As if to illustrate the fact that earthquake activity varies across a reasonably wide range, the week of 5-11 October 2017 showed a different pattern to previous weeks (which had showed a period of quiet following from several significant earthquakes and their aftershocks) without being exceptional.
This week, the United States Geological Survey’s real time earthquake map recorded a higher proportion of significant earthquakes (I define these as being at least magnitude 6) although the total number of tremors shown was more or less what we might expect.
The map, which isn’t comprehensive but which, broadly speaking, includes earthquakes of all magnitudes in the US and its territories and those of at least M4.0 elsewhere, recorded just over 1700 tremors, of which 22 were ≥M5.0 and 115 ≥M4.0. Of these, an unusual six reached M6.0, where we might usually see one or two.
The Week’s Biggest Earthquake: M6.7, South Atlantic
There are times when large earthquakes fill me with dread as I try to find something to say about them. These are the ones that occur in areas so remote and sparsely inhabited that there’s little instrumentation to record information and so few people likely to be affected that it isn’t worth seismologists publishing any data on them.
This week is one of those weeks. The largest earthquake this week, at M6.7, is remote almost beyond my comprehension, located in the South Atlantic with the nearest land worth noting (that is, apart from an odd uninhabited rocky island) the southern tip of Africa, almost 2,300 km away.
Remote instrumentation, however, is a blessing. Seismologists can pick up some information from large earthquakes at stations all over the world, and so there are some things we know about this event, other than its magnitude. We know its depth (around 10km); we know the nature of movement (lateral) and we know its tectonic setting (close to a fracture zone at the plate boundary between the African and Antarctic plates).
This isn’t much, but it’s enough to make a very simple assessment — that the earthquake resulted from movement along a strike-slip fault associated with the fracture zone.
M6.6 Earthquake, Alaska
Almost exactly on the other side of the planet, in the far north-west of the Pacific Ocean, the second largest earthquake of the week is slightly less remote (two people reported having felt it, whereas nobody at all claimed to have felt anything from its larger counterpart). We know little about this earthquake, either, although it was deeper (at around 110km) and also had a strike-slip motion.
The tectonic setting here is very different. Like most large earthquakes, it occurred at a plate margin and this one is a subduction zone, where the Pacific plate converges with, and descends beneath, the North American plate along the Aleutian Trench.
The evidence, however, suggests that this probably isn’t a direct result of subduction. Although the earthquake is 110km deep, its location relative to the plate margin (around 150km into the over-riding plate) is a little too far away to make me think it’s caused by movement at, or even very close to, the plate interface.
It’s possible that the descending plate may be at a relatively shallow angle at this point, but the second piece of evidence suggests otherwise. If that were the case, we might expect to see some kind of compressive motion — but the presence of lateral movement implies that the cause was deformation within the over-riding plate.
US Earthquakes: California
So, two remote earthquakes — and one that’s anything but (and, neatly, around half-way between them). The M4.1 which occurred 20km from San Jose might be less than one-hundredth the size of either of those reported above, but it attracted over 5,300 responses to the uses ‘did you feel it?’ page, and almost 3.5 million people probably experienced some kind of very light shaking.
It’s another shallow earthquake, and another with strike slip motion. Because of its location, in such a heavily-monitored area, we know more about it. It’s associated with movement on the notorious San Andreas Fault Zone, specifically the central section of the Calaveras fault, which is to the east of the main San Andreas Fault itself.
This week’s ‘quake is relatively small. There was an M5.4 nearby in 2007 and the USGS notes that: “The 2003 Working Group for California Earthquake Probability assigned an 11% probability that the Calaveras Fault would produce a magnitude 6.7 or larger earthquake in the next 30 years.”
Last Thoughts: Different Margins, Similar Mechanisms
I don’t usually choose my featured earthquakes on the basis that they illustrate a point (last week was an exception) but this week the three which I chose as being interesting do happen to do just that.
All three earthquakes share one common feature — they were all caused by strike slip movement. Yet all occurred at different types of tectonic setting — one in a collisional setting (Alaska), on in an extensional setting (South Atlantic) and one where the plates slide past one another (California).
The point, therefore, is that the nature of an earthquake is rarely as simple and obvious as its tectonic setting suggests, and the more information we have about an event, the more we can say about it.