I’ve been away for a week but don’t think I missed anything outstanding. In the past seven days, 13-19 July, 2017, however, we do have something that’s unusual but shouldn’t be — an earthquake of over magnitude 7.
This unusual-but-not-unusual event apart (I’ll explain later) the number of earthquakes recorded on the United States Geological Survey’s real time map was more or less what we might expect.
There were, perhaps, a few more than usual at the lower end of the scale, but there’s usually quite a lot of variation here anyway. The map (which includes earthquakes of all magnitudes in the US and its territories and those of at least M4.0 elsewhere) isn’t comprehensive for the smaller ones, but it does pick up those of significant magnitude.
The Week’s Biggest Earthquake: M7.7, North-East Russia
The largest earthquake this week struck in the very north of the Pacific at the western end of the Aleutian Trench. With a magnitude of M7.7 it was the second largest of the year (only slightly smaller than an M7.9 in Papua New Guinea in January).
The Aleutian Trench marks a subduction zone where the Pacific plot descends beneath the North American plate, producing one of the most seismically and volcanically active zones on Earth. Earthquakes here are regularly large though (owing to the remoteness) they cause little or no damage. This week’s tremor generated a tsunami but a very, very small one: the maximum height, according to the NOAA, was just 0.3 feet.
At M7.7, the earthquake is big enough to justify a detailed summary from the USGS — something I always welcome, as it saves me poking and prodding at data that may be incomplete. The USGS seismologists have access, in any case, to far more detailed information.
Even without this, it was immediately obvious that the earthquake wasn’t a subduction earthquake. Firstly, the movement was lateral rather than vertical, and secondly the scale of the tsunami was so small as to be negligible, when a large offshore earthquake in a subduction zone might have been expected to produce a tsunami of more significant scale.
If you want the details: “the earthquake occurred as the result of transform faulting on or near the plate boundary between the Pacific and North America plates in the Northwest Pacific Ocean. The focal mechanism solution of the event indicates the earthquake occurred either on a right lateral fault oriented NW-SE, or on a left lateral fault striking NE. At the location of the earthquake, the Pacific plate moves towards the NW at a rate of ~73 mm/yr with respect to North America, effectively moving horizontally past the North America plate at this the western extent of the Aleutian Trench.”
M5.4 Earthquake, Greece
The collision of Africa with Eurasia as part of large-scale continental movement has trapped oceanic crust between the two continents. The Mediterranean, which is the remnant of the ocean which once separated them, is a confusion of slivers of crust, a maze of different fault types and directions — and, although not generally subject to the larger earthquakes, is no stranger to seismic activity.
The consumption of crust takes place along subduction zones, and the largest in the Mediterranean runs south of the island of Crete, where an earthquake of M5.4 occurred this week. It’s too small to generate an earthquake summary but the data published by the USGS leads us to similar conclusions as the Aleutian earthquake, though on a smaller scale.
Continental convergence along the Hellenic Trench is ongoing but, like the Aleutian earthquake, the motion was lateral and the distance from the trench (around 60km) relative to the depth (8.7km) too large to imply a direct association with subduction. This implies some kind of shallow crustal deformation.
US Earthquakes: Oklahoma
Oklahoma seems to have gone quiet recently. I say ‘seems to have’ because I’ve noted above that the recording of the smaller earthquakes, even in the US, isn’t as comprehensive as that of the larger ones. It does seem that fewer earthquakes have been recorded in the state recently, but I can’t say for certain.
This week’s M4.2 was the joint largest for the year to date. By comparison, 2016 saw 16 tremors of at least that magnitude in the state, with three reaching magnitudes of M5.0 or more. Whether this indicates that seismic activity is slowing down, or whether it’s just natural variation, I wouldn’t like to say.
Where Have The Big Earthquakes Gone?
In the the introduction I said I’d come back to this week’s largest earthquake and what made it special or not. So what was so different about an earthquake larger than M7.0? Normally we’d expect to see one every month or so, but so far this year we’ve had just three. So we might consider that unusual and we might wonder why.
But that’s just over a period of six and a half (let’s say seven) months, and averages represent clusters of activity and inactivity. If you look back at 2016, there were 16 of these large earthquakes — more than one a month. Add the two together and in 19 months we have 19 earthquakes of at least M7.0. Which is exactly what we’d expect.
As you were…