Answer: A tsunami is a sea wave of local or distant origin that can be
generated when the sea floor abruptly deforms and
vertically displaces overlying water. Such a
displacement can occur when an earthquake ruptures
oceanic lithosphere. When the opposite sides of a fault
are inclined and have a vertical component of motion, we
have an earthquake with dip-slip
faulting. When the opposite sides of a fault are
vertical and move horizontally, we have an earthquake
with strike-slip faulting. Given two earthquakes of the
same size, the one that has greater vertical fault
motion is likely to displace a greater amount of
overlying water. Indeed, the
Sumatra and
Macquarie Ridge earthquakes occurred on different plate
boundaries and had different faulting mechanisms. The
Macquarie Ridge forms part of the Pacific-Australian
plate boundary and the faulting mechanism of this
earthquake is predominantly strike-slip. The Sumatra
earthquake occurred on the interface of the
India and
Burma plates and its faulting mechanism was
predominantly thrust with vertical slip.
However, tsunamis can also arise from strike-slip
earthquakes. A strike-slip Macquarie Ridge earthquake
on May 1989, which had a similar magnitude (Mw 8.1) to
the December 2004 earthquake, generated a small tsunami.
A strike-slip earthquake in the Gulf of Alaska
(November 1987, Mw 7.9) generated a 0.8 m tsunami while
a strike-slip earthquake off the coast of northern
California (Aug 1991, Mw 7.1) generated a 0.5 m tsunami.
Although the fault displacements produced by these
earthquakes were predominantly horizontal they may have
had a slight vertical component. A combination of
horizontal and vertical motion across a fault plane is
called oblique slip. Strike-slip earthquakes can also
cause underwater landslides that can generate tsunamis.
Thus, another major reason that the Sumatra earthquake
generated a tsunami is its sheer size, a magnitude (Mw
9.0) that was so much larger than that of the Macquarie
Ridge earthquake (Mw 8.1).
