There was relatively little significant seismic activity worldwide in the week of 19-24 October 2017. True, there was one indisputably large tremor — of magnitude 6.7, in Indonesia — on the United States Geological Survey’s real time earthquake map; but that apart, all was reasonably quiet.
The map, which broadly speaking includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of just under 1,650 tremors, of which just two reached M6.0, 28 were at least M5.0 and 87 were at least M4.0.
These numbers are a little lower than what we might normally expect, though well within normal bounds. The distribution was also pretty much as usual with most of the larger tremors being at or near plate boundaries, although this week there were three exceptions — one in the Bering Sea, one in the South Pacific and one, which I’ll discuss below, in Tanzania.
The Week’s Biggest Earthquake: M6.7, Indonesia
Having said that this week’s earthquakes are largely unremarkable, the biggest of the week did make me stop and think for a little. At M6.7, in an area where the crust is broken and the tectonic setting is complex, this M6.7 in the Banda Sea is, in terms of its magnitude, nothing out of the ordinary. But, as we’ve seen so often in the past, earthquakes are about more than just magnitude.
At the macro level, the Banda Sea is where the Australian plate collides with, and subducts beneath, the Philippine Sea plate, but a look at the detailed tectonics of the region shows that this is a simplistic explanation. As well as the main boundary there are contrasting directions of subduction along parallel thrust faults, and to the north there are major strake slip (laterally-moving) faults.
What caught my eye is the depth — the earthquake occurred at just less than 550 km, though its epicentre is less than 100km from the margin as marked on the map. That’s unusual. Looking at the USGS regional seismicity patterns in more detail shows that earthquakes occur regularly at depths of at least 300km along a band parallel to the trench.
In yet more detailed maps (I refer to seismologist Robert Yeats for these), we can see that the band of deep earthquakes is related to subduction of the older and denser oceanic crust at the northern margin of the Australian plate which, Yeats remarks, takes place at an unusually steep angle.
M5.2 Tremor, Tanzania
There is no plate boundary in east Africa — yet. We know, however, that continents break up and reform, and that the early stages of the process (which appear on the planet as major continental rifts) are occurring in the east of the African continent in the form of the East African Rift Valley, running from Ethiopia down to Mozambique. Its source lies below Ethiopia and to the north of this, the rifting process has already developed two new narrow seas — and possible future oceans — in the Red Sea and the Gulf of Aden.
There’s no information on the actual mechanism of the M5.2 earthquake which occurred in Tanzania this week, on the eastern flank of the rift above Lake Tanganyika. If I had to guess, I’d suggest that it was probably caused by normal faulting as a direct result of the extensional movement as the rift widens. That said, there’s also a chance that it might have been caused by lateral movement.
It’s worth noting that, although we don’t normally associate this part of Africa with major earthquakes, there is a bit of unwelcome history here. Yeats notes that, in 1910, an earthquake of M7.4 occurred on the eastern side of Lake Tanganyika — the area where this week’s earthquake also occurred.
US Earthquakes: Montana and the Rockies
The western US is an area best known for the drama-inducing San Andreas Fault Zone — but there are other major forces at play. Much of the western US has been shaped by extensional tectonic forces, giving rise to the typical mountain ridge and valley topography known as ‘basin and range’.
At the northern edge of this, before the mountains give way to the stability of the northern Great Plains and Canadian continental interior, an area of normal faulting gives rise to occasional small (and even more occasional large) earthquakes. This week Montana saw a tremor of M4.3 close to Lincoln. Again, though its too small to generate any information from the US as to its fault mechanism, my guess would be that it’s probably the result of extensional movement.
Last Thoughts: Extensional Earthquakes
So, I began by referring to the majority of earthquakes being at major plate boundaries and then went on to feature two which aren’t. What does that tell us?
It tells us that the Earth is dynamic. Continents break up and make up. They come together and they split apart. Earthquakes along continental rifts, such as that in East Africa, are the early signs of major tectonic evolution.
Africa as we know it, seemingly so solid and stable, is, like the rest of us, transient, although its timescales are much longer. There may be something which prevents the African rift growing into an ocean, or it may continue and split the content from north to south.
Whatever happens, it’s reasonable to suppose that a couple of million years down the line, the configuration of the continents will look very different to the way it is today — and the signs of change are there for us to see.