So, here we go again with another quiet week on the earthquake front, at least according to the United States Geological Survey’s real time earthquake map for 2-8 February, 2017. The map, which broadly speaking includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed a total of less than 1400 tremors, of which just one was larger than M6.0 — and that one, unusually, wasn’t in the Pacific region.
Although the total number of tremors of ≥M4.0 was around the same as usual (there were 97), there were just 22 of these larger than M5.0 — at the low end of what we’d usually expect. As usual, most of these were along the boundary between the Australian and Pacific tectonic plates in the western Pacific and that between the Australian and Sunda plates along the western margin of Indonesia.
Elsewhere, there was a scattering of earthquakes across central Asia, from the Caspian Sea to the Himalayas, but with over 2000 km separating the furthest apart of these, they aren’t directly connected.
The Week’s Biggest Earthquake: M6.3, Pakistan
The week’s largest earthquake, coming in at M6.3, occurred just off the coast of Pakistan. Technically, the setting is one of collision, with Arabia (and Africa behind it) closing on Eurasia. While much of this collisional zone is characterised by thrust belts and uplift on land, the south-eastern end is an offshore subduction zone (the Makran subduction zone), and it is in the (more seismically active) eastern sector of this that the earthquake occurred.
At M6.3, this earthquake was significant, and the USGS summary indicates that an estimated 2.5 million people may have felt some kind of shaking, though for most it would have been weak. Perhaps the most notable thing is not this week’s tremor itself — which wasn’t widely felt and where shaking was relatively limited, geographically — but the fact that it reminds us of how vulnerable some areas are to major earthquakes.
The Makran subduction zone experienced a tremor of M8.1 in 1945 and, as the USGS summary observes: “the population … resides in structures that are highly vulnerable to earthquake shaking”. Of particular concern is the fact that the city of Karachi lies just a couple of hundred kilometres further along the subduction zone.
Earthquakes along the Java Trench
The Java Trench is a much larger and much more notorious subduction zone than the Makran, largely because it was the source region for the Boxing Day earthquake and tsunami of 2004. It’s regularly active, although the megathrust earthquakes which have made it so infamous are rare, and this week was no exception.
The week’s earthquakes illustrate movement along the length of the trench, where the Indo-Australian plate descends beneath the Sunda plate. There are four of them, varying in magnitude from M4.7 to M5.2. The USGS has published no data on the direction of movement, but their location and depth (all roughly as far from the trench as they are deep) suggests that they’re all the result of movement at or near the plate interface.
Together, these intermediate sized earthquakes give a picture of movement along the length of a margin. Too often the focus of this digest is on a single larger event. Sometimes it’s useful to look at lower-level activity, too.
US Earthquakes: Um…
America, it seems, has been asleep this week, failing to register a tremor large enough to show on the main map (i.e. at least M4.5). The largest the superpower could muster was an M3.9 in Alaska — and that was as big as a tremor in Australia, which is normally pretty quiet, seismically speaking.
So what was there? The Lower 48 produced a scattering of small earthquakes, more or less where you’d expect — some in Oklahoma (not many); some in California (not many); and so on. But again, a map of all earthquakes from the USGS map does show the main seismic zones of the Rocky Mountains rather neatly.
Last Thoughts: Joining the Dots
Small earthquakes, as in the US, definitely produce patterns. If we were to look at all movement — something which would require a much greater level of detail than is shown on the USGS map — we’d be able to pick out some of the major fault lines as on a dot-to-dot drawing.