And so the pendulum swings back, from a busy week of earthquakes to a normal week and into a quiet one. The United States Geological Survey’s real time earthquake map wasn’t exactly congested this week.
The largest tremor on the map which, broadly speaking, includes all tremors in the US and it territories and those of at least magnitude 4 (≥M4.0) elsewhere, was just M5.9 and the second largest was just M5.5. In total there were 14 tremors of at least M5.0.
The tremors were small, but the distribution throws up some interesting insights. The largest occurred in Australia — pretty much unheard of — and a couple of earthquakes occurred in the very remote far north of the Arctic Ocean, so far north they were almost off the map. I’ve said it before and I’ll say it again. A quiet week can be fascinating.
The Week’s Biggest Earthquake: M5.9, Australia
Australia? I hear you ask. Did she really say Australia? Did she mean Australia? Not Austria or Argentina or Afghanistan?
I did mean Australia. I can’t recall an Australian earthquake previously claiming the honours as the largest in the week. Australia is one of the oldest, most stable continents on the planet; and yet we shouldn’t be surprised by this week’s tremor. Stable continental interiors, being old, are criss-crossed by ancient fault zones which can become reactivated.
Seismologist Robert Yeats describes earthquakes in Australia as “moderately common” and the USGS archive includes over 60 of ≥M5.0 in the past century. They include a series of 17 between 1987 and 2001 in a remote part of the Northern Territory, which included the largest, at M6.7; another collection in the Darling Ranges of Western Australia (maximum M6.5); and, smaller but more significant, an M5.6 in Newcastle which killed 13 people.
This week’s earthquake was truly in the middle of nowhere and the USGS event page estimates that barely 6000 people will have felt it. But it does remind us that Australia is like anywhere else — vulnerable to an earthquake in a populated region.
Earthquakes in the Eastern Mediterranean
On the other side of the world from Australia, the continents are anything but stable. The closure of the ancient ocean which separated Africa and Eurasia continues, leaving the Mediterranean Sea as a relic and with a complicated jumble of different type of fault zones as land is uplifted in some areas and subducted in others.
This week’s map shows a series of earthquakes in the eastern Mediterranean, in Greece, Albania, Macedonia, Bulgaria and Italy. While they are too small and too widely spread to be directly related — over 600km separates the furthest-flung — it’s reasonable to suggest that, though each has a local tectonic context, they share a single, large-scale source.
That source is continental collision as Africa moves northwards against Eurasia, folding and faulting the crust that lies between. Eventually it will lead to the closure of the Mediterranean Basin and probably, in the future, a new mountain range.
US Earthquakes: Oregon
The largest earthquake in the US wasn’t that big, either, in global terms, though it came in at a respectable M4.9. It occurred almost 300 km off the site of Oregon, so remote that the USGS event page includes no impact summary and we may therefore assume that no-one felt it.
The earthquake occurred at an active fracture zone, between two sections of ocean ridge. It’s part of the Juan de Fuca microplane and in this area earthquakes are frequent, moderately sized and, generally, pretty much harmless.
Last Thoughts: Plate Tectonics Never Dies
This week we learned a few things about plate tectonics. Oceans close and, in closing, generate earthquakes. Major plates are consumed until only remnants, like the Juan de Fuca plate, remain. This subduction causes earthquakes. But the constant cycling and recycling of crust, even when it eventually settles to an apparently stable state, leaves fractures which can be reopened under relatively minor stresses.