Source Mechanism of the Mw 6.4 Nicobar earthquake of July 28, 2025: constraints from GNSS and strong motion accelerograph observations

Deep beneath the surface of the Earth, massive tectonic plates grind against one another, storing energy that is occasionally released in sudden, violent shifts known as earthquakes. In the Indian Ocean, the boundary where the India plate dives beneath the Sunda plate creates a zone of intense geological activity. When these plates slip, they can generate powerful tremors that shake the ground and, in some cases, displace vast amounts of water to create tsunamis. For scientists, understanding exactly how a fault moves during an earthquake—whether it is a thrust, where one block pushes up over another, or a strike-slip, where blocks slide past each other—is critical. This knowledge helps them gauge the potential for a tsunami and assess the danger to nearby coastal communities. While seismometers, which record ground shaking, have long been the primary tool for studying these events, they have a limitation: near a powerful quake, the shaking can be so intense that the instruments become overwhelmed and stop recording useful data. In recent years, a different technology has emerged as a powerful partner to seismometers. Global Navigation Satellite Systems, or GNSS, which include the GPS we use for navigation, can now measure the ground's movement in real-time with high precision. Unlike seismometers, these satellite receivers do not saturate or break under intense shaking; instead, they track the exact position of the ground as it moves, offering a clear picture of the deformation even when traditional sensors fail.

On July 28, 2025, a significant earthquake with a magnitude of 6.4 struck the waters west of Great Nicobar Island in the Andaman and Nicobar archipelago. This region, already scarred by the devastating 2004 tsunami, remains a place of high seismic activity. The 2025 event occurred in the frontal part of the subduction zone, roughly 100 kilometers from the nearest town, Campbell Bay. Fortunately, the quake did not generate a tsunami or cause damage to the nearby settlements. However, for scientists, the event presented a unique challenge and opportunity. The area has very few local seismometers; the nearest reliable station was over 500 kilometers away, and only a handful existed within a thousand kilometers. This scarcity meant that standard methods for determining the earthquake's source mechanism—the specific geometry and direction of the fault slip—were difficult to apply using local data alone. To solve this puzzle, a team of researchers turned to a sparse network of high-speed GNSS stations and strong-motion accelerometers scattered across the islands. These instruments, operated by the Survey of India and the Indian National Centre for Ocean Information Services, recorded the event at a rate of one measurement per second, capturing the dynamic movement of the ground as the earthquake waves passed through.

The researchers combined data from four GNSS sites and five accelerometer stations to reconstruct the earthquake's source. They faced a significant hurdle: the stations were not evenly distributed around the epicenter. Instead, they were clustered mostly to the north and northeast, leaving a massive gap in observations from the other directions. In a typical scenario, such a large gap in coverage would make it nearly impossible to determine the fault's orientation with confidence. The team processed the raw data to filter out noise and isolate the ground displacement caused by the quake. They then used a computer modeling technique to simulate how the ground would move for different types of fault slips, comparing these simulations to the actual recordings. The goal was to find the model that best matched the real-world data. Despite the uneven distribution of sensors, the analysis revealed a clear picture. The earthquake was caused by thrust motion, where one block of the Earth's crust pushed up and over another along a gently sloping plane that runs parallel to the island arc. This finding aligned with the general understanding of the region's geology and matched estimates from global earthquake catalogs, even though those global estimates relied on data from much farther away.

One of the most striking aspects of the study was the ability of the GNSS data to stand on its own. When the researchers analyzed the satellite data separately from the ground-shaking sensors, they arrived at the same conclusion regarding the fault's movement. The GNSS stations, which measure position rather than acceleration, successfully captured the direction and nature of the slip. This is a significant validation for the use of satellite technology in seismology, particularly in remote or oceanic regions where installing a dense network of traditional seismometers is difficult or impossible. The study also explored the depth of the earthquake. While global agencies estimated the depth to be between 22 and 29 kilometers, placing the event on the main boundary where the two plates meet, the local data suggested a shallower depth, ranging from 6 to 10 kilometers. This shallower depth implies the quake might have occurred on a secondary fault branching off the main boundary, rather than on the main interface itself. The difference highlights how local data can offer a different perspective, though the lack of sensors in all directions means the exact depth remains somewhat uncertain.

The magnitude of the earthquake was also a point of discussion. The local analysis suggested a magnitude between 6.8 and 6.9, slightly higher than the 6.4 reported by global agencies. The researchers noted that this discrepancy likely stems from the limited number of sensors and the uneven angles from which they viewed the event, which can sometimes lead to an overestimation of the energy released when using local data alone. Nevertheless, the core finding regarding the mechanism of the quake remained robust. The study demonstrated that even with a sparse network of instruments and a large gap in coverage, high-rate GNSS data can reliably determine how an earthquake of this size occurs. This is a vital capability for the Andaman and Nicobar region, where the threat of future tsunamis is ever-present. By proving that satellite data can fill the gaps left by missing seismometers, the research offers a new, reliable tool for rapidly assessing the nature of future earthquakes. If a large event were to strike again, these satellite networks could provide immediate insights into the fault's movement, helping authorities determine the tsunami risk faster and more accurately than ever before.