One day when I’m bored I’m going to sit down and make a graph of earthquakes of at least magnitude 5 (≥M5.0), week by week, for a year. Then we’d surely get some idea of exactly how variable those numbers are.
What’s set me thinking along these lines? Well, this week (7-13 May, 2016) the United States Geological Survey’s real time earthquake map, which roughly includes tremors of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere, yet again showed… not a lot.
By ‘not a lot’ I mean that none of the almost 1,950 tremors was as large as M6.0, although there were a respectable 23 ≥M5.0.
The Week’s Largest Earthquake: M5.9, Mexico
The Pacific coast of Mexico is regularly subject to earthquakes in excess of M6.0, so it’s no surprise to find the largest earthquake of the week here, at M5.9. In fact, the last decade has seen the stretch of coast on which this week’s occurred producing an average of one a year, with the largest coming in at M7.2.
The source of these earthquakes, and of this week’s, is the subduction zone which runs along the Mexican coast. The Cocos plate, comprising dense, oceanic crust, is descending beneath the more buoyant rocks of central America along the Middle America Trench. Tension builds and earthquakes occur.
This week’s earthquake occurred at a depth of 24km, around 100km from the plate boundary. Normally this would suggest that the cause was deformation in stressed crustal rocks, rather than movement at the plate interface. But Mexico is different; subduction here occurs, for reasons unclear, at an unusually low angle (it’s known as ‘flat slab subduction’).
Maps of the descending plate, such as those referenced by Robert Yeats, show that the plate boundary beneath the epicentre of the M5.9 is, indeed, around 25km depth. So, despite its distance from the trench, the probability is that the earthquake was, indeed, a subduction earthquake.
M4.5 Tremor, Greenland
Sometimes a small earthquake catches the eye. This week that happened and so, unusually, I’ve allowed myself to stray into the realms of speculation. There were two of them, clearly related, an M4.4 and M4.5 off Greenland.
Greenland, as it happens, is arguably the most tectonically stable place on Earth, built on some of the oldest rocks on the planet. Most people don’t know that; the only thing they know about Greenland, in all probability, is that it’s covered in ice.
Greenland’s ice is melting. When ice sheets several km thick melt, they release the pressure on the land beneath. The land bounces (I use the term loosely as it’s inevitably a slow process, one known as isostatic rebound) upwards.
And when massive areas of rock experience changes in pressure, they can crack. Research published in 2008 identified a series of earthquakes accompanied deglaciation after the last ice age in Scandinavia — another theoretically highly stable area. Some of them may have been as large as M8.
I can’t find much information about these particular earthquakes, but I’m going to stick my neck out and suggest that there may be a connection here to the melting of the Greenland ice sheet. Of course M4.5 is nothing compared to M8 — but it’s still worth watching.
US Earthquakes: Nothing to See Here
I really am scratching my head about what to say about the US here. Because if it was quiet worldwide, the US — all of it, even Alaska — was positively comatose. The largest earthquake registered on the USGS map was M4.4. So one of the most turbulent seismic zones in the world couldn’t register a tremor as large as Greenland, one of the most stable. That’s your fact for the week.
Last Thoughts: Global Warming and the Dynamic Earth
I’m not going to get into a debate about climate change but I think it’s fair to say that most serious scientists accept that our climate is warming. And we’re all aware of the obvious effects — changes in sea level, changes in weather patterns and so on.
This week, if I’m correct, we’ve seen the effects of that warming in the shape of an earthquake. Even if I’m wrong about this particular event, there’s a strong body of evidence to suggest that melting ice sheets cause major earthquakes (and also volcanic eruptions, though that’s beyond the scope of this article.)
“Global warming causes earthquakes.” Now there’s something for the internet sensationalist to pick up.