GPS measurements show the fault is locked and loading. Deforming the seafloor at 40mm/year, equivalent to compressing a car-sized rock by 1 meter every 25 years.
USGS, Public domain, via Wikimedia Commons
A locked tectonic spring
Beneath the Mentawai Islands off the coast of West Sumatra, a section of a massive fault has been silent for too long. This is the Mentawai segment of the Sunda megathrust, the 5,500-kilometer-long fault where the Indo-Australian Plate pushes beneath the Sunda Plate. While the plates are converging at a rate of about 49-60 mm per year, this particular segment is not moving smoothly. Instead, it is "locked" by friction.
This locking causes immense strain to build up. Geodetic measurements from a network of GPS stations show the overriding plate is being squeezed and warped. The islands themselves are slowly subsiding at rates of around 5-11 mm per year as the tectonic spring gets compressed. This ongoing deformation builds up a "slip deficit"—the amount of movement that should have happened but hasn't. Scientists estimate that a potential slip of about 10 meters has accumulated since the last major rupture in 1833. This stored energy will eventually be released in a massive earthquake. The section of the fault beneath Siberut Island is of particular concern, as it has not ruptured significantly since 1797.
Reading the coral record
The history of this fault is written in the region's coral microatolls. These disc-shaped corals grow upwards to the lowest level of the tides. During an earthquake, if the seafloor suddenly lurches upwards, a portion of the coral is lifted out of the water and dies, leaving a permanent ring. If the land subsides, the coral will grow upwards to catch the new sea level. These growth patterns record tectonic movements going back centuries.
The coral record for the Mentawai segment reveals a history of "supercycles"—periods of strain accumulation that end in a series of major earthquakes every 200 years or so. The last major sequence of events to rupture this area occurred in 1797 and 1833. The 1797 event had an estimated magnitude of 8.4, and the 1833 earthquake was even larger, estimated between M 8.8 and 9.2. Analysis of coral uplift shows the 1833 event involved a slip of about 12 meters along the fault.
Having been over 200 years since the 1797 event, the accumulated strain is now at or beyond the level that was released during that earthquake. Scientists consider the Mentawai seismic gap one of the world's most hazardous, with the potential to generate an earthquake of magnitude 8.8 or greater. Such an event would also produce a major tsunami that could inundate the coast of West Sumatra.
💡Fun Facts
The 2004 Indian Ocean earthquake, one of the most powerful ever recorded, ruptured a different segment of the same Sunda megathrust to the north.
Coral microatolls act as precise natural recorders, allowing scientists to map vertical land movements from past earthquakes with astonishing accuracy.
The 1833 earthquake caused the ground in the city of Padang to shake for three minutes, while shaking in Bengkulu lasted for five minutes.
The network of GPS stations monitoring the fault, part of the Sumatra GPS Array (SuGAr), can detect coseismic and postseismic deformation of just a few centimeters.
This is an underwater geological feature. Its effects are studied through GPS data and the geology of the Mentawai Islands. Reaching the islands requires ferry travel from Padang and then local boats, often in remote and rugged conditions.
