The year rolls on. Parts of North America and Europe are experiencing unusually low temperatures; parts of Australia are baking in unusually high ones. But one part of the Earth system, at least, remains as normal: the earthquakes.
The week of 5-11 January 2017, as represented by earthquakes on the United States Geological Survey’s real time earthquake map, fell well within the boundaries of the expected. 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, showed just under 1500 tremors for the week, of which 34 were ≥M5.0, one ≥M6.0 and one ≥M7.0.
And there was little exceptional in the numbers, either, beyond a pair of earthquakes of M5.8 and M5.2 in northern Canada, so remote that there’s very little to say about them. The rest of the week’s major earthquake activity occurred pretty much where we’d expect it, around the margins of the planet’s tectonic plates with a concentration in the western Pacific.
The Week’s Biggest Earthquake: M7.3, the Philippines
The section of the western Pacific which includes the Philippines and eastern Indonesia is, if I’m honest, a mess.
The USGS and other tectonic maps of the world show major plate boundaries — most notably the Philippine Trench to the east and the Timor Trough to the south, but that’s by no means the whole picture.
Zoom in and look at more detailed tectonic maps and you’ll find a web of short subduction zones, thrust zones and normal faults.
This mega-scale crazy-paving results from the compression of the Pacific, Australasia and Eurasia: there’s a lot of crust caught up between these three tectonic giants. It’s one of the reasons why there are so many earthquakes in this part of the globe. An earthquake of the magnitude of this week’s M7.3, which had its epicentre in the Celebes Sea, is by no means exceptional.
What’s interesting about this tremor is its depth. At first sight, an earthquake of this magnitude in this location (even in this area, it’s a long way from any significant fault zone) might be ascribed to local crustal faulting and attributed to shallow deformation. But this is deep — very deep. To be precise, it occurred 617km inside the earth.
Faults viewed on maps are in two dimensions: they don’t show how and where they extend in the Earth. I’m grateful for the information provided by the USGS here, which indicates that the tremor was “the result of deep reverse faulting within the inclined seismic zone defining the deep limit of the Molucca Sea microplate beneath the Celebes Sea Basin.”
This is particularly interesting, because the Moluccan Sea plate is buried and even the more detailed maps don’t show it as an individual entity — illustrating the complexity of the tectonics in this area.
M5.3 Quake, Iran
I always worry when I see there’s been an earthquake in Iran. It’s one of those areas where it doesn’t take a big tremor to cause a lot of damage, down to the high population and lack of earthquake-resistant buildings. This week’s M5.3 in Iran cost at least four lives at the time of writing — but given that an estimated five million people or more felt shaking, this is, thankfully, a remarkably low figure.
Iran, trapped between the Eurasian, African and Indo-Australian plates, is another area where differing major plate movements cause complex fault zones — though in this case, the major expression of this coming-together is on land rather than beneath the ocean. Again, the fault-maps of the region produce a much more complicated picture than the broader maps — which tend to show a single collisional margin.
Unlike the Philippines earthquake, this one is, on investigation, pretty much what it looks like on first sight. A shallow earthquake in a zone of continental collision is most likely to be the result of thrust faulting as mountains build. The instrumental readings, the depth and the detailed fault map confirm this.
If it looks like a duck, walks like a duck and quacks like a duck, they say, it’s a duck. For once.
US Earthquakes: Alaska
I like Alaska (on a map, I mean: I’ve never been to visit). It’s easy on the eye, with that long finger of the Alaskan peninsular and the chain of islands stretching away.
Alaska looks good on fault maps, too, with parallel faults stretching along the same lines as the Pacific and North American plates come together. Unlike the other regions we’ve considered this week, it’s much more simple, much more… elegant.
A map of this week’s earthquakes in the peninsular and the Aleutian islands looks simple, too, and they are — the product of the collision, which produces the faults, which produce the topography we see on the map. Lovely.
Last Words: Those Arctic Earthquakes
Meanwhile, in the very middle of nowhere in the very far north, at an enormous distance from any known plate boundary, there were the two large earthquakes I mentioned in the introduction. I can’t let them pass without comment, because they are unusual. But I can’t garner enough information on them — so remote, so inaccessible and in an area little studied — so I can’t comment in detail.
Geologically, the area is very old and very stable, but such areas aren’t immune to earthquakes. The USGS notes: “Scientists who study eastern and central North America earthquakes often work from the hypothesis that modern earthquakes occur as the result of slip on preexisting faults that were formed in earlier geologic eras and that have been reactivated under the current stress conditions. The bedrock of Eastern North America is, however, laced with faults that were active in earlier geologic eras.”
Strictly speaking, the high Arctic isn’t the same tectonic province as eastern and central North America. But in the absence of any other information, this explanation seems equally applicable.