So here we are again, back in the realms of normal (whatever that means) earthquake activity. Or, if I’m honest, normal to below normal for the week of 14-20 July, 2016.
The United States Geological Survey’s real time earthquake map includes (roughly) earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere. Normally we’d expect to see at least one earthquake of ≥M6.0 and the chances are we’d see it somewhere in the Pacific. This week that’s precisely what happened.
If there’s anything unusual in the spread of earthquakes this week, it’s that there were so many in the range M5.0-M5.9. I haven’t done the statistical analysis but normally I’d expect somewhere between 20-30 and this week turned in 37. Even this isn’t really worthy of too much attention — eight of those were the largest earthquake of the week and its aftershocks.
The Week’s Largest Earthquake: M6.3, Near Raoul Island
Sometimes it must be hard being in charge of the USGS map. So often, the largest earthquake is so far from anywhere that it’s hard to define it in terms of land, which is more user-friendly than its latitude and longitude. This week they’ve placed the earthquake at 201km NE of Raoul Island, New Zealand, which is perhaps a little more helpful than its technically accurate 28.033°S 176.386°W. Either way — it was a long way from anywhere.
Tectonically speaking it’s a nice, simple earthquake with a series of aftershocks. The Tonga-Kermadec Trench, where the Pacific plate subducts eastwards beneath the Australian plate, extends for a couple of thousand km from Samoa to New Zealand, with not a lot between the two to define it save the occasional volcanic island.
Earthquakes here tend to be fairly straightforward and subduction related. This one seems to fit that bill nicely — the tremor was around 12km deep and maybe 50km from the trench boundary, so seems likely to have been caused by deformation in the overriding plate as subduction occurred.
M5.5 Earthquake, Chile
Over on the other side of the Pacific, and almost an order of magnitude smaller, the earthquake which struck off the coast of Chile was almost certainly similar in origin. In this case the Pacific plate subducts westwards beneath the South American plate (though because subduction takes place beneath the highly populated coastal margin of South America, you can’t call it remote and the USGS earthquake summary suggests that over three quarters of a million people may have felt something — though not much).
It’s the same story as above — almost weirdly so. At a depth of 12km and around 50km from the trench, with the epicentre (the point on the surface immediately above the point at which the earthquake occurred), this tremor looks likely also to be the result of deformation resulting from the subduction process, at or near the plate boundary.
US Earthquakes: Florida
Yes, okay. There was an M4.2 in California and an M4.1 in Oklahoma, but we’ve done those two states to death and I can’t rest the chance to type something new. This week there was an earthquake in Florida.
Well, all right. It isn’t quite what it seems and calling it an earthquake is probably a bit of Disneyfication. (And, um it wasn’t actually in Florida.) In fact the seismic event recorded on the map — a trifling little thing of M3.7 — appears to be the result of some weapons testing at an offshore range. The USGS describes it as an “experimental explosion” and some Floridians even reported having felt it. Excitement all round.
Last Thoughts: Subduction Zones
Subduction zones can be pretty complicated, especially when they’re associated with other plates or microplates or complex goings-on beneath the surface. Both of the week’s main featured earthquakes were anything but complicated. They were nice, simple, subduction earthquakes along a along destructive plate margin.