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.

This map shows the location of the 6.6-magnitude earthquake that struck Sumatra on Oct. 1, a day after the earthquake in Padang. This second earthquake occurred very close to the Dikit segment (Sieh and Natawidjaja, 2000) of the Sumatran fault, which extends 2,000 km from near Krakatoa in the south to Banda Aceh in the north. Past geological records indicate no previous ruptures on the Dikit segment of the fault, although the Kerinci segment, to the north, produced a 7.0-magnitude earthquake in 1995.

An Indonesian team, led by Dr. Danny Hilman Natawidjaja of the Indonesian Institute of Sciences (LIPI), plans to visit the Dikit section of the fault to determine whether the rupture that caused the Oct. 1 earthquake occurred at the surface.

If so, this will be helpful in determining more precisely where the fault associated with the earthquake is, since there is inherent uncertainty in the location of fault ruptures as determined solely by seismographic data.

This cross-section image shows the origin of the Sept. 30 earthquake (upper green star) and an aftershock. The dots (colored according to depth) represent previous small earthquakes in West Sumatra from April 2008 to February 2009, as located by Dr. Frederik Tilmann and others using portable land-based and ocean-bottom seismometers (triangles).

he diffuse band of earthquakes angling down beneath Siberut island and Sumatra reflect deformation within the descending Indian/Australian plate and illuminate its location. The top of this diffuse band of earthquakes helps define the Sunda megathrust, which is the boundary between the two plates. The part of the megathrust shown with a solid red line is the part that is locked and is therefore likely to produce a great earthquake within the next few decades. The part that is shown with a dashed red line appears from GPS measurements to be unlocked and slipping more or less freely; it is unlikely to slip abruptly and cause great earthquakes.

The magnitude 7.6 earthquake of 30 September 2009 appears to have been produced by the rupture of a fault within the descending plate, well below the megathrust. The large aftershock (lower green star) also appears to be within the descending plate. The two locations are from the U.S. Geological Survey.

This map shows the network of GPS stations arranged along a long stretch of Sumatra to collect satellite-based seismic data on earthquake activity in the region. Some of these stations are located very close to the Sept. 30 earthquake, but scientists at the Earth Observatory of Singapore and the Indonesian Institute of Science need several days in which to analyze the data so they can better understand details about the earthquake and possible later effects.

This image shows the relationship of the Sept. 30 earthquake in 2009 to earthquake ruptures of September 2007. The earthquakes two years ago were caused by fault ruptures farther to the south from Padang. The topmost red star shows the approximate location of the Sept. 30 earthquake. (Source: Konca et al., 2008)

This map shows the approximate epicenters of the Sept. 30 earthquake (red star) and a follow-up earthquake a day later (orange star), relative to undersea and geological features. The thin red contours show the depths to the megathrust, where tectonic plates collide and one of the slabs begins dipping deeper into the earth. The Sept. 30 earthquake originated about 80 or 90 km deep, probably below the megathrust. (Source: Sieh et al., 2000)