After a week away I’m back to write the digest and it appears that, in the week of 10-16 August, everything has settled down into a normal seismic pattern, just as I’ve settled into my post-holiday routine.
The earthquakes appearing on the United States Geological Survey’s real time earthquake map were pretty much what we’d expect them to be, in number and distribution. The map, which broadly includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, recorded a total of just under 1900 tremors in total, of which just two were ≥M6.0, 33 ≥M5.0 and 121 ≥M4.0.
As we might expect, most of the larger earthquakes occurred at or near the margins of the Earth’s tectonic plates, its the odd exception — most notably, a tremor in the normally-stable African continental interior — of which, more below.
The Week’s Biggest Earthquake: M6.4, Indonesia
Indonesia is big, and it encompasses one of the most dangerous seismic zones on the planet. You won’t need a long memory to recall the destruction caused by the Boxing Day earthquake of 2004 and there have been other significant events since then, along the Sunda Trench.
This week’s M6.4 earthquake was nothing like as large as that of 2004, but its tectonic setting was the same. Off the west coasts of Java and Sumatra, the Indo-Australian plate is subducting beneath the smaller Sunda plate along a margin thousands of kilometres long as the two plats converge — and the data available from the USGS about the source of this earthquake confirm that convergence is the main cause.
This particular part of the boundary has, in the past been the source of very large earthquakes indeed — a tremor in 1797, which predates seismic measuring techniques but for which seismologist Robert Yeats quotes estimates of M8.4, and a more recent event, in 2007, of the same magnitude. (The 2004 ‘quake occurred a little further to the north.)
To place this week’s tremor — the largest of the week — in context, the difference between it and its local predecessors is two in terms of magnitude. But the magnitude scale is logarithmic, so that it was just one-hundredth the size. And to illustrate further the high seismicity of this region, there have been over 40 tremors of at least this magnitude in the region since the beginning of the 21st century.
M4.8 Earthquake, Botswana
The planet’s large continental areas (known as cratons) are distant from major plate boundaries and generally free from large — or even intermediate — earthquakes. Eastern Africa, where a rift in the continent indicates the beginnings of continental break-up, is an exception to this and, on the occasions when a notable earthquake occurs in Africa, it’s usually in this area.
This week’s M4.8 in Botswana, however, is a little different; too far west to be explained by continental rifting. In so remote an area, it’s perhaps not surprising that the data relating to the quake itself are scarce, but we can look at the context and take a guess at its origins.
Yeats makes the point that: “Southern Africa contains faults that have been reactivated in the Quaternary” and though I don’t have access to fault maps of this area there’s every chance that such faults exist. A shallow tremor in an otherwise stable continental region far from seismic belts is most likely to be caused by movement on a very old fault and this does seem most likely to be the case here.
US Earthquakes: Oregon
What’s happening in the US this week? Not a lot in the lower 48, although the Earth shook a bit in Alaska. But I feel as if I’ve talked a lot about Alaska recently, so what about elsewhere?
Off Oregon, the once-large Juan de Fuca plate (remnant of the Farallon Plate) is slipping slowly into oblivion beneath continental North America along the Cascadia subduction zone. While Cascadia remains a (mercifully) sleeping giant among subduction zones, the Juan de Fuca’s drawn-out ending is marked by relatively high levels of seismicity (in terms of number of tremors not magnitude) along a number of fracture zones.
This week’s M4.6, off the coast of Oregon, was entirely typical — a shallow (10km) strike-slip earthquake resulting from internal deformation and (possibly) a component of rotational movement.
Last Thoughts: Breaking Up Isn’t Hard to Do — But It Takes a Long Time
Plate tectonics is a slow process. The breakup and reassembly of continents is a cycle (known as the Wilson Cycle) that takes hundreds of millions of years to complete. I was reminded of it by my (admittedly throwaway) comment about the Juan de Fuca plate being in its final throes. In geological time, that’s plain sloppy.
Major earthquakes at subduction zones such as the Sunda Trench are a reminder that the crust is being consumed there, and that the ocean is, however slowly, closing. Similarly, tremors at mid-ocean ridges (not featured this week) are a function of crust being created and an ocean opening. And although this week’s Botswana earthquake almost certainly isn’t anything to do with the east African Rift system, that system does illustrate another aspect of the Wilson Cycle — continental breakup.
Africa won’t break up for a long time (and it may never do so). The Juan de Fuca plate, that final fling of a much larger slap of the planet, will be with us for hundreds of generations. But in geological terms these things are very much on the move, Our planet is truly dynamic.