I have a feeling this article is going to be rather short and sweet, partly because the week of 26 January-1 February 2017 hasn’t produced a lot of significant earthquake activity, and partly because I find myself separated from my trusty earthquake reference, which is invaluable for weeks when I need to know about faults no-one else has ever heard of.
So, let’s look at the numbers. I don’t keep a running average of earthquake shown on my source, the United States Geological Survey’s real-time earthquake map, but I have a pretty fair idea in my head of what constitutes a normal week.
The map doesn’t show every earthquake on the planet, but roughly speaking it covers the major ones — usually those of at least magnitude 4.5 (≥M4.5) but a fair few of those of M4.0-M4.4 as well — along with those of all magnitudes (not all earthquakes) in the US and its territories. And this week, those earthquakes are a bit thin on the ground.
In total the map recorded just under 1300 earthquakes, of which 97 were in excess of M4.5 and 26 above M5.0. The largest was M5.9. In a typical week we might expect rather more, and at least a couple in excess of M6.0.
The Week’s Biggest Earthquake: M5.9, Vanuatu
One thing that continues very much as normal is the occurrence of earthquakes in the western Pacific. Here, the coming-together of the Pacific and Australian plates is complex, as the margin changes in nature and direction. But we would usually expect to find a significant earthquake (nearer M6.0 than M5.0) in this region, and this is the case this week.
The earthquake occurred at a relatively straightforward section of the plate boundary, where the Australian plate subducts beneath the Pacific plate along the north New Hebrides Trench.
The depth and location of the earthquake relative to the plate margin itself suggest that it’s associated with the actual plate interface. Normally we’d expect such an earthquake to be caused by collisional movement, but in fact the USGS data suggest that it was caused by lateral movement, probably along deformation in the upper plate.
It’s worth mentioning that, though this is the largest earthquake in a quiet week, it’s very much first among equals over a longer period. The area has experienced earthquakes of M5.6, M5.8 and another of M5.9 in the past 30 days.
Deep South: M5.4 Earthquake, Drake Passage
In the Drake Passage, between the tip of South American and the Antarctic Peninsula, waves can pass uninterrupted around the globe: it’s a savage, lonely place. Caught between the two lies the Scotia microplate, which features fairly regularly in this digest because of earthquakes along the eastern margin, which is a short subduction zone.
This week’s deep south earthquake, however, was at the other end of the microplate. Here, the margin is a little vague and not fully marked on the USGS map (although you can trace it by the bathymetry, which is the shape of the sea bed).
There’s little fully-cited information out there on this particular part of the world, but the USGS map identifies it as the Shackleton Fracture Zone. Fracture zones accommodate differential movement through a set of fractures perpendicular to the (notional) plate boundary, and are characterised by strike-slip earthquakes. The little we know for certain about this tremor — its location, its depth and the direction of movement — suggests that that’s exactly what it is.
US Earthquakes: Alaska
Earthquakes? Well, not really. I’m going a little off-topic here, because (although you won’t have noticed it) Alaska’s Bogoslof volcano has been going a bit crazy over the past six weeks — so much so, in fact, that it came in to carry of the award for volcano of the year for 2016 on the basis of a couple of weeks’ explosive action.
What’s interesting is that there’s virtually no earthquake activity in the area. Major subduction zones are characterised not just by earthquakes but by chains of volcanoes, and so you might expect the two to be connected. But the trench around Bogoslof remains relatively serene.
Last Thoughts: Earthquakes Without Volcanoes?
So, if volcanoes and seismic activity refuse to appear together in Alaska, what are they doing in Iceland? Iceland doesn’t get a lot of major earthquake activity, but it has plenty of volcanoes, and this week there was significant activity beneath Katla volcano — enough to prompt warnings of an increased likelihood of an eruption.
Alaska is a subduction zone: Iceland is an area of divergent movement. There are differences in the origins of volcanic activity and differences in tectonics. Small earthquakes are often the precursors for volcanic activity — and it may be that there was volcanic activity associated with Bogoslof but it simply doesn’t appear, for whatever reason, on the map.