A Locked Plate Boundary
Off the coast of Sumatra, the Indo-Australian tectonic plate dives beneath the Sunda plate in a process called subduction. This massive fault, the Sunda megathrust, is one of the most seismically active structures on Earth. Along this 5,500-kilometer boundary, a 700-km-long section below the Mentawai Islands has been identified as a "seismic gap." This means the fault here is locked, accumulating stress instead of releasing it through steady creep or smaller earthquakes.
The last time this specific segment ruptured in a major earthquake was on February 10, 1797. That event, estimated at a magnitude (Mw) of 8.6–8.8, caused widespread damage and generated a tsunami with a run-up of 5 to 10 meters near the city of Padang. The surge was powerful enough to drive a 200-ton English sailing ship a full kilometer inland up the Arau River. Another massive earthquake in 1833 (Mw 8.9-9.1) ruptured an adjacent segment to the south, but the main Mentawai patch has remained largely silent, building up strain for over 225 years.
Measuring the Strain
Scientists monitor the strain accumulation using a network of high-precision GPS stations. These instruments show the Mentawai Islands are being dragged toward the submarine trench at a rate of about 15 mm per year. The overall convergence between the Australia and Sunda plates here is about 60 mm per year. Calculations based on the trench-normal convergence rate of 46.7 mm/year suggest that a potential slip of nearly 10 meters has accumulated since the 1797 event. If this accumulated strain were released at once across the entire locked segment, it could generate an earthquake with a magnitude of 8.8 or higher.
A series of large earthquakes in September 2007, including an Mw 8.4 and an Mw 7.9, ruptured the southern part of the Mentawai segment. Analysis of ground displacement and uplift showed these events only released a fraction of the total accumulated energy, leaving the northern, larger portion of the gap intact and still under immense stress.
A 200-Year Cycle in Coral
The history of this fault is written in the region's coral microatolls. These corals grow up to the low-tide level, and their flat tops are sensitive natural recorders of sea-level change. During the long periods between earthquakes (the interseismic period), the overriding plate is dragged down, causing the islands to slowly subside and the corals to grow upward to keep pace. When a megathrust earthquake occurs, the islands suddenly uplift, killing the top layer of the coral and creating a distinct marker in its growth rings.
By studying ancient and living corals, scientists have uncovered a pattern of major seismic upheavals followed by long periods of quiet strain accumulation. These paleoseismic records show that this section of the megathrust has generated sequences of great earthquakes roughly every 200 years for at least the last 700 years. This historical record confirms that the Mentawai seismic gap is in the late stage of its seismic cycle, suggesting a future large rupture.