So, where are we this week? Pretty much in normal territory again for the week of 23 February-1 March 2017, if the earthquakes recorded on the United States Geological Survey’s real time earthquake map are anything to go by.
The map, which records earthquakes of all magnitudes (not the same as all earthquakes) in the US and its territories and those of at least magnitude 4.5 (with a few smaller strays) elsewhere, isn’t a complete record, but for the larger tremors it’s probably reasonably comprehensive.
This week it included a total of almost 1500 tremors, of which just one was at least magnitude 6 (≥M6.0), 32 were ≥M5.0 and 94 ≥M4.0.
As we would expect, most of the larger tremors occur at or in association with the margins of the Earth’s tectonic plates, although there are one or two outliers — a couple in Africa, which are associated with potential continental breakup and one in the High Arctic (cause unknown).
The Week’s Biggest Earthquake: M6.9, Fiji
We kick off once again in the western Pacific, where we so often find the largest earthquake in any given week. Here, the juxtaposition of the Pacific and Australian plates extends for thousands of kilometres and involves changes in both the direction and nature of movement.
This week’s largest earthquake measured M6.9 and occurred at great depth (over 400km) beneath the sea bed south of both Tonga and Fiji (and a long way from either). Its epicentre (the point on the surface immediately above the spot at which it occurred) lies between the Tonga Trench, where the Pacific plate descends beneath the Australian one, and the Hunter Fracture Zone, where the margin is more diffuse and movement tends to be lateral rather than vertical.
The relationship between depth and location of this ‘quake is the clue to its origin. The epicentre is roughly 400km from the Tonga Trench, the same as its depth. The Pacific plate descends at an angle of around 45 degrees, so that we can assume that the earthquake was probably caused by movement at or near the plate interface.
Depth, incidentally, is key in another respect. The deeper the earthquake, the more energy is lost before it reaches the surface. In an undersea location, this may make all the difference between an earthquake such as this triggering a tsunami or not. And in this case, it did make a difference — no tsunami occurred.
M5.2 Earthquake, off Vancouver Island, Canada
Meanwhile, in the north east of the Pacific, the earthquakes this week are rather smaller but no less interesting. Pretty much the whole of the boundary of the Pacific is seismically active, not just the Pacific plate itself but others within the boundary of the ocean. Up off the Pacific coast of the US and Canada there’s a small but complicated plate, the Juan de Fuca plate, which regularly generates earthquakes of intermediate size, mostly with lateral motion.
It’s interesting because it’s the remnant of the much larger Farallon plate, which has been largely subducted — in other words, it’s a tectonic plate at the end of its (many million year) lifespan. This week’s earthquake, an M5.2, was pretty typical of those which occur there, caused by strike-slip movement along one of the many fractures which characterise the fragment of the plate.
Seismologist Yeats describes this area as ‘relatively aseismic’, by which I can only assume that he means it doesn’t reach anything like its seismic potential (more on that below). It’s certainly active, regularly so, but its earthquakes, usually intermediate in size, go largely unremarked because they’re so far from land and don’t generate tsunamis.
US Earthquakes: Washington State
The subduction of the Juan de Fuca plate is the likely culprit, albeit indirectly, for an earthquake of M4.1 which occurred west of Seattle. I daren’t speculate too closely about the actual origin of this (not untypical) earthquake, because the tectonics here are complex with many folds and thrusts associated with subduction and uplift.
The available evidence does seem to suggest, however, that the tremor, which occurred at a depth of just over 15km, is probably the result of movement on one of the many faults associated with the uplift of the Cascade mountain range — itself a result of collision, subduction and, ultimately, uplift.
Last Thoughts: America’s Sleeping Giant
For all the rumbling and trembling within the Juan de Fuca plate and in the mountains of Washington and Oregon, the actual subduction zone, the Cascadia zone, is seismically very, very, quiet. The earthquakes tend to be like those outlined above, resulting movement within the Juan de Fuca plate itself, or from associated uplift.
Some might think this a good thing: others might consider it ominous.
We know that this subduction zone is capable of generating very large earthquakes indeed — most notably, an event in 1700 which seismologists have estimated, on the basis of geological investigation and oral history (it predated instrumental measuring) to have had a magnitude of M9 — which, if it were to be repeated today, would have a devastating impact across the Pacific.