I’ve said it before and I’ll say it again. Earthquakes don’t come regularly. Some weeks there are few, barely worthy of mention. Others there are more noteworthy earthquakes than you could shake a stick at. The week of 26-31 August 2016 was definitely one of the latter, with so much going on that it’s difficult to know where to start.
But of course I’ll start where I always do — with the numbers. The United States Geological Survey’s real time earthquake map offered up a total of almost 1800 tremors this week, though ti didn’t tell the whole story. The map, broadly speaking, includes earthquakes of all magnitudes (which is not the same as all earthquakes) in the US and its territories, and those of at least magnitude 4 (≥M4.0) elsewhere.
This week offered us three earthquakes of over M6.0, in the Atlantic, the western Pacific and the Himalayas; four along the spine of the Atlantic Ocean (of which more later); a collection of aftershocks following last week’s deadly tremor in Italy; and an outlier up in the Arctic wilderness of Baffin Bay. In terms of distribution they were pretty much where we’d expect them to be — earthquakes are predictable to a certain extent, at least — with the larger ones concentrated along the margins of the Earth’s tectonic plates.
The Week’s Biggest Earthquake: M7.1, Mid-Atlantic
I’ve observed before that we get, on average, one earthquake of ≥M7.0 around every month. There was one last week in the South Atlantic and another this week in the Mid-Atlantic. They weren’t connected — they were too far apart and in different tectonic settings — and the occurrence of the two large events so close together in time is, again, a matter of natural variation.
This week’s major earthquake, an M7.1 between Africa and South America, like so many along mid-ocean ridges, troubled no-one. No-one, according to the USGS event shake map, felt it. Despite its size there was no tsunami. It resulted from strike slip movement along a fracture zone associated with the Mid-Atlantic Ridge.
In the central Atlantic, the ridge is the axis along which rising magma creates new ocean crust and drives the Atlantic wider, separating the Eurasian and North American plates in the north and the African and South American plates in the south. It’s part of a continuous ridge system that passes through all the Earth’s oceans and is key to the system of plate tectonics. The ridge itself is offset by fracture zones and it’s at one of these, the Romanche Fracture Zone, that the tremor occurred.
In any week we might see several such tremors on the USGS map, although they’re typically not the largest earthquake, which tend to occur in zones of convergence. There were others this week along the mid-Atlantic ridge. Two are smaller and we can ignore them. The third is up next.
M4.6 Earthquake, Iceland
In terms of magnitude an M4.6 is really neither here nor there. There were many larger tremors on the map this week. But in terms of media attention, this one, like every other significant Icelandic tremor, caused its own minor media storm and sent my Twitter feed into complete meltdown as the media over-reported the possibility of a major eruption and the world’s volcanologists (with, one feels, a sigh of exasperation) attempted to play it down.
Iceland is unique; a place where the Mid-Atlantic Ridge makes landfall as the result of a certain set of physical conditions that exists nowhere else on Earth. The ridge crosses the centre of the island, where it is spectacularly visible. But ocean ridges are associated with volcanism as well as with earthquakes and, humans being what they are, an earthquake near a volcano in Iceland can be interpreted as meaning only one thing — an imminent eruption.
This week’s M4.8 (and a series of others too small to appear on the map) occurred beneath Katla, a volcano with a record of violent eruptions. Kata last erupted in 1918 and is (as far as we can judge) thought to be overdue for another eruption.
The interesting thing about this (and previous earthquakes) is that seismic activity associated with volcanism can be shown to have a seasonal pattern as ice melts and relieves pressure on the ground beneath. The most authoritative source, Iceland’s Meteorological Office, draws the following conclusion: “Our assessment is that the volcano is in a period of summertime unrest.”
Katla is still very much a grumpy old volcano as is bound to erupt at some point. But this week’s earthquake almost certainly doesn’t mean it’ll erupt immediately.
US Earthquakes: M4.8, Wyoming
And so to the States, where the largest earthquake in the lower forty-eight occurred in western Wyoming. Although Wyoming has some tectonic similarities with Iceland (yes, honestly — think geysers) this earthquake wasn’t associate with the underlying volcanic hotspot which has created another geothermal paradise at Yellowstone.
Rather, this one is the result of extension, where the uplift of the Rockies has led to the creation of parallel mountain ranges and intervening troughs, known as basin and range scenery. These are created by slippage along normal faults (downwards movement) and the evidence suggests that this is the main cause of the Wyoming earthquake.
Last Thoughts: What, No Subduction?
Most weeks I focus in at least one earthquake resulting from subduction — the descent of one plate beneath another as a result of collision between them. Subduction earthquakes tend to be larger and therefore often are more damaging and more worthy of comment.
In fact there were plenty of those this week — it’s just that, for once, the largest tremor wasn’t caused by subduction. Though something tells me it won’t be long before subduction earthquakes are the largest on the map once again.