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). The 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) 

This image illustrates the coupling, or “locking up,” of tectonic plates that lie beneath the Metawai Islands, to the west of Sumatra. This kind of coupling causes a buildup of strain in the plates, which eventually rupture and trigger earthquakes. Many earthquakes already have occurred as a result of ruptures at various points along this long stretch of plates, and scientists have predicted that a remaining patch is due to rupture. The resulting earthquake is expected to be quite large because of the intensity of long-accumulated strain.

(Source: Chlieh et al., 2008)

This image shows the section of the Sunda megathrust that broke in the 8.7-magnitude earthquake of March 2005, as represented by the red line. The pink shading shows the areas where the ground rose up to 3 metres because of the rupture; the blue shading indicates areas that sank (up to 1 metre) because of the rupture. The Sept. 30 earthquake occurred in the down-going slab at the lower right-hand corner.

(Source: Richard Briggs, USGS)

This map shows the Sept. 30 and Oct. 1 earthquakes and their geographic relationship to bigger Sumatran earthquakes of the past decade. In the section marked “Mentawai patch,” the unshaded part between the colored areas is the region of the underlying Sunda megathrust where scientists believe the next very large earthquake is likely to occur because that part has not yet ruptured and the strain has accumulated for a very long time.