I’m back after a week’s break and, to be honest, I don’t think I missed much. Looking at this week, there are one or two things that caught my eye, but probably the most significant one is the number of earthquakes that popped up on the United States Geological Survey’s real time map this week.
The map includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere. The numbers it produces are an underestimate — a very significant one, but more of that later — but it does include the significant events and it does give us an idea of what’s going on.
This week, the total number of earthquakes on the map was (very roughly) around a third higher than normal: there were just over 2200 when we’d normally expect there to be around 1500. But the numbers of larger earthquakes was pretty much what we might expect: three ≥M6.0; 24 ≥M5.0; and 113 ≥M4.0.
In terms of distribution, the earthquakes were more or less where we’d expect them to be, with the larger ones concentrated around the margins of the Earth’s tectonic plates. there were a couple of exceptions — in central Australia and the middle of the Indian Ocean.
The Week’s Biggest Earthquake: M6.2, Chile
There may have been a lot of (recorded) earthquakes this week, but there was nothing exceptional in terms of magnitude. This week’s largest earthquake occurred in Chile, at a depth of 135km, way beneath the Andes.
Tectonically, the Andean margin is the result of the collision between the Nazca and South American plates, with the former descending beneath the latter along the Peru-Chile Trench. The collision raised the Andean mountains and continues to cause significant seismic activity along the entire western margin of South America. This week’s earthquake, however, wasn’t the direct result of subduction, but of the uplift of the mountain range.
There are a couple of clues to this. One is the nature of movement — it was caused by extensional movement, rather than collisional, with rocks moving apart and slipping downwards. Secondly, although it was relatively deep it was nevertheless considerably shallower than the actual interface between the plates. USGS maps of the area indicate that the depth of the interface, in this location, is over 400km, so the 135km depth implies deformation within the over-riding (South American) plate.
M4.2 Earthquake, Iceland
Okay, so it wasn’t very big (around one-hundredth the size of that above) but this week’s earthquake in Iceland caught my eye. Iceland’s earthquakes always do, because they are often associated with volcanic eruptions and a volcanic eruption in Iceland has the potential to cause a whole lot of disruption, inconvenience and economic damage — though not, as if happens, large-scale loss of life.
Nobody seems to have much to say about this week’s M4.2 in southern-central Iceland. There’s no USGS earthquake summary and the Icelandic Met Office, which is the official source of Iceland’s earthquake data, confines itself to indicating the tremor on a map. But what we can see is that it’s one of a cluster in the recently-active (2014-15) Bárðarbunga caldera.
My guess would be that this earthquake is probably either readjustment of the crust as a result of tectonic forces (it’s under extensional pressure) or else some magma is either entering or leaving the chamber beneath the volcano. Whether or not it presages an imminent eruption isn’t possible to say, but my guess would be not. Nevertheless, it’s worth keeping an eye open.
US Earthquakes: The Wild and Wobbling West
I’m obsessing a little bit about numbers, this week, though I try not to. But I thought it might be instructive to look at a whole lot of earthquakes plotted on the map of the western US (excluding Alaska). In the past seven days the USGS map of the western half of the States shows over half of the total earthquakes on the map (remember that both figures are an underestimate).
What you can see from those included is that, while major earthquakes follow major margins, smaller tectonic margins — for example, old fault zones, mountain ranges — are marked by smaller earthquakes. It isn’t rocket science, so to speak, but it does remind us that it isn’t just major seismic zones that experience earthquakes.
Last Thoughts: How Many Earthquakes?
Words, words, words, sighed Hamlet; and this week I feel I could paraphrase and sigh: numbers, numbers, numbers.
The USGS has a whole page of disclaimers about its earthquake map, which is good enough for my purposes, though anything but complete. It isn’t laziness on my part not to include them every week, though perhaps I should always remember to include a link. But there’s the rub (dear me, that’s my second Hamlet quote in this article) — even the ‘complete’ list of earthquakes in the US is anything but complete.
So how many earthquakes are there in a week? The Incorporated Research Institutions for Seismology (IRIS) websites quotes the USGS as giving an average estimate of 1.3 million (yes, million) earthquakes larger than M2.0 in a year, which would mean logging 25,000 of them on a map and make the patterns hard to pick out.
Given that the earthquake scale is logarithmic, you can multiply that number by at least a hundred for those of at least M0. (And yes, you can have negative magnitude earthquakes.)
So, how many earthquakes? Maybe even more (to quote Hamlet a third and final time) than are dreamed of in your philosophy. No wonder they don’t all appear on the map.