The M7.1 Earthquake on the Southeast Indian Ridge, December 2015

The Southeast Indian Ridge forms a major divergent plate boundary separating the Antarctic Plate from the Australian Plate in the southern Indian Ocean. This mid-ocean ridge system extends more than 2,000 kilometers and exhibits typical characteristics of seafloor spreading, including axial volcanic ridges, transform faults, and hydrothermal activity. Spreading rates along the ridge average 6–7 centimeters per year, producing new oceanic crust through episodic magmatic and tectonic processes. On 4 December 2015 at 22:24 UTC, a magnitude 7.1 earthquake struck the ridge at a focal depth of 10 kilometers. The event occurred within a well-documented segment of the ridge characterized by active transform faulting and normal faulting associated with plate divergence. Seismic records indicate that the rupture involved strike-slip motion along a transform segment, consistent with the regional tectonics where the ridge is offset by numerous fracture zones. This earthquake belongs to a sequence of strong events recorded on the Southeast Indian Ridge since 2000. Another magnitude 7.1 event occurred on the same day approximately 21 kilometers away, highlighting the capacity of the ridge to host multiple large ruptures in close spatial and temporal proximity. Such clustering reflects the segmented nature of the ridge axis, where stress is released along discrete fault systems rather than in a single continuous rupture. Mid-ocean ridge earthquakes of magnitude 7 or greater remain relatively infrequent compared with subduction zones, yet they provide critical data on lithospheric strength and fault mechanics at spreading centers. The 2015 events occurred in a region with low sediment cover, allowing efficient seismic wave propagation and clear recording by global networks. Post-event analysis confirmed no significant tsunami generation, as expected for an oceanic strike-slip source at moderate depth. Geological studies of the Southeast Indian Ridge emphasize its role in the ongoing separation of Australia and Antarctica, which began approximately 80 million years ago. Magnetic anomaly patterns document symmetric spreading since the Eocene, with the ridge maintaining a steady configuration despite occasional ridge jumps and propagations. The 2015 seismicity aligns with this long-term tectonic framework, underscoring the persistent activity along the plate boundary. Continued monitoring of the Southeast Indian Ridge contributes to improved understanding of oceanic crustal formation and the seismic hazard potential of remote mid-ocean settings. Future deployments of ocean-bottom seismometers may refine hypocentral locations and reveal finer details of aftershock sequences following large ridge events.

References

United States Geological Survey Earthquake Catalog Global Centroid Moment Tensor Project Database National Oceanic and Atmospheric Administration Ocean Floor Data Center