Two earthquakes near Antarctica reveal activity along an ancient fault

• In 2025, earthquakes of magnitude 7.5 and 7.6 struck the Drake Passage, with epicenters 15 km apart
• Researchers linked the quakes to an ancient fault zone where strain continues to accumulate
• The discovery refines our understanding of seismic hazards, but it does not mean that all old faults are dangerous or that the next quake can be predicted

Two powerful earthquakes near the Drake Passage

In 2025, two powerful earthquakes of magnitude 7.5 and 7.6 struck the Drake Passage, between South America and Antarctica. Their epicenters were just 15 kilometers apart, and the events occurred 49 days apart. An international team of researchers found that the quakes struck an area containing an ancient tectonic structure that had previously been considered inactive. This links powerful earthquakes not only to present-day plate boundaries.

The findings were published on October 30, 2026, in the journal Communications Earth & Environment. The study was led by Emanuele Lodolo of the Italian National Institute of Oceanography and Applied Geophysics. The authors studied both earthquakes and compared the locations of their epicenters with an underwater geological structure. The match points to an ancient transform fault: it formed under a different tectonic setting, but, according to the study, strain continues to accumulate in this zone.

The main takeaway concerns the assumption that when a plate stops moving, its old faults also become permanently inactive. The study presents a more complex picture: a structure may cease to be a boundary between moving plates yet retain its mechanical weakness. Under certain conditions, that weakness can once again become important in the accumulation and release of stress. The authors do not claim that every ancient fault will inevitably cause an earthquake; rather, they point to a possibility that needs to be taken into account.

How an ancient structure survived in Earth's crust

Earth's surface is divided into tectonic plates that move slowly over the mantle. Stress builds up in rocks along their boundaries and is released from time to time in the form of earthquakes, which is why these areas are considered the main zones of seismic activity. However, today's map of the plates does not reflect the full geological history. Over millions of years, some plates were subducted beneath others, while others became part of larger tectonic systems, leaving structural traces behind.

One such structure is associated with the ancient Phoenix Plate, remnants of which lie near the Antarctic Peninsula. Researchers found that the earthquake epicenters coincided with the location of an old transform fault. Such faults form where sections of oceanic crust move in different directions and slide past one another. When this movement stops, oceanic fracture zones remain in its place. They have traditionally been considered tectonically inactive, but the new study calls the simplicity of this classification into question.

To reconstruct the structure of the area, the team combined seismic data with geophysical information about the seafloor. In particular, the scientists used seismic reflection profiles: waves travel underground, reflect off geological layers, and return, making it possible to study subsurface structures. This approach allowed them to compare the earthquakes with features of the ancient fault zone. The data indicate that deformation is still occurring there, even though the structure formed in the past and was not considered an active plate boundary.

Why an ancient rift may once again concentrate stress

Researchers propose two factors that may explain the renewed significance of the ancient rift. The first is its inherited geometry: features of the structure’s shape may concentrate stress in certain places. The second is the present-day interaction between the Scotia Plate and the Antarctic Plate. The forces they transmit may also affect more distant areas. In this way, an ancient fracture formed under different conditions has become part of the current stress system, even though it is not an active plate boundary.

The authors also found evidence that volcanic materials can penetrate the rock along weakened structures. They further suggest that the ancient rift now acts as a major shear zone within the part of the Phoenix Plate that continues to subduct slowly beneath the South Shetland Islands. These findings describe a possible role for the old structure in the modern movement of the crust. They do not mean that the study established every detail of the processes underway or determined the exact timing of future earthquakes.

An important implication of the work is the idea of geological “memory” in Earth’s crust. Changes in tectonic structure do not automatically erase pre-existing faults and weak zones. They can continue to influence the distribution of deformation even after the original movement has stopped. In the Drake Passage region, this idea helps explain the connection between major earthquakes and an ancient zone. But the age of a fault alone does not show whether it will be reactivated: current stresses, orientation, and the structure’s mechanical properties all matter.

How the discovery changes seismic hazard assessment

Seismic hazard models usually focus particularly on active plate boundaries, since strong earthquakes often occur there. However, events can also occur within a plate—these are called intraplate earthquakes. The work in the Drake Passage shows that an ancient oceanic fault zone can contribute to strong earthquakes even if it no longer serves as an active boundary between plates. Therefore, analyses of seismic risk should not be limited to present-day plate boundaries.

The practical significance of the findings is that the geological inheritance of oceanic crust needs to be taken into account more carefully when assessing where seismic energy may accumulate and be released. The study does not, however, claim that all old fractures are dangerous. Whether they can become active again depends on a combination of factors, including the current stress state, fault orientation, and rock properties. The findings broaden our understanding of where seismic sources might be located, but do not justify applying conclusions about a specific area to every ancient structure.

The study also cannot determine when the next earthquake will occur in the Drake Passage region. The authors emphasize that reactivation depends on current stresses and the characteristics of a particular structure, rather than offering a calendar-based forecast. It also remains unclear how widely this mechanism operates in other regions: the data presented concern the area studied near Antarctica. The reliable general conclusion is more modest, but important: tectonic history can continue to influence crustal behavior even after the former plate boundary has disappeared.

Source: UNIAN