And so, after a week of stillness, the week of 31 March-6 April 2016 saw normal service resumed. The earth kept shaking.
The United States Geological Survey’s real time earthquake map, which records (in broad terms) all earthquakes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, was a busy place.
In total there were over 1700 earthquakes on the map, of which five, all around the margins of the Pacific Ocean, were ≥M6.0 and 25 ≥M5.0.
As usual, most of the intermediate and larger earthquakes (for convenience, those of ≥M4.5) were associated with the margins of the planet’s tectonic plates.
There was just one outlier, off the west coast of Greenland.
The Week’s Largest Earthquakes: M6.9, Vanuatu
This week, two tremors tied for the title of the week’s largest earthquake. As the two occurred only around 40km apart, in a similar tectonic setting and within a few days, it’s reasonable to deal with them together as they are evidently part of the same earthquake series.
The first of the two occurred on 3 April; the second, to the south west, was three days later. Both occurred at a similar depth (35 and 34 km respectively). With differing magnitudes we might have defined one as the mainshock and one as a foreshock or aftershock; but in this instance it’s simpler just to treat them as two large earthquakes with the same cause.
And what was the cause? The complex tectonics of the western Pacific regularly give rise to noteworthy earthquakes as the boundary between the Australian and Pacific plates twists and turns, varying in nature and in the direction of subduction (where that occurs).
West of the islands of Vanuatu, subduction does, indeed, take place, with the Australian plate descending beneath the Pacific plate along the North New Hebrides Trench. The earthquakes (and the dozen or so aftershocks of ≥M4.0 which accompanied them) all appear on the map on the overriding plate at a distance from the trench which is roughly equal to their depth. This suggests that they are straightforward examples of fracture at or near the plate interface — in other words, classic subduction earthquakes.
M5.1 Tremor, Greece
The many subduction zones of the Pacific are a feature of a mature ocean which is beginning to close. On the other side of the world, the Mediterranean basin represents an example of a much more advanced stage in the closure of an ocean.
The former Tethys Ocean has been consumed by the coming together of Africa, Eurasia and Arabia.
What’s left of it is squeezed between the continents, twisted and uplifted. While not regarded as a major earthquake zone to compare with those of, say, the western Pacific, it nevertheless experiences regular seismic activity and is capable of producing much larger (and potentially damaging) tremors.
This week’s M5.1 occurred along the Hellenic Trench, which curves from the southern Aegean, south of the island of Crete. Here the African continent is subjecting beneath Eurasia. This week’s earthquake was almost on the trench itself and at a depth of 10km. These data aren’t clear enough to indicate whether, like the Vanuatu series, it was a direct result of subduction or whether it was the result of deformation. But we can be sure the continental collision had a hand in it.
US Earthquakes: Meanwhile, in Alaska
Most weeks when we discuss US earthquakes, we talk about one of three places — Alaska, California or Oklahoma. Well, here we are again, back up in the north of the Pacific. And once again we can look to a subduction zone as the source of an earthquake, this time an M6.2 on the Alaskan peninsula.
This earthquake is unlikely to have been directly caused by subduction. Further west, the Aleutian Trench is a large and straightforward subduction zone but the tectonics of mainland Alaska are more complicated, with many fault and fold belts running broadly parallel to the Pacific-North American margin.
This week’s earthquake was shallow (10km) and around 200km from the trench. That makes it almost certainly the rest of crustal deformation.
Last Thoughts: The Pacific Ring of Fire
The Pacific is surrounded by zones of earthquake and volcano activity. Major subduction zones occur not just around the Pacific plate itself but along the margins of the Cocos and Nazca plates, which subduct beneath Central and South America. The margins of this ocean include the subduction zones of Alaska, Cascadia, the Andes, New Zealand, Tonga, the Philippines and Japan, among others. Oh, and just for good measure, our old friend San Andreas (though not a subduction zone) is in there too.
It’s no surprise, then, that there’s barely a week when we don’t see a major earthquake (at least M6.0) in the Pacific, and it’s cause for comment when the largest earthquake occurs elsewhere.