Did the Earth move for you this Valentine’s Day? Sorry, that’s a corny joke and a very old one, but I couldn’t resist it; but the Earth doesn’t move or stop moving because of our calendar. It just keeps a-quivering, day and night.
This week, the numbers produced nothing outstanding, but that doesn’t mean there were no events worthy of note. In the week of 11-17 February 2016, the United States Geological Survey’s real time earthquake map included just two tremors of at least magnitude 6 (≥M6.0); 25 of at least M5.0 and 89 of at least M4.0.
The map includes earthquakes of all magnitudes in the US and its territories and those of at least M4.0 elsewhere, and showed a total of just over 1520. The distribution was nothing unusual with most of the larger ‘quakes around the margins of the planet’s major tectonic plates, along with a few outliers.
The Week’s Biggest Earthquake: M6.3, Indonesia
Located in the jumble of tectonic plates, microplates and crustal slivers that is the western Pacific, it’s unsurprising that Indonesia is so highly seismically (and volcanically) active. This week the largest earthquake, an M6.3, occurred south east of Java, in the Banda Sea.
At first glance tis earthquake appears as if it may be connected to the subduction zone which runs offshore from the islands of Java and Sumatra and which has been the source for some of the planet’s major earthquakes (including the Boxing Day earthquake of 2004). The epicentre of the earthquake was, indeed, located to the north of the subduction zone’s eastern end.
But plate tectonics is, as Oscar Wilde once said of the truth, rarely pure and never simple. The collision (at a very general level) of Australia and Eurasia has led to widespread folding, thrusting, subduction, extension and all sorts of other things as the crust caught between the continents has to find somewhere — up, down or sideways — to go.
The depth of the earthquake — just 28km — suggest that it isn’t the result of subduction. Yeats notes that this area is “the surface expression of a continental collision zone,” albeit one in its very early stages. This being the case, it’s most likely that the earthquake occurred as a result of uplift along one of many thrust faults, triggered by the process of collision.
M5.8 Tremor, Christchurch, New Zealand
It isn’t that long ago, in 2010, that the New Zealand city of Christchurch was struck by an earthquake of M7.2. A year later, a tremor of M6.1 in the same area killed almost 200 people. The city had experienced significant earthquakes previously, in the nineteenth century.
This week, an earthquake of M5.8 made Valentine’s Day memorable — and probably more than a little scary — for the residents of the city. Although no major damage was reported, the Internet is full of clips of collapsing cliffs, mud bubbling up in the streets and water slopping around in swimming pools. It was a very visible earthquake.
Its origins are less visible. The South Island of New Zealand is dominated, tectonically speaking, by a major strike-slip fault, the Alpine Fault, which runs along its west coast — many miles from Christchurch. So where do the Christchurch ‘quakes come from?
The 2010 and 2011 tremors were eventually determined to have been caused by movement on a previously unmapped fault. (Yeats observes that, following the nineteenth century tremors, “there was no sign of faulting … west of the city”. Without any further information, it’s impossible to say for certain whether the 2016 tremor occurred on the same fault or one close to it (either known or unknown). But the evidence suggests that it did.
US Earthquakes: Oklahoma
Anyone who’s been following this digest with any regularity will know how often the US earthquakes section comes back to the Sooner State. The ongoing sequence of anthropogenic earthquakes seems to be getting larger and larger — and this week reached M5.1.
The USGS has some interesting facts in relation to this tremor. It is: “One of 7 M4 or larger earthquakes within the last four months near Fairview, Oklahoma”. It’s the largest in the state since 2011 (when an earthquake of M5.6 struck near Prague. And, perhaps most interestingly — and disturbingly — it’s “currently the 3rd largest earthquake in Oklahoma state history”.
Note the word “currently”. And keep watching Oklahoma.
Last Words: Fault Zones
On a simple tectonic map, fault zones appear as single lines. It’s a kind of visual shorthand. When we talk about the San Andreas fault, we actually mean the San Andreas Fault Zone — a much broader area of faulting along a continental margin.
Both the Indonesian and New Zealand earthquakes this week show us the shortcomings of simplification. They remind us that there’s a lot going on that we can’t see and, in the case of the Christchurch ‘quake, perhaps just don’t know about.
As I said before — rarely pure and never simple.