Saturday 22 September 2012

Earthquake on the Gulf of Corinth.

On Saturday 22 September 2012, slightly after 6.50 am local time (slightly after 3.50 am, GMT) a Magnitude 5.1 Earthquake hit central Greece, slightly to the north of the Gulf of Corinth and roughly 100 km northwest of Athens, at a depth of about 10 km, according to the United States Geological Survey. This is a fairly large quake, and quite shallow, with the potential to cause serious damage to buildings and potentially fatalities (the USGS estimate that a quake of this scale in this location would have a 32% chance of causing the loss of at least one life), though no damage or casualties have been reported on this occasion.

Map showing the location of the 22 September 2012 quake, and the areas likely to have suffered the most severe effects. Damage to buildings is likely within the innermost, green, circle.  USGS.

The geology beneath the Gulf of Corinth is slightly complicated. The Gulf forms part of the boundary between the Aegean and Eurasian Plates. This is a divergent margin, with the two plates moving apart, causing sinking in the centre of the Gulf, with the rocks on either side sinking along a series of faults, a structure known as a graben to geologists.

The graben beneath the Gulf of Corinth. The drawing apart of the Aegean and Eurasian Plates causes the lithosphere under the Gulf to thin, and the rocks of the crust to sink, splitting along a series of concentric faults on either side of the Gulf. Moretti et al. (2003).

However this is not the whole story, as to the south of The Peloponnese the African Plate is being subducted beneath the Aegean Plate along the Hellenic Subduction Zone, passing under The Peloponnese and the Gulf of Corinth (note this is oceanic plate attached to the north of Africa, not the continental African Plate).

The subduction of the African Plate beneath the Aegean Plate and the Gulf of Corinth. Turner et al. (2010).

Thus there are two potential causes of Earthquakes beneath the Gulf of Corinth; shallow faulting associated with the graben caused by the drawing apart of the Aegean and Eurasian Plates, and a deeper zone where friction between the subducting African Plate and the overlying Aegean and Eurasian Plates can lead to quakes. However depth is not the only guide to the cause of a quake, as the friction of the African Plate passing under the Aegean and Eurasian Plates also exerts stresses on the rocks in the graben, and can contribute to quakes there.

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