The time scale of triggered seismicity is difficult to get a handle on. Some recent
studies have suggested that it may be related to wave-like movements in the
upper mantle triggered by sudden displacements. Since the viscosity of the upper
mantle is very high, the movement of such density variations could take months
years, even decades to move significant distances in the earth. An example is
triggering of earthquakes at high latitudes due to the removal of the ice sheet at the
end of the last ice age. While much of the vertical rebound occurred within the
first hundred years, it is still occurring, and earthquakes are still happening due
to readjustment of the earth's surface to the loss of this load over 10,000 years
ago. A similar mechanism may occur in regard to readjustments in the upper
mantle following major earthquakes. If so, wave-like phenomena may take years
or decades to move to the location and trigger distant earthquakes.
If this is the case, since the mantle is fairly homogeneous over long distance,
and pressure-related wave trains tend to be a series of dilatations and compressions,
it might be possible to look at long-term historical seismicity and see triggered
earthquakes over many years time. For example, a mantle pressure wave moving
at 1 km per day would take more than a year to go only 300 km. It seems unlikely
given the fact that the pressure wave from the unloading of the icecaps
is still around, that such waves will attenuate quickly, so this hypothesis
is quite reasonable.
The other aspect of this hypothesis is that it may be possible to give a general
time range in which triggered phenomena occur if and when the details of the
pressure wave motion can be worked out (provided there is such a thing).
Human beings live in a fast-paced world. We always assume if we don't
see quick results, then there are no results. The earth is a very slow system
not operating on human time tables. It seems unlikely to me that the only
triggering from large earthquakes occurs in times given in days or hours given
the geologic processes involved.