So, after last week’s Shakespearian extravaganza, I suppose it’s back to the cold hard Earth. Last week, I talked a lot about numbers and ended up by questioning what’s normal (Lord save me, I’m turning into a statistician!) but I’m back in control now.
I’m going to stick my neck out and say that this week’s earthquake numbers are more or less what we’d expect, other than a single event which skews the whole. More on that below.
The United States Geological Survey’s real time earthquake map showed a total of just over 1,650 events this week (subject to the usual caveats, most notably that it’s only really reliable for earthquakes larger than magnitude 4 worldwide, though more of the smaller ones are shown for the US and its territories).
The largest earthquake (which, with its associated fore- and aftershocks is the one that dominates the numbers) was the only tremor of at least M6 (≥M6.0); there were 28 of at least M5.0 and 135 ≥M4.0. All was normal in terms of location, too — all the larger earthquakes were associated with tectonic margins.
The Week’s Biggest Earthquakes: M6.9 and Friends, Chile
At M6.9, this week’s biggest earthquake is the largest in a continuing sequence just off the coast of Chile, at a depth of 25km. The sequence also includes: the second and third largest earthquakes this week (M5.9 and M5.6); a quarter of those of at least M5.0; and 45 out of 82 of at least M4.5.
Earthquakes in this area are not only very common but are often much larger than this. The reason is the coming-together of the Nazca and South American tectonic plates, with the former descending beneath the latter along the Peru-Chile Trench.
In the absence of detailed information, I would normally rely on the locational and tensor data supplied by the USGS and describe this as being an earthquake resulting from deformation in the over-riding plate. This is correct, although simplified. This week, however, I stumbled upon a much more detailed examination of this on a blog (Temblor) which is worth a look (see link in Resources section below article).
The blog was written prior to the largest earthquake in the sequence, but the explanation still holds. In particular, it includes a cross-section which shows exactly how and where this type of earthquake occurs at subduction margins.
M5.0 Earthquake, South-Western Turkey
I’ve said before that I watch out for seismic activity in Turkey with some trepidation, given the capacity for a major event to strike in a very-heavily-populated area (with catastrophic results). This week’s M5.9 is a reminder that there’s plenty of earthquake activity in Turkey away from the potentially deadly North Anatolian Fault.
The earthquake was not located on any of the active margins of what seismologists call the Anatolian Block, but it’s undoubtedly influenced by the tension and contrasting movements in the eastern Mediterranean.
Maps of earthquakes in the Aegean region show that extensional earthquakes such as this are typical along an east-west trending belt between the North Anatolian Fault and the subduction zones south of Crete.
US Earthquakes: Quarrying
This week I’m looking at quarries. Sometimes, particularly as we focus on seismic activity related to oil extraction, it’s easy to forget that human activity has been causing earth movements for hundreds of years — mining and quarrying are the obvious culprits, but dam building comes in there too.
This week I was reminded of this by the USGS map and a single Minnesota earthquake. I can’t remember the last time I saw an earthquake located in Minnesota and this one was immediately obvious as a quarrying event (not just because the USGS has a separate, diamond, symbol).
It’s worth looking at the earthquakes of all magnitudes for the whole of the US. Do so, and you’ll be able to identify several such tremors. Some are labelled as quarry blasts, some (more alarmingly) as explosions, though I haven’t been able to find a technical explanation on the map for the differences.
The Minnesota tremor stood out because of its location (all the others are in the west of the States). But these events happen all the time. We just don’t normally notice them.
Last Thoughts: No Common Theme
I usually try to link my featured earthquakes, but this time any coherent theme is beyond me. We have a typical subduction-related earthquake in a major subduction zone. We have a less-easily-defined tremor, caught between the jaws of conflicting plate movements. And we have an anthropogenic earthquake which has nothing at all to do with modern plate tectonics.
If I’d chosen a different set of earthquakes, we could have been looking at volcanic earth movements in Hawaii, or extensional faulting at mid-ocean ridges, or our old friends the shivering primaries of Oklahoma; and if I’d extended my search into last week I could have chosen an earthquake in a part of Western Australia which hasn’t seen a plate movement for probably billions of years.
Earthquakes play by the rules of nature, up to a point; but there are many, many variables within those rules.