Recent earth tremors around Victoria dam – a scientific explanation in relation to the tectonic history of Sri Lanka

By Dulip Jayawardena

(Retired Economic Affairs Officer United Nations ESCAP 1990 to 2003 Former Director Geological Survey Department- Present GSMB (1983-1986)

Earth tremors were reported in the print and electronic media on 29 August and also 2 September 2020 in an area surrounding the Victoria dam in Kandy. The Geological Survey and Mines Bureau (GSMB) the designated Agency to monitor such earth tremors have mobilized geologists to investigate the damages caused by such earth movements.

The Director General of the GSMB announced that there had been no records of such tremors in the Pallekelle seismic monitoring station which is close to the areas affected. However, a senior Director who is a geologist gave a different version and reported that tremors were detected at the monitoring station.

The reporting of earth tremors and earthquakes are on the Richter scale, which has been revised many times. According to this scale unit 1 records a micro tremor not felt or felt rarely and the frequency of occurrence worldwide is several million per year. The scale 2 -2.9 is felt slightly by some people and damage to buildings and the worldwide frequency per year is over 1 million. The magnitude 3 to 3.9 is a minor tremor often felt by people but very rarely causes damage. Shaking of indoor objects can be noticed. The frequency is over 100,000 per year. The magnitude 4.0 -4.9 is noticeable with the shaking of indoor objects and ratting noises and felt by most people in the affected area and slightly felt outside and generally causes minimal damage. Moderate to significant damage very unlikely. Some objects may fall off shelves or be knocked over and the frequency worldwide is 10,000 15 000 per year.

Tectonic evolution of Sri Lanka theories of three peneplains and upliftment and plate tectonics

The earlier theory of the three peneplains was proposed by Wadia in the late 1948 when he was the Director of the Mineralogy Department. His theory was based on the ring of waterfalls around the country that were caused by erosion of the rocks to the upper plain mid plain and the lower plain.

However, in 1974 after a gravity survey was conducted by the Geological Survey Department (Present GSMB ) covering the entire island and it was conclusively proved that the there was an upliftment of the Precambrian terrain composed of over 80 per cent of the land area which had earlier being recognized as the Highland and Vijayan Series.

The Gravity Map of Sri Lanka on the scale of 1:100000 was published with an explanation as Professional Paper No 3 with T Hatherton, D. B. Pattiarachchi and V. V. C. Ranasinghe. This was possible with technical cooperation by the Government of New Zealand under the Colombo Plan.

During the gravity survey, earlier trigonometrical stations established by the Survey Department on the triangular survey for the topographical maps were reopened. It was then noticed that some bench marks for these stations were uplifted especially in the hill country of the island.

The most significant revelation was a significant gravity low continuously identified between the eastern boundary of the Highland and Vijayan series rocks. This led to the theory of plate tectonics where the Vijayan series sank below the Highland series forming a convergent plate boundary.

The evidence for such activity can be seen in the change of course of the Mahaweli river from a direction of north east to north as indicated by some Buddhist shrines such as Somawathi and Deegawapi which were earlier on the banks of this river.

Earth tremors are noted frequently along this fault line and geologists together with geophysicists should constantly monitor such movements.

A new tectonic plate boundary between

Australia and India

Many researchers have carried out extensive studies on the development of a new tectonic plate boundary in the Indian Ocean which occurred over 40 million years ago. This boundary that runs east west is about 400-500 kms south of Sri Lanka. If this boundary is activated it will have devastating effects to the infrastructure south of Sri Lanka such as the Hambantota Port, Mattala Airport and industrial zones envisaged in the area.

Danger to Victoria concrete arch

dam from earth tremors

Victoria dam is the largest concrete dam in Sri Lanka built under the Mahaweli Project costing Rs 9.8 billion during 1980-1984 to solve problems as hydropower for the country diversion of water for agriculture, solving unemployment, encouraging inland fisheries and flood control.

“Victoria Dam is a double curvature (cupola) arch dam of 122 meters maximum height and 520 meters crest length. The arch dam is constructed with the individual crown cantilevers. A large amount of water load acting on the upstream surface of the dam is transferred to the abutment by arch action and a certain amount of water load is transferred to the foundation by cantilever action. The destruction of the Victoria dam can cause flooding up to Trincomalee affecting the livelihood of the area .crack was identified and it has been developing in the downstream face of the Victoria dam (Sri Lanka Head works Division of Mahaweli Authority 2014)Therefore the Victoria Dam is investigated to identify the effect of earthquake ” (Ref Assessment of Concrete Arch dam under Possible Earthquake Loading in Sri Lanka Case Study on Victoria Dam by Shobitha Tharamarajah and Kamal Karunananda International Journal of Scientific and Engineering Research Volume 8 Issue 10 October 2017 ISSN 2229-5518)”

Another article titled “Comprehensive monitoring of Victoria Dam /Civil + Structural Engineer magazine <https://csengineermag.com/comprehensive-monitoring-of- victor…. > should also be studied.

