The Earth shook more than a bit this week. Following last week’s M7.1 in Alaska, the other side of the North Pacific shook as well, with an M7.2 over in eastern Russia. It was as if the planet’s political, rather than physical, fault lines were indulging in an old-fashioned US-Russian episode of anything-you-can-do-I-can-do-better. It was almost like the Cold War all over again.
More on that later. The United States Geological Survey’s real-time earthquake map had a couple of other large events, too. The map, which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of over 1350 earthquake for the week of 28 January-4 February 2016.
Of these, three were ≥M6.0 (the M7.2 mentioned above, along with an M6.2 north of New Zealand and an M6.0 in the deep South Pacific). There were 27 tremors of at least M5.0, all of them associated with the major plate boundaries, and 99 of at least M4.0.
The Week’s Largest Earthquake: M7.2, Kamchatka
Russia’s Kamchatka peninsula is a restless place. Tectonically, it’s a southern extension of the North American plate, known to seismologists as the Okhotsk microplate. It extends southwards between the Pacific and Eurasian plates, with the former subducting beneath it from the east. It forms part of a major subduction zone extending from central Kamchatka down to Japan.
The Kamchatka peninsula is the northern part of this zone; to the south it finds surface expression as a chain of volcanic islands, the Kuril Islands. According to the USGS, this week’s earthquake was “the result of oblique-normal faulting at a depth of 180 km”, and resulted from the collision of the two plates. The USGS further notes that: “Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates.”
Kamachatka is capable of generating very large, and often very deep, earthquakes. The USGS archive includes eight tremors of at least M8.0 over the past century. The most recent of them, in 2013, is notable not just for its size (M8.3) but for its exceptional depth (around 600km).
M6.2 Tremor, North of New Zealand
Much further south, another of the Pacific’s margins was active. Between Tonga and New Zealand, the Pacific plate subducts beneath the Australian plate in what it is pretty much a textbook example of a subduction zone. It tends to give regular textbook examples of subduction zone earthquakes, too.
Earthquakes at the interface between the plates show a clear relationship to location, the distance from the boundary increasing with depth according to the angle of depth. (This zone of earthquake foci is known as the Wadati-Benioff zone.) Looking at the distance of the epicentre (the spot on the surface immediately above where the earthquake occurred) and its depth gives a clue as to whether the tremor occurred close to the boundary.
In this case, movement at the the plate boundary itself does appear to be the cause of the earthquake. The epicentre was (roughly) 350km from the plate boundary and its depth was 382km. A typical angle of descent for a trench is of the order of 45 degrees (they vary) implying a direct relationship between the subducting slab and the earthquake.
US Earthquakes: Oklahoma
Too often, the US earthquakes section focuses on anthropogenic earthquakes in Oklahoma; and too often, I remark that research is ongoing. This week, the USGS updated its information on the subject of that research — far too long to summarise here, but certainly worth a look (see resources).
The news release does, however, include a graphic that’s far more revealing and thought-provoking than any number of paragraphs detailing how many earthquakes appear of weekly maps. The numbers are stark. “Between the years 1973–2008, there was an average of 21 earthquakes of magnitude three and larger in the central and eastern United States,” says the USGS. “This rate jumped to an average of 99 M3+ earthquakes per year in 2009–2013, and the rate continues to rise”.
But if you want to get a sense of what that actually means, take a look at the graph.
Last Thoughts: Last Year
Earthquakes don’t occur to a strict pattern. Some weeks have plenty, some not so many. The USGS this week published a summary of 2015’s activity. Surprise surprise: “This worldwide number is on par with prior year averages of about 40 earthquakes per day of magnitude 4.0, or about 14,500 annually… In 2015, there were 19 earthquakes worldwide with a magnitude of 7.0 or higher. Since about 1900, the average has been about 18 earthquakes per year.”
All normal, in other words.