There are also reports of pretty interesting/odd electromagnetic phenomena reaching up to the ionosphere, which might be what animals are ultimately sensing: https://en.wikipedia.org/wiki/Earthquake_light
A lot of these videos might be transformers blowing out or swinging power lines touching, but given the ubiquity of surveillance cameras and dashcams, we're fortunately getting more and more data. If there's something there that can be used for prediction better than seconds ahead of the earthquake, the value would obviously be immense.
It might of course be that animals are reacting to something else, like sulphuric gases being released from the ground, infrasound of early earthquake acitivty etc. – we just don't know enough about it yet, but the fact that there's correlation between animal behavior and seismic activity seems worth studying.
