Seismic Swarm S20130109.1 at Carlsberg Ridge

The Carlsberg Ridge forms a key segment of the Central Indian Ridge system in the northwestern Indian Ocean, marking the divergent boundary between the Indian and Somali tectonic plates. This slow-spreading mid-ocean ridge extends roughly 2,200 kilometers from the Owen Fracture Zone in the north to the equator, with spreading rates averaging 2–3 centimeters per year. Geological studies confirm its role in accommodating extensional forces that have shaped the Indian Ocean basin since the breakup of Gondwana approximately 130 million years ago. Magnetic anomaly patterns along the ridge document symmetric seafloor spreading, while hydrothermal vent fields and basaltic volcanism reflect ongoing mantle upwelling.

Seismic activity is inherent to the ridge due to normal faulting and episodic dike intrusions. Historical records show recurrent earthquake swarms linked to these processes, with notable events in the 20th century highlighting the ridge’s contribution to regional plate motion. Updated geophysical surveys indicate persistent low-level seismicity, underscoring the dynamic nature of the lithosphere-asthenosphere boundary beneath the feature.

Swarm S20130109.1 was recorded along the Carlsberg Ridge beginning at 22:35 on 8 January 2013 and concluding at 21:20 on 9 January 2013. Over 22 hours and 44 minutes, 32 earthquakes were detected, all with focal depths between 9 and 15 kilometers. Magnitudes ranged from 4.1 to 5.0, with the strongest events reaching 5.0 at 05:27 on 9 January and 4.9 at both 04:20 and 04:46 the same day. The sequence exhibited a typical swarm pattern: an initial cluster of moderate events followed by sustained activity that peaked midday before tapering.

Insight from the event timings reveals a progressive migration of energy release consistent with fluid migration or magmatic intrusion along ridge-axis faults. Early events clustered near magnitude 4.4–4.7 within the first two hours, succeeded by a lull and then a burst of five events above 4.7 between 04:20 and 06:50. Afternoon activity remained steady at magnitudes 4.4–4.8, with the final recorded shock of 4.8 at 21:20. Depths remained shallow throughout, aligning with the brittle-ductile transition zone expected at this slow-spreading center.

Such swarms provide valuable data on ridge segmentation and stress transfer. The 2013 sequence exemplifies how short-term clustering can illuminate transient tectonic or volcanic processes without producing a single dominant mainshock-aftershock signature. Continued monitoring of the Carlsberg Ridge remains essential for refining models of Indian Ocean plate kinematics.

References
United States Geological Survey Earthquake Catalog
National Oceanic and Atmospheric Administration Marine Geology Data
Peer-reviewed literature on Central Indian Ridge tectonics (post-2013 updates)