Segmentation of the Andaman–Sumatra forearc and implications for great-earthquake rupture

The ocean floor is not a single, solid shell but a jigsaw puzzle of massive plates that slowly grind against one another. Where these plates collide, one often slides beneath the other in a process called subduction. This grinding creates immense friction and stores energy over centuries, which is eventually released in the form of earthquakes. When these quakes happen under the ocean, they can displace huge volumes of water, triggering tsunamis that travel across entire oceans. The region where the Indian Ocean meets the islands of Sumatra and the Andaman Sea is one of the most active and dangerous places on Earth. It is here that the plates move at an angle, or obliquely, to the coastline. This angle splits the movement into two parts: one pushing the plates together and another sliding them sideways. Scientists have long tried to understand how this complex motion is shared between the deep fault lines and the crust above them, and why the massive 2004 earthquake, which killed over 200,000 people, behaved so differently from one end of the rupture to the other.

For decades, researchers assumed that the entire stretch of ocean floor from the southern tip of Sumatra up to the Andaman Islands acted as a single, rigid piece of crust. They believed that the 2004 earthquake was a single, continuous event that broke this entire section at once. However, a new study by a team of geoscientists challenges this long-held view. By combining precise measurements of how the ground moves today with the directions of past earthquake slips, the researchers discovered that this assumption was incorrect. They found that the crust in this region is actually divided into two distinct, moving pieces, separated by a hidden boundary near the Nicobar Islands. This discovery changes how we understand the mechanics of the 2004 disaster and reveals a previously overlooked fault line that could pose a significant future threat.

The team built a detailed model of the region, stretching from southern Myanmar down to Java. They fed their computer model two types of real-world data: the slow, steady drift of the land measured by satellites, and the specific directions in which the ground shifted during thousands of recorded earthquakes. The model worked like a puzzle solver, trying to find the arrangement of moving blocks that best explained all these observations. The results were clear: a single, continuous block of crust could not fit the data. Instead, the model required a break in the crust near the Nicobar Islands, a location the authors call the "Nicobar Neck." South of this point, the crust moves in one direction and at one speed, while north of it, the crust moves differently. The 2004 earthquake did not simply break a uniform plate; it was a compound event that jumped across this hidden boundary, rupturing two separate microplates that were being loaded with stress in different ways.

This boundary is not just a mathematical result; it is supported by physical evidence hidden in the Earth's gravity. The researchers analyzed the gravitational pull of the crust in this area and found a distinct low point right where their model placed the boundary. This low gravity suggests the rock here is less dense than the surrounding areas, likely because it is fractured, filled with fluids, or chemically altered. It acts as a weak spot, a zone where the crust is mechanically weaker and easier to break. This weakness helps explain why the 2004 earthquake changed its character as it moved north. In the southern part of the rupture, the ground moved mostly straight toward the trench, but as the rupture crossed the Nicobar Neck, the movement became more sideways and the energy release became less intense. The boundary acted as a natural divider, separating the two distinct behaviors of the earthquake.

Perhaps the most surprising finding concerns a fault line that runs offshore, parallel to the Sumatran Fault on land. The model shows that this underwater fault, which the researchers call the Andaman–Nicobar Fault, is moving at a rate of roughly 39 millimeters per year. This is twice as fast as the famous Sumatran Fault on the island itself. Despite moving so quickly, this fault has never produced a large, recorded earthquake. This creates a mystery: where is the energy going? The fault could be slipping slowly and silently, a process known as creep, which releases stress without shaking the ground. Alternatively, the movement could be spread out across many smaller, unmapped cracks. Or, the fault could be locked tight, silently building up a massive amount of strain that has not yet been released. If the fault is locked, it represents a significant, uncharacterized hazard. A sudden rupture here could generate a local tsunami, as the shallow water of the Andaman Sea would amplify the effects of any vertical movement.

The implications of this study extend far beyond academic curiosity. For years, hazard models for this region have treated the entire margin as a single unit, assuming that a massive earthquake could rupture the whole length at once. This new framework suggests that the system is more complex. The 2004 event was not an anomaly that broke the rules; it was a demonstration of how the system actually works, crossing a boundary that had gone unnoticed. The existence of the Nicobar Neck means that the stress building up in the south is not necessarily the same as the stress building up in the north. It also highlights the danger of the offshore Andaman–Nicobar Fault, which carries a huge amount of movement but has been largely ignored in safety planning. By recognizing that this region is made of two distinct moving parts, scientists can now build better models to predict where the next great earthquakes might happen and how large they could be. The 2004 tsunami was a tragedy that reshaped our understanding of the world, but this new research suggests that even after two decades, there are still fundamental secrets in the deep earth waiting to be uncovered.