Never heard about EMP traveling thru the ground. It will travel down the long power lines. It will invade anything not shielded (as in a Faraday cage).
Even if it is unplugged.
The EMP flowing by a wire creates an electrical current. Just like wires spinning in an electrical field is a generator.
In the case of an EMP the pulse can be so strong that the current produced overwhelms the electronics in modern devices and basically fries them.
The device doesn't even have to be plugged into a wall socket, internal wiring creates enough to damage it.