Another week, another variation in the seismic statistics.
At one level, all is normal. The United States Geological Survey’s real time earthquake map, which, roughly, includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of almost 1600 tremors. That’s more or less what we’d expect.
The (rough) figures for different magnitudes are also nothing to raise an eyebrow — 104 of at least M4.0 and 23 of at least M5.0.
At the top end, however, there’s a little more seismic activity than in your average week, with four tremors ≥M6.0, one of them in excess of M7.0.
The Week’s Largest Earthquake: M7.2, South Sandwich Islands
On average, we can expect to see one earthquake of ≥M7.0 in a month. Sometimes there are more. Sometimes there are none. May 2016 saw just one, an M7.2 in the remote South Sandwich Islands, deep in the South Atlantic.
Large earthquakes are often subduction-related, and that appears to be the case here. The South Sandwich Islands mark one of just two active subduction zones in the Atlantic (the other is the Caribbean). Here, the South American plate subjects along the narrow eastern edge of the Scotia microplate, creating a chain of volcanic islands — and a seismic zone.
Because it’s so remote, the Scotia plate is easily overlooked — but the subduction zone, and the fracture zones to the north and south, are seismically active. There have been five tremors of ≥M7.0 in this area in the past decade. Probably, nobody felt any of them.
M6.6 Quake, East of Fiji
Another ocean, another subduction zone. Unlike the Atlantic, the Pacific Ocean has many subduction zones, most of them thousands of kilometres long. In consequence, it has many of the larger earthquakes. This week there were tremors of M6.6 and M6.4 west of, yet almost certainly associated with, the Tonga Trench. This subduction zone, with its southern extension, the Kermadec Trench, runs from Samoa to New Zealand and marks the boundary between the Pacific and Australian plates.
The Tonga Trench is one of the simpler subduction zones and, as such, often provides elegant examples of some textbook concepts. In particular, it offers us an illustration of something called a Wadati-Benioff zone. The theory is that, as a subducting slab descends, earthquakes at or near the plate interface occur at greater depths and their epicentres (the point on the surface immediately above where the earthquake occurs) occur further into the overriding plate.
Many subduction zones are complicated by other factors; but earthquakes along the (relatively simple) Tonga-Kermadec trench tend to follow this rule. The epicentres of this week’s pair of earthquakes lie 400-500km west of the trench. Their depths were 416km and 572km. They may not have been caused directly by movement at the plate interface, but they certainly follow the pattern of earthquakes associated with subduction increasing in depth.
US Earthquakes: Alaska
Alaska, another of the world’s major subduction zones, grumbles on and on, on a weekly basis, occasionally producing a major (and I do mean major) seismic event. This week an M5.8 occurred between the Aleutian Trench and the Alaska Peninsula. But the quake’s depth (35km) and its location (almost 300km for the trench) suggests that it was the result of crustal deformation rather than movement directly generated by subduction.
Last Words: Rarely Straightforward
It was Oscar Wilde who remarked that the truth is rarely pure and never simple. Sometimes I think he might have been talking about earthquake science. There are many complicating factors and the textbook situations rarely occur.
In the South Sandwich Islands, for example, the subduction zone is very short and sandwiched (pun intended) between two lateral faults; in Alaska there are conflicting directions of motion; in much of the western Pacific there are microplates popping up above subduction zones; and so on.
That’s why I find something pleasing about earthquakes like those in Fiji this week. Not too big, not damaging — and demonstrating a concept.