Earthquakes and tsunami - Not strangers anymore

by Justice P. H. K. Kulatilaka

In the wake of the massive earthquake which triggered off the Tsunami on 26th December, 2004, there were two earthquakes of high magnitude in the Sunda Strait, one on 28th March 2004 and the other on 12th September 2007. Though they did not trigger off massive tsunamis they did generate local tsunamis affecting Indonesia. These earthquakes were followed by a series of aftershocks. Prof. Kapila Dahanayake of the Department of Geology, University of Paradeniya has stated that the frequency of earthquakes of high magnitude has increased during the past three decades, and unfortunately it is not possible to give a clear indication of future occurrences for the reason that when an earthquake would strike cannot be predicted.

Earthquakes

Rocks in the deep interior of the earth undergo compression due to very strong pressure exerted on them by some geological processes as a result of which "rupture" occurs owing to the release of a large amount of energy equivalent to energy released when atom bombs are exploded. The abrupt release of energy results in a sudden oscillatory, some times violent motion or trembling in the earth's crust. This phenomenon is known as an earthquake.

Over 90% to 95% earthquakes are located at the plate boundaries. There can be inter-plate earthquakes. Those are the earthquakes which occur within the plate itself. Sri Lankan geologists have referred to an earthquake which occurred in 1615 which had affected Colombo killing about 2500 people. They have expressed the view that the epicentre would have been to the west Colombo.

After a massive earthquake aftershocks may give rise quakes of various magnitudes. Consequent to the earthquake of 26th December 2004, there has been occurrence of a number of earthquakes of different magnitudes, of which the most notable one was that of 28th March 2005 which recorded a magnitude of 8.3 Richter scale. It even gave rise to a Tsunami of minor scale which had taken lives of about 500 in Sumatra.

Geophysicists are of the view that these quakes are the result of the stress built up at the earthquake of 26th December 2004. Most recent earthquake occurred on 12th September, 2007 off the western coast of Sumatra. It generated a magnitude of 8.40 on Richter scale.

There is no mechanism to predict when an earthquake would occur i.e., date and time. However, within minutes of its occurrence it can be reported and warnings can be issued for the safety of the public.

Mechanism an earthquake

Vibrations caused by earthquakes referred to as seismic waves are recorded on an instrument called seismometer. Recording on the seismograph gives a zigzag tracing. The scale used to measure the magnitude of an earthquake is referred to as Richter scale. In order to get a proper analysis and accurate reading of the magnitude and location of an earthquake, seismographic data are obtained from at least three stations which are located at reasonable distances. There are two kinds of seismometers, mainly

Broadband or broad period seismometer

Short band or short period seismometer.

The former has the capacity to record major earthquakes whereas the latter has the capacity to record minor earthquakes and tremors.

Tsunami

The word "Tsunami" sounds soft, innocent and sweet. Ironically the impact of it on 26th December 2004 on our shores was cruel devastating and sore. Tsunami is a Japanese coinage meaning harbour waves. According to geologists and geophysicists the outer shell of the earth's crust is made up of 12 or 13 major tectonic plates, and they expressed the view that the 13th tectonic boundary is located just 350 kilometers to the south west of Sri Lanka. These plates move relative to each other. As plates move stress accumulates and eventually slippage could occur near plate boundary causing fault or a rupture in the earth's crust, eventually an earthquake. This would result in sea floor dropping en bloc, sea floor rising en bloc or a thrust upward. This large and instantaneous displacement of sea floor leads to the displacement of thousands of cubic kilometers of ocean water triggering off major ocean waves namely 'Tsunamis'.

If the knock at the plate boundary was horizontal no such displacement of ocean water would occur and hence such a condition would not result in a tsunami. To trigger a tsunami the rupture where the earthquake occurs must be underneath or near the ocean.

To be continued...