A quiet week, with just 92 earthquakes of at least magnitude 4 (≥M4.0) and only 18 of ≥M5.0, nevertheless produced three significant earthquakes of at least M6.0. The USGS real time earthquake map shows the pattern involves a concentration of tremors along the margins of the Earth’s tectonic plates and particularly around the rim of the Pacific Ocean.
The Week’s Largest Earthquake: M6.7 off Nicaragua
The subduction of the Cocos plate, which is moving eastwards against the narrow neck of Central America, provided the location for the week’s largest earthquake, an M6.7 which struck off the coast of Nicaragua on 15 June.
The crust of the Pacific sea floor is relatively old, cold and dense, composed of basalt, while the crust which makes up the Central American isthmus is younger, warmer and more buoyant. The result is that collision leads to subduction of the ocean crust, which is dragged downwards. Friction accumulates along the subducting plate and is released as earthquakes.
The M6.7 was a relatively shallow event, at a depth of around 38km, and its epicentre was on the overriding plate. Despite its offshore location no tsunami was generated, although a local alert was issued; nor were there reports of any injuries. The tremor was followed by several aftershocks of ≥M4.0.
Although it is on a subduction zone, Nicaragua has not in recorded history been subject to so-called megathrust quakes which have occurred at subduction zones elsewhere: the largest tremor recorded on the USGS historic earthquakes list for the country is M7.6 and the second largest M7.0. The earthquake of 15 June would rank third on this list.
M6.2, Crete: Earthquakes in the Mediterranean
The Mediterranean Sea saw a cluster of earthquakes – an M6.2 followed by over a dozen aftershocks of up to M5.8 – just to the south of the island of Crete. The Mediterranean is a remnant of a former ocean, the Tethys Ocean, which is in the final stages of closure as Africa moves northwards and collides with Eurasia: the coming together of these two continents has created a complex geological situation.
As the two continents collide, the areas of oceanic crust trapped between them are squeezed and jostled, some subducted and some accreted to the continents. To the south of Crete, the northern edge of the African plate is being subducted along the West Hellenic Trench and the Pliny Trench. The whole region has long been subject to earthquakes and the most recent, though relatively large, is not unexpected.