So, shall we have some numbers? What’s up on the United States Geological Survey’s real time earthquake map this week? Anything astonishing? Anything unusual? No to both of those. But anything interesting?
There’s always something interesting.
This week’s map, which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, offers no surprises for 16-22 June 2916.
The map’s total of just under 1700 tremors includes 4 of ≥M6.0, 31 ≥M5.0 and 112 of ≥M4.0. They’re all more or less where we’d expect them to be, too, with the larger ones round the margins of the Earth’s tectonic plates, with concentrations in the western Pacific and along the Andean margin of South America.
The Week’s Biggest Earthquake: M6.3, Vanuatu
As with last week, we have joint largest earthquakes and I’ve picked one. This week an M6.3 struck in the Vanuatu island chain, the largest in a series of almost twenty in excess of M4.5.
This area forms one of the more straightforward sections of the complex boundary between the Pacific and the Australian plates. Along the western Pacific margin the plate boundary twists and turns, with changes in both its nature and in the direction of subduction (in those sections where subduction occurs).
Overall it’s primarily a convergent margin, with the two plates coming together, and in the region of Vanuatu the Australian plate descends eastwards beneath the Pacific plate along het New Hebrides Trench.
Subduction causes earthquakes, either directly at the plate interface or in the fracturing of crust deformed by the dominant collisional movement. In this case the depth of the earthquakes series — most, including the mainshock, shallower than 15km — implies the latter.
M6.1 Earthquake, Mid-Atlantic
The Atlantic Ocean in bisected by a dog-leg of an ocean ridge, a margin between tectonic plates. The ocean ridge system circles the entire planet, though the fact that so much of it is on the deep sea bed means that it’s seldom visible.
The system is driven by upwelling of molten rock along the ridge, forcing the tectonic plates apart, generating strains and creating earthquakes. Intermediate-sized earthquakes are by no means uncommon, even if they pass largely un-noticed.
This week’s M6.1 in the mid-Atlantic, some 1800 km from land, was no exception.
There were three other earthquakes on the mid-Atlantic ridge, though smaller, from the far north to the far south, along with another in the Indian Ocean.
US Earthquakes: Oklahoma
Most weeks there’s some new research on the ongoing anthropogenic earthquake swarm in Oklahoma. Often these studies are highly specific and of marginal interest. This week, as the swarm continues, a piece of work published in the Journal of Geophysical Research addresses a key question — how large can these earthquakes get?
To date the largest has been the M5.6 Prague earthquake of 2011. Statistical analysis of the earthquake led the report’s authors to conclude that: “induced earthquake magnitudes should be treated with the same maximum magnitude bound that is currently used to treat seismic hazard from tectonic earthquakes”.
Last Thoughts: All Normal
Normal is the word of the week, or so it seems. The number of earthquakes? Normal. The geographical distribution? Normal. Our largest earthquake, our featured earthquake, our regular list to still-shaking Oklahoma? Nothing out of the ordinary.
But, as I noted in the introduction, normal doesn’t mean dull. In particular, the knowledge that something is not out of the ordinary can be valuable in terms of improving our management of normal events — as the Oklahoma study shows.