Last week was very quiet on the earthquake front, and this week, 25-31 May 2017, only a little less so. The United States Geological Survey’s real time earthquake map showed (overall) more or less the ‘normal’ number of earthquakes but there were fewer of them in the higher-magnitude categories — which, by the way, is where the map is more accurate.
The map includes earthquakes of all magnitudes (not the same as all earthquakes) in the US and its territories, and those of at least magnitude 4.5 (≥M4.5) elsewhere. It’s an underestimate, for all sorts of reasons, but it does include all the significant tremors.
This week there was just one earthquake larger than M6.0 — an M6.6 in Indonesia — and 33 of at least M5.0. As usual, these larger tremors were associated with the margins of the planet’s major tectonic plates.
The Week’s Biggest Earthquake: M6.6, Indonesia
In terms of natural disasters, Indonesia must one of the most dangerous places in the world. The subduction zone along the coasts of Java and Sumatra is capable of generating major (often tsunamigenic) earthquakes; the crumpled crust underlying the rest of the country is regularly seismically active; and on top of that the volcanoes associated with this crumpling of the earth are highly active (five currently erupting) and capable of massive explosive activity.
This week, then, it’s hardly surprising to find the week’s largest earthquake once again in Indonesia. In some ways the country is an accident of tectonics: like much of the rest of this area of south east Asia, it’s the product of an agglomeration of crustal pieces that are currently reshaping both on and beneath the surface, in a sequence of interrelated segments far too complicated to appear on any global map.
This week’s M6.6 earthquake (by no means unusual for its location) occurred in central Indonesia, on the island of Sulawesi. Tectonically, this area is monumentally complicated, with several different slivers of crust, their margins characterised by different directions of movement, jostling for space.
Once you focus on more detailed fault maps, however, it does seem that the direct cause of the earthquake is reasonably straightforward — movement along on or near a normal fault which crosscuts the island.
M5.8 Earthquake, Chile
Like Indonesia, Chile is no stranger to either earthquakes or volcanoes, though tectonically speaking it’s much less complicated. Only two major plates — and no minor ones — are in play for this week’s M5.8 in south-central Chile, and their relationship is about as straightforward as it gets.
The Nazca plate is moving eastwards against the South American continent and descends beneath it along the Peru-Chile Trench. Earthquakes, some of them major, occur along the length of this margin as a result of the subduction process. And the coming-together of the two results in the uplift of the Andes mountain chain, which runs along the western edge of South America.
There’s no earthquake summary available from the USGS for this earthquake, so we have to draw our own conclusions based on the available information. In this area, the angle of subduction is relatively steep, so that the depth of the earthquake (79km) and its distance from the trench (around 300km), taken together, indicate that it wasn’t the result of movement at the plate interface.
Add in what we know about the type of movement — somewhere between normal and strike-slip — and this suggests that this is earthquake isn’t the result of subduction as such, but of the mountain-building processes which accompany it.
US Earthquakes: Is All Okay in OK?
I haven’t revisited the earthquake swarm in Oklahoma for a while, largely because there hasn’t been much to say. But, in much the same way as Sherlock Holmes recognised the significance of the dog that didn’t bark, there’s a moment to come back and look at what isn’t happening.
I really am wary of the smaller earthquakes on the USGS map, because they are significantly under-reported. That said, I would expect the larger earthquakes (say, more than about M2.0) to appear on the map. In the past, these earthquakes have numbered several dozens in a week. This week — 22.
There are all sorts of reasons why this might be, and I really would hesitate to draw any definite conclusions. It may be — just may be — that this is the first sign that the swarm is calming down. But I wouldn’t bet my house on it.
Last Thoughts: No Such Thing as Simplicity
I think I may originally have been attracted to plate tectonics by the simple elegance of lines on a map. A line down the middle an ocean where the contents move apart, a line off the coast where a tech runs side by side with a mountain range; the curve of an island arc, thousands of kilometres long.
These things do exist; but they’re inevitably an over-simplification. In most cases, as the sheer madness of Indonesia’s geology tells us, plate tectonics is far more complicated than that and it isn’t just what’s going on at the surface but what’s happening well below it that causes earthquakes.
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