Residents sleep on the floor of Baiturrahman Mosque after a powerful 8.6 earthquake and a series of aftershocks hit Banda Aceh, in Indonesias Aceh province, on Wednesday.

Singapore: The powerful undersea earthquake off the Indonesian island of Sumatra this week was a one in 2 000 years event, and although it resulted in only a few deaths, it increases the risks of a killer quake in the region.

Wednesday’s 8.6-magnitude quake and a powerful aftershock were “strike-slip” quakes and the largest of that type recorded, Kerry Sieh, director of the Earth Observatory of Singapore, said.

“It’s an exceptionally large and rare event,” he said.

“Besides it being the biggest strike-slip earthquake ever recorded, the aftershock is the second biggest as far as we can tell,” Sieh said.

Strike-slip quakes involve the horizontal movement of colliding earth plates, and are less powerful than those where there is vertical movement.

A magnitude-9.1 quake in roughly the same region on Boxing Day in 2004 decimated Aceh province on Sumatra and killed more than 230 000 people in 13 countries around the Indian Ocean.

Wednesday’s quake caused few casualties and triggered very small waves, despite its magnitude. But the main problem is that it is likely to have increased stress on the plate boundaries near Aceh, increasing the risks of another major earthquake in the same area as the 2004 disaster.

In addition, research by Sieh and colleagues published in 2010 showed that the 2004 Aceh quake only relieved about half the stress that has built up over the centuries along a 400km portion of the faultline.

That makes another major quake in the area a matter of time. – Reuters