UPDATE: USGS appear to be revising the initial magnitude estimate downwards. Currently bouncing between 8.6 and 8.7. Definitely big though, as we saw a huge signal on our own seismometer here near London (see below).
UPDATE 2: After a series of ~M5 aftershocks, an M8.2 has just gone off. http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/usc00090da.php#detailsLive earthquake monitoring here http://earthquake.usgs.gov/earthquakes/map/
UPDATE 3: ongoing aftershocks, but large regions of the tsunami alert have been lifted. http://www.bbc.co.uk/news/world-asia-17675399#TWEET124665
It would appear that about an hour ago a huge earthquake occurred offshore of Banda Aceh, not a million miles from the location of the 2004 M9.1 event which triggered a tsunami killing almost a quarter of a million people across the Indian Ocean.
Initial moment tensor solution is in, and bizarrely looks like it might be strike-slip (i.e. a sideways movement on a transform fault, rather than the thrust fault you might normally expect from a subduction zone earthquake. A discussion of precisely why this is bizarre can be found here). There’s a tsunami watch notice for the entire region, with estimated arrival times here. A strike slip movement significantly reduces the tsunami risk, hence the ‘watch’ notice, rather than ‘warning’.
I’ll update as more info becomes available. It’s likely that as more seismic information comes in the magnitude estimate will be modified.
USGS earthquake report: http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/usc000905e.php