Tantrum tremors

by Sajitha Prematunge

Most people have no idea what's going on in the world until they are forced to turn their attention to it. Three earth tremors in Sri Lanka caught the attention of Geologists and the public alike in the past month.

The first of which was two earth tremors near Beralihela at around 12.01 am and 02.00 pm on 9th July 2007. The second with a 5.2 magnitude was an underwater earth tremor off Pothuvil, at around 9.57 am on 18th July 2007. The third was reported from the same area of Beralihela at around 5.50 am and 6.45 pm on 20th July 2007.

But very few people truly know the mechanism behind earth tremors. In fact earth tremors are different from earth quakes. Quakes are more severe and greater in magnitude than tremors. Earth quakes are induced when the movements of tectonic plates, whether horizontal or vertical, release sudden bursts of energy.

There are 12 main Tectonic plates in the world. Convergence zones form in the border of two intersecting tectonic plates and divergence zones form when two plates separate. These are referred to as faults; active regions where earth quakes and volcanos are most frequent.

The earth is covered by tectonic plates and there are 12 main plates. And the tremendous energy needed to move the mammoth plates originate from the core of the earth consisting of radioactive metals like uranium and thorium.

Consequently in a divergence zone new plate material is constantly produced to compensate for the loss caused by the convergence zone of the same plate. This is called Dynamic Equilibrium.

But because of friction a significant amount of energy has to build up, in order to move a plate. When the accumulated energy reaches its' climax the plate may move with a sudden jerk, that could trigger an earth quake. Consequently most of the major earth quakes of 6 - 9 magnitudes occur in the plate boundaries. Although very rare intraplate earth quakes (quakes inside of the border) of low magnitudes can occur, as in the case of Thissamaharama quakes.

The true cause or causes for the Thissamaharama earth tremors are still uncertain. They were intraplate earth quakes which would have been induced by a number of causes.

Near the Mid oceanic ridge, in the western border of the Indian plate, new plate material is incessantly being formed as the plate is pushed north-east ward. Adding to the elevation of the Himalayas, where the Indian plate meets the Eurasian plate.

In fact the indian plate used to be connected to the Antarctic millions of years ago, but the constant push upward has cast it towards the Eurasian plate. But the Everest has reached its maximum hight, because the two plates can not converge any further.

To compensate for the force from the plate formation zone and the Himalayas, the sediments and lithophere in the middle of the plate tends to bend. This is the area refered to as a defuse deformation zone.

Geologists of the Geological Survey and Mines Bureau believe that this could be the cause for the Thissamaharama tremors. They believe that the sudden release of stored energy by the defuse deformation zone might have caused the Thissamaharama earth quakes.

They also suspect that this phenomenon seems to be operating as an instigating factor in the formation of yet another tectonic plate, which has already been named "Capricorn" This is bordered by the mid ocean ridge from the west and deformation zone by the west.

On the other hand it's obvious that since the Tsunami Sri Lanka seems to be experiencing more earth tremors than usual. This could be a reality or could be that people are just more on the alert since the tsunami. But it was reported that the Tsunami triggering earth quake actually changed the spinning rhythm of the earth. This is due to the sudden emotion of an enormous amount of pent up energy. So the Tsunami triggering earth quake might have a lot more up its' sleeve yet.

Unfortunately the first two earthquakes were undetected by the Pallekele seismic station, since it was too small a tremor to be detected by its' seismograph. According to the intensity as reported by the locals, the epicentre seems to be in Beralihela area. But there are no facilities to detect the epicentre or magnitude.

The second two tremors, although, recorded by both seismographs in Peradeniya University and Pallekele station, the epicentre could be anywhere in a 120 mile radius from the Pallekele station. Generally at least three seismographs are required to determine the epicentre exactly.

In both cases however, the tremors were felt very strongly in a fracture or fault zone located in the area. Geologists say that if all these tremors originated from one fault, it could be associated with some kind of anomaly in that particular fault. If each of the tremors originated from different faults it could be a regional event which could have also caused the July 18th earth quake which occured in the Indian ocean east of Panama.

Geologists say that the Lunugamvehera reservoir could be another instigating factor. The Sri Lankan land surface is covered by hundreds of fractures. Another plausible explanation is that the tremendous pressure administered by colossal reservoirs like the Lunugamvehera, caused some moments in the faults, which in turn induced the tremors.

The first notable earth quake in Sri Lanka was in 1505. Since then more and more ever bigger reservoirs were built, imposing more and more pressure on the fault.

Apart from the movement of tectonic plates and faults, many other factors can provoke an earth quake. Subsurface magma flow and genesis of sinkholes are two of them. In the case of the Thissamaharama quakes subsurface magma flows can be disregarded, but sinkholes are a likely cause.

Geologists say Thissamaharama has a reserve of lime stone under ground. The constituents of lime stone, Calciumbicarbonate reacts with Carbonic acid and dissolve to make sinkholes. Other than limestone sinkholes can develop in carbonate rocks and salt beds. The vibration caused by the sudden generation of a sinkhole could very likely trigger an earth quake.

Even with the most advanced technology, it's almost impossible to foretell an earth quake. The trick is to be alert all the time. Even if direct damage by an earth quake in Sri Lanka is relatively little, the side effects can be very damaging. Geologists of the GSMB claim that a land slide in the areas subjected to the earth tremors, is highly likely.

Because the soil might have come lose due to the tremors, the likelihood of the impending rains inducing a land slide is a possibility. In which case the residents of the area, specially those who are directly below land slide prone slopes, are at great risk.

The most difficult obstacle faced by the Sri Lankan geologists is the lack of proper instruments. When these earth tremors took place there were five seismographs in the island but only one was in working. On the other hand USA has over a thousand functioning seismographs around the country.

They have made a general modal applicable for all earth quakes in the world. This is how magnitude of an earth quake is measured. Other countries like China is high on their heels.

In USA magnitudes of as little as 1.0 is recorded. But only earth quakes of magnitudes above 4 in Sri Lanka is recorded at the US Geological Survey (USGS). Therefore with out the proper instruments to detect our own earth quakes it's impossible to determine the magnitude of a below 4 earth quake.

The geologists also claimed that it's difficult to do any sort of proper study on earth quakes in Sri Lanka without the proper instruments. The establishment of a well maintained seismic network is of utmost importance, and for this they need functioning seismographs network to conduct a cluster study, determine epicentres, make a modal for determining magnitude, etc.... Moreover the geologists stated that the existing seismic stations have to be immediately reactivated.

The seismic station in the University of Peradeniya is of considerable importance in seismic studies.

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