Recent earthquakes trigger mixed opinions
More than 20 earthquakes have struck central Quang Nam Province's Bac Tra My District and Song Tranh Dam since the start of this month. The 4.8 Richter quake at 11 am yesterday lasting 10 seconds was the strongest one to hit the area so far. Viet Nam News reporters Ngoc Bich and Thu Hien interview experts and local people to help readers gain insight into this matter.
What is your assessment of the current situation at the Song Tranh Hydropower Plant area?
Cao Anh Dung, Deputy Director General of the Ministry of Industry and Trade's Industrial Safety Techniques and Environment Agency
According to reports by the independent consultant – Swiss' AF-Colenco – hired by the State Council for Assessment and Acceptance of Construction Works, the Song Tranh Hydropower Plant's dam is designed to resist an earthquake with a magnitude of 5.5 Richter and even higher. Theoretically, the sequence of tremors caused by water storage will continue recurring but is forecasted not to exceed 5.5 on the Richter scale.
|Cao Anh Dung
It is wrong to think that the bigger the hydropower plant, the more easily it causes earthquakes. The Song Tranh Hydropower Plant is the central region's biggest, with reservoir storage of more than 730 million cubic metres of water. But it is still small compared to plants such as the Hoa Binh Hydropower Plant on the Da River in northern Hoa Binh Province. Whether the earthquakes are strong or weak also depends on geological conditions in each area.
Actually, earthquake excitation is a common global phenomenon that occurs whenever hydropower plants come into operation.
Le Huy Minh, deputy director of the Geophysics Institute under the Viet Nam Academy of Science and Technology
Generally, hydropower plants cannot be built without earthquake excitation because the plants have to be constructed in mountainous areas where active faults often exist.
The Song Tranh 2 Hydropower Plant was also built on an area with active faults. These active faults caused earthquakes and tremors even when the plant's reservoir had not yet been built. According to the Institute's statistics, from 1715 to 1992 (before the plant was built), in the plant area and neighbouring area, there were only 8 earthquakes, all small.
However, when the plant reservoir was built, water stored in the reservoir was absorbed into the faults, weakening the rocks and soil beneath and causing the crust to move. These movement released energy and led to earthquakes.
Therefore, it can be said that earthquake excitation originates from the movement of active faults. This can happen immediately after power plants come into operation or after dozens of years. For example, the Kyona reservoir in India caused earthquakes after 20 years of operation.
Earthquakes caused by the excitation often occur sooner than tectonic earthquakes, and the magnitude of the former is never bigger than that of the latter in the same region. Scientists predict that the maximum magnitude of any tectonic earthquake in the plant area will be 5.5 on the Richter scale.
Dang Phong, chairman of Bac Tra My District's People's Committee:
Experts have concluded, based on their knowledge and a 10-day inspection, that the sequence of earthquakes in this area follows the rule of earthquake excitation set by the water storage of the hydropower plant reservoir of Song Tranh.
However, we do not place any confidence in this conclusion because these earthquakes and tremors have been happening while the water leakage at Song Tranh dam is being repaired. We have no idea whether the recent earthquakes will have a negative impact on the leakage of the dam or not.
I cannot believe in the safety of the dam at this moment!
No one can be completely sure of the magnitude of coming earthquakes or know whether they will exceed the reservoir's design limit. Moreover, the flood season is coming, doubling the risk to people's lives.
What are the losses that people face from the earthquakes?
Dang Phong: Initial statistics show that nearly 20 houses and two schools have been destroyed by the earthquakes.
However, the material losses are actually nothing in comparison with the negative impact on local people's morale.
They are living in fear. Production activities have been delayed. Many children are not allowed to go to school and have been taken to live in other areas by their parents. Many have no interest in their business anymore and have stopped all plans they may have had for building houses and establishments.
Their reason is that they don't know what their life will be tomorrow due to earthquakes. They have no peace of mind.
It seems that not only local people, but also central authorities and geological scientists, are very passive when earthquakes occur. Why?
Le Huy Minh: In Bac Tra My District, since last November, earthquakes have occurred with increasing frequency and intensity. In September alone, there were 20 earthquakes. In fact, the real number of earthquakes is probably even higher as there are only two seismometers installed in central Hue and Binh Dinh Province, which is 130-150km away from the district and can record earthquakes in the plant area with a magnitude of 3 and over.
Before constructing the Song Tranh 2 plant, no one thought earthquakes would happen with such frequency and intensity in this area of the province. Thus, we did not suggest the installation of seismometers to observe the tremors and earthquake situation. Accelerometers equipped for the plant only measure vibrations on the ground surface and fail to measure the intensity of quakes. We are really puzzled.
|Le Huy Minh
Local people do not know much about earthquakes. So of course they are frightened by them, especially those accompanied with explosions. Earthquake excitation often causes explosions due to its shallow seismic focus.
But looking on the bright side, if earthquakes hit the district frequently, this will gradually release energy stored in the crust. This is good because if energy accumulates in the crust for years, it can cause a strong earthquake with severe consequences.
