In a new study published in the journal Solid Earth, scientists confirmed the existence of a giant fault under Sulawesi Island (Indonesia), continuously extending from the mainland to the outer sea through the Celebes Sea. This discovery clarifies the direct link between earthquakes on land and seabed deformation - a key factor causing tsunamis.
Previously, this fault was mainly seen as operating on land. However, new research shows that it continues to extend deep under the seabed, forming a much larger geological system than previously perceived.
Prolonged breaks to the sea are of particular importance. When an earthquake occurs, not only the ground shakes, but the seabed can also be raised or subsided, directly triggering tsunamis.
This helps to explain more clearly the disaster of 2018 in Palu Bay, when a tsunami more than 10m high suddenly hit the shore even though the main fault mainly slid horizontally - a type of movement that usually causes few tsunamis.
The new discovery shows that the offshore fault may have contributed to the deformation of the seabed, creating larger waves than expected. Although not fully explaining the mechanism, it has removed an important "blind spot" in previous research.
Seismological data shows a clear difference in the Earth's crustal thickness under the Celebes Sea. Some areas are only about 8km thick, while others can be up to more than 30km.
This difference shows that Earth's shell blocks with very different properties are colliding and sliding through each other at the fault zone. This is where stress easily accumulates and releases suddenly, increasing the risk of strong earthquakes.
Scientists also discovered a "low-speed" zone - where earthquake waves travel slower - signs of rock cracking or containing fluid. Such areas are often weak spots, where breaks are prone to activation in the future.
One of the worrying points is that the slippage speed of this fault is relatively fast, about more than 3cm per year. This means that energy can accumulate silently for a long time before being released by a large earthquake.
In the 2018 earthquake, the fault spread at a "supershear" speed - faster than seismic waves, causing widespread damage.
Experts warn that the region may look peaceful for a long time, but in fact is accumulating great risks under the surface.
The new discovery poses an urgent requirement to update disaster risk maps, not only on land but also in offshore near-shore areas. This is especially important for coastal cities, seaports and residential areas located around narrow bays - where tsunamis can rise rapidly.
Strengthening the seabed monitoring system is also considered key to improving early warning capabilities and minimizing damage when disasters occur.
