Were tsunami alerts necessary?
|Residents moving out after the tsunami warning
The Meteorology Department, after this week’s earthquake, immediately issued a tsunami warning and subsequently the Disaster Management Center with the assistance of the police and other agencies evacuated the people living in coastal areas causing a lot of panic. The question that has to be asked from the experts is whether such a course of action was necessary.
I shall in this short paper scientifically prove that such action could have been avoided if proper records were kept and research carried out on plate movements in the region and the major subduction zone along the Sunda Arc an the boundary of the Indian and Burmese sub Plates by leading scientific institutions were followed since the major earthquake in 2004. There were three major undersea earthquakes that occurred in the area off the Sumatra Island since 2004. The coordinates of the epicenters of these earthquakes are,
1 Aceh Sumatra 9.3 (RS) Latitude 3.27 North 95.82 East Longitude Depth 30 kms (18.6 Miles): Date 26 December 2004 at 7.58 am local time: Region off the west coast of Northern Sumatra: Nearly 300,000 people killed and extensive damage to property and infrastructure: 4th largest quake in the world since 1900 and largest since 1964 Prince William Sound Alaska quake (9.2 RS): Felt in Indonesia, Malaysia, Singapore, Thailand, India, Sri Lanka.
2. Nias Island 8.7 (RS) Latitude 2.065 North 97.010 East Longitude Depth 30 kms (18.6 miles): Date March 28, 2005 at 1109 pm local time: Region Northern Sumatra:1000 people killed 300 houses destroyed extensive damage on Simeulue: Felt in Indonesia, Malaysia, Singapore.
3. Bengulu Southern Sumatra 8.4 (RS) Latitude 4.517 South 101. 382 Longitude Depth 30 kames (18.6 miles): Date September 12, 2007 at 6.10 .26 pm local time:Region Southern Sumatra: 9 people recorded killed so far and many buildings damaged or destroyed in Bengulu and Padang areas: Felt in Jakarta, Malaysia, Singapore, Thailand.
It is relevant to relate the location of Sri Lanka to the above 3 major earthquakes to learn more about the risk of generation of tsunamis and its landfall. Sri Lanka is between 6.00 and 9.30 North Latitude and 80.00 and 81.30 East Longitude approximately. According to the above coordinates the undersea earthquakes 2 and 3 described above did not generate a tsunami with landfall in Sri Lanka as the energy was disseminated to the sea as recorded in 1833.
There is a clear distinction between the devastating earthquake of December 26, 2004 that generated a massive tsunami unprecedented in the world causing widespread devastation and loss of life and the other two earthquakes that occurred within a time span of nearly 3 years. Two scientists from the Northwestern University Chicago Seth Stein and Emile Okal reported that the 2004 earthquake was caused by the subduction of the Indian Plate under the Burmese Plate and a rupture of the plate margin over a distance of 1200 km running from the coast of the Indonesian province of Aceh to the Andaman Islands. The shaking of the ground lasted for a record of 500 seconds (8 minutes) longer than the Chilean quake of 340 seconds in 1960. (9.5 RS). It was also estimated that the rupture of the Plate boundary was 11 metres deep and 200 km wide This is the explanation given as to why Sri Lanka and India were so badly hit by the ‘excited’ tsunamis and the big waves were not from the quake itself to the southeast but from the thrust of the ocean floor to the east. These scientists determined that the enormous strain accumulated on the northern part of the rupture has been released saying there appears to be no immediate threat of an ocean wide tsunami on this segment because such great earthquakes are typically at least 400 years apart. However there is danger of earthquakes on land to the north in Myanmar Bangladesh and the Himalayan foothills. The recent earthquake in Kashmir could be attributed to such release of strain in the northern part of the Indian Plate.
Now I will attempt to analyze the recent under sea earthquake. According to the USGS the magnitude of 8.4 to 7.8 Southern Sumatra earthquakes of September 12 occurred as the result of thrust faulting on the boundary between the Australia and Sunda Plates in contrast to the December 2004 quake. At the location of these earthquakes the Australia plate moves northeast with respect to the Sunda plate at a velocity of 60 mm/year.
The magnitude of 8.4 earthquake of September 2007 is the fourth earthquake of magnitude greater than 7.9 to have occurred during the past decade on or near the plate boundaries offshore of western Sumatra. It occurred north of the 7.9 earthquake on June 4, 2000 .The September 12 magnitude 7.8 earthquake occurred about 225 km northwest of the magnitude 8.4 earthquake at the northern end of the after shock region. There two quakes and the aftershocks overlay the southern portion of the estimated rupture of 1000 km in 1833 caused by a major quake of over 9. Accordingly the earthquakes now experienced in the southern Sumatra region are due to the rupture of the Sunda and Australia plate boundaries and the reactivation of the rupture in 1833 that generated a tsunami with no landfall in Sri Lanka. Accordingly I would predict that Sri Lanka would not be affected by a major tsunami in the near or distant future. However as explained in my article in The Sunday Times of August 5, 2007 the generation of tremors on land due to such undersea earthquakes causing aftershocks should be closely monitored as an earthquake of over 5.5 will be more catastrophic than a tsunami.
In conclusion I must stress that the government should take immediate action to train seismologists and geophysicists in interpretation of seismic data. The Disaster Management Centre in collaboration with international and bilateral donor agencies and other relevant scientific institutions such as the GSMB (Geological Survey and Mines Bureau) should take immediate action to embark on a Deep-Ocean Assessment and Reporting of Tsunami (DART) Project as seismic data alone doesn’t measure tsunamis and coastal sea level stations (tide gauges) do not provide direct measurement of deep ocean tsunami energy propagating towards land. Assistance from National Oceanographic and Atmospheric Administration (NOAA) of the United States may be sought for such a project in collaboration with the other Indian Ocean countries. The government should rethink its decision to designate the Meteorology Department as the focal point for tsunami and earthquake warnings as the subject is more complex and needs effective coordination and scientific inputs.
(The author is the Former Director of the Geological Survey Department (GSMB) and retired Economic Affairs Officer United Nations ESCAP and can be contacted on firstname.lastname@example.org)