Answer:
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 When
the opposite sides of a fault are vertical and move
horizontally, we have an earthquake.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).
