The issue with any manual wound or rotor based automatic watch movement (clock or wrist) is the hairspring getting magnetized, or damaged by magnetic pull, and to a lesser extent the balance shaft. If this occurs the watch will not keep accurate time. The solution is demagnetization.
The reason most of these watches reference trains is because an engineer of a train needed to keep accurate time to avoid collision. This was actually a problem. Later it became an issue with research scientists.
For reference, a MRI is between 5000 and 15000 gauss (some newer 30,000). This is why you can’t wear metal in them. 15,000 gauss is 30,000 times the strength of the earth’s magnetic field.
When a magnetic field is introduced to a watch the hairspring can freeze for a moment and the watch can loose time. The same can happen with shock or extreme temperature changes. In extreme cases the magnetic field will stretch the hairspring and damage it.
To combat this, new higher quality watch movements, have hairsprings made out of silicon and anti-magnetic metals, like Nivachron (Swatch Groups proprietary name for their product). Rolex‘s version is called Parachrom Bleu. These hairsprings come in various grades, depending on cost of the watch and build spec. Not only are they amagnetic, they are also more resistant to shock and extreme temperature fluctuations. They also increase cost, especially silicon.
Companies like Rolex (Milgauss rated to 1000 gauss) and Omega (Railmaster or Aqua Terra 15000) are rated to 15,000 gauss). These are build purposely built watches, designed to withstand gauss loads, for industries, that have their employees exposed to high magnetic fields. The watch cases are also constructed of anti-magnetic materials. The new Tudor BB METAS cert are also tested to 15,000 gauss.
These watches are tested by COSC and METAS for accuracy standards and are rated for the specifications. Most modern high end Swiss movement or Japanese movement (Grand Seiko) watches are at least 1000 gauss ratings. These ratings will be found in the specifications.
Like quartz, cheap mechanical automatic watches, with ferrous metal hairsprings, and watch cases will be damaged by high magnetic fields and electric pulse. Your cheap (and expensive) Sekio and Citizen automatic watches will be affected. The question is, how close to you have to be and for how long does the exposer have to be for damage to happen?
In reality, EMPs effects are not going to damage your clock/watch. It’s not going to short them out, digital, or damage the hairspring, magnetic field. If you’re close enough to the source, for it to affect your watch, the electro magnetic field is going to be the last of your worries. Radiation will be your issue.
If you don’t believe this, the Omega Railmaster is one of the best automatic ‘tool’ watches you can get. Well, use to get, as it was recently discontinued. They are around $3200 used and $5700 new (assuming you can find one). A Casio G Shock will also outlast you for thousands less. Buy a $150 cheap Orient mechanical automatic, put it in a cheap HF Apache box and bury it. When the would ends, you can charge people to tell them the time.
I collect watches and this crap interests me. I own a Railmaster. Not because I and afraid of EMP, but because it’s a great watch.
If you’ve ever wondered why a high end Swiss Watch cost what they do, here’s a small part of it: