It’s been an interesting week. I’ve observed before that normal statistical variation means that it isn’t unusual to get periods when the Earth experiences very few larger earthquakes and periods when it experiences many. The week of 18-24 August definitely fits in to the latter category.
The United State Geological Survey’s real time earthquake map went into meltdown this week. The map is broadly defined as including tremors of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere. This map isn’t a complete record, especially of the smaller tremors, but it’s pretty comprehensive for larger earthquakes. There were more than a few of those.
There was one very large earthquake — an M7.4 in the South Atlantic — which, together with its aftershocks, accounted for 24 tremors in excess of M5.0. A smaller earthquake series off Japan accounted for another eight. All told, the map recorded 55 tremors ≥M5.0. In a typical week we might see 20-30.
The Week’s Biggest Earthquake: M7.4
As we’ll see later, earthquakes make headlines according to how many people they affect, not how large they are. An earthquake of M7.0 or more is to be expected, on average, a dozen times or so each year. This week’s largest tremor wasn’t one that made many headlines, occurring as it did in a very remote part of the planet — the South Sandwich Islands in the South Atlantic.
Tectonically speaking, the South Sandwich Island arc is a short subduction zone, one of just two in the Atlantic. It forms the eastern edge of the Scotia microplate, a sliver of crust between the South American and Antarctic plates, the former of which is being subjected beneath it. All the available data indicate that it was the result of compression, resulting from movement at or near the plate boundary.
Large earthquakes along this margin aren’t unusual — there was an M7.2 along the same subduction zone in May 2016 — but because of their remote location, very far from population centres, they pass largely unnoticed.
The earthquake satisfied the conditions to generate a (localised) tsunami but nothing has, at the time of writing, been recorded or reported, probably because there’s no-one around to do so.
M6.2 Earthquake in Central Italy
If an M7.4 in a remote location can pass unnoticed, an M6.2 in a developed, populated region certainly cannot. This week an M6.2 in central Italy made headlines with reported deaths — at least 37 at the time of writing, just a few hours after the tremor occurred and with rescue work going on — and extensive damage. The USGS Pager alert suggests an 80% chance that there could be up to 1,000 deaths resulting from the tremor and notes that: “Significant casualties are likely”.
Western Europe is often perceived as seismically stable; but the collision of Africa and Eurasia means it’s anything but. With many slivers of crust being caught in the Mediterranean, there’s a huge complexity of movement with subduction zones, zones of extension and areas of strike slip faulting.
This week’s deadly earthquake struck in Umbria, in central Italy and, perhaps counter-intuitively in an area dominated by continental collision, was the result of normal faulting (extensional tectonics). That’s because there’s so much going on in the region that there’s relative differences in movement locally. Or in this case, as the USGS puts it: “primarily a response to the Tyrrhenian basin opening faster than the compression between the Eurasia and Africa plates.”
Although the population in the immediate area surrounding the epicentre is relatively sparse, the shaking was locally severe and strong shaking was felt in the city of Perugia (around 150,000 people) with the major cities of Naples and Rome both affected by light shaking.
M5.7 Quake, Queensland
This week, for the first time, our third featured earthquake isn’t in the US but in Australia. Recently I’ve covered a number of Australian earthquakes, even though the area is regarded as tectonically stable. This week there was a tremor of M5.7 off the Queensland coast.
Though not the largest to have occurred in the past year, this one made more headlines because, unlike May’s M6.0 in the Northern Territories, it was in a populated area rather than the back of beyond. “Most public interest, however, came from relatively weaker earthquakes in south eastern Australia and other populated centres,” noted The Canberra Times, going on to quote seismologist, Daniel Connolly on the subject.
“‘As far as earthquakes near populated centres go, they tend to be our more impressive ones,’ Mr Connolly said. ‘If an earthquake is big enough to be felt by a human, it’s probably going to be a decent earthquake.’”
Last Thoughts: Never Mind the Magnitude: Feel the Shaking
The major earthquake of the week, more than an order of magnitude larger than a deadly one in Italy, passed unnoticed. Smaller earthquakes in populated parts of Australia make news where larger, remote ones don’t. We don’t really need an illustration of this but it keeps coming up.
Earthquake hazard isn’t about the magnitude of the earthquake. It’s about the numbers of people affected and the vulnerability of the environment in which they live.
Decoding Science. One article at a time.