Generally speaking, earthquakes occur in clusters, rather than
just a single tremor. The mainshock is, by definition, the largest
earthquake in a series – and as such it may be the first
(followed by a series of aftershocks) the last
(preceded by a series of foreshocks) or somewhere in between.
Geological Precursors to Earthquakes
Using detailed instrumentation, it’s possible to identify and measure
minute changes in the immediate area of a fault which may occur
before an earthquake. These changes are many and varied, including
emission of radon gas, alterations in the local magnetic field,
emission of radio waves, changes in levels of groundwater and
observed changes on the ground.
As with foreshocks, the problem here is twofold. In a summary
of the literature on the subject Cicerone et al remark that
“while each of these phenomena has been observed prior to certain earthquakes,
such observations have been serendipitous in nature.”
In other words, sometimes the signs will occur and sometimes they won't.
In one of the best known prediction experiments, at Parkfield
in California, when an earthquake eventually occurred, researchers
observed none of the precursors – despite extensive scientific
scrutiny of the area.
Animal Behaviour as an Earthquake Precursor
There’s certainly evidence that animals can tell whether
an earthquake is imminent: the United States Geological Survey puts
such evidence as far back as 353 BC when animals reportedly disappeared
‘several days’ before an earthquake struck in Greece. And John McPhee,
in his Pulitzer Prize-winning study of American geology,
refers to the behaviour of wildlife immediately before a major
earthquake struck Montana in 1959.
“Birds of every sort had made a wholesale departure from mountain.
It would be noted by others that bears had taken off as well,
while bears that remained walked preoccupied in circles.”
No-one knows where the bears went.
There is an explanation for this: that animals are more sensitive
to earthquake waves than humans and might feel the first tremors sooner.
But that’s a matter of seconds, not days or even hours:
so there’s plenty of uncertainty here too.