One
of the many
tectonic plates that make
up the Earth's outer shell descends, or "subducts,"
under an adjacent plate.
This kind of boundary
between plates is called a "subduction zone."
When the plates move
suddenly in an area where
they are usualy stuck, an earthquake happens. |
Stuck
to the subducting
plate, the overriding
plate gets squeezed.
Its leading edge is dragged down, while an area
behind it bulges upward.
This movement goes on for decades or centuries, slowly
building up stress. |
An
earthquake along a subduction zone happens when the
leading edge of
the overriding plate breaks free and springs seaward,
raising the sea floor and
the water above it.
This uplift starts a tsunami. Meanwhile, the bulge behind
the leading edge collapses, thinning the plate and lowering
coastal areas. |
Part
of the tsunami races toward nearby land,
growing taller as it comes close to shore.
Another part heads
accross the ocean
toward distant shores.
This explains how a
tsunami may occure many hundreds of miles from where
it originates. |
