Edit: Two new items may shed more light on this question:
- BBC: Gravity signals rapidly show true size of giant quakes links to:
- Science: Observations and modeling of the elastogravity signals preceding direct seismic waves
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?