It’s another quiet week in numbers terms, seismically speaking — but the week of 23-29 March, 2017 has a few tasty tidbits to offer for anyone looking for an interesting earthquake. Let’s crunch the numbers, first.
The United States Geological Survey’s real time earthquake map shows a total of almost 1650 earthquakes, of which two are of at least magnitude 6 (≥M6.0), 25 ≥M5.0 and 114 ≥M4.0.
The map includes, broadly speaking, earthquakes of all magnitudes — but not all earthquakes — in the US and its territories and those of at least M4.0 elsewhere. The figures are an obvious underestimate of total activity, though they do pick up the major seismic events.
A look at the featured map, which shows the distribution of the earthquakes ≥M4.5, shows that most major earthquakes are associated with the margins of the Earth’s tectonic plates. The outstanding exception is that in the middle of the Pacific, in Hawaii — and we’ll come to that one later.
The Week’s Biggest Earthquake: M6.6, Kamchatka
Russia’s Kamchatka peninsula is a strange place, noted for its chain of volcanoes (many of them active) and for regular seismic activity. Tectonically, it sits west of the margin along which the Pacific plate descends beneath the Okhotsk microplate (a southern extension of the larger North American plate), with the two coming together along the Kuril-Kamchatka Trench at a rate of 76-88mm per year.
It’s a subduction zone, which means that most larger earthquakes are going to be associated with movement at or near the plate margin, although the major forces involved also cause deformation and seismic activity within both the over-riding and descending plates.
This week’s M6.6 bears a little investigation, as it occurred a couple of hundred km or so to the north of the end of the trench, at a depth of around 20km where the USGS maps suggest the plate interface lies at a depth of around 60km. This suggests that deformation, rather than subduction, is the primary cause.
Kamchatka has a history of very large subduction zone earthquakes (including, interestingly, very large earthquakes at very great depths). The largest on record in this section of the trench is an M9 from 1932, and in this context, this week’s tremor is nothing out of the ordinary.
M5.3 and M5.1 Tremors, Azores
Technically the two earthquakes which occurred this week in the mid-Atlantic were not in the Azores, but in the middle of the ocean 140km west of the island chain. This matters when it comes to determining their origins.
The Azores themselves are a volcanic island chain resulting from a mantle plume, or hotspot — but they also lie close to a triple junction which marks the boundary between the North American plate, the African plate and the Eurasian plate — indeed, they lie astride the latter boundary, which is (mostly) transform in nature.
There’s a lot going on here, tectonically — far too much to describe in detail — but the limited available evidence suggests (if I may mix my metaphors) that the Azores hotspot and the transform fault are a little bit of a red herring. The location and depth of the two earthquakes recorded here this week suggest that they are more likely to be associated with the separation of North America and Europe along the mid-ocean ridge.
US Earthquake: Hawaii
As it happens, the US earthquake featured this week is that wild card I mentioned above, floating about in the middle of the map of the Pacific. Hawaii regularly experiences small earthquakes as its volcanic magma chambers fill and empty — imagine the rise and fall of an animal breathing (only very much more slowly).
This may or may not have been the cause of this week’s earthquake — the data from the USGS aren’t enough to be certain. It’s also possible that the tremor may have been caused by movement along one of the many normal faults associated with the growth of the island chain.
Last Thoughts: Multiple Layers
The Azores always gives me pause for thought. There’s a lot going on — a mantle plume within reasonably close proximity to an ocean ridge, and the additional complication of a major lateral boundary. In Kamchatka, too, the junction of the Kuril-Kamchatka Trench with the Aleutian Trench means that earthquakes there bear closer inspection.
In all three of this week’s featured earthquakes, there’s a degree of ambiguity, although there’s enough evidence to hazard a reasonable guess. It’s a lesson — the processes of earthquake genesis are much more complicated than they might at first appear.