Not a lot stirred in the week of 7-13 January 2016 — not in seismological terms, at least. In fact, it was probably as near an average week as is possible given that there’s no such thing as an average week.
Better, perhaps, to say that the United States Geological Survey’s real time earthquake map showed nothing particularly unusual.
The map, which records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed a total of just under 1600 tremors worldwide, of which just two were larger than M6.0 and 25 at least M5.0.
If there was an anomaly, it was the M4.7 in the deepest South Atlantic, but information on tremors in such remote locations is scarce and the USGS event page is eloquently blank. In this case I won’t comment because I’d rather stay silent and be thought a fool than express an opinion and prove myself to be one.
The Week’s Largest Earthquake: M6.5, Philippines
At least there’s information about the week’s largest earthquake, an M6.5 event in the Philippines. The western margins of the Pacific are tectonically congested as the Pacific plate converges on the Eurasian plate (in the north) and the Australian plate (in the south) with all manner of crustal slivers, or microplates, caught up in the resulting crush. The result, inevitably, is earthquakes.
This week’s largest tremor took place in the Philippines. If the tremor’s broad tectonic context is the collision between the Pacific and Australian plates, we can break that down into a more specific collision between two of the planet’s smaller plates, the Philippine Sea and Sunda plates.
At first sight, the cause of the earthquake seems most likely to be subduction along the Philippine Trough. If you dig a little deeper, however, there’s yet another layer of tectonics which doesn’t appear on the map.
Seismologist Robert Yeats remarks on the convergence of the two minor plates and notes the existence of a yet smaller, buried, plate — the Molucca Sea plate. It’s likely that the earthquake is related to the convergence of the two larger plates. The Molucca Sea plate, subducting both east and west, looks to be on its way out.
M6.2 Earthquake, Japan
Further to the north the nature of the Pacific margin is less chaotic and therefore a little easier to understand, if not without its complications. Japan overlies the junction between four plates, the Pacific, Eurasian and Philippine Sea plates, with the addition of a long finger of the North American plate extending southwards.
It isn’t always easy, in this context, to find the source of an earthquake. Our knowledge of what happens beneath the sea, and deep beneath the sea bed, is anything but complete. Yeats, in his description of the tectonics of the area, draws attention to what he calls a ‘diffuse plate boundary’ and the differing interpretations of what lies beneath.
Most significant earthquakes in this area (west of Hokkaido) are shallow; but this week’s occurred at a depth of 239km, implying that it’s not related to the putative thrust belts which form the diffuse boundary to which Yeats refers and which are the likely source of the shallower events, but rather to subduction of the westward-moving Pacific plate.
US Earthquakes: M4.8 Oklahoma
In the introduction I mentioned that there wasn’t much unusual on this week’s map. That still holds — but a few years ago I might have regarded the largest earthquake in the lower 48 as anomalous. Yes; it’s Oklahoma again. The only thing that makes this latest in the ongoing, human-induced earthquake series noteworthy is that it’s slightly larger than usual.
Last Thoughts: Complications
Buried tectonic plates…competing theories about plate motions…deep subduction zones underlying narrow thrust belts…it isn’t easy explaining what’s going on. Our planet is complex, and even those bits of it that are well-studied are full of surprises, with several possible mechanisms for an earthquake which appears on a map.