Numerically speaking, the United States Geological Survey’s real time earthquake map didn’t provide any talking points in the week of 20-26 July 2017. The map, which shows (broadly speaking, and subject to a lot of caveats) earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere, came up with numbers that were just about what we’d expect.
There was one earthquake of at least M6.0, 22 of at least M5.0 and 121 of at least M4.0 in the total of just under 1950 tremors shown on the map. And most of them were where we’d expect them to be, with the larger ones at the margins of the planet’s tectonic plates and the smaller ones picking out broader seismic zones.
Nevertheless, the larger earthquakes featured this week aren’t in places we would regularly see major tremors.
The Week’s Biggest Earthquake: M6.6, Aegean Sea
I’m always interested in earthquakes in the eastern Mediterranean and so probably feature them more often than someone else might. But it’s actually not that usual for the largest tremor of the week to be in this area, and this week’s M6.6, which killed two people, is the largest in the Aegean since 1914.
The tectonics of the Aegean are complicated.
To the south lies the subduction zone of the Hellenic Trench (compressional, north-south, with Africa converging on Eurasia).
To the north lies the dangerous North Anatolian Fault (lateral, broadly east-west, with the Anatolian Block sliding past Eurasia’s southern margin).
These two dominate but there are other faults and motions in play in this area, too, with the result that there’s a lot of movement, of different types and in different directions, around the Aegean Sea.
The USGS notes that the central part of the Aegean is subject to extensional movement and that it’s this that caused this week’s earthquake, which, it says, occurred: “as the result of normal faulting at a shallow crustal depth within Eurasia plate lithosphere. The focal mechanism solution indicates that the earthquake occurred on a moderately dipping fault striking either eastward or westward.”
The tremor was felt along much of Turkey’s Aegean coast and the Greek islands in the area, with the USGS summary indicating that around 40 million people may have felt the earthquake — including in major cities such as Athens and Izmir — although for only very few of these would the shaking have been strong.
M5.9 Tremor, Central Indian Ocean
Larger earthquakes tend to occur at subduction zones and it’s unusual for me to be talking about a mid-ocean ridge earthquake as the second-largest of the week. This week’s, an M5.9 on the mid-Indian Ocean Ridge, is large but not exceptionally so. The truth is that this kind of earthquake is a regular feature of the USGS but because they are so remote, and because most weeks see several larger earthquakes, these events tend to slip under the radar.
Mid ocean ridges are where the Earth’s tectonic plates are driven apart, fuelled by rising hot rock caused by convection within the planet. With that upwelling comes extensional movement and the oceans are separated by deep rift valleys where submarine volcanism occurs. As the rocks get pushed apart, they fracture and earthquakes occur — which is what has happened this week, deep below the surface, 400 km north east of Mauritius.
This isn’t the only earthquake of its type this week — there were others in the South Atlantic and South Pacific — but it’s the largest.
US Earthquakes: All Quiet
Sometimes I’m struck by how earthquake-free the US actually is, given its enormous size and the major fault zones which lie along its western and Alaskan boundaries. We talk a lot about these faults, and even minor earthquakes on them generate a lot of chatter — rightly, because even an earthquake of intermediate magnitude in, say, Los Angeles, has the potential to cause enormous human and economic damage.
But this week, the USGS map showed not a single earthquake in the US — including Alaska and Hawaii — of M4.0 or more. The largest, in Alaska, was just M3.8. And that’s something I find unusual.
Last Thoughts: What’s Unusual
So there was nothing unusual in the numbers this week, but what makes something stand out is context. Large earthquakes in the eastern Mediterranean aren’t unexpected but they aren’t (usually) very large — so we notice them when they are. Similarly, we tend to ignore the larger earthquakes at mid-ocean ridges, mainly because they’re so remote that they don’t affect humans and so aren’t considered worth discussing.
As for the US — yes, the absence of even an intermediate-sized earthquake for seven whole days may not be earth-shattering news, but I did raise an eyebrow.
In other words, what makes an event unusual isn’t the event itself, but its context.