The Indonesian island of Sumatra has been the scene of devastating earthquakes in recent years, and the risk of another powerful earthquake occurring is extremely high, a recent geological survey shows. California Institute of Technology scientists published their findings in a letter in the December 4 issue of Nature, and this week in Science.

Four years ago, a magnitude 9.1 or 9.3 Indian Ocean earthquake with the epicenter off the west coast of Sumatra, the second largest ever recorded, triggered a devastating tsunami that killed over 225,000 people in 11 countries. In Indonesia alone, at least 170,000 people were killed. The event was triggered by the rupture of a 1600-km-long piece of the Sunda megathrust, in its northern region.

Less than four months later, an 8.7 magnitude aftershock made another 1,300 victims, after a south piece also ruptured, although the tsunami generated this time was far less destructive than the previous one. The earthquake was followed by several other aftershocks, magnitudes 5.5 to 6.0. In 2007, another sequence of earthquakes occurred in the Mentawai Islands area.

The team of Caltech researchers from the Tectonics Observatory warned that recent seismic activity was not enough to release the tectonic strain accumulated in the area’s subduction zone for the past two centuries, and therefore we should expect intense seismic activity to hit again. As Jean-Philippe Avouac, director of Tectonics Observatory and one of the lead authors of the study, explained, the large earthquakes in Sumatra are far from being over.

The scientists noted that the 2005 earthquake has managed to “unzip” the stuck area in that patch of the zone and relieve the stress accumulated since 1861. What this means is that this area in particular is unlikely to produce another large quake any time soon.

But that cannot also be said about the area further south, near the Mentawai islands, hit in September 2007 by two major earthquakes in the same day, an 8.4 and a 7.9. These events followed the 8.8 magnitude quake in 1797, and the 9.0 magnitude quake in 1833, but they’ve only ruptured a fraction of the area ruptured by the 1833 earthquake.

As scientists explained, the 2007 events were not at all a surprise. Based on coral reef patterns, they were able to predict that the young population of Sumatra is likely to experience another big earthquake in their lifetime.

Corals have given scientists a lot of details about the ongoing and possible seismic activity in the area. In 2007 for example, the two earthquakes that occurred at a 12-hour time difference came as no surprise, considering the area here (and therefore the corals) had not suffered any major rupture for over 150 years.

According to scientists, corals tend to grow outwards as sea levels are falling, and fossilized corals here hold evidence of the major ruptures taking place in the Mentawai region in 1797 and 1833. Considering the pressure that has accumulated here since 1833, the Sumatra Island will be hit by at least an 8.8 earthquake.