The week of the 11-17 May 2017 saw an apparent uptick in the number of earthquakes recorded on the United States Geological Survey’s real time earthquake map — a total of almost 1800.
I have to add the caveat that this is a huge underestimate — the map includes earthquakes of all magnitudes in the US and its territories, but it includes by no means all of them, and it only includes earthquakes of at least M4.5 (sometimes a little smaller) elsewhere. So we can’t take the total figure as an increase in earthquake activity per se, but rather as an increase in recorded activity.
The map is, however, generally reliable in terms of the larger tremors and these are fairly constant — three of at least M6.0 and 41 of at least M5.0. And, as usual, all of these larger earthquakes occurred in association with the margins of the Earth’s tectonic plates.
The Week’s Biggest Earthquake: M6.5, South Sandwich Islands
I have a soft spot for the South Sandwich Islands. Not that I’ve ever been there — there can’t be many people who have — but these remote islands in the deep south of the Atlantic fascinate me. Week after week there’s an earthquake there and no-one feels it. But I find this archipelago — one of only two short sections of subduction zone in the Atlantic Ocean — fascinating.
Tectonically, the islands have certain things in common with the other length of the subduction zone, the Antilles in the Caribbean (there are obvious differences in climate!). Both are short island arcs at the eastern end of a plate bounded to the north and south by transform margins. But a series of earthquakes of the magnitude of this week’s (the largest was M6.5) in the Caribbean would have made news headlines worldwide. Not in the South Sandwich Islands.
The earthquake was the largest in a series of 16 in the past week of at least M5.0 (there will have been many more, smaller, ones) and all the evidence suggests that it is closely related to the subduction zone itself (though it seems unlikely to have taken place at the plate interface).
At this point, the South American plate subducts beneath the Scotia plate along the South Sandwich Trench. This week’s tremor is anything but abnormal — the last half century has seen 24 earthquakes of this magnitude or larger, with seven of them at least M7.0.
M6.2 Earthquake, El Salvador
I mentioned the Caribbean plate above, and the second of the featured earthquakes this week occurred at the western margin of this plate. Unlike the Scotia plate, the western edge of the Caribbean plate is marked by a subduction zone, where the Cocos plate subducts beneath Central America along the Middle America Trench.
The data for the origins of this earthquake are a little counter-intuitive: although on the surface (pun intended) it looks as its it’s caused by subduction (and therefore will have compressional motion) the USGS data show the opposite.
This M6.2 earthquake, 10km deep and around 50km from the actual trench, had an extensional cause.
This implies that it’s most likely to be the result of deformation at shadow depths in the over-riding plate — possibly associated with movement on the (lateral) northern margin of the Caribbean plate, which terminates onshore in Guatemala a few hundred km to the north.
US Earthquakes: California
Most Californian earthquakes are (perceived to be) associated with the San Andreas Fault Zone. This week, an M4.1 rattled the windows of Santa Barbara, north of LA. It wasn’t a San Andreas earthquake, strictly speaking, and it certainly wasn’t the still-awaited Big One, but it was associated with movement along one of the many faults which mark the wider margin between the Pacific and North American plates.
It isn’t clear exactly what type of fault this was, and the USGS ‘beachball’ diagram (which indicates direction of movement) is a strange one, somewhere between lateral and extensional movement.
Last Thoughts: Nothing Classic This Week
The earthquakes this week are a funny lot. They’re all sort of what you might expect, but not quite. At first glance we should have subduction earthquakes in the South Atlantic and off El Salvador, while California’s tremor should be caused by lateral movement in the San Andreas fault Zone.
But it isn’t like that. The South Sandwich one is a little too shallow to be right at the plate interface. The El Salvador one has the opposite direction of movement to what the location would suggest. And the California one has the strange beachball diagram that suggests it’s neither one thing nor the other.
If earthquakes were really as simple as they are often represented, it would be much easier to predict their movements and impacts. This week’s tremors indicate that there’s plenty going on and that there are very many different factors at play in all three of these — apparently straightforward — earthquakes.