I was away last week and so missed my normal weekly article. My original thought had been to pick up on a theme of all things Mediterranean (I was in Greece) for the week of 18-24 October, but every so often something crops up that really shouldn’t be ignored. One of these was a series of three earthquakes off the west coast of Canada which benefits from some explanation.
The Queen Charlotte Sound ‘Triplets’
The unusual thing about this week’s earthquakes is not the location (shallow depths some way to the west of the Cascadia subduction zone) nor the magnitude (M6.5, M6.6 and M6.8) but that they occurred very close together in space and time. All three occurred within less than three quarters of an hour and the maximum distance between their epicentres was around 30km.
First up, let’s consider the location. The earthquakes occurred at shallow depths (10-11km) just to the east of a transform boundary, the Sovanko Fault Zone, which marks the trailing edge of the Explorer microplate. This microplate is subducting beneath North America along the Cascadia subduction zone at around 50mm pa.
Cascadia is a sleeping giant of a subduction zone. It’s been quiet for over 200 years but is capable of producing very large earthquakes indeed. When this week’s earthquakes were first reported I saw a flutter of alarm on Twitter, but the important thing about these ‘quakes is that they aren’t actually associated with the subduction process.
All three have a strike-slip mechanism (lateral not vertical movement) and are associated with the Sovanco fracture zone rather than the Cascadia subduction zone. And earthquakes of this nature — and magnitudes larger than M6.0 — are by no means uncommon.
I sometimes mention doublet earthquakes, which are close together in time, space and magnitude, but these three, so similar in so many ways, represent a step further, effectively a triplet earthquake. There are other ways of describing it — Dr Wendy Jones, one of my favourite (and most authoritative) Twitter sources describes it, in terms of the area of fault ruptured, as “like a M7.0 that got stuck on the way and broken into 3 pieces”.
Interestingly, this applies not just to area but to magnitude. Because the magnitude scale is logarithmic, the combined magnitudes of these three earthquakes is also roughly around what we’d get in terms of energy release from an M7.0.
Sea Level Rise and the Impact on Historic Sites
And so to the Mediterranean. Earth science includes climate science, and one of the main impacts of global warming is the sea level rise that comes from the melting of the ice caps. This week, a study published in the online journal Nature reported on the potential impacts of sea level rise on World Heritage Sites around the Mediterranean Basin.
The Mediterranean has been the centre of sea-borne trade for thousands of years, and has the historic coastal settlements to prove it. But change in climate threatens them.
People talk of sea level rise as if it was a very specific thing — a few millimetres every decade, for example — but the reality is different. Sea level is not constant, because it varies with winds and tides, and rising sea level is probably most likely to manifest itself in increasingly high tides, higher and more frequent floods, and increased coastal erosion.
The study reviewed increased flood likelihood and erosional damage for various different levels of sea level rise, and the conclusion is alarming for anyone who values the Mediterranean heritage. Of 49 cultural World Heritage Sites on the shores of the Mediterranean, 37 are deemed to be at risk from flooding and 42 from coastal erosion today, with the risk only likely to increase.
I was in Athens, which is sufficiently far above the sea to be safe, but in Greece the sites of Rhodes and Corfu are threatened. Other iconic monuments and destinations are also at risk — including Dubrovnik, Ephesus, Carthage and Venice.
Vesuvius, 79AD…But When, Exactly?
We almost certainly all know about Vesuvius and its eruption of 79AD. It’s erupted several times since, most recently in 1944, and may well erupt again in the not-so-distant future, but it’s the eruption of Roman times that everyone knows about, with its cataclysmic devastation of Pompeii and Herculaneum, and the contorted figures of humans and animals cast in ash in the museums.
Conventional wisdom has it that the eruption took place on 24 August, based upon the written account of Pliny the Younger, who watched the whole thing from a safe distance. Recently, however, archaeologists have uncovered graffiti from the walls of a house in Pompeii which pinpoints a later date.
The BBC website reports the inscription as dating from “16th day before the beginning of November (that is, October 17)” and the fragility of it (it was written in charcoal) suggests that it couldn’t have been written more than a few days before it was covered by ash. In other words, the date of the eruption would be around 24 October.
There’s some debate as to how useful this kind of information actually is, and my own view is that it’s more significant for other disciplines — archaeology, for example — than for Earth science, where the precise date of a specific event isn’t necessarily much help in understanding where and when the next will occur.
What I would note, however, is that Pompeii and Herculaneum are among those sites at risk of increased flooding and erosion. And so our understanding of the physical processes might help us to protect, and learn still more from, some of the older classic sites.