This diagram, provided by Fred Taylor of the University of Texas-Austin, shows the location of the Fiji Islands, at lower right centre, where a magnitude 7.2 struck on 9 Nov. The small circles represent GPS monitoring stations that collect a steady stream of tectoseismic data in the Fiji-Tonga area, where spreading seafloor ridges of the earth’s crust and a major subduction zone contribute to extremely complex tectonic motions. The Tonga trench (far lower right) is where the Pacific plate subducts westward beneath the Australian plate in the west.
A magnitude 7.2 earthquake that occurred beneath the Pacific island nation of Fiji on Nov 9 was centred at a depth of 585 km (366 miles), according to the US Geological Survey. That depth is not far from the depths of Earth's deepest earthquakes, and is far too deep to generate a destructive tsunami. Earthquakes at this depth are generally believed to be caused by sudden changes in the chemical structure of minerals in oceanic plates that have descended along subduction zones to hundreds of kilometres beneath Earth's surface. These sudden seismic increases in mineral densities are due to the exceedingly high pressures at these great depths. In this particular case, the earthquake appears to have occurred within the Pacific plate, which descends from the location of the Sept 30 magnitude 8.0 Samoan earthquake to about 600 km (372 miles) beneath Fiji.
This map shows the location of a 6.7-magnitude earthquake that struck near the northern coast of the Indonesian island of Sumbawa on Nov 8. The centre of the quake was about 300 km (186 miles) north of the Sunda megathrust and about 70 km (43 miles) east of the caldera of Tambora volcano, which in 1815 produced the largest volcanic eruption in recorded history. From data available on the USGS website, the Sumbawa quake appears to have been caused by slip on a thrust fault, which crops out on the sea floor about 46 km (28 miles) north of the earthquake's centre. It slopes gently from the seafloor under Sumbawa. Geologists call this sort of structure a "backthrust" because it moves in a direction opposite that of the megathrust. In 1992, slip on this same megathrust a couple hundred kilometres to the east caused a 7.9-magnitude earthquake and tsunami on the north side of Flores island, with significant loss of life and property.
This diagram and the map that follows show where a magnitude 7.8 earthquake struck Vanuatu on Oct. 8. It occurred about 2,200 km (1,243 miles) west of Samoa, where a magnitude 8.0 earthquake struck a week earlier, on Sept. 30. This region of the Pacific is home to one of the most complex arrangements of tectonic plates on earth. The Oct. 8 earthquake appears to have occurred on the megathrust that separates the Australian plate, which is diving eastward, from the overlying plate on which Vanuatu sits. The Sept. 30 earthquake off Samoa was caused by rupture of the Pacific plate, at a position just before it begins its descent westward beneath the islands of Tonga. The map below is a Google Earth image combined with epicentres as recorded by the U.S. Geological Survey.