Looking at the USGS Earthquakes site I noticed they had a list of all recent mag 2.5+ quakes and there is a magnitude 7.3 one reported for Nicobar Islands at 04:21 GMT (3.5 hours after the 8.9 one). It was at a similar depth (10kms) to the Sumatran Quake, although no idea whether the Nicobar one is likely to generate the same impact.

You can get an RSS feed of earthquake information from the USGS here.