It’s been a while. I stopped writing a purely-earthquake roundup because it was becoming repetitive and there was very little new to say, but, as they say, what goes round comes round and so here I am again, though not with an exclusively earthquake article (let’s not forget Kilauea). Not because there is anything particularly special about the recent crop of earthquakes, but because some people might think there is and this requires further examination.
The Randomness of Earthquakes
There have been a lot of large earthquakes recently.
I’ve said before that I’m not a statistician, and even if I was I have neither the time nor the access to the required data to carry out an exhaustive assessment of the patterns of earthquake activity. People who know more than I do have crunched the numbers , at some level at least — and the graphic (actually a screenshot from a Tweet by seismologist Dr Wendy Jones) summarises them beautifully.
Probably the most important point in the Tweet is the last one: “Definition of random distribution”. Dr Jones goes on to note in another Tweet that: “For the last couple of months, people have been worried that the world wasn’t having enough quakes. Now people are worried we are having too many. Human beings continue to try to make patterns in random distributions and ascribe meaning to those illusory patterns.”
In a sense, of course, earthquakes aren’t random, because we know the seismic zones where the larger ones will occur, even though we don’t know when or precisely where or how large the next earthquake might be. But that’s the limit of their predictability. So the recent spate of large earthquakes isn’t something we should be concerned about in the same way that we might be concerned about, say, rising sea level.
What and Where: The Earthquake Roundup
In the last seven days (16-22 August, 2018) there has, indeed, been a spate of earthquakes and I shall attempt a short summary of the most significant ones. The United States Geological Survey’s real-time earthquake map shows 53 in excess of M5.0, including an M8.2 in Fiji, an M7.3 in Venezuela, an M6.9 in Indonesia and an M6.6 in Alaska.
We can reasonably expect to see one earthquake of M8 or more every year. The one that struck 563km beneath the Pacific in the Fiji region this week, though large, caused no damage because of its considerable depth. It occurred at the boundary between the Pacific and Australian plates and is remarkable as being the largest on record in this area, although large earthquakes are by no means unknown here (and recording is a relatively new science so its perfectly possibly that larger events may have occurred in the past).
Just off Venezuela, where the Caribbean plate abuts the South American plate, there was an earthquake with a magnitude of M7.3. The depth (123km) and location (east of a subducting section of the margin) suggests that the earthquake was probably the result of subduction rather that movement along the shallower transform faults which lie above the earthquake at the surface.
In Indonesia, an M6.9 earthquake marks the latest in a series of devastating tremors on or near the island of Lombok which have, to date, costs hundreds of lives. This sequence of earthquakes began on 28 July and the latest, though overshadowed by larger events elsewhere, is equal in size to the largest preceding it, and it may well be that there is further significant earthquake activity to come.
In Alaska there was an earthquake of M6.6 along the active subduction zone between the Pacific and North American plates. This remote area of the North Pacific is frequently the location of major earthquakes (including the second largest earthquake on record) and these often go remarked upon.
I’d also like to give an honourable mention to another Alaskan earthquake that occurred on 12 August and which would have been in last week’s article had I not been on holiday. This one, an M6.3, was not significant in the general context of Alaskan earthquakes — but Alaska is a very big place and not all of it is produces large earthquakes. This particular tremor has the distinction of being the largest recorded in Alaska’s North Slope.
There have been plenty of other earthquakes this week, and there is plenty more to say about those I’ve considered. The important message is that while some of them may be remarkable in certain specific ways, they don’t collectively represent any kind of trend in the scale or intensity of earthquake activity.
In recent weeks the volcanic activity at Kilauea has slowed, although it hasn’t been possible to say for certain whether that represented a pause in the current eruption or whether the event was moving towards its close.
While it still isn’t conclusive, the reports coming from Hawaii’s Volcano Observatory have a breath of finality about them. They have reduced in number and they are considerably shorter. The meat of the most recent is worth quoting in full: “Seismicity and ground deformation are negligible at the summit of Kilauea Volcano. On the volcano’s lower East Rift Zone (LERZ), only a few ocean entries are oozing lava; laze plumes are minimal. Sulfur dioxide emission rates at both the summit and LERZ are drastically reduced; the combined rate is lower than at any time since late 2007.”
It looks as though the latest phase of the current eruption (which, remember, has been going on since 1983) may be over.