I go away for a week, and the world goes quiet. Very quiet — the United States Geological Survey’s real time earthquake map failed to rustle up anything larger than a couple of tremors of magnitude 5.8 for the week of 6-12 October, 2016. Normally we might expect to see two or three of at least magnitude 6 (≥M6.0).
The map, which includes (broadly speaking) tremors of all magnitudes in the US and its territories and those of at least M4.0 elsewhere, recorded just 21 earthquakes of ≥M5.0 and 90 ≥M4.0 in its total of just under 1800.
With a couple of exceptions, one in the North Sea (considered below) and one in the Indian Ocean, these larger earthquakes were pretty much where we’d expect them to be — around the margins of the Earth’s tectonic plates. As usual, the greatest concentration was in the western part of the Pacific.
The Week’s Biggest Earthquake: M5.8, Taiwan
In fact, Monday’s M5.8 in Taiwan was the joint largest of the week (with one in Indonesia); but I comment more regularly on Indonesia and so Taiwan is where we begin. Tectonically speaking, the island is caught, as you might say, between a rock and a hard place, at the northern end of the boundary between the Philippine Sea and Eurasian tectonic plates.
This sounds fairly straightforward but, in reality, the situation is more complicated. The boundary isn’t simple: it consists of two subduction zones, one trending roughly east-west and the other roughly north-south. The island of Taiwan is largely formed by the latter with the former connecting around halfway down its length. To make matters worse, as Robert Yeats observes, the two subduction zones exhibit different behaviour.
The details are too complex to go into here, but the available evidence (location, depth and style of movement) suggests that this week’s earthquake is the direct result of neither of these, but rather of a general collisional movement. That it is the result of compressional movement is beyond doubt, but its shallow depth (14km) and location in a no-man’s-land between the two subduction zones imply that its source is broader deformation, rather than movement at or near either of these two zones.
M4.8 Tremor, Norwegian Sea
Earthquakes — intermediate ones at least — can, and do, occur where they aren’t expected. This week saw a tremor of M4.8 pop up in the icy Arctic waters of the Norwegian Sea, almost 200km from the closest land. Unusual, yes — unprecedented, no.
The North Sea (I simplify) is a sedimentary basin where subsidence is taking place – that’s where an area is dropping down. Subsidence goes hand in hand with extensional tectonics and results in normal faulting, with one side of a basin slipping downwards.
There’s very little information available on this week’s M4.8, and what there is is conflicting. (The USGS has its depth at 17km and the European Seismic Network lists it at 2km). But it’s a reasonable bet, given the geological setting, that it’s a shallow earthquake resulting from normal faulting in one of the North Sea’s many buried rifts.
This is unusually large for the North Sea, but it isn’t the largest on record. In 1931, an earthquake further south, on Dogger Bank (between the UK and Denmark) came in at M6.1, and remains the United Kingdom’s largest earthquake. Norway’s largest, an M5.8 which occurred in 1866 in the same region as this week’s, was ten times the magnitude.
US Earthquakes: Back to the Salton Sea
A few weeks back, I commented on an earthquake swarm in the Salton Sea, at the southern end of the San Andreas Fault Zone. The swarm seems to have quietened down now, if the USGS map is anything to go by (though it doesn’t include all the smaller earthquakes), but I did spot a snippet of news about it this week in Scientific American.
It isn’t much, but I found it interesting. The article quoted seismologist John Vidale as saying that: “in the Salton Sea, the earthquake swarm last week changed the odds of an earthquake of at least M7 on the San Andreas fault to between 1 in 3000 and 1 in a 100 for the following week”.
It didn’t happen. (The article was dated 6 October). But that doesn’t mean it won’t.
Last Thoughts: All Kinds of Everything
An earthquake caused by compressional forces. An earthquake caused by extensional forces. And the (varying) possibility of a large earthquake caused by two plates sliding past one another. None of these are particularly large — but they do show that, even in a quiet week for earthquakes, there’s always something worth looking at.