31 October, 2009 - No human casualties were reported as of yesterday but initial information suggest that structures damaged by the September 21 earthquake may have been further weakened by the 5.2 magnitude tremor which struck eastern Bhutan around 11 pm on Thursday.

With its epicenter in Mongar, close to the Trashigang border, the quake was also felt 180 km away in Thimphu. “The house shook for 5 seconds and there were around three aftershocks,” said home minister, Lyonpo Minjur Dorji who is in Trashigang. Thursday’s quake is about 10-15 km south of the September 21 earthquake’s epicenter in Narang.

Reports from Trashigang dzongkhag says that 72 houses were affected, most of which were damaged by the September quake. Gewog wise, 50 houses were damaged in Yangneer, 17 in Udzorong of which seven are new houses and one has collapsed. Another five were damaged in Lumang. Reports from other dzongkhags were still pouring in.

According to Mongar dzongda Sherab Tenzin 34 houses were affected in his dzongkhag so far. Of these 11 are in Thangrong, six in Sheri Muhung, five in Mongar, five in Tsakhaling, four in Narang and three in Ngatshang.

The Pemagatshel dzongda, Gholing Tshering, said that around 10 structures were affected. A primary school in Chimung gewog damaged earlier was damaged badly with its classrooms and staff quarters now made unsafe for living according to the dzongda. In Lhuentse four houses in Sengor gewog were damaged of which two were new.

Department of geology’s seismologist, Dowchu Dukpa, said, “the 5.2 earthquake has caused less damage because it was around 32 times weaker than the 6.1 earthquake.” Both quakes took place close to one another, but were caused by entirely different reasons.

The 5.2 quake was caused by a ‘strike slip fault’ where the Indian and Eurasian plates move sideways away from each other instead of against each other. “This is usually more destructive but it was of a smaller magnitude and occurred at a depth of 26.5 km below the surface,” he said. The 6.1 earthquake which occurred at a depth 14 km is of a more normal ‘thrust’ earthquake where the Indian and Eurasian plates move directly into one another.

Dowchu said the 6.1 earthquake was caused by the ‘main center Thrust’ fault line and a branch of it may have caused the 5.2 quake. This fault line runs across the Himalayas and in Bhutan it runs close to the southern dzongkhags before rising up sharply covering eastern dzongkhags like Mongar and Trashigang.