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.
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.
