After a hectic — and sometimes deadly — few weeks of seismic activity, the earth seems to have quietened down, in earthquake terms at least. In the week of 23-29 November 2017, the United States Geological Survey’s real time earthquake map wasn’t exactly overpopulated.
In fairness, the map doesn’t show every earthquake but it does show the most significant ones, including those of at least magnitude 4 (≥M4.0) worldwide and tremors of all magnitudes — which is not the same as all tremors — in the US and its territories. The largest earthquake recorded just crept up to M6.0, and there were just 16 tremors of M5.0 or larger in the total number shown (which was just under 1500).
With nothing unusual in the numbers, was there anything special in the distribution? No — the larger earthquakes, as usual, were concentrated at and in association with the margins of the Earth’s tectonic plates.
The Week’s Biggest Earthquake: M6.0, Papua New Guinea
The largest earthquake of the week is often in the Western Pacific. There’s a reason for this — the margin between the Australian and Pacific tectonic plates is long and complex, with changes in the nature and direction of movement. Around Papua New Guinea it comes close to the Philippine Sea and Sunda plates, and the complex relationship between these has generated what is, frankly, a tectonic mess of small crustal slivers. New crust is being created in some areas and old crust consumed in others.
This week’s largest earthquake has its epicentre (the point on the earth’s surface immediately above the point where it occurred) somewhere in this muddle. In this area, the exact location of a plate margin is unclear and not marked on the USGS map, although Robert Yeats notes that it is the ‘northern limit of Australian continental crust’. This suggests that it is probably at the boundary between the two larger plates — although in actual fact this northern Australian crust lies on the South Bismarck plate.
Confused? To complicate matters further, it also lies close to a lateral fault between the South Bismarck and Pacific plates, and the point at which the Solomon plate is subducting beneath the Pacific margin. In other words, there’s a lot going on in this area and it isn’t always easy to pick out a single cause — if, of course, there is one.
Fortunately the USGS also publishes additional information, and from the location, depth (about 60km) and direction of movement (compressional) I would be reasonably confident in suggesting that the earthquake is related to the subduction of the Solomon plate, rather than lateral movement at the South Bismarck-Pacific boundary.
The Eastern Mediterranean
With no other major earthquakes on the map this week, there’s an opportunity to look at wider groupings of seismic activity, and this week we’re in the eastern Mediterranean. Like the Western Pacific, this is a bit of a tectonic jigsaw puzzle, with different directions of movement and many slivers of crust.
This week there were three smallish earthquakes in this region — two in south-western Turkey (M5.0 and M4.9); one in the Ionian Sea (M4.7); and one in Bulgaria (M4.6). While these may well have had different mechanisms in terms of the type of fault on which they occurred (the USGS doesn’t provide these data) they are all a product, at the macro level, of one major tectonic process — continental collision.
Continents break apart and come together; oceans open and close. The Mediterranean is the last remnant of a major ocean and as Africa and Eurasia come together, the crust between them is squeezed and uplifted as if caught in the jaws of a massive trap. It’s a complicated and ongoing process and we can expect to see many more minor — and the occasional major — earthquake in this region.
US Earthquakes: Alaska
Unlike the other two areas we’ve considered, Alaska is reasonably straightforward. Most of its earthquakes are the product of subduction as the Pacific plate descends beneath the North American plate along the Aleutian Trench.
That said, there are complications here, too, and this week’s M5.3 in the Prince William Sound is an example. There’s a sharp bend in the plate margin at this point, with a couple of crustal slivers caught up in it, and one of these — the Southern Alaska microplate, is the location for this week’s tremor.
M5.3 isn’t big for an earthquake in Alaska — and it’s worth noting at this point that this microplate, and the rotational movement that accommodates the changing stresses as the plate margin changes direction, isn’t far from the epicentre of one of the largest earthquakes on record — the M9.2 of 1964.
Last Thoughts: Our Dynamic Planet
There may not be much going on in earthquake terms, but our planet is dynamic. Plate margins aren’t just about earthquakes but about volcanoes, too. A feature of subduction zones are the line of volcanoes which lie behind them, forming chains such as the Andes and the Cascades, and volcanic island chains.
This week media focus has been on volcanoes rather than earthquakes, with what appears to be an imminent, possibly cataclysmic, eruption of Mount Agung. Mount Agung is a classic of its kind, and is the product of a subduction process.
The Australian plate subducts beneath the Sunda plate along the Java Trench and Mount Agung is one of literally dozens of volcanoes, many of them currently active, which lie behind it — perhaps the most famous (or infamous) of them being Krakatua. Earthquake watchers may recall that this subduction zone is the one which generated the Boxing Day earthquake of 2004.
This digest is an earthquake digest, but it’s mistake to assume the earthquakes operate independently of other earth processes. When the planet moves, it doesn’t do so in isolation.