After a hectic — indeed, traumatic — few weeks of major earthquakes, the week of 28 September-4 October, 2017 was mercifully quiet. The United States Geological Survey’s real time earthquake map showed a total of just over 1800 maps, but few of them were significant.
The map isn’t comprehensive, though it does include the larger earthquakes. Broadly speaking, it includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0 elsewhere) but it considerably underestimates the many thousands of small earthquakes which shake the planet on a daily basis.
This week the largest earthquake shown on the map is just M5.8 when usually we would expect at least one, and probably two or three, in excess of M6.0. There are just 21 tremors of at least M5.0, and of the 117 recorded as being at least M4.0, 21 are aftershocks from Mexico’s M8.1 earthquake in September.
A quiet week is both a relief and an opportunity. Serendipitously, this week marks the 50th anniversary of the publication of what some people think is the seminal paper in plate tectonics, McKenzie and Parker’s Plate Tectonics on a Sphere. And plate tectonics is, of course, the theory that underpins our modern understanding of earthquakes.
With this in mind, this digest is what we might loosely call a special edition. I’ve picked the three featured earthquakes to illustrate different aspects of plate tectonics.
The Week’s Biggest Earthquake: M5.8, Mid-Atlantic
Chance would have it that this week’s biggest earthquake is pretty much nondescript and there’s not a lot to say about it, other than we know where it occurred (around 1000km from the nearest land, French Guinea) and that its key motion was lateral.
Ideally I’d have liked it to occur a little closer to the mid-Atlantic Ridge (it was about 300km distant) so that I could classify it easily as extensional tectonics, but despite its lateral motion, it is associated with the creative element of plate tectonics.
Upwelling magma from the mantle fuels the creation of new crust along a submarine mountain range that runs around the planet. This extension, which in this case is widening the Atlantic, has impacts at a great distance from the ridge itself, as the shifting crust cools and fractures (along lateral zones) as the newly-created crust pushes it outwards.
This week’s M5.8, in the middle of nowhere and on a remote and un-named fracture zone, looks to have been a result of just such growth of new crust.
M5.2 Earthquake, South Atlantic
So, we have extensional tectonics, where new crust grows — and we have compressional tectonics, where old crust is destroyed. Most of the planet’s major earthquakes occur at subduction zones, where crustal plates collide and, because something has to give, one of them descends beneath another.
As it happens, because the Atlantic is growing, it has very limited subduction activity, but one area where it does occur is in the extreme south, where the South American plate descends beneath the Scotia microplate along the island arc of the South Sandwich Islands. This week, this island arc saw an earthquake of M5.2. There’s no information on its fault mechanism, but in an overall compressional setting there’s every chance that its main motion would also have been compressional.
This area is remote and its regular intermediate-to-large earthquakes (there have been 25 of at least this week’s magnitude so far in 2017, including one of M6.5) trouble nobody compared to those at the major subduction zones, most notably those around the Pacific. But what those ‘quakes, and this one, do, is show us that the planet is dynamic and they may well mark the beginning of the eventual closure of the Atlantic as the plates reconfigure over hundreds of millions of years.
US Earthquakes: Puerto Rico
I’ve included Puerto Rico as a US earthquake because, for this purpose, it falls under the USGS’s umbrella of recording more than just the major earthquakes. The territory hasn’t had its troubles to seek recently, and you might think an earthquake was the last thing it needed. But this one, fortunately, is small enough, at M4.4, to be harmless.
At a very broad level, I’ve chosen this earthquake because it illustrates the third element of plate tectonics, where two plates slide past one another. In this case I’m being a bit disingenuous, because the actual tectonic setting of Puerto Rico is much more complex than that — but it’s an earthquake that has occurred close to the point where the North American and Caribbean plates move laterally past.
Last Words: Happy Anniversary, Plate Tectonics
Okay, I’ve simplified for the purposes of illustration, choosing this week’s earthquakes to demonstrate the key features of plate tectonics rather than looking at them on their own merits. But it’s useful as a starting point to what is a complex and increasingly-developing branch of Earth Science.
Of course, plate tectonics wasn’t revealed in a single stand-alone paper (for a start, McKenzie and Parker’s short article in Nature included references to 26 others, and the idea of plates was first proposed way back in 1915 by Alfred Wegener). Scientific development is usually incremental and builds on the work of others.
That said, it’s always helpful to take a moment to stop and think about how we understood the workings of our dynamic planet before such a theory as plate tectonics became widely accepted. And the answer is — we didn’t.