The week of 17-23 November was a haunting one. Although the total number of earthquakes recorded on the United States Geological Survey’s earthquake map wasn’t anything out of the ordinary, one, at least brought with it a haunting memory.
Following on from last week’s New Zealand earthquake, which reminded us of the disaster in Christchurch in 2011 which killed 65 people, we had a major tremor of M6.9 in Japan, where a tremor of M6.9 brought echoes of the Tohoku-Oki earthquake of the same year, with its death toll of an estimated 15,000.
These events remind us that while earthquakes can’t be accurately predicted in terms of timing and magnitude, they do occur in roughly the same places. The USGS map, which includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) was broadly as expected, with the largest tremors concentrated around the margins of the planet’s tectonic plates, and the usual high levels of activity around the Pacific margin.
The Week’s Biggest Earthquake: M6.9, Japan
The largest earthquake of the week, variously reported at magnitudes up to M7.4, eventually turned out to be a relatively minor affair, with the USGS recording it as M6.9 and not considering it worthy of a separate earthquake report on the event page associated with its real time map. But it was what it could have been, as much as what it was, which raised concerns and made the news.
The earthquake occurred in broadly the same location as the 2011 tremor, off the island of Honshu where the Pacific plate subducts beneath the Okhotsk microplate. Like its notorious predecessor, it generated a tsunami — though at a maximum of 1.4 metres it was, like the earthquake itself, much smaller than its predecessor.
There were, however, significant differences between the two, quite apart from the size. (For the record, this week’s tremor was over two orders of magnitude smaller.) The 2011 event was a classic megathrust earthquake, occurring at the plate boundary.
This week’s event occurred over 100km from the boundary at a shallow depth, and seems most likely to have resulted from crustal deformation rather than direct subduction: the available data show that it resulted from extensional, rather than compressional, movement.
As a rule of thumb, a tsunami will occur if an earthquake exceeds a magnitude of (around) M7.0, is at a shallow depth and involves vertical movement. In this case, all of these conditions were (just about) met. In the great scheme of things, this week’s event was perhaps not significant — but it does serve as a sobering reminder of the potential of dynamic natural processes to cause damage.
M6.4 Earthquake, Argentina
On the opposite side of the Pacific Ocean, the second largest tremor of the week, an M6.4 beneath the Andes in Argentina, provided an interesting counterpoint to the Japanese event. As in Japan, the tremor resulted from subduction, this time along the Peru-Chile reach, where the Nazca plate moves eastwards beneath the South American continent, resulting in the uplift of the Andes. And the available data also indicate that its primary cause was extensional, rather than compressional, movement. Both occurred at a considerable distance from the margin itself.
In this case, however, the focus of the earthquake was deep — over 110km — which places it in the subducting, rather than the over-riding plate. This is not atypical of earthquakes of this depth in this region which, as the USGS notes, occur “within the Nazca plate as a result of internal deformation within the subducting plate”.
US Earthquakes: All Quiet (For Once)
With so much going on elsewhere, it was an unusually calm week for earthquakes in the US. The largest, an M5.4 in the Aleutian Islands, is pretty much par for the course. And even in Oklahoma the swarm of earthquakes associated with wastewater disposal seems to have settled down after a couple of significant recent earthquakes — the largest registered M4.0 and the number appearing on the map is much lower than usual.
Last Thoughts: Meanwhile in Canada
The association between earthquakes and fracking seems always to come up in the context of Oklahoma — thought in fact these earthquakes are not the result of fracking, per se, but of wastewater injection following on from it. The verdict on fracking itself, in this area, has not, to date, been conclusive.
This week, however, results of a study in Canada do appear to indicate that small earthquakes in Alberta have resulted from fracking itself. The implications of these findings for the fracking debate as a whole, however, remain to be ascertained.
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