An aftershock measuring 8.8 on the Richter scale struck Sumatera mere hours after a powerful quake sent residents running for higher ground in Banda Aceh after a tsunami warning was issued for western Indonesia on Wednesday.
The aftershock, the latest in a series of quakes, struck Sumatera at 5:43 p.m., the Meteorology, Climatology and Geophysics Agency (BMKG) reported. The region was placed on tsunami alert after a magnitude 8.5 earthquake struck some two hours earlier 500 km southwest of the city of Banda Aceh, on the northern tip of Indonesia's Sumatra island, at a depth of 33 km, the US Geological survey said.
Indonesia's Meteorology and Geophysics Agency (BMKG) issued a tsunami warning for Aceh, West Sumatra, North Sumatra, Lampung and Bengkulu immediately after the quake was reported.
The Pacific Tsunami Warning Center said a tsunami watch was in effect for the entire Indian Ocean. It later said the threat of a big tsunami had receded, although the warning remained in place.
"It doesn't look like a major tsunami. But we are still monitoring as tsunamis come in waves," Victor Sardina, a geophysicist on duty at the Hawaii-based institute, told Reuters. The BMKG reported that small tsunami waves hit the coasts of Sabang and Meulaboh, in Aceh. The waves measured between 10 and 80 centimeters in height.
"The potentials for a tsunami is not as big as in 2004," Sutopo Purwo Nugroho, spokesman for the National Disaster Mitigation Agency (BNPB).
Tsunami warnings would stay in effect for Indonesia. "The tsunami warnings along the western coasts of Aceh and North Sumatra will last for about an hour, in West Sumatra for about one-and-a-half hours and in Lampung between two and two-and-a-half hours," Sutopo said Wednesday afternoon.
Sutopo said power was down in Aceh province and people were gathering on high ground as sirens warned of the danger. "The electricity is down, there are traffic jams to access higher ground. Sirens and Koran recitals from mosques are everywhere," he said.
Residents in Banda Aceh reported the ground shaking violently. "It began as a mild tremor and then got stronger," an AFP correspondent in Banda Aceh said. He said the ground shook for about five minutes, and telephone connections went down briefly.
"There are people trying to evacuate, some are praying and children at a school were panicking as teachers tried to get them out," he said. "There are traffic jams everywhere as people are trying to get away from the coast – many are on motorcycles," the correspondent said.
There were no immediate reports of casualties or damage in Aceh, President Susilo Bambang Yudhoyono said in a speech from the Presidential Palace in Jakarta.
"There is no tsunami threat although we are on alert," said he said at a joint news conference with visiting British Prime Minister David Cameron, who said Britain was standing ready to help if needed. "The situation in Aceh is under control, there's a little bit of panic but people can go to higher ground," Yudhoyono added.
He said he had ordered a disaster relief team to fly to Aceh, which was devastated by the 9.1 magnitude 2004 quake, which sent huge tsunami waves crashing into Sumatra, where 170,000 people were killed, and across the Indian Ocean. In all, the 2004 tsunami killed about 230,000 people in 13 Indian Ocean countries, including Thailand, Sri Lanka and India.
'horizontal shift'
Wednesday's quake was felt as far away as the Thai capital, Bangkok, and in southern India, residents said. Hundreds of office workers in the Indian city of Bangalore left their buildings while the Indian port of Chennai closed down because of the danger of a tsunami, the port said.
The quake was in roughly in the same area as the 2004 quake which was at a depth of 18 miles (30 km) along a fault line running under the Indian Ocean, off western Indonesia and up into the Bay of Bengal. One expert told the BBC the Wednesday quake was a "strike-slip" fault, meaning a more horizontal shift of the ground under the sea as opposed to a sudden vertical shift, and less risk of a large displacement of water triggering a tsunami.
The quake was also felt in Sri Lanka, where office workers in the capital, Colombo, fled their offices, and in Phuket, both of which were hit hard by the 2004 tsunami.
Mahinda Amaraweera, Sri Lanka's minister for disaster management, called for calm while advising people near the coast to seek safety.
"I urge the people not to panic. We have time if there is a tsunami going to come. So please evacuate if you are in the coastal area and move to safer places," Amaraweera told a private television channel.
In Bangladesh, where two tremors were felt, authorities said there appeared to be no threat of a tsunami. Australia also said there was no threat of a tsunami there.
Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.
A giant 9.1-magnitude quake off the country on Dec. 26, 2004, triggered a tsunami in the Indian Ocean that killed 230,000 people, nearly three quarter of them in Aceh. (JG/Retuers/AFP)