and elegant - is there any serious discussion or speculation that something like this - a prompt gravity signal - could somehow be actually useful as an early warning system?

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If you have a sufficient number of sensors, e.g., land-based or ocean-bottom seismometers, in the vicinity of where strong earthquakes typically happen, then it will take a few seconds for seismic waves to reach them, and a few hundred milliseconds for the electric signal to get to a place where a warning could be triggered.

On the other hand, the gravimeter would provide essentially instant warning. So you save a few seconds. At the same time, it takes many many seconds for earthquake waves to travel to the places where the warning would matter. In fact, the earthquake rupture itself may be many many seconds long. And it will take minutes for tsunamis to reach shore.

In other words, what you gain in advance warning time is not actually very large, and it may not be worth the effort. I would also not be surprised if it takes tens of seconds or minutes to actually see the change in gravity in noisy gravity measurements, and/or to take the same amount of time to just process the data.

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  • $\begingroup$ Thanks for your perspective, it makes a lot of sense. I'm also wondering if this is a new idea, or if it has been considered previously, or if this is pretty much a new idea prompted by the opportunity that the 2011 Tohoku-Oki earthquake provided by happening so close to this facility. $\endgroup$
    – uhoh
    Nov 26, 2016 at 0:13
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    $\begingroup$ I'd like to add the the "data process" part: this is a huge field of research, to understand where are the most damaged area from the seismic signal. During an earthquake, you don't always know where the damage is because of communications failure and overall chaos. Proper analyses can take weeks to months, and research is focused on attempting to make good enough estimates in minutes to hours. Several milliseconds do not matter in this case. $\endgroup$
    – Gimelist
    Nov 26, 2016 at 2:17
  • $\begingroup$ I've edited the question, added two new links. $\endgroup$
    – uhoh
    Dec 2, 2017 at 16:45

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