Tsunami!
Why is water pulled out of bays and beach fronts as a tsunami
approaches? --R. D., Folsom, PA
Heave a rock into a calm lake. Kersplash! Waves emanate in widening circles from the
disturbance, eventually hitting the shore. When they reach land, what happens first? Does a wave
crest fall upon the land causing lake water to wash up the beach or does the water pull away? It
depends on which part of the wave reaches land first: the crest or the trough.
 Tsunamis are no different. A cosmic body
crashing into an ocean can trigger a
tsunami, so can volcanoes erupting under
the sea or landslides. Usually, however, an
underwater earthquake starts the event.
The seafloor buckles where drifting plates
that make up the Earth's outer shell slowly,
over millions of years, collide.
The heavier oceanic plate slides beneath the
continental plate but not always smoothly.
"While in the deepest part of the subduction
zone," says geophysicist Eric Geist, a
tsunami scientist at the U.S. Geological
Survey in Menlo Park, California, " the
oceanic plate creeps along as it sinks
beneath the continental plate into the
mantle. At shallow depths the oceanic plate
'sticks' to the continental plate until an earthquake occurs."
Then the oceanic plate plunges down in a sudden lurch that snaps the continental plate like a
cracked whip.
The plate part (B) next to the fault (A) thrusts up; the adjacent rock (C) pulls back down in
compensation. See Figure 1. The resulting continental-plate rock has a wave shape with a crest at
B and a trough at C. The rock holds this new shape.
The up-thrusting and down-dropping rock moves the water above it into a similar shape. The
rock movement pushes and pulls a column of water from the floor to the surface of the sea. The
push bulges the sea above the surface maybe a foot high; the pull drops the water into a trough.
See Figure 1. The buckled floor has moved an entire column of water into a trough-first
waveform.
Waves emanate from the disturbance just as they do when a rock hits a lake. The local tsunami
travels trough-first. The wave going in the opposite direction--out to sea--travels crest-first.
They travel in widening circles across the oceans of Earth for thousands of miles nearly as fast as
a jet airliner. When the waves hit the shore, they hit hard with almost all of their initial energy.
 The wave pattern changes upon reaching
shallow waters near the shore. The shape
of the land--both near-shore floor and
coastal contours--reflects and refracts the
waves. See Figure 2. As a result of this
jumbling, a crest of a local tsunami may
occasionally reach land before the trough.
"On the average, however," says Geist, "the
trough reaches land first." Then the waters
rush out and expose the shallow seafloor.
Next the crest deluges the land.
That's why approaching local tsunami
waters recede: the trough reaches land first.
Just the opposite happens for distant
tsunamis. Occasionally a trough arrives
first; then the waters recede first.
Otherwise, the crest floods the shore
without a warning.
Further Surfing:
The
highest tsunami, Alaska, 1958
Tsunamis
in a river?
Tsunamis: lake
water sloshing like bathtub water US Geological Survey "Tsunamis and
Earthquakes" Great site. Gives the basics
of how tsunamis happen, some wonderful
tsunami animations, research, and modeling.
University of Washington Geophysics
Department "Tsunami!" Another great site.
Explains the physics of tsunamis, discusses
warning systems, surveys of recent
tsunamis, and research.
PBS Online, Savage Earth, Waves of
Destruction: Tsunamis, "Surfs Up!" by Daniel Pendick An absorbing account of how tsunamis are
formed and the damage they cause. Plus animation!
US Geological Survey "Understanding plate motions" Excellent discussion of how moving
tectonic plates cause earthquakes. |
