It was an interesting week this week, tectonically speaking, even if the numbers weren’t particularly outstanding.
As usual, the United States Geological Survey’s real time earthquake map, which shows earthquakes of all magnitudes in the US and its territories and at least magnitude 4 (≥M4.0) elsewhere, shows only the bare figures.
There was just one earthquake of ≥M7.0, so remote that it affected no-one and went largely unnoticed in the media, and just two others of ≥M6.0, both in the western Pacific.
Twenty earthquakes were larger than ≥M5.0; 112 ≥M4.0; 230 ≥M2.5; and there were over 1500 of all magnitudes. And the distribution was much as we would expect, around the margins of the Earth’s tectonic place.
So far so normal. But let’s take a look at what I like to think of as the seismological small print.
The Week’s Largest Earthquake: M7.0, South Atlantic
In most weeks, the occurrence of an M7.0 earthquake would justify an article all to itself, but this week’s occurred in the middle of nowhere. More specifically, the quake occurred on an un-named fracture zone in the south Atlantic, a few hundred miles from the also extremely isolated island of St Helena.
The USGS shake map includes only the wide, wide ocean and no-one, so far at least, has reported having felt it. For such a large earthquake, it seems to have concealed itself pretty well.
For all that, it’s an interesting and unusual beast. The USGS map regularly shows earthquakes on mid-Atlantic ridges but generally speaking they don’t exceed M6.0. So what can we say about this one? We can generalise about the origin of such earthquakes, which are caused by the divergent movement of tectonic plates at ocean ridges causing fracturing and faulting, and which are typically shallow, involve strike-slip faulting and don’t cause tsunamis.
The only source of specific information on this one comes from the United States Geological Survey, which suggests that the most recent tremor conforms to all those generalisations with the exception of its magnitude, and that this area tends to produce large(ish) earthquakes of this nature, though none recorded to date has been as large as this.
“Moderate-to-large earthquakes in this region of the south Atlantic are not uncommon,” the USGS notes. “Over the past century, sixteen other earthquakes of M 6.0-6.6 have occurred within 250 km of the June 17, 2015 event, both along the same fracture zone and in association with other shorter fracture zones to the south. The June 17, 2015 event represents the largest in the region over that time period.”
M4.5 Quake, South Africa
Another earthquake of interest, in southern Africa, was rather smaller than the largest of the week, at just M4.5. This tremor, like that in the Atlantic, was apparently barely felt, or more likely barely reported. What makes the quake interesting is the fact that it’s located at very great distances from any plate boundary and is even far south of the (potentially) developing rupture of the East African Rift.
Southern Africa contains some of the oldest rocks in the world, up to a staggering 3.6 billion years old in the Kaapvaal craton, in which the epicentre of this earthquake lies. Earthquakes of this magnitude may not be large or common in such old and stable continental interiors but they do occur.
Robert Yeats, who comments only in general on southern Africa in his book Active Faults of the World, observes that “Southern Africa contains faults which have been reactivated in the Quaternary” and it does seem that the most likely cause in this case is normal faulting along very old fault lines.
US Earthquakes: Tennessee and the Reelfoot Rift
Two small earthquakes — just M2.6 and M2.7 — occurred this week in western Tennessee. Both were shallow and the largest generated only weak shaking in the immediate area. These were almost certainly the result of further reactivation of ancient faults, but in this case the faults belong to a deeply-buried ancient rift valley, the Reelfoot Rift.
The Reelfoot Rfit underlies the New Madrid Seismic Zone, which produced a series of very large earthquakes (possibly as large as M8) in 1811-12. The most recent earthquakes testify to the fact that the region is still seismically active.
Last Words: Old Faults Don’t Die
Normally we associate earthquakes with currently active boundaries — such as subduction zones and mid-ocean ridges. The earthquakes in Africa and Tennessee are a reminder that faults which existed millions, and possibly even billions, of years ago, may be reactivated and cause earthquakes.
Decoding Science. One article at a time.