Hi Michael. In 1855, there was a Japanese businessman, who ran a spectacle shop. He used to attract customers with a horseshoe magnet, which attracted a lot of needles. But, on that day when an earthquake happened to his city, the iron needles and other small objects fell on the ground abruptly and he was quite shocked. Two hours later, a destructive earthquake happened. After the earthquake, the magnet started to attract iron again.
Mankind got to know the change of magnetism fields before earthquakes quite early.
Earthquakes can result in the change in electromagnetic fields. Generally, there were two reasons for such changes. One is that, under the reaction of the earth stress, the effect of "piezomagnetic effect" would happen to rocks, thus, result in the partial change in geomagnetic field; the other reason is that, the earth stress would condense or extend the rocks, which would also results in partial changes in electrical resistivity, which would in turn cause partial change to the electromagnetic field. The change of temperature in rocks could also bring changes to the electromagnetic property. Therefore, to observe the change of electromagnetic field is also one on the major method to forecast earthquakes.