The Sunda Megathrust's silent secret
Off the coast of Bengkulu, the Indo-Australian Plate dives beneath the Sunda Plate at a rate of about 60 millimeters per year. This process, known as subduction, builds up immense stress in the Earth's crust. This is the Sunda Megathrust, a massive fault that has produced some of the most powerful earthquakes ever recorded, including the devastating 2004 Mw 9.2 event and the 2007 Mw 8.4 Bengkulu earthquake. These events release centuries of accumulated strain in a matter of minutes.
However, the megathrust also has a quieter, slower way of releasing energy. Instead of sudden, violent ruptures, some sections of the fault move in what are called slow-slip events (SSEs). These are essentially earthquakes in slow motion. They release a comparable amount of energy to large earthquakes—sometimes equivalent to a magnitude 6.5 or greater—but they do so over weeks, months, or even years. This gradual release means no seismic waves are generated, so the ground does not shake and the events are completely imperceptible to humans. They are silent earthquakes, detectable only by meticulously tracking the subtle deformation of the Earth's surface with high-precision GPS arrays.
Reading the rocks and corals
Detecting these faint movements is a major challenge, especially in a region with such high seismic activity. The constant deformation from numerous smaller quakes and their afterslip can easily mask the millimeter-scale signature of an SSE. In fact, a decade-long analysis of data from the Sumatran GPS Array (SuGAr) found no definitive evidence of ongoing slow-slip events, suggesting they may be too small or infrequent for the current network to resolve.
Geological records, however, tell a different story. By studying the growth patterns of coral microatolls in the Banyak Islands, scientists have uncovered evidence of an astonishingly long slow-slip event. Between 1966 and 1981, the coral records show a 15-year period where the ground, which had been subsiding, reversed direction and began to rise. This subtle uplift was the surface expression of a massive, 15-year-long silent earthquake on the Sunda megathrust below. Another study of corals near Simeulue island revealed a 32-year-long SSE during the mid-1800s, which culminated in a powerful Mw 8.5 earthquake in 1861. These findings show that the fault experiences a wide spectrum of slip behaviors and that stress can be redistributed in complex ways, potentially loading adjacent locked sections of the fault and lead to future destructive earthquakes.