So here we are again. Another week, another deadly earthquake, another earthquake roundup written with a heavy heart. The United States Geological Survey’s real time earthquake map for 14-20 September, 2017 doesn’t show anything particularly out of the ordinary, but that’s because it’s based on numbers of earthquakes, not their impacts.
The map, which broadly speaking, shows earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, offers more or less its usual spread of tremors. There was one larger than M7.0, two larger than M6.0 and 28 in excess of M5.0 in ints total of just over 1850 recorded events.
The Week’s Biggest Earthquake: M7.1, Mexico
So here we are again in Mexico, the same but different. On 19 September, exactly 32 years after another major earthquake killed thousands in Mexico City and less than two weeks after another deadly seismic event in the country, Mexico yet again features as the location of the largest earthquake of the week. You don’t have to look far on the media to find terrifying footage of the event as buildings shook in Mexico City, and the current death toll — over 200 — is expected to rise.
So what’s going on in Mexico? Although first impressions might suggest that this M7.1 event is an aftershock of September 7s M8.1, that isn’t the case. Although the statistical formula of Bath’s Law suggests that an aftershock of around M7.0 might be expected after an earthquake of M8.0, this isn’t it — its epicentre is too far away from the mainshock (at over 600km) to be part of that series.
The tectonic setting is the same as for the preceding earthquake, with the convergence of the Cocos plate and Central America leading to the subduction of the former beneath the latter. In terms of the earthquake itself, the USGS has all the data and this (like its predecessor) is not the product of of compressional movement but of deformation: “The location, depth, and normal-faulting mechanism of this earthquake indicate that it is likely an intraplate event, within the subducting Cocos slab, rather than on the shallower megathrust plate boundary interface”.
This earthquake may not be an aftershock, but is it linked to the event further south? There’s no concrete evidence for the closeness of these two (in terms of time) being anything other than a terrible coincidence.
In an excellent article in Nature, Alexandra Witze looks at the possibilities and quotes seismologists who find the second earthquake to be too distant in both time and space from the first for the two to be conclusively linked — although one of those seismologists, the USGS’s Gavin Hayes, describes the relationship as “pretty suspicious.”
Linked or not, the second, smaller, earthquake caused more damage and death than the first despite being just a tenth of the size. A major reason for this is that it was much closer to Mexico City, where many deaths have been reported — and Mexico City itself is a victim of both history and geography, made vulnerable to earthquake damage by its location on a former lake bed.
The properties of the ground here are enough to amplify the earthquake waves, increasing the shaking and causing a phenomenon called liquefaction, in which the ground becomes considerably weaker (as the name suggests, becoming, in may cases, like a liquid) and the likelihood of building collapse increases.
M6.1 Tremor, South of New Zealand
What a contrast the second earthquake this week is to the first. Around 600km from the southern tip of New Zealand, close to an ocean ridge in the far south of the Pacific Ocean, an earthquake of M6.1 registered as the second largest of the week (and just a tenth the size of the largest).
In the middle of nowhere, it seems to have troubled no-one. (Just two people have reported that that they felt it, and I’ve no idea where they could have been.) There are no detailed data, other than that it had a lateral movement: in all likelihood it occurred at a transform fault associated with the ridge itself.
US Earthquakes: Idaho
As well as revisiting Mexico, we’re popping back to Idaho. But this time the activity that’s been going on this week — the largest of 45 minor earthquakes registered M4.0 — is related to the mainshock of M5.3 near Soda Springs on 2 September. The earthquake itself wasn’t particularly unexpected in this area of relatively high seismic hazard, but the ongoing sequence to date has included more than 470 tremors of at least M2.5.
While earthquake swarms are by no means unusual, this one is more sustained than most and so the USGS has produced seismic forecasts for the area. By far the most likely outcome — assigned a probability of 90-95% — is that the sequence will decline gradually. At the other end of the scale is the remote possibility — smaller than 1% — that what we currently consider the main shock in the sequence might be the foreshock to a much larger one.
Last Thoughts: Why Do Any Others Matter?
Writing about any secondary earthquake while the news channels are running stories of dead, injured and missing in Mexico isn’t easy. Frankly, why do they matter? In Idaho, the possibility — remote though it is — of a larger shock is one that warrants coverage, but who is really interested in an earthquake, even a large one, in the middle of nowhere?
In the context of the news from Mexico, but this earthquake and most of the others on the map is effectively irrelevant. But it’s good to remember that most of the seismic activity on the planet does no-one any harm, and that deadly events are, in actual fact, relatively few.