I was away last week and so, strictly speaking, shouldn’t be reporting on past events this week, but it’s coming up to the holidays and the rules in all other areas of my life are getting a little more flexible. So this week I’m going to stretch the boundaries just a wee bit, and the roundup for the week of 6-12 December is going to include a couple of events from last week, too. Why? Because they’re noteworthy.
In terms of what else to include, I’m a bit spoiled for choice this week. The American Geophysical Union (AGU) is having its Fall Meeting at the moment, so my timeline is full of intriguing Earth science snippets. The downside is that most of them are presentations of yet-to-be-published research so there’s not a lot (yet) for me to read up on. But if nothing else, it’s well worth giving them a follow on Twitter to get a flavour of what’s coming up.
Three Big Earthquakes
Though I would never call myself a statistician I talk a lot about normal distribution of large earthquakes. I confess to being a little surprised (and grateful) not to have seen anyone suggest that the three large earthquakes that occurred in the fortnight since the large digest might be connected, and the fact that I’m dealing with them together isn’t intended to suggest that they are. It’s just a convenient grouping of three major events that I would normally cover.
We’d expect an earthquake of M7.0 around once a month — maybe slightly less often, maybe slightly more so. The three earthquakes of this magnitude in the past fortnight — M7.0 in Alaska, M7.5 off New Caledonia and M7.1 in the South Atlantic — take the total so far this year to 15, which is certainly nothing to get nervy about.
Each one deserves more than the paragraph or two I have to give them, so do follow the links to the United States Geological Survey’s event pages (in the resources section) to find out more.
The first, and most widely reported, was that which struck just north of Anchorage, in Alaska, the result of the coming-together of the Pacific and North American plates. The area is a major subduction zone but the USGS report indicates that the tremor occurred: “within the subducting Pacific slab, rather than on the shallower thrust-faulting interface between these two plates”.
You don’t have to look far on the internet to find video image of the shaking caused by the event, but despite the magnitude, and the closeness to the city of Anchorage, there were no reported deaths or serious injuries. That’s a measure of preparedness. Alaska, which experienced the second largest earthquake on record (M9.2) is ready for large earthquakes, and so damage is limited.
The second earthquake was the largest, an M7.5 in New Caledonia, where the Australian plate descends beneath the Pacific plate, and it was the result of normal faulting as a result of deformation. The USGS summary indicates that this: “is sixth M 6+ earthquake to occur in this region over the past three months, and is part of an active sequence of events that began on August 29th, 2018 with a M 7.1 interplate thrust faulting earthquake to the east of the South New Hebrides Trench”.
The third earthquake, at M7.1, was the most remote, occurring in the South Sandwich islands. Like the either two, this was an earthquake at a subduction szone, in this case where the South American plate subducts westwards beneath the Scotia microplate. In contrast with the Pacific, the Atlantic has only two, shot, subduction zones and the South Sandwich islands regularly experience large, though not enormous, earthquakes.
So the week brought us three large earthquakes, all at subduction zones and none (as far as I can discover) causing any injuries or deaths. It’s a reminder that earthquake activity — significant earthquake activity — is a normal part of living on a dynamic planet.
Seismic Hazard — Where Earthquakes are Dangerous
Following neatly on from these three relatively harmless earthquakes comes a new study of seismic hazard. I’ve commented on hazard before: it’s not a measure of the size of the earthquake but of the damage it causes, and as such is influenced by a number of factors other than magnitude. These include (crucially) the number of people likely to be affected and the vulnerability of the places in which they live and work.
This week the journal Nature reported on the release of three new interactive maps which attempt to plot seismic hazard as a tool for civil authorities and others involved in managing the potential impacts of earthquakes.
The maps show three key things. Firstly, the actual likelihood of an earthquake occurring — the seismic risk. This is a detailed map of where earthquakes are most likely to occur. Secondly, they show where buildings are most likely to be damaged (remember the old seismologist’s adage that earthquakes don’t kill people: buildings do.) Thirdly, they show the number of buildings around the world.
Taken together, these present some idea of hazard — in other words, in areas where earthquakes are more likely to occur, where there are many buildings and where those buildings are more vulnerable to damage, the overall potential for damage is greatest. As the journal points out: “risk and exposure maps explore what the chances of earthquakes mean for human lives”.
Automated Volcanic Eruption Warning System
The greatest hazard mitigation — for any natural hazard — is, of course, avoidance. We’re a long way from predicting the occurrence of specific earthquakes or volcanic eruptions in any meaningful way, but this week a piece of research was published that indicates another small step on the route to mitigation. Not prediction — but warning.
The distinction is an important one. Prediction indicates when an event is likely to happen, rather than the earliest possible notification that it is happening. This week a paper presented to the AGU includes a new early-warning system for volcanic eruptions.
The volcano in question is mount Etna, in Italy. and the summary of the paper notes that: “We present the first early‐warning system based on the acoustic waves generated by volcanic eruptions,” but makes a point of stressing that: “it is not based on probabilistic consideration”.
The warning system is automatic and has been in process for some time. In 2015, Nature reports, “a warning was automatically issued to the Italian government almost two hours before an eruption, which allowed the government to activate emergency plans about one hour in advance of a volcanic eruption for the first time”.
This is a breakthrough in management, rather than in forecasting — but hours or even minutes can be crucial in terms of reaction and result in lives saved.