Generally speaking, the overall number of tremors recorded on the United States Geological Survey’s real time earthquake map (which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) varies relatively little.
In terms of notable and newsworthy earthquakes, however, there’s considerable variation — some weeks the Earth shakes a lot, and some weeks it shakes very little.
The week of 21-27 August 2015 was one of the latter weeks.
Of nearly 1500 earthquakes on the map, the largest were just magnitude 5.7 (M5.7) whereas in a typical week we’d expect to see at least one and probably several tremors of M6.0 or more (most of them somewhere in the Pacific). The 15 which reached at least M5.0 certainly fulfilled this pattern, with all but two located around the margins of the Pacific Ocean.
A quiet week gives us the opportunity to leave the Pacific alone for a while. This week’s featured earthquakes are all in the Atlantic/European hemisphere — an area which, while it can never be said to be stable, generally doesn’t produce the size of earthquakes which we normally see around the Pacific — and eastern Indian — Ocean.
The Week’s Largest Earthquake: M5.7, South Sandwich Islands
Adrift at the bottom of the world, the remote and barely-sturdied South Sandwich Islands are one of just two sections of well-developed subduction zone on the margins of the Atlantic Ocean. Subduction zones are regularly the source of major earthquakes and their absence is one of the reasons why the Atlantic is generally overshadowed, seismologically speaking, by the Pacific.
This week the South Sandwich Islands produced the week’s joint largest earthquake. At M5.7, it isn’t large, either in overall terms or even in its tectonic setting. Hundreds of kilometres from the nearest occupied land (South Georgia) the volcanic South Sandwich chain is the result of the subduction of a short length of the South American plate beneath the Scotia microplate. And though too remote to be newsworthy, it’s active.
This week’s earthquake was probably the result of crustal deformation rather than movement at the subduction zone itself, given its depth and distance from the trench. Though often overlooked, this short section of plate boundary is more than capable of producing significant earthquakes.
A quick look at he USGS earthquake archives reveals almost 40 tremors larger than M6.0 in this region in the past two decades, along the trench itself and the plate margin a little further south.
We may not notice, but this part of the planet is definitely moving.
Earthquakes in the Mediterranean
Caught in the convergence of Africa and Eurasia, the Mediterranean Sea is being crushed out of existence. Slowly — very slowly — as the two continents converge, the Mediterranean Basin is being squeezed and contorted, with fragments of crust being broken and twisted. In this overall convergent zone, local variations produce extensional tectonics and continental uplift. It’s a recipe for earthquake activity.
In most weeks, the USGS map shows something happening in the Mediterranean — usually a medium-sized tremor in the central or eastern basin. Occasionally something bigger — or more newsworthy — might happen, say in Spain or Italy or Greece, a reminder that this continental collision involves mighty forces.
This week there were earthquakes in northern Italy (M3.5); Greece (M4.8, M4.3, M4.2) and Turkey (to of M4.2). On a map they are broadly indicative of the zone along which the contents are closing, millimetre by millimetre.
None of them was particularly large, but as the Scots say, ‘many a mickle make a muckle’. Or, in more commonly understood parlance, it all adds up — to continental collision.
US Earthquakes: The Puerto Rico Trench
Strictly speaking, the Puerto Rico Trench and the associated islands to the south aren’t all part of the US, as they include territories associated with many different nations. Geographically speaking, however, they make up a unit — which, as it happens, is part of the second subduction zone in the Atlantic.
Like the South Sandwich islands, this area is capable of producing the occasional large earthquake; but unlike them, the proximity to the US and the inclusion of some US territories means that more of the earthquakes which occur here are included on the map. As a result, we have a clearer picture of the many minor earthquakes here (over 70 greater than M2.5 recorded this week).
Like the Mediterranean, this area is characterised by both extension and convergence and the earthquakes shown on the map will have a variety of different sources.
Last Thoughts: Subduction in the Atlantic
The Atlantic Ocean is free from major subduction zones — at the moment. But oceans open and oceans close and the development of subduction zones is a key part of the later phase. This week we looked at earthquakes in the Caribbean and South Sandwich Islands, demonstrating that both are active. It’s worth noting that some seismologists also think that there may be a third subduction zone developing west of Gibraltar. This implies that the closure of the Atlantic may, very slowly, be getting under way.