There was a lot of seismic activity going on from 2-8 June, 2016. The week produced nothing spectacular — the largest earthquake on the United States Geological Survey’s real time earthquake map was M6.6 — but there was a lot of action elsewhere and the planet is best described as ‘busy’.
The map, which (broadly speaking) includes most tremors in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included five tremors of ≥M6.0, 29 of ≥M5.0 and 129 of ≥M2.5 in its total of over 1600. All of these figures are larger than we might expect in a normal week.
Quite why this might be isn’t clear. It may be that, for some reason, more of them were reported (remembering that the map is not complete and draw on external sources, especially for intermediate-sized events outside the US). My guess would be that it’s actually our old friend, statistical variation that’s responsible.
There are no surprises in the distribution, though. The larger earthquakes occur pretty much where we’d expect — around the margins of the the Earth’s tectonic plates.
The Week’s Biggest Earthquake: M6.6, Indonesia
The largest earthquake of the week occurred of the Indonesian island of Sumatra, in the eastern Indian Ocean. Although Indonesia regularly produces the largest earthquake, you’re more likely to find it in the more tectonically-complex east of the country rather than the west.
This week’s M6.6 may have been the largest of the week but it was small in terms of its tectonic setting. The Java-Sumatra subduction zone, where the Indo-Australian plate descends beneath the Sunda microplate, is one of the longest and most active on the planet, capable of generating some of the largest and most deadly earthquakes. It was the source of the Boxing Day earthquake and tsunami of 2004.
The most recent earthquake occurred some way to the south of the 2004 event and did not generate a tsunami. It was on the small side, and (although the USGS doesn’t have a summary report available) the data suggest that it wasn’t caused by faulting at or near the plate boundary, but is more likely to be the result of deformation. The former is more likely to involve a significant component of vertical displacement — and that’s what causes tsunamis.
M6.1 Quake, Kermadec Islands
Last week we looked at earthquakes in the north of the subduction zone that runs from Samoa down to New Zealand, the Tonga-Kermadec Trench. This week the activity was further south, in the remote Kermadec Islands.
This, too, is a relatively simple subduction setting, with the Pacific plate subducting westwards beneath the Australian plate. And again, we see that a large earthquake on a weekly digest is a small one in a larger context. This subduction zone, too, has a history of large earthquakes.
They aren’t generally as large as those in the Sumatran subduction zone, however. The largest along the Kermadec trench is a mere M8.2, back in 1917, while the 2004 earthquake was almost ten times the size (the magnitude scale is logarithmic). And the remoteness of the location and scarcity of population tend to mean that earthquakes in this area can go largely unnoticed.
It’s still an active, and potentially dangerous, subduction zone.
US Earthquakes: Checking in on Oklahoma
We haven’t looked at Oklahoma for a while. There’s been no significant earthquake activity and, as far as I’ve seen, nothing especially noteworthy in terms of research findings emerging, although the fracking debate continues (not just in the US).
Oklahoma does seem to have gone quiet. A snapshot comparison (admittedly, not a particularly scientific one) between this week and the same week last year, shows there were just 22 tremors of at least M2.5, the largest of them M3.6, compared with 69 and a maximum of M4.1 in 2015.
I wouldn’t dream of drawing any firm conclusions from this; but it does look like a lull, even if only temporary.
Last Thoughts: Indonesia Shaking
The four largest earthquakes on this week’s map all occurred in Indonesia. You might think this means there’s a connection — a mainshock and aftershocks, perhaps, or a series of noteworthy tremors presaging some regional-level seismic activity. But in fact only two were geographically close.
Indonesia is big. The distance between the M6.6 off Sumatra (in the west of the country) and the M6.3 much further east, is of the order of 2,500 km. The tectonic settings, too, are different. Sometimes we have to use political geography as a descriptor — but it’s always worth remembering that physical geography, and in this case specifically plate tectonics, runs by its own rules.