Seychelles

The Seychelles is a remarkable geological entity — the only mid-ocean archipelago in the world that sits on a fragment of ancient continental crust rather than oceanic crust or volcanic hotspot material. The main granitic islands (Mahé, Praslin, La Digue) are composed of Precambrian granite approximately 750 million years old, representing a microcontinent that split off from India during the opening of the Indian Ocean roughly 65 million years ago. These ancient granites are geologically stable and produce very little seismic activity.

The outer islands of the Seychelles — the Amirantes, Farquhar, and Aldabra groups — are low coral atolls sitting on the Indian Ocean oceanic crust, geologically distinct from the granitic inner islands. These low-lying atolls face tsunami risk from distant sources rather than local seismicity.

Instrumentally recorded seismicity in the Seychelles is very low. The ancient granitic basement has been mechanically welded for hundreds of millions of years, and the modern stress field of the Indian Ocean plate — driven by spreading at the Carlsberg Ridge to the north and the Southwest Indian Ridge to the south — does not generate significant intraplate earthquakes near the Seychelles.

The most significant seismic hazard faced by the Seychelles is not from local earthquakes but from distant large tsunamis. The 2004 Indian Ocean [[tsunami]], generated by the magnitude 9.1 Sumatra earthquake, reached the Seychelles and caused wave runup of 3 to 5 metres on some islands, damaging coastal infrastructure and killing approximately two people. This event demonstrated that the low-lying outer islands are particularly vulnerable to long-distance [[seismic-wave]] tsunamis from major subduction events along the Sunda Arc or other distant sources.