My apologies for being unclear. I'm not trying to be difficult, just genuinely curious about the differences in sensitivities /capabilities between something like fiber optic DAS and the OTDR device you mentioned previously.

Like I discussed earlier, I am familiar with fiber optic DAS interrogators and how they can be used to detect minute changes in strain across a specific location along a fiber optic cable (using Rayleigh backscattering), but what remains a question is if other telecom related hardware (e.g., these OTDR devices that seem to be geared more towards finding line segments permanently damaged by high strain events) are sensitive enough to detect changes associated with much smaller levels of strain (that don't damage the cable)? That's the question. Like for one of these subsea cables, do they have to alter the way they send/receive signals through the fiber optic lines depending on if it is low vs. high tide? Could you hook up an OTDR device to one of these subsea fiber optic cables and (without looking at the water level) tell if it was high vs. low tide? I believe you can do this using fiber optic DAS, but I don't have experience with other telecom-specific hardware/diagnostics and if they are designed with that level of sensitivity.

I did read the article, but it lacked important details on how the data were acquired. Here's the link to the actual journal article:

https://www.nature.com/articles/s41598-023-27444-3

They used a distributed acoustic sensing (DAS) interrogator unit (hooked up to underwater telecom-grade fiber optic cables) to collect the data. I am more familiar with DAS, having processed seismic data derived from the "raw" strain-rate measurements it produces (one could also derive temperature changes along the fiber in other configurations), but in my case the DAS interrogator was hooked up to a fiber optic cable specifically designed for that use case. My understanding is there are quite a few differences between those types of cables and the ones used by the telecoms (in terms of glass quality in the core as well as the thickness/type of materials used to protect the core). My understanding is there are also quite a few differences between the hardware used to "interrogate" the fiber (e.g., the DAS unit vs. the OTDR unit you mentioned earlier). All the differences amount to some kind of loss in sensitivity to the smaller amplitude seismic waves.

So my question (from before) is more related to how sensitive the hardware/diagnostics are on your end? I would have assumed they are geared more towards identifying the outcome from a high-strain event (e.g., one large enough to at least slightly damage the core or materials protecting it). Would you be able to identify lower strain events (ones that do no permanent damage) with your type of hardware/diagnostics? Take my earlier example and assume you had >10km of subsea telecom fiber optic hooked up in a typical configuration for two-way communications (say between an island and the mainland). If somewhere in the middle, you had 1000s of scuba divers lightly squeezing (or gently tapping) a 500m section of cable for a few seconds (or a few minutes even), would that be enough to disrupt communications? Or would it introduce a negligible amount of noise on the signals?

I ask because that hypothetical level of strain is probably still an order of magnitude greater than the strain imposed on the cable from a passing P wave (which would be detected in the early warning system and used to warn about potentially damaging waves arriving 10s of seconds later). If the answer to the former is "no" and the latter is "yes," then I suspect that advancements in algorithms alone would not allow someone using typical telecom hardware to detect these subtle changes. New hardware (i.e., DAS interrogators) would be needed for that application and would likely need to be hooked up to spare/dedicated fibers (assuming there are multiple glass fibers in a single cable) not actively being used for communications

I have no doubt they have some pretty sophisticated algorithms combing through their signals looking for anomalies. However, I'm not talking about strain/strain-rates that are at levels capable of potentially damaging the lines (permanently) or even close to it. I'm talking about strain levels on the order of holding the fiber in your hand and lightly squeezing it. Assuming the hypothetical city isn't located directly above the earthquake, the initial seismic waves that would propagate across a communications network would be lower amplitude P-waves (with wavelengths >2km and speeds ~2km/s for the dominant energy). So like, 1000s of hands lightly squeezing the entire fiber network at nearly the same time once every second for a few seconds. Correctly identifying/detecting these waves before the more destructive surface waves is what would give people seconds (or 10s of seconds) to prepare for the stronger, more destructive shaking. Unless some sort of specialized system is in place specifically for earthquakes (which some places probably already have), I would doubt that the initial wave (of lower strain-inducing energy propagating across the network) would cause things to exceed the tolerances/thresholds set for detecting a disruption of service using "normal" diagnostics. Or would it?

For sure it's more sophisticated. What you are talking about (and what I mentioned for measuring seismic wave propagation) requires a single (very specialized, very expensive) piece of equipment on one end of the fiber that does the sending/receiving and initial processing. However, in the context of using an existing communications network (that is also actively being used with the equipment they already have) I was speculating on how they could possibly detect a "big" disturbance without the same sort of specialty equipment (that we use for precise measurements) already in place. In the case of an early warning system, the measurements need not be that precise when it simply needs to alert folks that a large disturbance has been detected somewhere along the line.

This is basically what happens, but there is also a special "interrogator" unit that sends pulses of light and converts the back-scattered light to seismic data associated with a specific location along the fiber. Not sure how communications companies could use their existing technology to do the same - possibly by sending "known" signals from one communications hub to another and then if the receiver detects a significant change in the signal amplitude/phase (compared to some average over time), then "something big" happened "somewhere" along the cable.