So, here we are again, a week (16-22 March 2017) when the planet seems to be in one of its quieter periods and there’s not a lot of earthquake activity to report. But with over 1600 earthquakes recorded on the United States Geological Survey’s real time earthquake map, there’s still plenty to look at.
The map isn’t by any means exhaustive — it includes earthquakes of all magnitudes, though not all earthquakes, in the US and its territories, and those of at least magnitude 4 (≥M4.0) worldwide — but it does pick up the most significant activity, so we can be reasonably certain that we haven’t missed much of note.
This week there was no shortage of earthquakes, though nothing large — the largest of the week was just M6.0 and there were relatively few tremors (27) in excess of M5.0. The numbers of smaller earthquakes shown on the map was a bit larger than normal, but given the caveats about under-reporting, I wouldn’t attach too much significance to this.
The Week’s Biggest Earthquake: M6.0, Solomon Islands
Strangely, I was having a conversation with a neighbour last week about the Solomon Islands. She’s the first person I’ve ever met who’s planning to go there, and I’m the first person she’s ever met who knows where they are, let alone knows anything about them.
In fact, my knowledge of the Solomons comes almost entirely from what I know about earthquakes — they regularly feature as the location for the week’s largest seismic event, and that’s what’s happened again this week.
The biggest earthquake of the week isn’t, at M6.0, that big, but I’m not at all surprised to find something of this magnitude in this location — indeed, I’d be rather more surprised if there wasn’t an earthquake of at least M5.0 here every week.
Why is this? Well, the Solomon islands lie along the margin between the Australian and Pacific tectonic plates, where the two are coming together. The former descends beneath the latter along the South Solomon Trench and, as the USGS notes: “Seismicity along the trench is dominantly related to subduction tectonics and large earthquakes are common: there have been 13 M7.5+ earthquakes recorded since 1900”.
This week’s earthquake is also the result of compression, although too shallow (10km) and too far from the trench itself (around 200km) to be the direct result of subduction. Instead, it’s likely to be the result of crustal deformation, probably in association with the shallow North Solomon Trench which lies on the eastern side of the archipelago.
M5.3 Quake, South Sandwich Islands
I have a bit of a soft spot, seismically speaking, for the South Sandwich Islands. They’re remote, have a population which is possibly reaches single figures very occasionally (scientists get everywhere), and they’re lashed by gales and high seas. But they’re interesting.
They’re interesting because they’re one of just two short areas of subduction in the Atlantic Ocean (the other is in the Caribbean). The Atlantic is still growing and unlike the Pacific, its crust is not being consumed at enormous lengths of subduction zones. But it does have these two short areas of subduction, both where smaller plates abut the larger ones (in this case, the Scotia microplate and the South American plate) — and both produce earthquakes.
The data, such as they are — location around 100km from the trench, depth around 200km — suggest that this week’s South Sandwich tremor also occurred as a result of deformation rather than at the plate interface, though this time the deformation is likely to be in the descending plate. But with such scarce information, I’m hesitant to say more.
At the moment, the subduction margin of the Scotia plate is pretty active — in the past year there have been 88 earthquakes of at least M5.0, with two of them larger than M7.0. I bet you never read about that in the papers.
US Earthquakes: Alaska
So what about America? Well, this week we have another subduction zone and another earthquake that would be much more significant if it occurred in a highly populated area and which is relatively small for the tectonic setting in which it occurred. (It’s that kind of week.)
The second largest earthquake of the week occurred at the western end of Alaska’s Aleutian Island chain, so far from the North American continent that it was almost in eastern Russia. It’s fairly typical for its setting, caused by the coming together of the Pacific and North American plates. And (if you like a mind-blowing statistic) it was around one two-hundred-and-fifty-thousandth the size of Alaska’s largest recorded earthquake, in terms of energy released.
Last Thoughts: Island Chains
This week’s featured earthquakes have several things in common. Most notably, they’re all in association with island chains running along subduction zones. There is, as you might expect, a reason for this. Subduction of crust along a trench causes descending crust to melt, molten rock rises and erupts as volcanoes to form new land.
When ocean crust meets ocean crust, these volcanoes form island chains a little way from the trench, in the over-riding plate. You can see them in the earth’s oceans, particularly in the Pacific. Where you see such a chain you can expect to find volcanoes — and earthquakes.