The December 8, 2016, M 7.8 Solomon Islands earthquake occurred as the result of shallow, slightly oblique reverse faulting on or near the plate boundary between the Australia and Pacific plates. Focal mechanism solutions indicate that rupture occurred on either a northwest or north-south-striking, moderately dipping reverse fault. At the location of the earthquake, the Australia plate subducts beneath the Pacific plate towards the east-northeast at a velocity of about 96 mm/yr. The location, depth, and focal mechanism solutions of the December 8th earthquake are consistent with its relation to under-thrusting of the Australia plate beneath the Pacific plate (the northwest-striking plane of the focal mechanism solution).
While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse faulting events of the size of the December 8, 2016 earthquake are typically about 120x55 km (length x width).
The December 8th earthquake occurred about 100 km northwest of where the Australia:Pacific plate boundary transitions from thrust to transform tectonics between the New Britain Trench to the northwest and the New Hebrides Trench farther east. A pair of large earthquakes (M 7.6 and M 7.4) occurred in that plate boundary transition region in April 2014. More broadly, the Solomon Islands Arc is very seismically active, with 53 earthquakes of M 6.5+ occurring within 250 km of the December 8th event over the preceding century. The largest of these was a M 7.9 earthquake about 50 km to the southwest in October 1931, near the New Britain Trench. In 1979, a M 7.1 earthquake occurred in an almost identical location (8 km northwest) to the December 8, 2016 event. None of these historic earthquakes are known to have caused shaking-related fatalities, likely because of their remote location far from population centers that might be vulnerable to earthquake shaking.
The September 1, 2016 event was preceded by a M 5.8 foreshock, approximately 18 hours earlier and 30 km to the north, at a similar depth and with the same faulting mechanism as the subsequent M 7.1 earthquake.