The numbers are playing tricks on me again. I’ve lost count of the number of times I’ve pointed to the (general) average of one earthquake of at least magnitude 7 (≥M7.0) every month. In actual fact it’s a little more than that — 15 per year, according to that most impeccable of sources, the United States Geological Survey.
As it happens, this week, the 1-7 September 2016, is the fourth consecutive week when the USGS earthquake map has recorded a tremor of at least M7.0. The map — which (roughly speaking) includes events of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere, showed just one other earthquake larger than M6.0 and a plethora of intermediate earthquakes — 59 of ≥M5.0 and 164 of ≥M4.0. Many of these are aftershocks to the largest event.
The distribution of those larger earthquakes was pretty much as normal, with a concentration around the margins of the Earth’s tectonic plates. There was one notable exception — but we’ll come to that later.
The Week’s Biggest Earthquake: M7.1, New Zealand
The largest earthquake of the week, an M7.1, struck off the North Island of Mew Zealand on 1 September. So large a tremor so close to land is bound to make the news and so it proved — though in fact the USGS earthquake summary indicated that less than half a million people are likely to have felt any shaking.
For those who did, there was concern. The earthquake caused some local damage and residents were evacuated from some coastal areas in anticipation of a tsunami. The threat did, indeed, materialise but only in the shape of a 21cm (8 inch) wave.
Large earthquakes in populated (and developed) areas are worthy of reporting so the USGS does much of my job for me in its detailed earthquake summaries. I can’t, therefore, do better for scientific accuracy than quote directly from the USGS summary itself.
“The September 1, 2016 M 7.1 earthquake northeast of Gisborne, New Zealand occurred as the result of shallow oblique-normal faulting near the plate boundary between the Pacific and Australia plates. Preliminary focal mechanism solutions indicate oblique rupture occurred on either a steep dipping, southwest striking normal fault, or on a shallow dipping, north-northeast striking fault,” the summary states.
It goes on to note that: “The depth, location and focal mechanism of the earthquake all indicate this is most likely an intraplate event within the subducting Pacific slab, rather than being an interplate thrust earthquake on the overlying subduction zone interface”.
M6.3 Earthquake, Aleutian Islands
On the northern edge of the Pacific Ocean, the Pacific plate subducts beneath the North American plate in the long and highly seismically (and volcanically) active Aleutian island chain. This subduction zone is capable of generating mega earthquakes and regularly produces events of M5.0 or M6.
This week’s earthquake was at the eastern end of the island chain and occurred at a depth of 12km, with its epicentre around 90m north of the Aleutian Trench, in the overriding (North American) plate.
Unlike the New Zealand earthquake, this one doesn’t justify an event summary from the USGS. But the available information on depth, location and direction of movement suggests pretty much the same origin — movement within the plate rather than at the plate interface.
In this case, however, the movement appears to be strike-slip (lateral) rather than normal (vertical) movement.
US Earthquakes: Oklahoma
And so the the outlier mentioned above, the significant earthquake away from plate boundaries. Once more we’re in Oklahoma and this time we’re talking something big. At M5.6, this week’s earthquake near Pawnee equalled the largest earthquake previously recorded in the state, the 2011 Prague earthquake.
This earthquake has been coming, as the number and magnitude of earthquakes in this area has increased. Again the USGS event page, listed in the resource section, is the place to go for full details, but possibly the most significant observation is that this sequence of events is almost certainly not over.
“The magnitude 5.6 earthquake near Pawnee, OK, on Sept. 3, 2016 will continue to produce aftershocks. Residents in that area can expect to continue feeling earthquakes and there is the possibility of additional earthquakes capable of doing damage. As of 7PM CDT on September 4, 2016: There is a 12% chance of an aftershock of magnitude 5 or greater within the next month, and that likelihood increases to 29% if the time window is extended to include the next year.”
Recently, scientists have announced a new geological era — the Anthropocene. It’s defined by human influence. This earthquake, like so many others, is the result of human activity — in this case, wastewater injection. In the immediate aftermath, state authorities ordered a cessation of such activities in the surrounding area. But it’s most unlikely that will solve Oklahoma’s earthquake problems.
How Many Big Ones in a Row?
You’d be forgiven for thinking that the recent run of earthquakes of at least M7.0 means that something terrible is happening in earthquake terms — but, believe it or not, we’re still pretty much on target for our annual 15. This week’s tremor in New Zealand was the twelfth of the year.
Just for interest, we can look at the pattern of larger tremors. They aren’t evenly spread — two in January; one in March; three in April; one in May; one in July; three in August and one, so far in September.
We can expect more before the end of the year.