I'm not actually talking about an EMP, I believe it's more of a CME[0]. Coronal mass ejections would have more of an impact on the long lines, so pretty much the core power and networking infrastructure.
But yes, this would affect the ability to take out cash from ATMs and banks wouldn't be able to check your account balance, though most have local generators or they would pretty quick, so local credit unions might still be operational. However, anyone with cash on hand would be able to utilize it to make purchases and the local/state/federal government would fly into emergency mode to protect the ability for hard currency to be used.
They would also make sure that financial institutions are the first ones back online in order to get the economy flowing again and the Fed would make sure that a dollar was worth the same after the lights came back on as it was before. Laws exist which ensure that manipulation of the currency and even opportunistic price gouging would be punished afterwards.
Not so for bitcoin or any other cryptocurrency... no government guarantees it and if the network went down unevenly who knows what sort of manipulations would be attempted on those networks that now had players with over 51%...
Since Chinese mining companies control well over 60% of the network, would the remaining miners be able to handle the load? Especially since many miners would just keep rolling in isolation and immediately start publishing simultaneous blocks when they reconnected with each other...
If intercontinental infrastructure weren't repaired for many months, would Europe, US and Asia all end up with their own branches of the blockchain? Has the bitcoin community even run a model of what to do in a similar disaster situation? I know that after 9/11 every bank has super rigorous disaster recovery plans and backup mechanisms, so they for sure have hardened backups and power generators at core locations.
I mean, this is ridiculous, bitcoin can't even be used to buy something and isn't accepted anywhere useful and probably never will. So if this scenario happened, the first thing that would happen is the price would collapse as everyone waited on Coinbase's doorstep to sell their coins to get cash so they can actually buy something useful.
[0] https://sciencing.com/effects-solar-flares-technology-20402....