Repeaters are powered by a constant direct current passed down the conductor near the centre of the cable, so all repeaters in a cable are in series. Power feed equipment is installed at the terminal stations. Typically both ends share the current generation with one end providing a positive voltage and the other a negative voltage. A virtual earth point exists roughly halfway along the cable under normal operation. The amplifiers or repeaters derive their power from the potential difference across them. The voltage passed down the cable is often anywhere from 3000 to 15,000VDC at a current of up to 1,100mA, with the current increasing with decreasing voltage; the current at 10,000VDC is up to 1,650mA. Hence the total amount of power sent into the cable is often up to 16.5kW.
Would a solar flare induce enough current/voltage on these cable to actually cause a problem? Also by their nature of being under an ocean of conductive salt water wouldn’t there be some form of grounding providing protection?
I’m not an expert but I know enough of the physics I’ll stick my neck out and share some relevant info.
So the effects of salt water are pretty strong, but not impossible to overcome. Extremely Long Wave radio systems are actually in use to provide submerged submarines with signals indicating they need to surface and use higher bandwidth communication channels. https://en.wikipedia.org/wiki/Communication_with_submarines
Now since there’s a level of RF penetration, you’ll get some antenna effects inducing fluctuating currents which will vary along the cable depending on the depth.
On top of that, the largest CME events can and do affect the earths magnetic field, squashing and warping it. This “moves” the earths magnetic field, and electromagnetism 101 is “conductor + magnetic field + one of these two things moving = induced currents”. So the movement of the earths magnetic field will be able to induce current all the way along the cable regardless of depth, unaffected by the seawater.
How significant the induced current and voltage are... and how vulnerable the powered repeaters and head end equipment are... is something I’ll defer to appropriate experts on.
All submarine cables laid down in the last 20 years use fiber-optics, which isn't susceptible to solar flares like copper cables are.