A lot happened this week, if you look at the United States Geological Survey’s real time earthquake map.
The map, which records earthquake of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, is pretty congested for the seven days from 5-11 November 2015.
Although there were no large earthquakes (≥M7.0), there were seven of ≥M6.0 and 45 ≥M5.0.
Where were they all?
It isn’t that the planet has suddenly started shaking like a jelly: Much of this activity is focused on two particular earthquake clusters.
One, off the coast of Chile, included four ≥M6.0 and nine ≥M5.0; for the other, off the northern tip of Sumatra, the figures were two ≥M6.0 and 10 ≥M5.0.
Taking these out there was no shortage of activity elsewhere, but the tremors followed the usual pattern of activity, most of it focused on or near the margin’s of the Earth’s tectonic plates.
The Week’s Largest Earthquake: M6.9, Chile.
Big earthquake. Offshore. Chile. Have we been here before? Well, yes.
A couple of months back a major earthquake (M8.3) occurred off Chile, generating a tsunami but causing, in the circumstances, surprisingly little damage.
As large tremors tend to do, that earthquake generated a sequence of aftershocks, which had more or less died down until this week’s minor flurry of four large and many smaller ones. It’s interesting to note that a comparison of the larger earthquakes in the region since September (see map) shows that the most recent earthquakes (orange and yellow) were a couple of hundred kilometres further north than the September 16 ‘quake.
All of these earthquakes occurred in the same tectonic setting; on a subduction zone where the Nazca plate descends beneath the South American continent. Without detailed information, it’s impossible to say how (or whether) the two are related.
It does seem, however, that the second, smaller series may have been triggered by the first, possibly as a result of the first one increasing the strain further along the plate margin.
M6.4 ‘Quake and Aftershocks: North of Sumatra
‘Sumatra’ and ‘earthquake’ in the same sentence usually add up to one thing — the Boxing Day earthquake of 2004. As in the case above, this week’s cluster of earthquakes occurred in the same tectonic setting as the more notable megathrust ‘quake, along a major subsection zone.
The week’s tremors — M6.4 and a cluster of medium-sized aftershocks — occurred just off the northern tip of Sumatra, at shallow depths (between 10-50km). It isn’t clear, at first sight, what caused them, but the deepest of them are certainly deep enough to be associated with subduction.
That said, there is a major strike-slip fault extending along the island of Sumatra which may also have influenced the earthquakes.
The fact that the USGS map includes no notification of any tsunami warning implies that the latter explanation is more likely, because tsunamis result from vertical, not lateral displacement.
US Earthquakes: Alaska
Not to be left behind, Alaska produced its largest earthquake for some time — an M6.2 with three associated medium-sized aftershocks. And it’s the same story.
A major subduction zone, where on plate (the Pacific) subducts beneath another (the North American) generating earthquakes, along a stretch of trench capable of creating far larger seismic events.
In this case the depth of the tremors relative to the plate margin suggests that, of all of those considered, these are most likely to be created by movement at or near the plate boundary.
Last Words: Variations on a Theme
The largest earthquakes of the week are all associated with subduction margins — three different margins, involving six different plates. Serendipitously, those three subduction zones have generated the three largest earthquakes ever recorded — in Chile in 1960 (M9.5), Alaska in 1964 (M9.2) and Sumatra in 2004 (M9.1).
There are (possibly) subtle differences: only one looks to be a clear subduction earthquake, though another may be. That other might, alternatively, be the result of strike-slip deformation within the tectonic setting. And the third looks as if it may have been triggered by a larger earthquake close by. All the same — and yet all different.