As far as I can determine, GPS systems are at times being used to measure TEC, the Total Electron Content. And there are probably good GPS records available for locations around the world.

So, if this is the case then in theory anyway, it should be possible to look at older GPS / TEC records and see how well they did with spotting the approach of past powerful earthquakes occurring around the world. If the results looked pretty good then we might then have our first fairly global method for detecting the approach of some of our significant earthquakes. GPS systems are probably widely spread around the world.

If GPS / TEC can be used for forecasting, then I would expect that the approach of earthquakes occurring far out in the ocean might not be so easily detected. I think that sea water might have a buffering effect on whatever fault zone electromagnetic activities were causing the electron count to go higher in the atmosphere above the fault zone.

At this point, good questions would be:

Is TEC related to the formation of Earthquake Clouds?

Is TEC related to human and animal types of earthquakes sensitivity? They would include human headaches and Ear Tones, and toads and snakes leaving locations in the ground or water and coming out into the air when an earthquake is approaching?