Tectonically speaking, things are calming down. In the week of 8-14 October, even the reverberations from the Chilean megathrust earthquake had died down.
The Chile aftershock sequence contributed just three to the total of 23 earthquakes of at least magnitude 5 (≥M5.0) which appeared on the United States Geological Survey’s real time earthquake map for the week, and just eight of the 65 which registered as ≥M4.5.
The map, which includes earthquakes of all magnitudes in the US and its territories and those of at least 4.0 elsewhere, showed around 1450 tremors this week; the largest of the earthquakes recorded at M5.9.
Of the 65 ≥M4.5, the distribution was, as expected, largely confined to the margins of the planet’s main tectonic plates (some of which are narrow and some diffuse).
There was one outlier, though — an M4.5 in central Oklahoma. More of that anon…
The Week’s Biggest Earthquake: M5.9, South of Kamchatka
Things have been relatively quiet in the north-western Pacific recently, certainly compared to what’s been happening on the other side of the so-called Ring of Fire, in Chile. This week the largest earthquake was an M5.9 which occurred around 250km off the southern tip of the Kamchatka peninsula.
The margin between the Pacific plate and the Okhotsk microplate runs in an arc from the Kamchatka down to Japan and, as recent history shows us, is capable of generating very large earthquakes (the southern part of this boundary was the location for Japan’s devastating earthquake of 2011). So the largest earthquake of the week was, in fact, a relatively small one for its context.
The Kuril-Kamchatka Trench is a subduction zone where the old, cold, dense oceanic crust of the Pacific plate descends westwards beneath the Okhotsk microplate. The Kuril Island arc is the surface expression of this. And subduction zones produce earthquakes as strain builds up and is released.
It’s worth noting, however, that though this week’s largest earthquake is relatively small, it is the second largest to strike the Kuril-Kamchatka margin this year (outdone by an M6.3 in July).
M5.4 Earthquake, New Zealand
Further south the boundary between the Pacific and Australian plates is also (largely) a subduction zone. The southern section, the Hikurangi Trough, runs offshore along the eastern coast of New Zealand’s North Island.
Most plate tectonic maps (including the USGS interactive map, from which the images in this digest are taken) are simplified. While some areas are, in fact, straightforward, others are not. To the east of the Hikurangi trough lies an area of extensional and strike-slip tectonics.
A closer look at this complex situation suggests that this week’s M5.4 tremor, around 30km from the town of Castlepoint, was not in fact a subduction earthquake but is more likely to have been caused by movement along one of the shallower fault systems further to the west.
US Earthquakes: Oklahoma
After a while when nothing new came up in the Oklahoma earthquake swarm, it’s back. This week there was the coincidence of an M4.5 (the largest for a while) near to the town of Cushing, and the publication of a piece of work on the Cushing earthquake swarm.
Without going into details, it’s enough to note (as the research paper, from McNamara et al summarises) that the earthquake swarm in Oklahoma can be attributed to the injection of wastewater deep underground which reactivates long-dormant buried faults.
And it’s not good news for local residents. As the study notes: “it is reasonable to conclude that the Cushing and Wilzetta-Whitetail fault zones are critically stressed in a region sufficient enough to increase the likelihood of a large and damaging earthquake.”
Last Words: One Size Doesn’t Fit All
A recent comment on an earlier digest suggested (flippantly, I assume) that scientists might attribute Chile’s major earthquake to fracking. This week’s featured earthquakes are very different and they illustrate three different situations in which earthquakes may occur (there are, of course, others).
The Kamchatka temblor looks to have been the product of movement at a subduction zone. The New Zealand quake probably resulted from a more complex set of natural fault movements. And the Cushing earthquake, away from all plate margins, is almost certainly caused by man’s activity – anthropogenic quakes have the potential to cause damage, just like those that take place independent of human activity.