Answer:
The occurrence of two great earthquakes within such a
short space of time is indeed striking. However, even in
retrospect, we do not yet see evidence for a strong
causal relationship between the two earthquakes.
It seems clear that long-term stress changes associated with
one earthquake may trigger other earthquakes on the same
fault or on nearby faults. In fact, the aftershocks that
occur around the source of a large earthquake are
triggered by such stress changes. But the long-term
stress changes caused by an earthquake decrease rapidly
with distance away from the earthquake source. The
Macquarie Ridge earthquake was very far from the site of
the yet-to-occur
Sumatra-Andaman
Islands earthquake, and occurred on a different plate
boundary. The hypothesis that long-term stress changes
associated with the Macquarie Ridge earthquake triggered
the Sumatra-Andaman Islands earthquake therefore does
not seem compelling.
There is also strong evidence that the shaking of the
ground caused by a great earthquake, such as the
Macquarie Ridge earthquake, can trigger small
earthquakes in sensitive tectonic environments at large
distances from the great earthquake. The evidence for
such triggering is most convincing when the earthquakes
that are thought to be triggered occur near the time of
strongest shaking from the triggering earthquake, which
would be within several hours following the triggering
earthquake. However, the Sumatra/Andaman-Islands
earthquake occurred about two-and-a-half days after the
Macquarie Ridge earthquake.
An alternative to the hypothesis that the Macquarie
Ridge and Sumatra/Andaman Islands earthquakes are
causally related is that the occurrence of the two,
widely separated, great earthquakes within three days
was a probabilistic coincidence.
