So here we are, stepping over the threshold of the old year into the new. Just as you’d expect, the week of 29 December, 2016 – 4 January, 2017 provided, in earthquake terms, a seamless transition from one year to the next.
In total, the United States Geological Survey’s real time earthquake map included just shy of 1900 tremors for the week, of which three were at least M6.0, 37 at least M5.0 and 80 at least M4.5.
Why pick M4.5? At this point it’s worth a quick word about the map, which is my primary source of information. The USGS has recently changed the map legend — it’s too complicated to summarise here, but in essence the map includes earthquakes of all magnitudes (which is not the same as all earthquakes) in the US and its territories, and those of at least M4.5 (≥M4.5) elsewhere.
It isn’t that simple, however, because the map also includes some, but not all, earthquakes of between M4.0 and M4.5 outside the US. In essence, the message (which I suppose I should repeat with every digest) is that the week’s count of earthquakes, especially smaller ones, is incomplete.
The Week’s Biggest Earthquake: M6.9, Fiji Islands
Initially assigned a magnitude of M7.2 (later revised downwards) the Fiji earthquake of 3 January (UTC) was large enough to trigger a tsunami warning for local areas, although no tsunami occurred. It was followed by a number of significant aftershocks.
Tectonically, earthquakes in the Fiji-Tonga area of the western Pacific are difficult to interpret. The area falls between the Pacific and Australian plates but USGS map shows no defined boundary between the two, from the south New Hebrides Trench and the northern edge of the Tonga Trench (which, confusingly, begins at Samoa); but something must be happening there. Other earthquake maps show a fracture zone extending south and west of Fiji, but the boundary between the two remains both diffuse and obscure.
This doesn’t mean there’s no boundary — and, clearly, it doesn’t mean there are no earthquakes. The available evidence suggests that this week’s shallow earthquake occurred as a result of extensional tectonics and the triggering mechanism was normal faulting (one piece of land slips downwards relative to another). This vertical component explains the initial tsunami warning.
In all probability, the earthquake resulted from the complex interactions between the two plates, causing shallow deformation broadly associated with the generalised boundary. More than that I can’t say.
M4.5 Earthquake, Central Russia
The larger earthquakes in the world tend to occur at plate boundaries, so it’s always interesting to see one occurring a long, long way away from any seismically active margin. Central Russia is about as far away from those plate margins as it gets — but it isn’t unusual to see seismic activity here.
Continental drift involves the breaking up and coming together of continents — and it follows that break up must be initiated somewhere. Rift zones such as that in East Africa are the early stages of continental breakup (though not all rift systems complete the process) and the Baikal Rift, in central Russia, is another area where extensional tectonics dominate.
Perhaps surprising, the little-known (and remote) Baikal Rift is, according to Yeats, “the most seismically active of the world’s rift zones.” (The chances are that many earthquakes aren’t recorded on the USGS map — see above.) In 1957, for example, the area suffered an earthquake of M7.8 — huge for its setting — and others above M7.0 are on record.
US Earthquakes: Last Week in Nevada
Even earthquake-watchers take holidays. If I’d been around last week, I’d certainly have commented on the three tremors which shook western Nevada — earthquakes of between M5.5 and M5.7, which were felt as far away as San Francisco.
The Nevada Seismological Laboratory makes it clear that earthquakes are a regular occurrence in Nevada as a result of the uplift of the Rocky Mountains and Sierra Nevada. The area shaken by this week’s tremors is by no means immune. “Recent activity, since 2015, has included earthquakes greater than 4.0 just west of Walker Lake that were also strongly felt in Hawthorne.”
In other words — we can expect more.
That Was the Year, That Was
The year 2016 had its moments and will go down in history for more than just its earthquakes — but it’s worth a quick look back. If the USGS map and archive is to be believed (and, for the larger tremors, it is) then there were 145 tremors of at least M6.0 and 16 of at least M7.0.
Sometimes these earthquakes occur in clusters and result in many deaths, so that it feels as though something apocalyptic is happening. When nothing happens, of course, no-one notices. And, in fact, if anything, this year was less active than normal — no earthquake of M8.0 or larger (we’d expect to see one), and one fewer than we’d expect for those of at least M7.0, though there were a few (11, to be precise) more in the ≥M6.0 category.
So, in one sense at least, 2016 was refreshingly normal.