Sunday 27 May 2018

Pair of Earthquakes to the east of the Mayotte Islands.

The United States Geological Survey recorded two Earthquakes to the east of Grande-Terre (or Maore) Island in the Mayotte Archipelago, an overseas Department of France in the Mozambique Channel between the Comoros Islands and Madagascar, on Saturday 26 May 2018. The first had a Magnitude of 4.9 and occurred at depth of about 10 km roughly 48 km to the east of the island slightly after 2.30 am local time, and the second had a Magnitude of 5.2 and occurred at a similar depth about 31 km to the east of the island slightly after 11.30 am local time (slightly after 9.30 am local time). Both were felt on the island, but neither caused any damage or casualties.

The approximate location of the 26 May 2018 Mayotte Earthquakes. USGS.

Geologically Mayotte forms the oldest part of the Comoros Island Chain, which are hotspot volcanoes, located over a magma plume which originated deep in the Earth's mantle which is rising through the overlying African Plate, though they may be connected to the East African Rift Zone, which extends from the Red Sea to the coast of Mozambique, roughly a thousand kilometres southwest of the Comoros. Volcanism is thought to have first started beneath what is now Mayotte about 15 000 years ago, and persisted until about 5000 years ago, with the islands rising above sealevel about 7000 years ago, and continuing to grow after the cessation of volcanism due to the accretion of Coral reefs. Although all volcanic activity on the islands ceased thousands of years ago, the islands are still subject to occasional seismic events (Earthquakes), associated with the movement of magma deep beneath the hotspot. These can be potentially hazardous, due to the nature of the rock which makes up the island, mostly poorly consolidated volcanic ash, which is prone to liquefaction when sufficiently shaken; a Magnitude 5.0 Earthquake in 1993 is thought to have caused about €1.7 million in damage, mostly to privately owned homes.
This year has seen a series (or swarm) of Earthquakes in the area to the east and southeast of Mayotte, which may indicate the movement of magma beneath the surface, and possibly a future eruption somewhere in the Commoros. Such an eruption would not necessarily occur in the area where the Earthquakes are happening, as magma can migrate significant distances horizontally as well as vertically before erupting or becoming emplaced beneath the surface (i.e. setting to become hard rock without ever erupting).
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt these events; if you felt the first quake then you can report it to Earthquake Report here, and if you felt the second you cab report it here.
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