The 2015 M7.1 Earthquake on the Southeast Indian Ridge

On December 4, 2015, at 22:25 UTC, a magnitude 7.1 earthquake struck the Southeast Indian Ridge at a depth of 35 km. This event occurred in a remote oceanic setting and was followed shortly by another M7.1 shock approximately 21 km away, highlighting clustered seismic release along the ridge axis. The Southeast Indian Ridge forms a divergent boundary separating the Antarctic Plate from the Indo-Australian Plate. It extends from the Rodrigues Triple Junction in the west to the Macquarie Triple Junction in the east, with a full spreading rate of roughly 6–7 cm per year. The ridge axis is segmented by transform faults and exhibits typical mid-ocean ridge morphology, including axial valleys and volcanic constructs. Seismicity here is driven by normal faulting associated with plate separation and occasional strike-slip motion along transforms. Geologically, the ridge originated during the Cretaceous breakup of Gondwana and has recorded continuous seafloor spreading since the Eocene. Magnetic anomaly patterns document symmetric crustal accretion, with the ridge migrating northward relative to the Antarctic Plate over time. Hydrothermal activity and basaltic volcanism are persistent features, contributing to the oceanic crust that underlies much of the southern Indian Ocean. The 2015 events fit within the ridge’s long-term pattern of moderate-to-large earthquakes. Since 2000, several M7+ shocks have occurred along the same segment, underscoring ongoing tectonic extension. Depths around 35 km remain within the brittle upper lithosphere of young oceanic crust, where cooling allows stress accumulation before failure. No significant tsunami or damage resulted from these offshore events, consistent with their remote location and focal mechanisms. Continued monitoring by global seismic networks provides data that refine models of ridge segmentation and plate motion.

References

USGS Earthquake Catalog Global Centroid Moment Tensor Project Marine Geoscience Data System (seafloor spreading records)