The Carlsberg Ridge: Tectonic Setting and the 2003 Magnitude 7.6 Earthquake

The Carlsberg Ridge forms a key segment of the global mid-ocean ridge system in the northern Indian Ocean, specifically within the Arabian Sea. This divergent plate boundary separates the Indian Plate to the east from the Somali Plate to the west, accommodating seafloor spreading at rates of approximately 2–3 cm per year. As part of the larger Central Indian Ridge, the Carlsberg Ridge trends northwest-southeast and is characterized by a rugged axial valley, frequent transform offsets, and basaltic volcanism typical of slow-spreading ridges. Geologically, the ridge originated from the breakup of the supercontinent Gondwana and the subsequent opening of the Indian Ocean, with seafloor spreading initiating around 60–70 million years ago. Magnetic anomaly patterns along the ridge record symmetric spreading and provide evidence of changes in plate motion over time, including adjustments linked to the India-Eurasia collision. The region remains seismically active due to ongoing extension, with most events occurring along the ridge axis or associated fracture zones. Hydrothermal activity and associated mineral deposits have also been documented, reflecting heat flow from the underlying mantle. On 15 July 2003 at 20:27 UTC, a magnitude 7.6 earthquake struck directly on the Carlsberg Ridge at a focal depth of 10.0 km. This event represents the strongest earthquake recorded in the region since 1 January 2000. The shallow depth and location on the divergent boundary are consistent with normal faulting driven by plate separation. No other events of comparable magnitude have occurred in the immediate vicinity since that date according to available records. The 2003 earthquake underscores the persistent tectonic forces shaping the ridge. Such events contribute to the incremental widening of the ocean basin and can trigger aftershock sequences along adjacent ridge segments. Monitoring by global seismic networks continues to track activity, aiding in the refinement of plate-motion models for the northwest Indian Ocean.

References

United States Geological Survey Earthquake Catalog National Oceanic and Atmospheric Administration Marine Geology Data International Seismological Centre Event Bulletin