At least 1100 people are confirmed to have died and almost 2,400 people have been injured, in the powerful quakes that struck the Indonesian island of Sumatra. Thousands more are missing or trapped in rubble. The quakes destroyed about 75% of buildings in and around Padang, the capital of West Sumatra province which has a population of over 900,000.

According to the US Geological Survey, the first earthquake measuring 7.6 on the Richter scale struck at 1716 local time (1016 GMT) on Wednesday, some 85 km (55 miles) under the sea, north-west of Padang. It was followed by a second tremor, of magnitude 6.8 on the Richter scale, that hit close to Padang at 0852 local time (0152 GMT) on Thursday, about 150 miles further south-east.

Part of the Pacific's Ring of Fire, a horseshoe-shaped area of seismic activity ringing the Pacific Ocean basin, Indonesia is in the world's most seismically active region, registering 6,000 to 7,000 earthquakes with a magnitude of 4.0 or above annually.

The Indonesian earthquake struck nearly 12 hours after a powerful quake in the South Pacific that triggered a devastating tsunami that hit struck American Samoa and Tonga. But experts said the two events were not related. "They were 10,000km (6,200 miles) apart," New Zealand seismologist Bill Fry told AFP news agency. "You can get quakes that are close temporally and spatially as one transfers stress to another place against the fault, but that's not possible this far apart."

The Indonesian earthquake brought down hundreds of buildings, including hospitals, mosques, hotels, a mall and homes, and left thousands trapped under rubble. The quake cut power lines, snapped phone lines and communication networks, and triggered landslides. Many large buildings in the town had been severely damaged and some buildings were on fire. However, many smaller residential properties had escaped the damage. Power has now been restored to some parts of Padang.

In Padang residents fought some fires with buckets of water and used their bare hands to search for survivors, pulling at the wreckage and tossing it away piece by piece.

There are huge cracks in the concrete roads, a few feet wide, showing the full force of the tremors. Rescue officials have told reporters that some of the worst damage is in the heart of the city where they say a shopping mall has collapsed. Another worst disaster appeared to be the collapse of a school in Padang.

Wednesday's quake was along the same fault line that spawned the 2004 Asian tsunami (Boxing Day tsunami) that killed more than 230,000 people in a dozen countries. Padang lies on the same fault line as Indonesia's Aceh province, which was devastated in the 2004 tsunami with 130,000 dead. Geologists have long warned that Padang could one day be completely destroyed by an earthquake because of its location in a highly active seismic zone.

Military planes were flying medical equipment and personnel to the affected region. Government health ministry teams and Indonesian soldiers have arrived in the city to aid the search for survivors. Military personnel, as well as police and health ministry workers, were in the affected area, but they urgently needed heavy machinery to lift the rubble.

Stocks of emergency shelters, hygiene kits and clothing are ready to be distributed by aid teams funded by British charity Oxfam once workers can get through to the worst affected areas. Food, medicine and body bags have begun to arrive. Tents and blankets were also on their way to help the homeless, the health ministry said.

Australia is among the countries that have offered to send emergency assistance to Indonesia if needed. Bob McKerrow, Red Cross head of operations in Indonesia, told it had more than 400 personnel on the ground, including 50 doctors flown in on Thursday morning. "But it's just such a vast area to be working in with such bad infrastructure," he said. The roads and bridges have all been damaged, so there is a huge challenge ahead of rescue workers.

British aid is being sent to areas of western Indonesia, where the tremors have caused widespread landslides and at least two hospitals, mosques and hotels to collapse. The British Red Cross has launched a fund-raising appeal to help those caught up in both the earthquake and the tsunami in American Samoa and Tonga.

Titi Moektijasih, of the UN Office of Coordination of Humanitarian Affairs, told AFP that emergency efforts so far were insufficient, and added, "Compared to the extent of the damage, you see there should be more equipment, more people to do this."

In recent times Indonesia and the surrounding regions are hit by earthquakes and tsunamis. Asian tsunami of 26 December 2004 killed 170,000 in Indonesia alone. On 28 March 2005, about 1,300 people were killed after a magnitude 8.7 quake hit the coast of Sumatra. On 17 July 2006 a tsunami after a 7.7 magnitude quake in West Java province killed 550 people.

However, it is a double devastation now, two disasters in fewer than 24 hours, earthquakes in Indonesia and the tsunami in Samoa, and experts are trying to determine whether the quake was linked to the one earlier in the South Pacific.