Science extends our senses

We can't see the slow, gradual movements of the Earth over decades or centuries leading up to a large earthquake. To detect these movements, scientists use tools such as GPS and coral records, much like mobile phones allow us to hear the voice of someone thousands of kilometers away.

GPS measures the current movements of the plates of the Earth

A GPS receiver is a device on the ground that receives signals from satellites and uses them to calculate its position on the Earth.

There is a network of high-precision GPS receivers installed across Sumatera and the western islands; these receivers show scientists how much the rocks around the major faults are moving. GPS measurements show that the segment of the Sunda megathrust along North Pagai, Sipora, and Siberut Islands is locked (stuck) and continuing to build up energy toward a great earthquake.

GPS site near Air Bangis, West Sumatra

GPS receivers:

• are not earthquake or tsunami detection devices
• will not sound any warnings
• cannot stop earthquakes
• cannot cause earthquakes

However, GPS receivers do help scientists to find which areas are more or less at risk of large earthquakes.

Corals record earthquake history

Coral that was uplifted in the 2005 Nias earthquake

On the islands west of Sumatra, the west coasts of the islands move very slowly downward in the centuries between large earthquakes. During a large earthquake itself, the west coasts of the islands suddenly spring back up, sometimes by many meters.

(click image for larger version)

1. Corals grow up to the low-tide level.
2. If the land under a coral is slowly sinking, the coral grows upward to stay at that low-tide level.
3. In an earthquake, the coral is lifted out of the water, the part of the coral above the new low-tide level dies.
4. A single coral may show evidence of many earthquake cycles.

Scientsts have looked at thousands of individual corals throughout the islands west of Sumatra. This history shows that the Sunda megathrust near the Mentawai Islands has a series of large earthquakes about every ~200 years, the last series happened about 200 years ago, and each series lasts for a few decades.

The 2007 Bengkulu earthquake was the beginning of the modern series of large earthquakes; sometime in the next few decades, one or more large earthquakes are likely to happen near the Mentawai Islands.

Science extends our senses

Our eyes and ears are good at warning us of immediate dangers, like a truck approaching on a road that we are trying to cross, or dangerous wild animals. But we can't see the slow, gradual movements of the Earth over decades or centuries leading up to a large earthquake. To extend our senses to detect these movements, scientists use tools such as GPS and coral records, much like mobile phones allow us to hear the voice of someone thousands of kilometers away.

GPS measures the current movements of the plates of the Earth

GPS (Global Positioning System) satellites orbit the Earth, each sending a distinct radio signal down to Earth. A GPS receiver is a device on the ground that receives these signals and uses them to calculate its position on the Earth.

There is a network of high-precision GPS receivers installed across Sumatera and the western islands; these receivers show scientists how much the rocks around the major faults are moving. GPS measurements show that the segment of the Sunda megathrust along North Pagai, Sipora, and Siberut Islands is locked (stuck) and continuing to build up energy toward a great earthquake.

GPS site near Air Bangis, West Sumatra

GPS receivers:

• are not earthquake or tsunami detection devices
• will not sound any warnings
• cannot stop earthquakes
• cannot cause earthquakes

However, GPS receivers do help scientists to find which areas are more or less at risk of large earthquakes.

Corals record earthquake history

Coral that was uplifted in the 2005 Nias earthquake

Because the rocks bend as they are deformed near the Sunda megathrust, some parts of the islands west of Sumatra move up and down. In general, the west coasts of the islands move very slowly downward in the centuries between large earthquakes. During a large earthquake itself, the west coasts of the islands suddenly spring back up, sometimes by many meters, while the east coasts of the islands can move downward a little bit. These up-and-down movements affect the growth of corals.

(click image for larger version)

1. Corals grow up to the low-tide level.
2. If the land under a coral is slowly sinking, the coral grows upward to stay at that low-tide level.
3. In an earthquake, the coral is lifted out of the water, the part of the coral above the new low-tide level dies.
4. A single coral may show evidence of many earthquake cycles.

Scientists have looked at thousands of individual corals throughout the islands west of Sumatra, searching for these tell-tale signs of ancient earthquakes. They have combined their observations to produce a history of large Sumatran earthquakes going back more than 700 years.

This history shows that the Sunda megathrust near the Mentawai Islands goes through ~200-year earthquake cycles: the rocks slowly bend near the fault for a few centuries, then release that built-up energy through a series of large earthquakes at different sections of the fault within usually just a few decades.

The cycle then repeats, as another few centuries of slow plate movement eventually bends the rocks enough to again set off another sequence of large earthquakes.

The 2007 Bengkulu earthquake was the beginning of the modern series of large earthquakes; sometime in the next few decades, one or more large earthquakes are likely to happen near the Mentawai Islands.