In contrast to last week’s apparent worldwide calm, there’s an embarrassment of interesting earthquakes to choose from for this week’s digest. With the United States Geological Survey’s real time earthquake map (which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) showing one tremor ≥M7.0, five of ≥M6.0 and 36 ≥M5.0 – we can’t cover all those of interest.
In summary, the larger earthquakes were, as usual, mostly associated with the planet’s tectonic plate boundaries, though there were one or two eye-openers. By no means least among them were an M4.9 in Queensland, Australia, and a slightly smaller tremor (M4.6) on the northeast coast of Canada’s Baffin Island — both areas usually considered tectonically stable. And there were several along mid-ocean ridges, including the largest of the week — so more on mid-ocean ridge earthquakes later.
The Week’s Largest Earthquake: M7.1, Mid Atlantic Ridge
It’s rare for the largest earthquake of a given week to be at a constructive margin, and when that does happen it’s usually because of limited activity elsewhere. Not this week — the tremor which shook in the mid-Atlantic, over 1000km from anywhere (and by anywhere we mean Greenland) registered M7.1, which is unusually large for such a setting.
Most such earthquakes go largely unreported because of their remoteness and so we can only guess at the exact mechanisms involved, but the magnitude of this week’s means that there are some published data available.
Mid-ocean ridges (broadly) comprise spreading centres offset by lateral transform faults known as fracture zones and the USGS attributes this week’s tremor to strike slip faulting on just such a fracture zone, the Charlie Gibbs Fracture Zone. The lateral movement explains why, despite the magnitude, there was no resultant tsunami; tsunamigenesis requires vertical movement.
Although such tremors aren’t all that uncommon, the scale of the most recent is unusual. The USGS notes that: “Over the past century, five other earthquakes of M 6.3-7.0 have occurred within 250 km of the February 13, 2015 event, likely along the same fracture zone. The largest of these was a M 7.0 event on the same day in 1967, 48 years before M 7.1 earthquake”.
M6.7 Earthquake, Argentina
The Andean margin pops up regularly in the digest, a product of the collision between the Nazca plate and the South American continent.
Subduction of cold, dense oceanic crust beneath warmer, more buoyant continental material has led to the uplift of the Andes and earthquakes in this region may be either subduction-related (at the interface between the two plates) or as a result of the uplift, when they are more likely to be the result of shallower, reverse faulting.
At a depth of 223km and approximately 400km from the Peru-Chile Trench itself, the earthquake of appears, at first sight, to be anomalous — too deep to be caused by mountain building and too shallow for a subduction zone earthquake.
In this area, however, the angle of subduction is unusually low, which implies that the likely cause is, after all, movement along the interface between the two plates and can therefore be considered a subduction earthquake — although in the circumstances it would be hardly accurate to call it ‘typical’.
US Earthquakes: Oklahoma (Yes, Again)
Although the rash of Oklahoma earthquakes this week didn’t include the largest in the lower 48 (we can look to Arizona for that one, an M4.8) it is increasingly difficult to ignore the trend. This week there were 31 tremors of M2-5-M3.9.
On the basis of USGS research these tremors can be attributed to human activity (specifically, wastewater disposal).
At Decoded we regularly end up attributing earthquakes to slippage along unmapped faults, and now new research seems to confirm that this is the case, and that the swarm of Oklahoma quakes are occurring along ancient, previously unmapped, and newly reactivated, faults.
Quakes and Continental Interiors
The Oklahoma earthquakes show that sleeping seismic giants can awake if you poke then with a stick, just like the ones in fairytales. Sometimes they wake without help. Although there’s no information on the quakes this week in Australia and Canada, it’s a reasonable assumption that they, too, are the result of movement on faults that are very, very old indeed.