There are some fascinating events on the United States Geological Survey’s real time earthquake map for the week of 27 July-2 August 2017 — and it’s a pity that my chosen format doesn’t allow me to cover them all. There’s nothing that’s unique, however, and we can be reasonably certain that the ones that miss out on inclusion this week will reappear another.
So what are the numbers? The map shows, broadly speaking, earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere. In total, it showed just over 1800 events, of which none reached M6.0, 32 were of at least M5.0 and 103 of at least M4.0.
The distribution was broadly as normal, with the majority of the larger tremors concentrated at or near the margins of the Earth’s tectonic plates. There were a few that were slightly unusual in location — a couple in the North Atlantic, further away from the central ridge than we might normally expect, and one in central Africa.
The Week’s Largest Earthquake: M5.9, South of Japan
In fact there were two earthquakes vying for the crown as the largest in the week and both, as chance would have it, were remote and, as they affected very few people, largely passed unnoticed.
I’ve chosen to go with one — an M5.9 some way south of Japan — rather than the other — in the North Atlantic — on the grounds that the shaking from it was felt by somebody rather than nobody. In the great scheme of things, though, the 1.5 million people (or thereabouts) who may not have realised they were experiencing an earthquake doesn’t render the featured largest earthquake particularly significant.
The submarine tremor occurred in the Philippine Sea plate, close to the margin along which it subducts beneath the Eurasian plate. At first sight, at 10km depth and 10km from the trench, you might be forgiven for thinking that this is a subduction earthquake, but the direction of subduction is in the opposite direction, meaning the the most likely cause is deformation in the overriding plate. This is borne out by the fact that the direction of movement is extensional, rather than the compressional movement we would expect from a subduction event.
This part of the planet doesn’t usually feature in the digest, because of its remote location. In fact earthquakes of roughly this size are relatively common along this section of the plate boundary, with 16 larger than M5.0 in the last year, the largest being an M6.0 in May 2017.
M4.5 Earthquake, Iceland
All right, it’s only an M4.5, but forgive me. The tremor that shook parts of southern Iceland may — and I emphasise may — indicate that something is about to happen at Katla volcano.
Earthquakes are common under Iceland, partly because of the movement of magma and partly because the island lies across the Mid-Atlantic ridge. Seismologists are able to distinguish between the two types and this week’s M4.5 beneath the Mýrdalsjökull ice cap (which overlies Katla) bore the signature of mama movement.
In itself this means nothing, although it’s worth noting that here are other signs — a meltwater flood emanating from under the ice cap among them — which suggest increased activity, and possibly (who knows?) even small subglacial eruption.
Katla’s aviation code moved briefly from green to yellow (indicating an increased level of activity) earlier this week, but is back to normal again now. We won’t know whether Katla is going to erupt until she does, but the earthquake is certainly a sign of something happening. But it may already have passed.
US Earthquakes: Alaska
There were earthquakes in Alaska this week — a lot of them, which is hardly surprising since it’s one of the most seismically active places on the planet. None of them were large, but the Aleutian Trench, which separates the Pacific and North American plates, is capable of some very large earthquakes indeed.
There’s always a lot to learn about earthquakes. This week my eye was caught by an article in Columbia University’s State of the Planet blog, covering research into a section of that margin (shown, with this week’s earthquakes, in the image).
I won’t go into too much detail and I’m trying to avoid being sensationalist, but in essence, the research has identified similarities between this part of the trench and the northeastern Japan subduction zone, which ruptured to such deadly effect in 2011. The indications the new research are that there is the potential for rupture resulting in a tsunami rather larger than might otherwise be expected.
We’ve always known that Alaska can generate large tsunamis. But this new research offers food for thought as to where the next one might originate, and hat its effects might be.
Last Thoughts: Remote Earthquakes Have Implications For us All
So why should you care about an earthquake beneath Katla volcano? What does it matter if there’s a large earthquake in Alaska, so remote that no-one feels it?
Earthquakes don’t occur in isolation: they’re part of the earth system and they interact with other natural processes. Sometimes the relationship is causal — an earthquake generates a tsunami. Sometimes it’s effective — it might result from magma movement and is a precursor to an eruption.
Large tsunamis and large volcanic eruptions can have an effect for thousands of miles beyond the immediate surroundings of the earthquake with which they’re associated. The better we understand these processes, even in remote areas, the better prepared we can be for when something does affect us.