With the festive season coming up, and a lot going on in the news, no-one seems really interested in earthquakes — except people like me, of course. The week of 7-13 December, 2017 wasn’t outstanding in terms of the numbers, magnitudes or locations of the earthquakes which appeared on the United States Geological Survey’s real time earthquake map, but as always, it produced some interesting features.
The map, which includes earthquakes of all magnitudes — not the same as all earthquakes — in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of just under 1800 earthquakes, the largest of which came in at M6.4. There were five of at least M6.0, three of which were connected (of which more later) and 22 reached at least M5.0.
In terms of distribution there was nothing to catch the eye, bar one slightly unusual (but not exceptional) M4.5 in Poland. The rest of the larger earthquakes were, as always, associated with the boundaries of the Earth’s major tectonic plates.
The Week’s Biggest Earthquakes: M6.4, West-Central Pacific
I’m at a bit of a loss how to describe this week’s largest earthquakes, a pair of M6.4 tremors, in such a way that anyone will know where they were. The USGS puts them at M 6.4 – 50km NW WNW respectively of Fais, Micronesia, which didn’t leave me any the wiser. But a look at a map shows that the really are in the middle of nowhere. Think a couple of thousand kilometres east of the Philippines and about half as much again south of Japan and you’ll be in the right area.
So what happened? Tectonically, this pair of matching earthquakes (and the handful of aftershocks that also appear on the map, one of them only just a little smaller at M6.1) occurred near the leading edge of the Pacific plate close (relatively speaking) to where it collides with the Philippine Sea plate.
Make no mistake — this is the coming together of two big beasts and it takes place at depth. When continental plates collide they build mountains and when oceanic plates collide, they drag one another down. The margin between these plates (its the Pacific that loses the battle and is subducted) has created the Marianas Trench, thousands of kilometres long and with the distinction of being the lowest place on Earth and almost 11,000 metres below sea level.
That said, you might think that these earthquakes are the result of subduction, but the evidence suggests otherwise. They occur at the southern end of this section of the plat margin, where it turns through a right angle. To accommodate the lateral strain caused by this movement, the crust moves along strike-slip faults — which is what happened in this case.
M6.0 Tremor, Iran
I’ve talked a couple of times recently about the prospect of an earthquake in Iran. This week saw the second M6.0 in the central part of the country so far this month — very similar to its predecessor in size. When I talked about the earthquakes in the Pacific, I spoke of oceanic plates coming together and in this case it’s the giants of Arabia and Eurasia — and the result is uplift, not subduction.
Like its predecessor, this week’s M6.0 was caused by compression, even though most of the faults in this part of the area are lateral. In fact, the two are very similar, so much so that rather than describe them as fore and aftershocks, they’re best regarded as a ‘doublet’ — a pair of earthquakes occurring close together in time, space and magnitude. (Not unlike the two occurring in the Pacific and discussed above.)
With identical magnitudes and depths, very similar direction of movement and just a few days and around 15km between them, these two earthquakes are a classic pairing and, as such, almost certainly related.
US Earthquakes, California
Way down in southern California, the notorious San Andreas Fault zone changes its nature and becomes a constructive boundary along the middle of the Gulf of California. It was in one of the last shouts of this major strike-slip fault system, the Elsinore Fault, that there was an earthquake of M4.0 this week.
Such things aren’t unusual, as many sections of the San Andreas are creeping along quietly, so that movement is low level and fairly constant, though in other areas strain is building up at higher levels with the prospect of a major earthquake some time in the future.
As an aside, when people talk about the prospect of earthquakes ripping California apart so that half of it falls into the sea, they aren’t havering (as we say in Scotland) as much as you might think. The separation of Baja California from the mainland of Mexico is the beginning of the process: the chances are that that margin will propagate northwards and, in many millions of years the western side of the fault zone will drift off into the Pacific.
So there’s some truth in the rumour after all. But I wouldn’t lose any sleep over it.
Last Thoughts: It’s All Relative
So this week we saw another (in fact, another two) of those large earthquakes that no-one notices, no-one reports, no-one is affected by and, bluntly, no-one cares about, and illustration of the fact that the Earth quietly goes about its business for most of the time without anyone giving a toss.
We also saw an earthquake in an area where one of the biggest scare stories around — the break up of California — is actually happening, under our feet.
But it’s all relative. Continental breakup really is major change — but it’s so major that it isn’t achieved by any single event, but by creeping movement of the Earth’s tectonic plates.
The collisions between plates that have created earthquakes in Iran, in California and in the middle of nowhere this week aren’t in themselves, significant, but just minute steps in a cycle of continental break up and reassembly that lasts millions of years.
It’s a matter of perspective.