When a tsunami hits shallow water close to shore
When a tsunami hits shallow water close to shore, it begins to
interact with the land. Part of the wave is refl ected back offshore,
like a ball hitting a wall. Turbulence and friction slow down the
part that is not refl ected, but because the wave has so much energy
to begin with none of these processes signifi cantly decrease its
overall energy.
When a tsunami slows down but doesn’t lose a lot of energy,
what do you think happens to the wave physically?
Tsunami 5
Physics tells us that when the energy in a system remains constant, but velocity decreases, the mass in the system
must increase. A slower moving tsunami, is a physically higher tsunami; all the water in the wave “scrunches together
like the ribs of an accordian and heaves upward” (Waves of destruction).
In other words, when the front part of the wave starts to slow down, all the mass at the back of the wave catches up
with it, and creates a MUCH bigger wave. As they reach land, very large tsunamis have been known to reach heights
of 30 meters. This measure is known as the run up height.
When a tsunami enters
shallow water it loses
velocity, but gains height.
Coming ashore
How a tsunami comes ashore really depends on how it begins and
how the land is shaped. In some cases, water will actually withdraw
from sheltered harbours, leaving boats stranded, shells exposed,
and fi sh gasping for breath on the exposed ground. In other cases,
the ocean just rises and rises without warning and a wall of water
fl oods onto shore.
The force with which the waves hit lands is tremendous. It is enough
to wipe away beaches, knock down trees and crush structures. The
water will continue inland for hundreds of meters, pushed along by
the huge mass of water behind it. To make matters worse, the wave
can be just as destructive as it withdraws, smashing together objects
caught in its wake, and dragging debris far out to sea.
In 1964, a magnitude 9.2 earthquake off the
coast of Alaska spawned a tsunami which
did extensive damage to many coastal
towns including Kodiak, AK (above) and
Port Alberni, BC.
Photo courtesy NOAA.
http://www.photolib.noaa.gov/historic/c&gs/theb1341.htm
Predicting tsunamis
Since most tsunamis are caused by earthquakes, being able to predict tsunamis means being able to predict earthquakes.
Unfortunately, predicting earthquakes is extremely diffi cult. As much as we do know about the Earth, there are too
many variables that contribute to earthquakes to know with any certainty when one will occur. The best scientists
can do is provide probabilities that tell us how often, on average, earthquakes of various sizes will occur.
For instance, the most geologically active region of Canada (and the continental United States) lies along the west
coast. In this region there are about 1000 small earthquakes each year, most so small that no one feels them. There
have been about 100 earthquakes of magnitude 5 or more in the last 70 years. Once every 10 years or so, there is an
earthquake of magnitude 7 or bigger – these are called crust damaging quakes. Big megathrust quakes (magnitude
9 or greater) occur only once every 300-500 years.
If we can’t know for sure when an earthquake and
tsunami will occur, how do probabilities help?
Posted by Aneka Tips
on 17.06. Filed under