It’s normal, but it’s not normal. The Earth’s seismic activity for the week of 31 August – 6 September, 2017 makes interesting reading.
On the surface (pun intended) there’s nothing too much out of the ordinary. The United States Geological Survey’s real time earthquake map, which includes tremors of all magnitudes in the US and its territories and those of at least M4.0 elsewhere, doesn’t depart from what we might expect in terms of numbers and magnitudes.
Three earthquakes out of the total of just over 1650 reached a magnitude of at least M6.0. There were 26 of at least M5.0 and 107 of at east M4.0. And, as we might expect, most of the earthquakes were associated with the planet’s active plate margins.
I say most. Let’s look a little more closely.
The Week’s Biggest Earthquake: M6.3, North Korea
All right: I called it an earthquake but it isn’t. Technically I should probably describe it as a seismic event, but if you’ve seen anything on the news this week you’ll know what I’m talking about, and the USGS pulls no punches in describing the joint largest feature on the earthquake map (the same magnitude as a ‘real’ earthquake in Indonesia) as an explosion.
In one sense I’m leading you up a blind alley, here, because not only is this not an earthquake, but it’s actually something we have relatively little information about. The USGS event page is quite clear in its caveats: “Possible explosion, located near the site where North Korea has detonated nuclear explosions in the past. If this event was an explosion, the USGS National Earthquake Information Center cannot determine its type, whether nuclear or any other possible type”.
What do we know and how can we tell that it isn’t an earthquake? Well, the USGS summary poster clearly shows that it took place in a location where the natural seismic hazard is very low: in itself, that suggests a deliberate element to it. Other than that, distinguishing between different types of seismic event is complicated — far too much so for this digest — and continually evolving.
If you’d like to know more, I strongly recommend that you read an article on the subject (predating this week’s explosion) from the Science News online magazine: How earthquake scientists eavesdrop on North Korea’s nuclear blasts.
M6.0, South Sandwich Islands
In the South Atlantic, way away from anywhere, nature is asserting herself. The M6.0 earthquake in the South Sandwich Islands is, unless there’s something very sci-fi going on, completely natural.
The Atlantic Ocean is nothing like as seismically active as the Pacific, having very little in the way of plate margins (where most significant seismic activity occurs). There’s the mid-ocean ridge, which trembles regularly, and the transform fault separating Africa and Eurasia, but beyond that, there are only two short sections of subduction margin.
Being so short, these don’t tend to generate the major earthquake events that we see at the much longer zones, but they are regularly active and capable of significant activity.
This week’s M6.0 in the South Sandwich Islands has passed largely unremarked upon, given its remoteness, but in fact it’s another in a series of large earthquakes that occur along this island arc as the South American plate descends beneath the Scotia microplate.
If we dig into the archives, we find not only that this earthquake isn’t the largest of the year (there was one of M6.5 in May) but that there have been 21 of larger magnitude since the beginning of the century.
US Earthquakes: Idaho
In the opening paragraph, I talked about the week’ seismic events looking normal but being otherwise. Here I’m going to turn that statement on its head. An earthquake of M5.3 which shook Idaho generated plenty of news headlines — but in the great scheme of things, it isn’t that unusual.
Central Idaho is part of the so-called basin and range topography of the Rockies, where extensional movement is taking place. As a result of this, blocks of terrain slip downwards along normal faults — and that’s an earthquake.
Admittedly, the earthquake was large for this area, but it isn’t unprecedented. There was an earthquake of similar magnitude a little further north back in 2001. And the USGS doesn’t even have an event page for it — so it can’t be that significant.
Last Thoughts: What Seismographs Tell Us
Seismometers measure the movement of the Earth. Okay, so most of the seismic waves they record are from what we might consider ‘proper’ earthquakes, but there are plenty of other things that appear on their seismograms (the graphical output from the seismometer) too.
This week I’ve noted how they’ve recorded an explosion in North Korea. In the past I’ve commented on how they’ve picked up quarrying activities and even the roar of a football crowd when their team scores. And today, the Independent newspaper reports that in the Caribbean: “Hurricane Irma has become so strong it’s showing up as an earthquake on seismometers”.
Useful things, seismometers.