There are quiet weeks, and there are very quiet weeks. I don’t keep a week-on-week tally of how many earthquakes appear on the United States Geological survey’s real-time earthquake map, though maybe I should. But I do have in my head an idea of what ‘normal’ activity is (bearing in mind that the map is anything but complete) — and this week was definitely not normal.
The map (broadly speaking) includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere. The number of smaller earthquakes was pretty much as I’d expect — a total of about 1600 — as, indeed, was the number of larger ones (two of M6.0 or more). But there were just 15 tremors of M5.0 or above when I’d normally expect to see 25-30, and just 79 of at least M4.0, compared to a normal week when there might be around 100.
Whether this is down to under-reporting or just a particularly quiet period is unclear, and in a sense that doesn’t really matter. The pattern of earthquakes, which is determined by the underlying geology and plate tectonics, remained much the same as ever.
The Week’s Biggest Earthquake: M6.8, Alaska
I talked above about the pattern of earthquakes being broadly the same, week in, week out. The largest earthquake of the week exemplifies this. In most weeks (not all) I’m talking about a large earthquake in a subduction zone, and Alaska is a regular location. So, here we are again.
This week’s leaderboard is headed by the M6.8 which struck in the western section of Alaska’s Aleutian Island chain. The islands are formed by the coming-together of the Pacific and North American plates, with the former descending beneath the latter. The submarine trench (several thousand kilometres long) along which the descent takes place has a parallel line of volcanic islands behind it. Earthquakes are common.
There’s a lot going on in these seismic margins, and not all earthquakes take place at the interface between the two plates. The depth and location of this tremor — less than 10km deep and around 150km from the actual trench — are indicators that this particular event is the result of deformation in the over-riding plate. This fact is borne out by the USGS data summary, which indicates that the movement was lateral, rather than vertical (the latter being what we would expect to find if movement were along the margin).
M4.4 Earthquake, Azerbaijan
I like a quiet week. It gives me the opportunity to look at earthquakes that wouldn’t otherwise get any attention. This week there was an M4.4 at the eastern end of the Caucasus Mountains, the range which separates the Anatolian Block (Turkey) from the main stable continental areas of central Asia and which cuts across between the Black Sea and the Caspian Sea.
This is an area of continental convergence and, in consequence, of uplift. (Yeats puts the figure at over 10mm per year). This is accommodated partly by reverse faulting and partly by lateral movement along strike-slip faults.
The USGS map has no details of the direction of movement and the depth (10km) might relate to either of the above. A look at more detailed tectonic maps of the area doesn’t help: both thrust and strike-slip faulting is present in the area. So we can say little specific, other than that the earthquake is the result of collisional processes.
US Earthquakes: Ohio
Yes, you read that correctly. There was an earthquake, albeit a small one (M3.4) in Ohio, in the usually-stable continental region. It’s too small for any earthquake summary from the USGS, so anything I have to say is speculation, albeit educated speculation, and I’m more than happy to be proved wrong.
The chances are that it has one of two sources — anthropogenic, as a result of human activity relating to the extraction of oil and gas; or natural, as the result of movement on ancient, deeply-buried faults. If I had to guess I’d say the latter, purely because there’s no evidence of induced earthquake activity on the USGS maps to match up with what we see in Oklahoma. But of course, that may just mean that previous activity has gone unrecorded.
Last Thoughts: The Things We Don’t See
We know the USGS map is incomplete. If it were to include every earthquake in the past seven days it would be difficult to load and the data would be almost impossible to handle, at least at my level of technology. But this week’s latter two featured earthquakes raised two issues.
That single earthquake in Ohio poses questions. Because it’s an area where fracking occurs, that doesn’t necessarily mean it’s a fracking-related earthquake — but if it is, I’d expect to have seen more of them. And it may well be that there have been many smaller ones, probably, natural, that don’t appear on the USGS map.
The Azerbaijan earthquake, similarly, can be understood within its broad context but the map doesn’t include information on the movement of the fault and so we can’t draw a definite conclusion as to the earthquake’s origin.
The recording of earthquakes is, of necessity, incomplete. In itself that isn’t a problem — as long as we remain aware of that fact.