Trouble is, no matter where you go in today's world there is sufficient EM noise
to wash out any realistic EM signal precursory to an earthquake. It is a daunting
task to remove all sources of noise. A meeting was held several years ago
in which a number of scientists were invited to talk about all the known sources
of noise which would affect this type of system (not to mention the unknown
sources). There were over 50 types of noise identified which were usually larger
than any expected earthquake-related signal.
In the mid-1800's (according to an issue of the Encyclopedia Brittanica from 1880),
a strong electrical signal was observed in the hills of Italy. This signal would always
precede earthquakes in the area and was thought to be a reliable precursor. Then
the area was electrified and the signal was lost because it was smaller than
the electrical leakage into the ground from electrical systems. Now that the whole
world is electrified, it's going to be a herculean task to see these signals in real
time. Of course, like nearly all other precursory phenomena, they will always
be identified after the earthquake occurs without any difficulty.