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Hill country tremors: ‘Unusual’ quakes baffle Geologists

  • Diversity of location and daytime occurrence raise questions

 

Earth tremors are a seldom occurrence within the landmass of Sri Lanka, and the recent microearthquakes reported in various places in the island constitute an unusual situation, according to geologists.

Most of the reported earth tremors have been mainly in the 2.5 or lesser range in the Richter Scale, which measures the magnitude of earthquakes.

“The scale of these microearthquakes were 2.5 or less on the Richter Scale. This is considered negligible in terms of impact, but can be recorded by a seismograph. The only question is how these earthquakes were reported in different areas of the country. However, the country has felt a few tremors in the past, of which the amplitude and intensity are very low,” Professor of Geology at the Open University Rohan Fernando claimed speaking to The Morning yesterday (4).

According to Prof. Fernando, minor tremors were reported from Tissamaharama, Embilipitiya, and Ampara years ago, and recently from the Victoria dam and Walapane area.

“I remember the tremors reported from Ampara. This occurred when the Deyata Kirula National Exhibition was ongoing, during the daytime. We were not allowed to go to the area due to tight security by the Army. The tremors stopped after the exhibition concluded.“

Speaking further regarding the tremors reported in recent times, he noted that to his knowledge “this is not a natural process”. He cited the differences in the locations from where the tremors were reported, and the fact that they were reported during the daytime, as unusual phenomena. “Tremors can take place day or night but most of these microearthquakes reported from various places in Sri Lanka happened in the daytime. This is unusual.”

According to Prof. Fernando, an earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at the edges due to friction. When the stress on an edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth’s crust and cause the shaking that we feel. Earthquakes can and do happen at any time, day or night, he added.

Speaking to The Morning, Professor of Geology of the Department of Geology of the University of Peradeniya Atula Senaratne also noted that earthquakes have been rare in Sri Lanka, and that the earth tremors reported in the country may have been caused by reasons other than due to a natural process.

“Minister of Environment Mahinda Amaraweera has requested an investigation report on the occasional minor tremors that occurred around the Victoria Reservoir recently. An Experts Committee on seismic conditions was appointed to look into earth tremors and the possible causes of minor earth tremors”.

The Committee Members are the Chairman of the Geological Survey and Mines Bureau (GSMB), Engineer Anura Walpola, Professor Atula Senaratne, the Director-General of the Disaster Management Centre, Major General S. Ranasinghe, the Engineer-in-Charge of the Victoria Reservoir Project, Wasantha Ihalapitiya, Prof. Jagath Goonetileke, Prof. L.R.K. Perera, Dr. O.K. Dissanayake, Dr. Gamini Jayatissa from the National Building Research Organisation, and from the GSMB, Nalin de Silva, Nilmini Thaldena and Mahinda Seneviratne.

“We observed five different reasons that may have caused these earth tremors including human activities as well,” he added.

Mining activities can produce a wide variety of induced ground motions, including seismicity. At the mine site and the surrounding areas, blasting produces local vibrations though mine operators and explosives engineers strive to minimise many of the effects of these vibrations by careful blasting designs. In some cases, ground motions or seismic waves may be detected at long distances, Prof. Senaratne pointed out.

He added that the pressure wave from an underground explosion will propagate through the ground and cause an earthquake. Theory also suggests that a nuclear explosion could trigger fault ruptures and may cause a major quake at distances within a few tens of kilometres from the blast point.

“Small explosions are used for mining, quarrying, road excavating, and other constructional applications, as well as in natural resource exploration and crustal studies. Larger sources include underground nuclear tests, which produce waves strong enough to be observed on the opposite side of the earth. Naturally occurring explosive or implosive sources are rare, but some may occur with meta stable mineralogical phase transitions or magmatic processes at depth”.

However, according to Prof. Senaratne, Sri Lanka needs more equipment to ensure proper research on earth tremors.

“We have only four measuring equipment but we need more high resolution ones to get more conclusive information on the causes. Otherwise, in the long run, we might have to face consequences,” he added.

The GSMB Chairman Anura Walpola, when contacted by The Morning, said that initial attention has been focused on several incidents which are suspected to be the reason for these tremors occurring.

“Our investigating team is concentrating on finding out whether these tremors took place due to setting bores at a stone quarry or the breaking of limestone layers. If it is an explosion, the explosives used should be 56 kg in weight in order to have a minor tremor with a magnitude of around 2.0 on the Richter Scale.”

When queried whether they were mine related explosions, he added that investigations are presently ongoing in this regard and that by next week, they might be able to provide a proper answer regarding these minor tremors and associated speculations. The tremor could happen at the time of the explosion or even after the explosion, depending on the geological condition, he added.

“I can assure there is not a major risk in the future,” he added.