Answer:
Of
major concern following the 26 December 2004 magnitude 9.0
earthquake is the potential of another damaging tsunami
being generated by an aftershock. The only way to know for
certain if a tsunami has been generated is to directly
measure the height and propagation of the ensuing wave
using ocean pressure sensors and tide gauges. However, a
system of such instruments does not exist in the Indian
Ocean. Absent of a system of instruments, we must defer to
historical earthquake-tsunami records to calibrate our
thinking. For reasons summarized below, we conclude that
the chance of a tsunami resulting from an aftershock is
small, but finite. Nevertheless, we emphasize that the
great tsunami of 26 December is extremely unlikely to
reoccur in the near future.
Although magnitude is one factor that does affect tsunami
generation, there are other important factors to consider.
The earthquake must be a shallow marine event that
displaces the seafloor. Thrust earthquakes (as opposed to
strike slip) are far more likely to generate tsunamis, but
small tsunamis have occurred in a few cases from large
(i.e., > M8) strike-slip earthquakes. Although a number of
the Sumatra-Andaman Islands aftershocks are thrust events,
many are not. Similarly, the depth of these aftershocks is
quite variable. This variability (which is common in
aftershock sequences) is an important consideration when
evaluating the potential of any given aftershock
triggering a tsunami. With these caveats, we offer the
following general guidelines based on historical
observations
Magnitudes
below 6.5
Earthquakes of this magnitude are very unlikely to trigger
a tsunami.
Magnitudes
between 6.5 and 7.5
Earthquakes of this size do not usually produce
destructive tsunamis. However, small sea level changes may
be observed in the vicinity of the epicenter. Tsunamis
capable of producing damage or casualties are rare in this
magnitude range but have occurred due to secondary effects
such as landslides or submarine slumps.
Magnitudes
between 7.6 and 7.8
Earthquakes of this size may produce destructive tsunamis
especially near the epicenter; at greater distances small
sea level changes may be observed. Tsunamis capable of
producing damage at great distances are rare in the
magnitude range.
Magnitude
7.9 and greater
Destructive local tsunamis are possible near the
epicenter, and significant sea level changes and damage
may occur in a broader region.
Note
that with a magnitude 9.0 earthquake, the probability of
an aftershock with a magnitude exceeding 7.5 is not
negligible. To date, the largest aftershock recorded has
been magnitude 7.1 that did not produce a damaging tsunami
