Begining with a foreshock of mag 6.5, a bigger shock of 7.2 and then a series of aftershocks of 5+; all within several hours of immense rumbling and shaking in the Solomon islands at the junction of the Australian plate and the Pacific plate. In the map below the plate boundary trends NW-SE just south of the islands.
Here is the tectonic summary from USGS:
The Solomon Islands earthquake of January 3, 2010, likely occurred at the boundary between the Pacific and Australian plates, where the Australian plate subducts beneath the Pacific towards the northeast at a rate of approximately 95 mm/yr. The mechanism of the January 3rd earthquake is consistent with its occurrence in relation to underthrusting of the Australia plate beneath the Pacific plate, as part of this subduction process.
The Solomon Islands arc as a whole experiences a very high level of earthquake activity, and many shocks of magnitude 7 and larger have been recorded since the early decades of the twentieth century. The January 3rd, 2010 earthquake nucleated approximately 50 km to the southeast of a M8.1 earthquake in April 2007, which with an associated tsunami caused at least 50 fatalities and destroyed several coastal villages on nearby islands. An M6.5 foreshock occurred less than one hour before this main shock, in approximately the same location.