Some weeks there’s not much to say about earthquakes. Some weeks there’s a lot. This week, 2-8 November 2017, is definitely one of the latter, certainly from the point of view of earthquakes occurring on the United States Geological Survey’s real time earthquake map where we wouldn’t normally see them.
In part, this looks to me as if it’s not so much a change in what happens as how they’re recorded. The USGS map records earthquakes of all magnitudes in the US and its territories — which doesn’t mean it records all earthquakes — and those of at least M4.0 elsewhere. This week, a few strangers seem to have gatecrashed the party.
This week, the map’s total of 1,850 has two earthquakes of at least M6.0, 41 of at least M5.0 and 136 of at least M4.0. The first glance at the map shows a smattering of earthquakes I wouldn’t expect to see recorded — an M3.9 in Torremolinos, in southern Spain, another in Zambia, and M3.5 in Austria and even an M2.6 in the UK.
This demonstrates the limitations of the USGS map as a source, because small earthquakes in most of these areas aren’t actually uncommon — they happen regularly and do appear on realtime maps provided by national or regional seismological organisations. Maybe their equivalents will be there next week, or maybe they won’t, but this week at least we can recognise that they exist and give them their hour — or their week — in the sun.
The Week’s Biggest Earthquake
Whatever happens, I’ll bring you the biggest earthquake of the week. This week it was an M6.8 which occurred in the south west Pacific around 200km south of Samoa. The Pacific and Australian tectonic plates come together along a margin thousands of kilometres long, from far south of New Zealand to Indonesia, and at this point the nature of the margin changes from a straightforward subduction zone (the Tonga-Kermadec trench) to a boundary so diffuse that isn’t marked as a margin on tectonic maps.
This week’s M6.8, and its four accompanying shocks of between M4.8 and M5.6, were at the northern end of the subduction zone in the overriding plate, at the point where the trench curves westwards south of Samoa and before it peters out. All were shallow, but the USGS event summary shows the direction of movement only for the two larger ones, indicating that the movement was largely compressional but with a lateral component.
This is more or less what we might expect, and given the shallowness of the tremors and their location relative to the trench, it seems likely that they are the result of deformation in the over-riding plate rather than directly the result of movement at or near the plate interface.
M2.6 Tremor, Scotland
You must forgive me for being a tiny bit parochial at this point. I’ve never yet had the chance to comment on an earthquake as close to home as this, and it was a revelation to see and earthquake of M2.6 occurring in Argyll, on the west coast of Scotland. Scotland’s rocks are old and deeply deformed, with many ancient faults crosscutting them. They are not, however, close to a plate boundary. So why do earthquakes occur?
The answer may lie in more recent geological time — the last ice age, during which Scotland, like much of the northern hemisphere, was buried under a thick layer of ice. This was at its thickest over western Scotland. The weight of the ice depressed the crust and after it melted, the crust rebounded — and continues to rebound — very slowly. So it does seem that this movement — termed isostatic rebound — may have been responsible for this earthquakes.
To place this earthquake in context, the British Geological Survey’s earthquake map of the UK shows 14 tremors in the last 50 days, most of them with a magnitude less than M1.0, and and M3.6 tremor in Fort William (some way further north) made headline news back in August.
That’s a classic example of what it means to be tectonically stable.
US Earthquakes: Wyoming
Forgive me again, for a double dose of parochialism. I’ve picked this week’s US earthquake, an M4.0 in Thermophilis, Wyoming, because it’s one of only two US states that I’ve visited. Wyoming is geologically fascinating, lying partly in the Great Plains and partly in the Rockies, and it was in one of the outlying ranges of the Rockies (the Bighorn Mountains) that the event occurred.
The main movement, according to the USGS, was extensional, which is more or less what we’d expect. Much of the landscape of western North America reflects large-scale extensional movement, with wide rift valleys and uplifted blocks of what is known as basin and range topography.
Last Thoughts: Significance is a Matter of Context
I’ve talked about Scotland’s earthquake this week, not because it happened, but because it’s recorded on the map which is my primary source.
There’s no complete publicly available map of world earthquakes and I’m in no way criticising the USGS map, which is the best available, certainly for my purposes and especially for the larger tremors. But there’s no denying that it underestimates those of smaller magnitudes — it would be almost impossible not to when, as the Incorporated Research Institutions for Seismology (IRIS) notes: “Magnitude 2 and smaller earthquakes occur several hundred times a day world wide”.
In a way it’s a pity to be tied too much to the larger magnitude map, so I welcome the appearance of smaller tremors in the USGS map. Small earthquakes can sometimes be as fascinating as larger ones, depending on their context.