I mutter a lot about numbers of earthquakes: This week, 22-28 June 2017, the United States Geological Survey’s real time earthquake map has produced a set that must be pretty much (subject to the usual caveats) what I would consider average. The map, broadly speaking, includes earthquakes of all magnitudes in the US and its territories and those of at least M4.5 elsewhere.
This week, in its total of 1720 tremors, the map recorded two earthquakes of at least M6.0; 22 of at least M5.0 and 101 of at least M4.0.
And if the numbers are normal, so is the geographical distribution, with most of the larger tremors occurring at or near the margins of the Earth’s tectonic plates. There are a couple of exceptions — one in Africa, of which more later, and one in Hawaii — but again, these are only slightly unusual.
The Week’s Biggest Earthquake: M6.8, Guatemala
If I were feeling lazy, I’d be tempted to refer you to the digest of two weeks ago, when the largest earthquake was an M6.9 in Guatemala, not that far away from this week’s largest, which came in at M6.8.
The two earthquakes are the same in terms of the tectonic setting but there are some differences. Tectonically, both are products of the complex interaction between the Cocos, North American and Caribbean plates. The former subducts beneath the latter two along the Middle America Trench, and those latter two slide past one another along a transform boundary. Where the three come together, there are competing stresses and plenty of faulting.
Two weeks ago I speculated that the M6.9 earthquake might be the result of that strike-slip movement rather than subduction. The USGS have subsequently produced a diagram showing that the movement was extensional, which confirms my view in general, if not specific terms.
This week’s earthquake was shallower (46km rather than 94km) and included a significant component of compressional movement. It was located around 200km from the first, and much closer to the offshore trench.
Taken together, these suggest that the earthquakes are only loosely related, in the sense that they are products of the same setting rather than movement on the same fault or series of faults. This week’s is probably related to movement at, or more likely in the plate above, the subduction zone than the earlier event.
M5.6 Earthquake, Mozambique
Africa is old and generally stable, but it’s anything but immune from earthquakes, even if they don’t reach the magnitude of those at active subduction zones. Within the continent, the dominant tectonic feature is the East African Rift, where extensional movement is associated with possible continental break up — but earthquakes do occur elsewhere.
This week there was an M5.6 event in Mozambique. This one is a little bit mysterious, since there isn’t extensive information on the tectonics here. Robert Yeats notes that “seismicity in the channel between Africa and Madagascar may indicate a plate boundary” and the earthquake certainly lies close to this channel.
A look at maps of historical seismicity in the area, however (also in Yeats’ book) indicates that the earthquake occurred at the southern end of the rift zone. Once again, then, we have an earthquake which might be associated with either one of (or interaction between both) two different mechanisms.
The dominant motion is strike-slip, which doesn’t really help very much because it could relate to either or both. It’s an illustration of the complexities of earthquake interpretation. Like the truth, seismology is never simple.
US Earthquakes: California
Last week I talked about a swarm of earthquakes in Yellowstone, and noted that this kind of activity is normal, if not always easily explained. this week, to prove my point, there’s another swarm of minor earthquakes, this time in California.
The USGS map shows 40 earthquakes, the largest of them an M4.0, and these can be associated with tectonic movements within the Rockies — specifically, with the Mohawk Valley Fault Zone, which forms the eastern margin of the Sierra Nevada. The fault zone shows a strike slip boundary, and a series of earthquakes with strike slip motion.
Did I just say seismology is never simple?
Last Thoughts; Anyone Can Get It Wrong
I’m the last person to criticise anyone for a mistake, so don’t misunderstand me for dwelling on someone else’s: there’s a reason for it, and that’s that I want to emphasise — that the data on which this digest is based is incomplete and can change.
I probably wasn’t the only one to sit up when I got the notification of a major earthquake — M6.8 — in Santa Barbara, California, earlier this week. You’d have been particularly surprised if you lived anywhere near Santa Barbara, because you wouldn’t have felt it. It didn’t happen. Or rather, it did, but…um…it happened in 1925.
The USGS removed the notification, which was the result of a computer glitch, in double-quick time, but not before the media had had a bit of fun over it. The point, though, is that when we talk about earthquakes, we aren’t always dealing with the whole truth. Sometimes an earthquake magnitude is adjusted. Sometimes an earthquake is missed off the list. And sometimes, something pops up that just isn’t there.
We all make mistakes. I’m a little relieved to find I’m not the only one.