All interested parties should access these excellent articles as it covers all aspects of strengthening the Victoria dam to avoid any major catastrophe.

Reservoir induced earthquakes –application to arch and gravity dams in Sri Lanka

As stated above, the only large concrete arch dam in Sri Lanka is the Victoria dam, which was built mainly for the supply of hydroelectricity. Another concrete arch dam is Canyon producing hydroelectricity.

Apart from the Victoria dam there are dams built for irrigation and hydroelectricity throughout Sri Lanka which are gravity concrete dams. There are also embankment dams constructed mainly for irrigation.

The gravity dams constructed mainly for generation of hydroelectricity are at Bowatenna, Broadlands, Castlereigh Dyraaba, Kalu Ganga, Kotmale, Kukule Ganga,Laksapana, Maskeliya, Moragahakanda, Moragolla Norton ,Nilambe, Polgolla Puhulpola, Rantambe, and Upper Kotmale Ganga,

(Ref Dams and reservoirs of Sri lanka –Wikipedia< https://en.wikipedia.org/wiki/List_of _dams_and reservoirs_in _…)

It is asked whether the gravity dams constructed mainly in the hill country are being monitored for any tremors using the latest seismic equipment as at Victoria as reservoirs of these dams will also be subject to reservoir triggered seismicity giving rise to earth tremors or earthquakes.

It is also noted that most of the above reservoirs are located in areas with less inhabitants and the tremors will not be felt as at Victoria.

Dams in Sri Lanka are built on valleys that have formed by active erosion with recent theory of upliftment as indicated by the gravity survey covering the island. It is also known that under compressional tectonic force reverse or thrust faults produce upliftment giving rise to tremors

It is known that reservoir induced seismicity can also occur after a few years and will depend on the permeability of the rocks beneath the reservoir.

It is also noted that indused earthquakes at reservoirs experience delayed triggering at depths of 10 to 20 kilometers. Such earthquakes may occur 10 to twenty years after filling the reservoir. In this regard it is noted that the Victoria reservoir was filled in 1985 which is 35 years ago.

Further the other gravity dams also should be subject to regular monitoring for any tremors due to reservoir triggered seismicity. However, it is not easy to predict reservoir induced seismicity due to the state of stress and the rock strength at earthquake depths are not subject to direct measurement.

Ref : Dams and earthquakes –Seismology Research Centre

(https://www.src.com.au/earthquakes/seismology-101/dams-eart .. )

Recommendations

In the above article I have attempted to explain the recent earth tremors experienced in the valley surrounding the Victoria Dam as well as the high ground which occupied by villagers. These tremors were felt on two occasions and the GSMB has mobilized a number of geologists to investigate what damages have been caused to the dwellings etc. A retired Professor of Geology of the Peradeniya University has attributed the tremors to the extensive quarrying operations carried out for limestone in an area above the Victoria dam.

The GSMB is also investigating the triggering of these tremors due to the Victoria reservoir indused seismicity which I have also dealt above.

With my past experience as a geologist who mapped the area in the late 1970s there are many fold belts with prominent fault zones that could be activated with gradual upliftment of the terrain as proved by the gravity survey conducted in 1975.

It is important to monitor the other gravity dams I have listed as with age of these dams the pore pressure will increase at depths to kilometers beneath the reservoir.

I urge the government to appoint a multidisciplinary team of experts comprising of geologists , geophysicists, hydrologists, civil engineers and those from the NBRO as well as the universities to study the state of the Victoria dam as well as other gravity dams and recommend an effective method of monitoring such tremors.

I had been involved in advising the late Dr A. N. S. Kulasinghe on the problems involved in the construction of the Upper Kothmale gravity dam in a landslide prone area. However it is now opportune to monitor any seismicity and take remediable measures.

As revealed above the Victoria dam developed a crack on the downstream part in 2014 and is important that constant monitoring should be carried out .

I was also involved in following up the construction of the Victoria dam in early 1980 when the then Professor of Geology of the Peradeniya University revealed that a major fault zone runs along the axis of the dam. However, the consultant engineers responded that the Victoria dam was designed to take a wave crest of 10 meters over the dam and there is no danger to its stability.

I hope this article will be helpful to the experts to draw up a comprehensive long term plan to monitor our gravity dams that mainly generate hydroelectricity as well provide water for irrigation.

Last but not least the effects of climate change on these reservoirs and the surrounding areas should not be ignored .

(The author can be reached at fasttrack@eol.lk )