What do you think about the possibility of a dam breach and the local authority's petition to stop the plant's operation?
Cao Anh Dung: It is understandable that local people and authorities were so panicked, as the plant just began operation one year ago and already faces the problem of water leakage. However, stopping the plant's operation is unnecessary and wasteful, as both a domestic consultant – the Power Engineering Consulting Joint Stock Company No 1 – and a foreign consultant affirmed that the plant is safe.
In addition, according to Electricity of Viet Nam (EVN)'s report, the leakage, which was discovered in April, has been repaired successfully. The level of leakage was reduced by 89 per cent. In some places, the rate was more than 90 per cent despite the set requirement of 80 per cent. Now, when the water level reaches 140m, the water flow through thermal gaps is just 3.2 litres per second. Therefore, the dam will likely not be breached.
Le Huy Minh: Actually, most dam breaches are not because of earthquakes, but technical errors during the construction process; and the breach often happens to earth dams, which are built by the compaction method. None of the 300 concrete dams in the world, constructed by Roller Compact Concrete technology, has ever been breached.
However, to ensure the safety of local people, the plant's reservoir should return to operating at full capacity only when a comprehensive assessment of the impact of earthquakes on the plants is completed. We suggest the plant's owner actively keep track of the water height, water flow and the width of thermal gaps on the dam.
What do you think about the Government's decision to not let the water storage in the plant reservoir reach the highest level? Do the ministry and local authority have any short – or long-term plan for fearful local people?
Cao Anh Dung: The decision was issued based on the ministry's proposal to the Government to allow the continuance of water storage to check the leakage when the water level is higher. The ministry also asked EVN and Song Tranh Hydropower Plant's Management Board to intensify observation and immediately fix any leakage detected if the rate of water flow increases.
In the proposal submitted to the Prime Minister, we suggested the storage should be divided into two phases. The water level will be raised gradually from 140m to 156m, and then from 156m to 175m, depending on the situation of water leakage and earthquakes. This is also a measure to calm local people down.
In fact, the ministry has also instructed not only Song Tranh plant's management board but also the boards of other plants to promptly set up projects to prevent floods. The boards are also required to create plans to address a dam breach.
Le Huy Minh: I think that the Government's decision is suitable under the current situation. We have to put the plant into operation, but the operation must be considered carefully so as to ensure the safety of local people. Therefore, water level should only be 20m higher than the current "death" level (140m).
Geologists must also study the physical relationship between earthquakes and water storage. From last November to this March, there were several earthquakes, but this June and July, no tremors were recorded. Meanwhile, in September alone, a series of earthquakes struck the region. There have been cases in which the region was shaken by two or three earthquakes in one day. Clearly, the relationship is very complicated. We hope that the 50 seismometers to be installed around the plant and its neighbouring areas next month will provide accurate and specific statistics so that future earthquakes can be predicted more precisely.
Dang Phong: The situation has confused the district-level authorities. In the short-term, financial support of VND3 and 1 million (US$142-47) will be given to those whose houses were either completely or partially destroyed, respectively. They will also receive assistance in stabilising their production activities.
This week, people, especially children, will be shown how to protect themselves during earthquakes.
At this moment and with our capability, this is all we can do.
All these support activities are funded by local authorities alone. We have not got any assistance from Song Tranh Hydro-power Jointstock Company, which claimed that its owner [Electricity of Viet Nam] has yet to provide any guidance or decision.
Le Thi Huyen, a resident in Bac Tra My District
Some experts said local people don't understand earthquakes and that's why they complain about the current earthquakes. However, they do not live here, and they cannot understand our worries. They should spend some time in our shoes and see how for us, everything - even our lives - hangs on a thread.
But why do they, as experts, forecast the possibility of earthquakes in this area but still decide to build the dam? Why do not they give us any information about this so that we can prepare for it? No, they do nothing.
If earthquakes keep striking this area, I might move to another place. I am worried about my family's safety every day. I don't want to do anything except for check for new information about earthquakes. My house now cannot be sold to anyone.
Water storage should be stopped in this flood season and checked carefully. They should at least do that to show their sense of responsibility for us.
The local authorities have suggested that water storage in the reservoir of the dam should be delayed in this flood season. The water storage is currently at the lowest level, and earthquakes still hit the area.
If it reaches the maximum level, nearly four times higher, what will happen?
Any decision should take into consideration the safety of the thousands of people in Bac Tra My District and the four neighbouring districts.
If they really want to store the water, they should be careful when pumping water in the reservoir and do it little by little. After pumping in a specific amount, they need to stop to check whether any leakage appears or anything bad happens.
They should also find out whether there is any relationship between earthquakes and dam leakage.
From now on, if any dam is built anywhere, the investors should ask local authorities and people for their opinions because although they do not have expert knowledge, they have lived there for dozens of years and have the most comprehensive understanding of location, land, geography and geology. — VNS