manoj
30th March 2005, 04:33 AM
http://www.rediff.com/news/2005/mar/29quake3.htm
Waves of Destruction
Experts claim that an undersea earthquake can produce a tsunami if the local plates
that form the earth's crust there move up or down sharply, causing water to shift
suddenly.
If the recent quake resulted essentially in plates rubbing parallel to each other -- and
not lifting or falling -- it could no more than cause local damage and some
large-scale agitation of the water, the experts explain.
The location and depth of the epicentre of the quake, the magnitude, the amount of
sediments available, the depth of the waters around the quake zone and the
movement of sediments underwater, can all play a role.
The United Nations Educational, Scientific and Cultural Organisation International
Tsunami Information Centre describes the worst tsunamis as being generated from
large, shallow earthquakes with an epicentre or fault line near or on the ocean floor.
According to the US Geological Survey, even though the epicentres of both the
December quake and the current one were 30 km deep, the current one was 8.7 on
the Richter scale while the earlier one measured 9. Given that the scale is a
logarithmic one, making 9.0 on the scale 10 times stronger than 8.0, the difference is
significant.
But most seismometers measure only higher frequencies of energy; but most energy
in an earthquake comes from low frequency sources. So what is measured on the
Richter scale cannot quite explain the effects seen.
While the December quake ruptured almost a quarter of a million square kilometres,
it is not clear if the current one covered anywhere as much ground.
Slippage of heavy sediments - often in the form of landslides -- can also cause water
to move forward in a rush. This is one of the primary reasons for large tsunamis after
even a small earthquake.
Waves of Destruction
Experts claim that an undersea earthquake can produce a tsunami if the local plates
that form the earth's crust there move up or down sharply, causing water to shift
suddenly.
If the recent quake resulted essentially in plates rubbing parallel to each other -- and
not lifting or falling -- it could no more than cause local damage and some
large-scale agitation of the water, the experts explain.
The location and depth of the epicentre of the quake, the magnitude, the amount of
sediments available, the depth of the waters around the quake zone and the
movement of sediments underwater, can all play a role.
The United Nations Educational, Scientific and Cultural Organisation International
Tsunami Information Centre describes the worst tsunamis as being generated from
large, shallow earthquakes with an epicentre or fault line near or on the ocean floor.
According to the US Geological Survey, even though the epicentres of both the
December quake and the current one were 30 km deep, the current one was 8.7 on
the Richter scale while the earlier one measured 9. Given that the scale is a
logarithmic one, making 9.0 on the scale 10 times stronger than 8.0, the difference is
significant.
But most seismometers measure only higher frequencies of energy; but most energy
in an earthquake comes from low frequency sources. So what is measured on the
Richter scale cannot quite explain the effects seen.
While the December quake ruptured almost a quarter of a million square kilometres,
it is not clear if the current one covered anywhere as much ground.
Slippage of heavy sediments - often in the form of landslides -- can also cause water
to move forward in a rush. This is one of the primary reasons for large tsunamis after
even a small earthquake.