This week, the week of 25-31 October, 2018, I’m looking at volcanoes and earthquakes, with a side-serving of things we don’t know. We’re looking at an earthquake that’s been in the news, a report on volcanism that’s also been in the news when it probably didn’t need to, and we’re taking a journey into the deep ocean (because there are volcanoes down there, too).
Greece’s M6.8 Earthquake
You may recall that last week I didn’t write the Mediterranean-themed round-up I’d planned because I couldn’t overlook a triplet of earthquakes off the coast of Oregon. Not long after the article went live, there was a large earthquake in Greece (strictly, Greek waters) which I would certainly have covered and which would rounded off the article nicely. But here it is, a week late.
Although it leaps to mind more as a holiday hotspot than an earthquake one, the Mediterranean basin is very much seismically active. That’s the result of the coming-together of Eurasia, Africa and Arabia in which the Mediterranean Sea is being slowly squeezed out of existence as part of the continuing (and very slow) cycling and recycling of the continents.
Within it, there’s a lot going on and this week’s earthquake occurred in the Eastern Mediterranean, in an area where the dominant tectonic action (though not the only one) is subduction. Here, the African plate descends beneath Eurasia along the Hellenic Trench, which curves from (roughly) Zakinthos in the west to Rhodes in the east.
The M6.8 earthquake was caused by “oblique slip on a near-vertical west-northwest striking fault or a moderately dipping north striking fault” according to the United States Geological Survey’s summary, and is consistent with what we might expect.
Earthquakes of this magnitude in this area aren’t unprecedented, though, nor are they that common. A search of the USGS interactive map shows just nine of at least M6.8 in the last century along the Hellenic Trench and the area behind it, beneath which it is subducting.
Scary Volcanoes…or Not
It’s Hallowe’en and in my experience people love to be scared — not just on one day of the year but, judging by the scary headlines I’ve been seeing this week, at any other time of the year.
This week the USGS released an update to its assessment of national volcanic threat which, in some areas, was reported as a forecast of likely eruptions. (For example: “Yellowstone Super Volcano Threat just Increased to High” is one that was floating around on Twitter).
If you read the report, however, you’ll find that this is not the case. Not only has the status of Yellowstone not changed, but the report itself does not set out to predict the likelihood of an eruption. It’s an assessment of the risk should an eruption occur, and is based on a ranking of 24 different factors relating not just to the volcano itself (the nature of any possible eruption is significant here) but to “exposure of people and property to those hazards (independent of any mitigation efforts or actions).”
In itself the report isn’t especially newsworthy — it’s an update on an existing document, with not really much changing from the original. The 18 volcanoes listed as very high risk in the updated report are the same 18 that were listed in the original. Yellowstone was ranked number 21 before and still is, was classified as high risk before and still is. A few minor volcanoes have come in to the list and a few have dropped out. A few others have moved categories.
The reason I’m commenting on it here is twofold. Firstly, if you have the time and are interested in volcanic hazard, it’s actually an interesting read. Secondly, the miscommunication of an updated report is symptomatic of how easy it is to mislead, and misinterpret scientific information — and how important it is to get it right.
Deep-Sea Volcanoes in the Marianas Trench
The USGS report doesn’t just look at the 50 US states: it also includes other territories, including the Commonwealth of the Marianas Islands. The Marianas are a volcanic island chain generated above the Pacific plate where it subducts beneath the Philippine Sea plate. In terms of risk, its volcanoes aren’t serious contenders, largely because they’re so remote, but they have made the science news for another reason this week.
Research published in Frontiers of Earth Science shows the results on investigation of volcanic activity in what’s known as the back-arc spreading centre — in layman’s terms, an area of submarine volcanism resulting from submarine rifting (rather than the subduction processes which gave generated the Marianas volcanic arc itself, which lies to the west).
Rift systems, whether as part of the central ocean ridge system which runs round the planet or as part of smaller sections within larger plates, are where new crust is made. Almost all of them are underwater. In this case, the research is ground breaking in that it “is the first known historical eruption on the Mariana back-arc, and the deepest one documented anywhere on Earth.”
In a sense, the specifics of the research aren’t important. But they do illustrate how much goes on at depth that we aren’t aware of, especially in areas such as submarine volcanism, most of which takes place at great depth, and how much more we have to learn about the processes of the dynamic planet on which we live.