Current thought is that the EM field effects that you mentioned can be detected in the area of the fault zone. However, they are so weak that by themselves they don’t travel outside that area.

The EM Signals that I myself and some other researchers are working with appear to have the ability to travel around the world without too much trouble. And current thinking is that they might represent some type of interaction between the global geomagnetic energy field and earthquake fault zones. That interaction could also be linked with those fault zone area EM field effects. But the signals that we are working with could not travel around the world without being associated with some other type of energy source such as solar storms.

The research being done in this area appears to be years more advanced than you are probably aware. You would need to do a lot of reading and communicating with the researchers involved to stay current with this. I myself cannot even read all of the papers that are being generated related to this area of science.