There was a lot going on in the week of 16-22 November, 2017, most of it in the western Pacific. The United states Geological Survey’s real time earthquake map, which records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed almost 1800 earthquakes for the period across the globe.
Of these, six were at least M6.0 with one reaching M7.0, and 52 reached a magnitude of at least M5.0. This is unusual but not unprecedented, and is explained (as such high numbers often are) by an ongoing series of foreshocks and aftershocks in one sequence — in this case, in New Caledonia.
The New Caledonia earthquake series accounted for 49 of the 125 earthquakes of at least M4.5 and over half of those reaching M5.0. It also included five of the six reaching M5.0 — so it’s hardly surprising that the numbers are skewed.
If we take this out, seismic activity was relatively limited, with nothing significant beyond the usual smattering of earthquakes around the western rim of the Pacific Ocean.
The Week’s Biggest Earthquake: M7.0, New Caledonia
It’s probably simplest to discuss the New Caledonia earthquake series as a whole, rather than just the M7.0 which is the largest of the series so far — and the series has been going on for rather longer than the one week covered in this digest.
Firstly, let’s outline the tectonic context. In this area the westwards-moving Australian plate descends beneath the Pacific plate, along the South New Hebrides Trench, and it’s along a relatively short area of this trench that the earthquake series is concentrated.
If the context sounds familiar, it’s because I wrote about it a couple of weeks ago, describing an M6.8 earthquake which occurred just on the Pacific side of the margin and which was probably a subduction earthquake. This earthquake was followed by many others (121 are recorded on the map for the last 30 days) of which eight were of at least M6.0.
By definition, the mainshock in any series is the largest, regardless of where in the series it occurs, which makes this week’s M7.0 earthquake the most significant. Looking back, however, we see that the initial M6.8 seems to have been the initiating earthquake at the subduction zone and those which followed it — mostly on the other side of the margin and with an extensional component to them — are actually the result of the crust adjusting to the main ‘quake.
The earthquake series has been going on since the end of October and may not be over; but I would hazard a guess that most subsequent activity will also be crust deformation in response to the initial subduction ‘quake.
Earthquakes in Iceland
I’ve talked a lot about earthquakes in Iceland and their potential role as indicators of a further volcanic eruption. This fascinates me because a volcanic eruption in Iceland has the potential to cause enormous disruption on a regional scale and shouldn’t be ignored.
One of the less well known of Iceland’s volcanoes, Öræfajökull, has been rumbling recently — not at high magnitudes, but at significant levels. By significant, I mean that in a typical year seismologists record around 125 earthquakes and this year there have been over 500, with around 150 in the past month alone.
Of course, earthquakes don’t necessarily imply volcanic activity, but they can — and history tells us that we have good reasons to be very wary of Öræfajökull. It doesn’t erupt often but two of its known eruptions, in 1362 and 1727, have been significant and, if repeated today, would have significant implications for local residents and possibly for air travellers. And that’s why a lot of respected volcanologists are paying it a lot of attention.
US Earthquakes: All is Quiet
All right — all isn’t entirely quiet in the US, seismologically speaking, but there certainly isn’t anything that stands out on the USGS map this week. With that in mind, it’s always worth going back to look at what isn’t happening and wondering why.
It’s been a while since we looked at Oklahoma, where there’s been a long-lasting swarm of earthquakes resulting from the injection of waste water deep into the ground. There was a point where I was commenting on on this almost on a weekly basis, with dozens of minor tremors appearing on the map.
This week there are 19 of them, the largest coming in at M3.7. I’m not fully au fait with any local regulations on fracking in Oklahoma, and I’m always conscious that the USGS under-reports its smaller earthquakes, so that the full extent of the tremors, especially the smaller ones, isn’t always clear. That said, I’m an optimist — and I cherish the hope that it may be human action that has mitigated the extent of these tremors, just as it was human action that caused them.
Last Thoughts: What Lies Ahead
If you have an eye for an apocalyptic headline, you might appreciate this from the UK’s Daily Star: “Earthquakes in 2018 could kill BILLIONS as Earth slows on axis”. If you prefer a more measured approach, I can offer you The Guardian: “Upsurge in big earthquakes predicted for 2018 as Earth rotation slows”.
The source for these stories is a paper presented at the Geological Society of America’s annual meeting, in which two seismologists proposed the theory that changes in the Earth’s rotation might lead to an increase in major earthquakes in 2018. The Guardian quoted the report’s co-author, Roger Balham, as saying: “The correlation between Earth’s rotation and earthquake activity is strong and suggests there is going to be an increase in numbers of intense earthquakes next year” (2018).
It’s worth pointing out that the analysis is a purely statistical one and we’ve often noted that there’s considerable variation in earthquake activity anyway. And there’s no indication of where and when such earthquakes might occur. But as theories go, it’s certainly one worth keeping an eye on.
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