Leenia, there are a number of ways in which animals can behave unusually before earthquakes.

The obvious instance, of dogs barking etc a few seconds before an earthquake strikes, is related to the p-wave event, a compression wave which travels faster than the slower, larger, ground waves which are the ones generally felt and which do the damage.

A p-wave is often felt as a sharp blow, as if someone had dropped a heavy object. In a busy urban setting, it can often be mistaken for something like that. Dogs and other animals, with their lower-frequency aural threshold of hearing (i.e. they can hear lower notes than we can), hear a p-wave as thunder, and know it doesn't belong in the background noise.

In the case of shallow-focus earthquakes along transverse faults (such as the San Andreas), odd behaviour has often been reported weeks before large earthquakes. One possible cause is the small swarms of slight precursor earthquakes preceding the main event, which animals can hear (see above).

Another possibility is the increased emission of gasses from below ground as a result of dilatation (opening of fissures, cracks etc as rocks and joints fail under tension or torsion). This can be used as a predictive sign by measuring the concentration of Xenon in wells or groundwater. Xenon, being a short-lived radioactive gas derived from the radioactive decay of elements such as uranium or thorium in rocks, is detectable and measurable by geiger counter. With their more sensitive noses, dogs and other animals can smell other gasses which belong underground, go "pew" and also know that the smell doesn't belong.

There are other, less well-documented or explained phenomena such as birds flying in odd directions, cats starting at imaginary threats etc. I'm sure there's plenty of literature on these.