The United States Geological Survey’s real time earthquake map for the week of 24-30 November, 2016 shows that things have settled down a bit, after a few turbulent weeks.
While the overall number of tremors shown is around what we might expect, the pattern isn’t confused by series of aftershocks, as we’ve seen in recent weeks.
That said, there was certainly plenty of noteworthy activity on the map, which includes (broadly speaking) tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere.
In total there were just under 1600 reported events, with two of ≥M6.0 and 29 ≥M5.0.
The distribution was also broadly as we might expect, with the larger earthquakes concentrated around the margin of the earth’s tectonic plates.
The Week’s Biggest Earthquake: M7.0, Offshore El Salvador
This week’s largest earthquake occurred where the Cocos Plate subducts beneath Central America, along the Middle America Trench. The overall situation is complicated by the fact that the Cocos plate actually subducts beneath not one but two tectonic plates (the North American in the north and the Caribbean in the south); but the earthquake epicentre is very definitely along the southern part of this boundary.
There’s no USGS event page available for this event, although the broad regional summary notes that in this area: “subduction results in relatively high rates of seismicity and a chain of numerous active volcanoes; intermediate-focus earthquakes occur within the subducted Cocos plate to depths of nearly 300 km. Since 1900, there have been many moderately sized intermediate-depth earthquakes in this region.”
From this, we might infer that this offers an adequate explanation for this week’s M7.0. The available data, however, suggest otherwise. The earthquake was shallow — at just 10km. It’s close to the plate boundary and the combination of death and location suggest that it might have been caused by compressional movement at or near the plate boundary i.e. a classic subduction event.
The moment tensor diagram produced by the USGS, showing the direction of movement, again suggests differently. This implies an extensional origin — suggesting that the earthquake was the result of shallow deformation, probably in the over-riding plate, rather than subduction.
M6.6 Tremor, China
The USGS map lists this week’s central Asian earthquake as being East North-East of Karakul, Tajikistan, but in fact its epicentre lies over the border — just — in China. It’s difficult to pin down a specific origin from the available information, given that the region is one of very complicated tectonics.
At the macro level, it’s dominated by the collision between India and Asia, which has led to the uplift of the Himalayas and the Tibetan plateau; but the scale of this uplift has caused multiple major fault zones — normal, reverse and strike-slip — which are capable of generating (and frequently do generate) earthquakes.
Maps of active fault zones in this area, reproduced by Robert Yeats, indicate that the region of the epicentre has both reverse and strike-slip faults, with the latter perhaps marginally the more probable candidate (the scale of Yeats’ maps is too small to be specific). Again, the USGS offer a clue through its moment tensor diagram — and this confirms that movement was predominantly strike-slip.
US Earthquakes: Offshore Oregon
There wasn’t a lot of significant earthquake activity in the US this week, but one that stands out is the M5.5 off the coast of Oregon (and by that I mean 200km off the coast). Although in itself not especially significant — with just 10 people reporting they felt it and no reported damage — this strike-slip earthquake is a reminder of a significant threat to America’s west coast.
The earthquake occurred at the western edge of the Juan de Fuca plate, the remnant of a much larger plate which has been subjected beneath the American continent — and continues to be so. The Cascadia subduction zone, at the eastern edge of the plate, has been quiet for centuries — but we know that it can generate huge subduction earthquakes with significant consequences.
Last Thoughts: Not What They First Seem
This week’s earthquakes are a reminder that there are many components to determining the origin of an tremor and that it’s always worth looking at the scientific detail. Depth and location alone would imply a collisional source for the El Salvador ‘quake, whereas the direction of movement shows it to have been extensional. Similarly, in China, there were a number of possibilities on the map, and it took the recorded data to suggest one rather than another.
We simplify tectonics because, to some extent, we have to. But what’s going on in the Earth can be very complicated indeed.