So, another week, another roundup of the usual suspects. The United States Geological Survey’s real time earthquake map for this week included three earthquakes of at least magnitude 6 (≥M6.0), all of them around the margins of the Pacific plate.
The map includes, broadly speaking, earthquakes of all magnitudes in the US and its territories and those of ≥M4.5 elsewhere (the usual caveats about underestimates of the smaller-magnitude tremors apply). It may not be a complete record, but it does give us a clear idea of where on Earth the larger tremors will occur.
All of the 34 tremors of at least M5.0 and most of those larger than M4.5 were associated with tectonic plate margins of some sort, while the smaller magnitudes of earthquakes tended to be more widely scattered — which is exactly what we’d expect.
The Week’s Biggest Earthquake: M6.8, Vanuatu
I’ve never been to Vanuatu, but I’ve spent so much time there, virtually speaking, that I feel I know it well. Another week, another week’s biggest earthquake in this chain of volcanic islands that lies along the margin of the Pacific and Australian plates.
It’s a long and often highly complicated margin, but this section of it is relatively straightforward — a subduction zone — and this week’s M6.8 earthquakes is typical. The epicentre of the ‘quake (the point on the surface immediately above the earthquake itself) was around 100km in the overriding (Pacific) plate and the event itself took place at a depth of around 169km.
From available earthquake maps, it isn’t clear exactly what the depth of the plate interface is at this point, but it does seem to be around the 150km mark. Bearing this in mind, and taking into account that the movement is, again as we would expect, compressional, it seems that this earthquake results from movement at, or possibly just below, that plate interface.
M4.6 Earthquake, The Red Sea
Well away from the Pacific, interesting things are happening. By ‘interesting’ I mean a small (M4.6) shallow (11km) earthquake in the centre of the Red Sea.
The Red Sea is an area of extension. The axis of this long, narrow gulf between Africa and Arabia is marked by a plate boundary and the two sides of it are moving apart on either side of a central rift. Eventually, this narrow sea may become an ocean.
Although the USGS doesn’t have any detail about the type of movement, it’s a pretty safe bet that this is caused by extensional movement as the plates move apart and the crust between them drops. A quick look at earthquakes in this region over the last 50 years shows that shallow earthquakes in the central Red Sea are actually very common — and can regularly exceed M5.0.
US Earthquakes: Alaska (Again)
I’ve talked a lot about Alaska recently: if it wasn’t for that, I’d have been tempted to make this, an M6.2 in the Aleutian Islands the second of my featured earthquakes, rather than the third. But as it’s by some way the largest in America and the largest in Alaska since…um…last week’s M6.3, I couldn’t in all conscience ignore it.
Like so many others in the Aleutian Islands chain, this earthquake seems a pretty straightforward product of the collision between the Pacific and North American plates along the Aleutian Trench, with the former subducting northwards beneath the latter. At 10km depth and 150km from the trench, the most likely cause is deformation in the overriding plate as a result of the collision.
Last Thoughts: Continents Moving Apart
I said at the beginning that most of the larger earthquakes were associated with plate margins. I should add that the majority of the largest ones are clearly associated with subduction, where continents are coming together.
But continents break up, too, and as we’ve seen with the Red Sea, that break-up leads to regular, intermediate earthquakes (and occasionally very large ones) along ocean ridges as the plates move apart.
If you look at the USGS map, there’s a particularly neat illustration of this in the North Atlantic, where three tremors of exactly this nature occurred along the Mid-Atlantic Ridge, north of Iceland. The Pacific, with its many subduction zones, is an ocean on the decline, with more crust being consumed than created. The Atlantic is still growing — and the Red Sea is just at an earlier stage in the process.
The Earth’s crust is constantly being consumed and recreated. Our planet is the ultimate recycler.