There wasn’t a great deal of earthquake activity in the week of 20-26 October 2016: the largest tremor to feature on the United States Geological Survey’s real time earthquake map, and the only one larger than magnitude 6, was an M6.2 to the west of Japan.
The map, which includes tremors of all magnitudes (though not necessarily all tremors) in the US and its territories and those of at least M4.0 elsewhere, recorded just over 1550 seismic events, of which 17 were ≥M5.0 and 92 ≥M4.0. As usual, the larger ‘quakes were concentrated around the margins of the planet’s tectonic plates, with occasional outliers — including, this week, an M4.7 in the Arctic Ocean, so far north that no information is available other than its location and its depth.
The Week’s Biggest Earthquake: M6.2, Western Japan
This week’s M6.2 tremor in the west of the Japanese island of Honshu was a reminder that even relatively small earthquakes (in the context of Japan, this was minor) pose a threat to life and to property. Media reports indicate that at last seven people were injured — but that’s remarkably few considering that the USGS summary page suggests that over 170,000 people experienced strong, very strong or severe shaking as a result of the tremor.
Japan is no stranger to earthquakes, many of them much larger and much more deadly than this week’s. The reason is that the country lies across (and is the product of the relationship between) four of the earth’s tectonic plates — the North American, Pacific, Eurasian and Philippine Sea plates.
This week’s tremor occurred on the Eurasian side of the boundary between the latter two, roughly 300km form the actual margin. The depth of the earthquake (10km) and tectonic maps of the area indicate that it was the result of movement along one of the many lateral (or strike-slip) faults, which characterise the plate margin at this point. This fact is borne out by the available evidence from the USGS.
M5.8 Tremor, Kuril Islands
The next largest earthquake also took place in the western Pacific, though this one was a little further north, just over 200km from the Japanese island of Hokkaido. At a regional level the same tectonic structure applies, with the four plates coming together: but this time the earthquake occurred where the Pacific plate collides with, and sinks beneath, a southern extension of the North American plate.
At around 30km depth and roughly 100km from the plate boundary, the earthquake is associated with subduction, though it seems unlikely to be generated by movement at or very close to the boundary. Rather, it’s likely to be caused be deformation within the over-riding plate.
Unlike the (not much) larger earthquake, which occurred around 1500km away, this tremor (a long way from major centres of population) caused no reported damage or injuries. At the time of writing the USGS event page showed the just one person had logged a report that they had felt it — indicating once again that it isn’t just size but location and density of population that determine the impact an earthquake has.
US Earthquakes: Alaska
In the US, the two largest earthquakes (by almost an order of magnitude, at M4.8 and M4.9) occurred way up in the north of the Pacific, in Alaska. Here, again, it’s the Pacific and the North American plates interacting; here, again, the former subducts beneath the latter. At around 300km apart these may well be related.
Both are at roughly the same distance from the plate boundary (150km) with their epicentres in the over-riding plate. Both are at roughly the same depth (140km). And while neither of them is significant enough to warrant a moment tensor diagram (which shows the direction of movement) it’s a reasonable guess that both are the result of subduction at or near the plate margin.
Last Thoughts: The Ring of Fire
All the earthquakes featured this week occurred in the north western Pacific — and in fact 12 of the 17 tremors of at least M5.0 were Pacific earthquakes. This isn’t really surprising: the Pacific Ocean, surrounded as it is by lengthy and active subduction zones, not only produces the chain of volcanic islands and mountain ranges which give it its picturesque name, the Ring of Fire — it’s also highly seismically active.
These subduction zones produce some of the largest earthquakes on the planet, accounting for seven of the top ten since 1900. The Ring of Fire is a dynamic, and deadly, beast.