It’s a year since I abandoned my weekly earthquake roundup in favour of a more general geoscience article. On the whole I think the experiment worked, but there’s always a place for information on earthquakes. So I thought I’d use this week’s article to have a look back at the earthquakes that shook us in 2018.
Earthquakes are a source of endless fascination in a way that (say) climate change, important though it is, is not. It’s because their impact is unequivocal: they are sudden and can’t be confused with anything such as (say) seasonal variation. When an earthquake occurs, it’s immediately obvious in the area where it is felt. The ground shakes. Things fall off shelves. Buildings collapse. In a major earthquake many people will be hurt or killed. And large part of the planet is vulnerable to damage from the earthquake itself or from a potential subsequent tsunami.
The greater the impacts of an earthquake, the more attention it gets: a moderate earthquake in a populated area will generate much more media coverage than a large one on a very remote area. For this reason it can sometimes seem that earthquake activity has increased.
This week I want to look at last year’s earthquakes — how many there were of larger magnitudes — and see how they compare to an average year.
2018: How Many Earthquakes, How Big?
So let’s begin with a very quick run through the numbers. The United States Geological Survey’s real-time earthquake archive allows us to look back over 2018 and map (and list) the earthquakes that occurs. The archive includes earthquakes of all magnitudes, though not all earthquakes. With this in mind it makes sense to consider only the larger events — those of at least magnitude 5 — where the listing is most likely to be complete and therefore comparisons wth previous years are meaningful.
In 2018, the archive tells us, there was just one earthquake of at least M8. That was a deep earthquake of M8.2 in the Pacific, close to Fiji. Thee were are further 16 tremors of between M7.0 and M7.9, all but two of which occurred in the Pacific Ocean’s so-called Ring of Fire. The two exceptions were a tremor of M7.1 in the South Sandwich Islands and one of M7.3 in Venezuela.
2019 also saw 134 tremors of at least M6.0 and 1800 of at least M5.0. These (M5.0 and more) are the ones shown on the map, which indicates that the majority of earthquakes are located, as we would expect, around the margins of the Earth’s tectonic plates. The larger earthquakes tend to be focussed along subduction zones, but there’s also significant activity along constructive margins at mid-ocean ridges.
How Does 2018 Compare With Previous Years?
The USGS helpfully has a table of earthquake numbers for each year from the beginning of the century. There’s considerable variation but I’ve summarised the table in the graphic to show average figures and compare them against 2018.
In fact, 2018 seems to have been close to average. The single tremor of M8+ was in line with the expected figure. There were slightly more earthquakes in the ranges M7.0-M7.9 (16 compared to 14) and M5.9-M6.0 (1800 compared to 1609) but there were slightly fewer in the mid-range — 134 earthquakes of M6.0-M6.9, compared to 140 on average.
The USGS figures also include earthquake fatalities, though these only go up to 2015 and are, in any case, only an estimate. They are, nevertheless, useful. If you look at the full table you’ll see immense variation and the average of over 50,000 deaths per year is as high because of two very large events — the tsunamigenic earthquakes in Indonesia in 2004, which killed almost 300,000 people, and the 2011 Japan earthquakes which killed over 225,000. By contrast, in 2000 only 231 earthquake deaths were recorded.
There’s no available death toll for 2018, but the most deadly event was that which, with its accompanying tsunami, ravaged part of the island of Sulawesi and killed over 2,000. Other earthquakes this year have killed mercifully few people.
What About the Continental Interiors?
I’ve noted above that major earthquakes are largely confined to the margins of the planet’s tectonic plates. that’s hardly surprising — it’s where most of the strain variations occur, either by compression as the plates come together or extension as they fall apart.
The map does show a few large earthquakes where we might not expect them, in the continental interiors — in the centre of Eurasia, in Africa and in Australia. Such earthquakes are relatively infrequent but not necessarily unexpected. In the former two cases, there are major rift valleys — potential precursors of new oceans, where the content is beginning to split.
In Australia, earthquakes of moderate magnitude (M5.0-M5.9) do occur despite the stability of the ancient continent. Continental collision has its impact, eventually, at very great distances from continental margins and areas of historic faults may be reactivated by such stresses — something which may account for such apparently anomalous events.
A Quick Word About Volcanoes
I didn’t mention December’s tsunami in Indonesia as a major event. That’s because, although earthquakes are the major cause of tsunamis, they aren’t the only one and the Indonesia tragedy was the result of a volcanic eruption, not an earthquake.
That said, earthquakes and volcanoes are inextricably linked. Volcanic activity changes the shape of the land, other catastrophically as it did in the example above with the collapse of a volcanic slope, or more gently as the magma chamber falls and empties, cracking the surrounding and overlying rock.
The eruption of Kilauea volcano is an example of this. It generated 54 earthquakes of at least M5.0 in 2018 associated with the explosive activity at its summit. In 2017, by contrast, there were no earthquakes of this magnitude in Hawaii.