After a busy couple of weeks the Earth has quietened down a little. I say a little because it certainly isn’t true to say that nothing happened — the United States Geological Survey’s real-time earthquake map shows four earthquake of at least magnitude 6 (≥M6.0). It’s probably more accurate to say that nothing significant happened — though, of course, not significant doesn’t necessarily mean not interesting.
In fact, three of those four larger earthquakes were of M6.0 and the largest, in the middle of the Atlantic, was M6.5. The map, which includes earthquakes of all magnitudes (not all earthquakes) in the US and its territories and those of all magnitudes elsewhere, recorded a total of just under 1750 tremors in the week of 30 November-6 December 2017, of which 26 were at least M5.0 and 132 at least M4.0. These numbers are nothing out of the ordinary.
All but one of the larger tremors are associated with the boundaries of the Earth’s major tectonic plates, which is something else we would normally expect. The one exception is a tremor of M5.0 in central Africa, and that’s not desperately unusual either, given that a few million years down the line this part of the world may, indeed, become a major plate boundary in its own right.
The Week’s Biggest Earthquake: M6.5, Central Mid-Atlantic Ridge
In an otherwise unexceptional week, it’s perhaps a little unusual to find the largest earthquake of the week in the middle of the ocean. Unusual — but not unprecedented. In fact, earthquakes along mid-ocean ridges are common, although because they occur where plates are moving apart, the tend to be smaller than those at collision zones (either subduction or continental collision).
That doesn’t mean they can’t be significant — the largest on record occurred in 1942 in the Pacific Ocean, and had a magnitude estimated to have been around M7.9.
Generally speaking, however, such earthquakes are innocuous and pass almost unnoticed. They are so remote that no-one feels them, and the nature of the movement, which is usually lateral as plates slide past one another to accommodate divergence of oceanic plates, isn’t the type of movement that would cause tsunamis, even if the magnitude of the earthquake were sufficiently large.
This week’s tremor, on the Romanche Fracture Zone (a section of conservative margin offsetting spreading section of the bridge) is a classic of its kind. No-one reported feeling it, and it occurred in quiet isolation around 1500km from land.
As a footnote, it isn’t the only earthquake in the Romanche Fracture Zone in the past year, although it is by some way the largest. The others, for interest, came in at M5.5 and M5.0.
M6.0 Earthquake: Central Iran
I talked a couple of weeks about the dread prospect of a major earthquake in Iran. This week, an earthquake of M6.0 serves as a reminder, if we need it, that the collision of continents — in this case, Arabia and Eurasia — and the mountain building processes which they initiate, continue to be active and to provide a potential threat.
When continents collide, the effects are felt over a very wide area, and although this week’s earthquake is at some distance from the topographic expression of the boundary, the crumpled scenery of Iran bears witness to the overall compressional forces.
Compression isn’t often direct (plates generally come together at an angle) so that movement along strike-slip faults accommodates the angle of compression.
Interestingly, however, although the area in which the earthquake occurred is dominated by major strike slip faults, the major motion recorded by the USGs was compressional.
US Earthquakes: Delaware
I confess: if you said “earthquakes” and “the US” in the same breath, Delaware is one of the last places I’d expect to hear mentioned. In fact, there’s so little earthquake activity that there’s very little available information even on the USGS website about historic events, which is why this week’s M4.1 may have set many First Staters trembling.
Anywhere where the underlying rock is old and faulted, no matter how stable it may seem, is liable to the odd, small earthquake, so perhaps at one level it isn’t really surprising that such an earthquake occurred. But a quick look at seismic hazard maps of the US confirms that Delaware is among the least likely parts of the US to experience any such event.
The scarcity of information is such that I’m reliant on local media quoting the USGS on numbers of earthquakes that are too small to show up on the interactive map (which, by the way, shows literally nothing of M2.5 or more in the last century). This source quotes just 59 tremors since 1871, most of them very small.
At M4.1, this week’s was the largest in over a century, which is probably why the USGS spokesman quoted in Delaware Online described it as ‘wild’.
Last Thoughts: Never Mind the Magnitude, Feel the Shaking
If you were one of the estimated 59 million or so who may have felt the shaking from the Delaware earthquake, you’re probably still comparing experiences with your neighbours and telling your friends in California that now you know how they feel. But, as always, I like to put these things in context, and the context is the relationship between an earthquake, its magnitude, and the potential it has for causing damage.
The earthquake scale is logarithmic. That means that the largest earthquake of the week was over 250 times the size of the Delaware tremor and released almost 4,000 times as much energy — but because no-one felt it, it’s effectively irrelevant. The Iranian earthquake was 79 times larger and released over 700 times as much energy, but was felt by fewer people — an estimated 16.5 million.
As a footnote, there’s another element to hazard, and that’s vulnerability. Even in somewhere like Delaware, where earthquakes aren’t a fact of life, buildings are generally more resistant to earthquakes than in rural Iran. And that’s a big factor in the impacts of any earthquake.