This is how it goes: one week I’m scratching around for something interesting to say about earthquakes and the next there are more fascinating seismic events than you can shake a stick at, and I’m struggling to pick just three to talk about. The week of 26 October-1 November, 2017 was definitely one of the second.
I’ll begin, as I always do, with the numbers. The United States Geological Survey’s real time earthquake map is bristling with tremors this week — almost 1750 of them. That’s an underestimate because the map isn’t comprehensive, showing (broadly speaking) earthquakes of all magnitudes — which isn’t the same as all earthquakes — in the US and its territories and those of at least magnitude 4 elsewhere.
Underestimate or not, this week there was a flurry of activity. The map included six earthquakes of ≥M6.0, four of them in New Caledonia, one in Indonesia and the sixth so far north it doesn’t appear on the map. Along with that there were 41 of at least M5.0 and 139 recorded as being M4.0 or above.
They were widely distributed, too. As usual, most of the larger tremors are at or associated with major plate boundaries, but there are some exceptions. An earthquake in Tanzania marks the East African rift; the Arctic’s mystery earthquake probably startled a few polar bears at 85 degrees north; there was a minor earthquake in central China; and in the next tier downwards, there were some spellcheck-busting tremors (mercifully small) in Bosnia and Herzegovina.
The Week’s Biggest Earthquake: M6.8, New Caledonia
As always, I begin with the biggest earthquake of the week — and it’s one of a series that includes four of the earthquakes of at least M6.0 this week, and over 20% of those registering at least M4.5. So while I might discuss this M6.8 tremor as a single earthquake, it’s really just one of a whole series of them.
There are not really any surprises about this earthquake: all the signs point towards it being a fairly typical subduction zone event. Although part of the monumentally complicated tectonic boundary between the Pacific and Australian plate, which twists and turns its way for thousands of miles with many a surprise along the way, the New Caledonia section is fairly simple.
The Australian plate subducts northeastwards beneath the Pacific plate along the New Hebrides trench. The earthquake occurred in the overriding plate, at a depth of 10km and about the same distance from the trench. This immediately suggests movement at or near the plat interface, and the data on direction of movement — which is compressional — confirm that it is almost certainly a subduction event.
Where available, the equivalent data for the rest of the earthquake series imply a different source — extensional movement — and as most are located on in the descending plate and all occurred after the mainshock, this suggests that they are effectively the crust’s response to the initial movement, rather than a continuation of it.
This time three years ago, scientists and non-scientists alike were watching in awed fascination as Iceland’s Bárðarbunga volcano produced a fabulous display of lava fountains and gave birth to a new lava field. The Holuhraun eruption died down in February 2015, but the Earth beneath remains restless.
In Iceland, as in many other places, earthquakes and volcanoes are inextricably linked. Here, a magmatic hotspot overlies an ocean ridge — two areas where magma rises the surface. Mid-Ocean ridges, whether volcanic centres or not, are seismic zones and volcanoes produce earthquakes by virtue of the rise and fall of the molten rock which feeds them. Bárðarbunga has been rumbling on for a while at a low level, and this week there were two tremors large enough to make it onto the USGS map — both of them with a magnitude of M4.6.
What this means is open to question, as by no means all earthquakes presage eruptions, but tremors can be a valuable clue. I wouldn’t like to put my house on whether or not the volcano — or any other in Iceland — is going to erupt in the near future. But I would say it’s wise to treat some of the more excitable headlines (such as yesterday’s “Iceland’s BIGGEST volcano ‘ready to ERUPT’ any minute”) with a touch of caution.
US Earthquakes: Hawaii
Sometimes I think of Hawaii as being just like Iceland, but with sun. Of course there are many more profound differences than that, but both are volcanic islands resulting from hot spot magmatism — although that’s exclusively the case in Hawaii, whereas we’ve seen that Iceland also overlies an ocean ridge.
Hawaii didn’t match Iceland in the magnitude of its earthquakes this week — the largest recorded was M3.4, on Mauna Loa — but it’s rumbling away. Mauna Loa isn’t erupting, but it’s unquestionably active at low levels, and its neighbour Kilauea, which is currently erupting, is also generating small earthquakes as the crust compensates for moving magma below.
Last Thoughts: A Bonus Earthquake
I really wanted to write abut the M6.0 earthquake in the Arctic this week, but it defeated me. A pity — because an earthquake in this area, of this magnitude is definitely unusual.
At 85 degrees north, it’s impossible to zoom in on it on the USGS interactive map — and the image I’ve included shows you exactly what happens if you try.
Nobody reported feeling it. The shake maps show nothing but a blue sea. The USGS summary page tells us that it was north of Franz Josef Land but doesn’t tell us how far (my best guess is that it’s at least 1000km). All we know is that occurred at a depth of 10km and was caused by extensional movement.
If I had to make a call, I’d say it was probably caused by normal faulting of an existing fault, but that would be little more than an educated guess.