Geophysicists and geologists on Saturday visited the central province of Quang Nam after local authorities suggested region's largest hydropower dam is not the main culprit in earthquakes that have shaken the area recently.
The experts were told that some of the 11 earthquakes recorded between Monday and Friday have occurred in other places around the province, and not in the vicinity of the Song Tranh 2 reservoir.
Thorough surveys have been requested to see if the quakes were induced by the reservoir. Results are expected to be announced on September 12.
Le Huy Minh, deputy director of the Institute of Geophysics, said the delegation will study the exact points where the earthquakes occurred.
Many residents of Bac Tra My District below the dam were terrified when furniture in their homes shook and fell during the quakes that hit at night, with the strongest measured at 4.2 on the Richter scale.
The scientists on Saturday also visited a kindergarten and a secondary school in the district, where the quakes had caused cracks on the walls and floors. A wall at the kindergarten also lost a big piece of concrete.
After the first earthquakes in late 2010 when the dam was completed, scientists said water from the reservoir could have been absorbed into earthquake faults in the area, triggering seismic activity which could be heard as underground explosions occurring just below the earth's surface.
But Tran Van Hai, a dam manager, said water levels has been kept at its lowest for the past three months to fix the cracks and water leaks spotted in March. The provincial government on August 30 said the problem was completely fixed.
And although the Geophysics Institute said as many as 58 quakes have been recorded near the dam since 2011, local officials pointed out that the quakes have not been limited to the dam area.
Dinh Van Thu, vice chairman of Quang Nam administration, said the district used to experience earthquakes before the dam was built.
"And earthquakes have increased in the whole area, not just near the dam."
Dang Phong, chairman of Bac Tra My District, said officials from other districts "are also really worried" as "some vibrations have been felt in their areas as well."
Phong said scientists once estimated that the area is prone to earthquakes of the magnitude below 3 on the Richter scale. But now that the magnitude is 4.2, he was worried about unexpected disasters.
Quakes that measure 5 on the Richter scale can cause local houses to collapse, scientists say.
He said it's hard for local officials to calm the residents.
"We were calm and patient for a long time. But there has been a succession of earthquakes. I myself am feeling uneasy, how could I make people believe that things are going to be okay?"
The Institute of Geological Sciences said it will lend the provincial government five mobile earthquake measuring machines to place around the dam. There were plans to build measurement stations in the area but they are yet to be implemented.
The Song Tranh 2 hydropower plant was built with an investment of more than VND4.15 trillion (US$197.53 million). It began operations in December 2010 with an expected output of 190 megawatts.
In November 2011, the plant made headlines, along with other hydropower plants in the region, for worsening floods that killed 24 people. They were criticized for releasing excess water from their reservoirs without giving local residents enough notice.
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