The largest earthquake by the United States Geological Survey was M6.6 and there were just 17 of ≥M5.0 and 84 of ≥M4.0. As usual, these occurred mainly along boundaries between the planet’s tectonic plates, with the greatest concentration along the highly active margins of the western Pacific.
The Week’s Biggest Earthquake: M6.6, Kamchatka
The M6.6 which struck just off Russia’s Kamchatka peninsula in the north-western Pacific was the largest of the week’s earthquakes. Its epicentre was at the northern end of the Kuril Trench, roughly 100km from its junction with the Aleutian Trench. The Pacific plate is subducting beneath the North American plate at both of these locations and the location and depth of the tremor (around 50km deep and with its epicentre in the overriding plate) suggest that it was caused by movement at the plate interface.
Tectonically speaking the area is highly active and both the Kuril and Aleutian island arcs regularly experience major earthquakes as a result of subduction. Though the largest earthquake of the week, the M6.6 was not the largest to occur in the region in November (an M7.1 occurred further south off Japan). The Kuril Trench is, in fact, the source of many very large earthquakes including the notorious Tohoku-Oki earthquake of March 2011 and the largest recorded to date in 2013 – an M8.3 which struck in May on the other side of the Kamchatka peninsula and which is noted for being the largest deep earthquake on record (at over 609km).
The Mediterranean and the Closing of an Ocean: M5.1, Greece
Associated as it is with the relationship between ocean and continental crust, plate tectonics is tied to the opening and closing of oceans: oceanic crust is created at mid-ocean ridges and consumed at subduction zones and when continents meet, oceans close. We can see the closing stages of this cycle in the present-day Mediterranean, where the collision of Africa and Eurasia has created a complicated tectonic pattern along the Mediterranean and its margins.
This week the collision resulted in a minor earthquake of M5.1 on mainland Greece. Although this lies to the north of a minor subduction zone, the epicentre was in an area characterised by extensional rather than compressional tectonics and was therefore associate with normal faulting rather than being a direct result of subduction.
With little in the way of tectonic activity on the mainland United States, it’s worth taking a look at activity elsewhere. The US Virgin Islands is part of the Lesser Antilles island chain, product of one of just two small area of subduction associated with the Atlantic Ocean (the other is the Scotia Arc in the South Atlantic). These volcanic islands are the result of the convergence of the Atlantic and Caribbean plates and, like the Mediterranean, include zones of extension as well as compression. These areas appear to be the focus of the swarm of small earthquakes which occurred this week around both the British and US Virgin Islands.
Extensional Tectonics in Compressional Zones
The earthquakes in both the Mediterranean and the Caribbean this week demonstrate how complex the mechanics of plate movements can be. Though both appear associated with subduction zones, their local tectonic setting is extensional and appears (subject to further detailed information) the most likely cause of seismic activity in each case.
USGS. Real time earthquake map. (2013). Accessed 13 November 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.