I’ve had a bit of fun this week. Sometimes new and fascinating things pop up in the world of seismology — not necessarily earth-shaking (pun intended) but things which just make me look at the world and think: That’s why I’m interested in earth science. And the week of 30 March-5 April 2017 was full of those wonderful things.
First up, the numbers. The United States Geological Survey’s real-time earthquake map (which, incidentally, is one of my very favourite seismological things) recorded a total of over 1500 seismic events this week.
This isn’t comprehensive — it includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere — but it picks up the most significant tremors.
There were two tremors larger than M6.0, five of at least M5.0 and 127 of at least M4.0. In terms of numbers this was more or less what we’d expect, but the distribution of the larger tremors (≥M4.5) certainly raised my eyebrows.
Usually they would all be associated with the margins of the planet’s major tectonic plates, but this week there were no fewer than five located well away from the plate margins — though in fairness, two of them are associated with the uplift of the Himalayas which has an impact over a very large area of Asia indeed.
The Week’s Biggest Earthquake: M6.5, Botswana
When I first saw this earthquake notification I sat and looked at it for a while, wondering if it could be mistake. I suspect I’m not alone in that, judging by the period of silence as Earth science bloggers also presumably thought about it, and then the rash of articles about it which sped through my Twitter feed.
Why so surprised? Well, the African continent — the southern and western part of it — is among the oldest and most stable continental areas on the planet. Africa does experience notable earthquake activity, but that’s largely associated with the East African Rift Valley and major earthquakes in central and south west Africa are pretty much unheard of.
To quote the UC Berkeley blog: “hardly any quakes were recorded in the last 70 years at all. Botswana had only two noteworthy quakes, one magnitude 5.8 event in the northwest of the country in 1952 and another in 2002 about 80 miles southeast of Monday’s temblor. That event registered with a magnitude of 4.5.”
So what’s going on? Well, the USGS notes that: “the earthquake occurred as the result of slip on a northwest-trending fault centred in the lower-crust.” The data suggest it has an extensional origin, caused by sections of crust moving apart. The absence of any obvious plate boundaries nearby leads to the conclusion that what we are seeing is an expression of very large-scale processes.
To go back to the USGS, probably the authoritative source, the cause is likely to be: “broad-scale regional tectonic stresses that are similar to those responsible for producing the East-African rift system”.
M6.1 Earthquake, Iran
I was probably being a bit misleading by including the M6.1 in Iran among those earthquakes located at a significant distance from an active plate boundary. Though it’s around 1000 km from the nearest margin (between the Arabian and Eurasian plates), the collision of these two continents, like that between India and Eurasia, has impacts over very large areas of central Asia.
The collision zones are so broad that they overlap in the area of this week’s Iranian quake, which took place in a region where fault maps show (unsurprisingly) a complicated mixture of fault types, mainly strike-slip or thrust faults.
There’s no USGS summary of this event, and the available fault maps aren’t detailed enough to pin it down exactly, but it looks to me as if it occurred either on, or very close to, a reverse fault called the Kashafrud fault, one of several parallel-trending faults that are associated with the major uplift caused by continental collision.
US Earthquakes: Oklahoma
I’ve wasted a lot of time this week playing with a new toy. It’s a new website (not fully functioning today, alas, or I’d have produced a few natty graphics for you) from the Office Of The Secretary Of Energy & Environment in Oklahoma, and it includes an interactive map which allows you to look at earthquakes in Oklahoma for different years.
This gives a very clear indication of how earthquake activity has increased in the state. And here’s the fun bit — you can overlay those earthquakes on a map of waste water disposal wells. The correlation isn’t perfect (sometimes there are wells and no earthquake) but it’s sufficiently close to be convincing.
The link is in the resources but I’m adding it here, too. If you’re an earthquake geek with an urge to procrastinate, then, once it’s working fully… this is for you.
Last Thoughts: Not An April Fool
I keep a close eye on what’s happening in the earthquake world, and much of what I learn is too complicated or too niche to reproduce here. But something that caught my eye over weekend was so joyous that I had to check it wasn’t an April Fool. It wasn’t, so I’ll share.
As part of its engagement in schools, the British Geological Survey has joined forces with a primary school in Leicestershire and they are recording the earthquakes generated by the local football (soccer) team. Leicester City caused seismic waves by winning England’s Premier League and now their supporters are making their own.
The BGS updates the seismograms every time Leicester City play at home and you can see the tiny earthquakes the fans generate when their team scores. And do you know the best bit? The seismometer is made of Lego.