Sumatra earthquake at 7.7M

The Sumatra earthquake struck at a magnitude of 7.7M on 25th October 2010 at 21:42 local time (22:42 Malaysian time) off the western coast of Sumatra, Indonesia. The earthquake occurred on the same fault that produced the 2004 Indian Ocean earthquake. It was widely felt across the provinces of Bengkulu and West Sumatra and resulted in a tsunami that struck the Mentawai Islands.
Many villages on the islands were affected by the tsunami, which reached a height of 3m (9ft) and swept as far as 600m (1800ft) inland. The tsunami caused the widespread destruction that displaced more than 20,000 people and affected about 4,000 households. Around 300 people were reported to have been killed, with hundreds more still missing. The relief effort was hampered by bad weather and the remoteness of the islands, which may delays in reporting the casualties.

It's epicenter location was at 3.464ºS 100.084ºE.Why a tsunami warning failed?

According to Indonesian officials, a tsunami warning was set based on seismographic data. They found out that a tsunami warning system installed following the 2004 Indian Ocean earthquake and tsunami had failed to function properly. Two of the system's buoys out at sea had been vandalized and were inoperative. However, the epicenter of the earthquake was so close to the islands that a warning would probably have been too late in any case, as the tsunami took about 5 to 10 minutes to reach Pagai.