It can seem, especially for those of us suddenly balanced on the outer edge looking in, as if most of the seismic shifts in the week of 23-29 June 2016 were geopolitical rather than geological. Certainly there was nothing seismological to distract the eye from the momentous shifts taking place in Europe.
This week, the United States Geological Survey’s real time earthquake map had nothing especially earthshaking on it for the week. The map which — broadly speaking — includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, recorded a total of 1700 tremors, just one of which was ≥M6.0. There were 31 ≥M5.0, 137 ≥M4.0, and a further 241 of ≥M2.5.
The distribution was as normal, with the larger tremors all either on, or close to, the boundaries between the earth’s major tectonic plates. And, as usual, most of the activity was around the Pacific and the western margins of Asia.
The Week’s Biggest Earthquake: M6.4, Kyrgyzstan
Continental collision was the underlying (pun intended: sorry) cause of this week’s largest earthquake. The convergence of the Indian subcontinent with Eurasia has been ongoing for a long time (50-70 million years) and continues today at the relatively rapid speed (in geological terms) of 35-50 mm per year.
This collision has an effect across a very wide area indeed, breaking and uplifting the crust from Myanmar in the east to the central Asian republics in the west and raising the Himalayas and the Tibetan plateau. Depending on the angle of convergence, the nature of the faulting will vary, but the typical mechanism for earthquakes caused by contingent-continent collision is thrust, or reverse, faulting. This occurs as the crust breaks and one part slides over another, effectively stacking slices of crust on top of one another.
Thrust faulting was the mechanism behind this week’s largest earthquake, which occurred close to the western edge of the head-on convergence zone, close to the point where the angle of convergence changes and the dominant motion becomes, as a result, lateral. The USGS, in its summary of the event, notes that: ‘The tectonics of the epicentral region are complex, and both strike-slip and reverse faulting earthquakes have occurred in the vicinity’.
Earthquakes in Europe
There are limitations in a map that shows only those events above a certain magnitude — the USGS map shows only a handful in Europe. This week a lot of people might be thinking more about Europe than usual — so why don’t we?
There is, I suspect, a perception that Europe, certainly the western part of it, is highly seismologically stable; and to an extent that’s true. But the southern margins (along the Mediterranean) are approaching continental collision themselves, as the Mediterranean closes and Africa edges slowly closer to the Eurasian content, generating (in the not too distant past) large and damaging earthquakes in Spain, Italy and Greece.
Further north, the Alps originate from massive continental upheaval hundreds of millions of years in the past — but are still capable of producing earthquakes. And further north still, an extensional rift system not dissimilar in style to that in East Africa, though on a very much smaller scale, generates earthquakes in Germany, Belgium and the Netherlands. It’s only when you reach the continental core of Scandinavia that we can claim any real stability.
For the record, the European Seismological Centre map recorded over 330 earthquakes of at least M2.0 in Europe last week — tremors which are too small to appear on the USGS map.
US Earthquakes: Alaska
It’s a relief to move on from the mangled complications of Europe’s geography and topography to something that is, on the surface at least, rather easier to understand. The largest earthquake in the US this week was an M5.3 in Alaska, along the Aleutian subduction zone.
All the evidence suggests that this tremor, around 200km deep and a little over 250km north of the actual margin, was almost certainly a fairly straightforward subduction earthquake, caused by movement along the interface between the Pacific and North American plates as their coming together causes the former to descend beneath the latter.
Alaska has its tectonic complications, of course, like everywhere else. But for once it’s nice to look at something and find that it’s relatively straightforward.
Last Thoughts: Coming Together
All three featured earthquakes (or tectonic settings) are dominated by collision — coming together of continents. But it’s worth noting that the simple collisional tectonics is often (even usually) complicated by generating different types of movement or but complicating existing situations. And, particularly in continent-continent collisions, the tectonic outcome at any stage of the process is often much more complex that we might at first expect.