The week of 29 July-4 August 2015 was an interesting one, seismologically speaking.
In a typical week we would expect to see most, if not all, of the larger earthquakes in a given week at or near the boundaries of the earth’s tectonic plates, and to a certain extent that was true, with the largest tremor of the week striking in Alaska.
The 26 earthquakes larger than magnitude 5 (≥M5.0) recorded on the United States Geological Survey’s real time earthquakes map (which shows all tremors in the US and its territories and those of at least M4.0 elsewhere) threw up just one notable anomaly. This anomaly was in the form of an earthquake and two aftershocks off eastern Australia.
Looking at the next layer down, however, showed up some interesting, if smaller, events, with tremors in the stable continental areas of northern Europe (both north and south of Sweden) and across several states of America.
The Week’s Biggest Earthquake: M6.6, Alaska
Alaska, at the top of the world, and at the mercy of the subduction of the Pacific plate beneath the North American plate along the several thousand kilometres of the Aleutian Trench, shakes regularly and often.
Sometimes Alaska shakes exceptionally long and hard, laying claim to the second largest earthquake on record (the M9.2, so-called Great Alaskan Earthquake, of 1964). With this in mind it’s not surprising to find the largest tremor of the week in this remote region, even if at M6.6 it hardly qualifies as a giant.
The earthquake occurred in the eastern part of the subduction zone, where the oceanic crust subducts beneath continental crust.
The situation is complicated in this area by the existence of two smaller slivers of crust, or microplates, and by the shift to subduction from the lateral motion between the two major plates in the east.
A look at a fault map suggests that the motion may have been caused by movement along a major crustal fault, the Bruin Bay Fault. The depth of the tremor (118km) suggests, however, that it may have been the result of movement at or near the plate interface — in other words, directly related to subduction.
M5.7 Quake, Eastern Australia
If there’s a surprise among this week’s earthquakes, it comes in the shape of three tremors. All three earthquakes were around 10km deep, ranging from M5.2-M5.7, off the Eastern coast of Australia around 200km north east of Brisbane.
Australia is composed of cold, old solid rock and, although it does experience earthquakes, they are relatively few in number and small in magnitude — although Robert Yeats, who covers the entire continent in just four pages (including maps) in his 500-page book on fault zones, records several in excess of M6.0.
Rocks in stable continental interiors which currently exist far from plate boundaries have almost always been exposed to significant stresses in the past and can still, on occasion, generate earthquakes. With no detailed information on this week’s three tremors in Australia, we can look to the rest of the continent as a guide — and conclude that they, too, are almost certainly the product of movement along old faults.
US Earthquakes…Well, Everywhere Really
All right, so ‘everywhere’ is an exaggeration. But the map of earthquakes in the mainland US in excess of M2.5 shows a scattering of tremors in more states that we might normally expect.
Of course there are the usual suspects (California, Oklahoma, Kansas) but this week there was a respectable showing from elsewhere, with tremors in Oregon, Washington, New Mexico, Texas, Missouri and Montana. Many of these, like the Australian events, will have occurred on old faults.
Earthquakes: Rockin’ All Over the World
Like old rockers, old fault zones keep on moving. Sweden, Australia and much of the US demonstrated that this week, with their sprinkling of small-to medium earthquakes reminding us that, while stable continental interiors may not (with the occasional exception) have the potential for major earthquakes that’s present at plate margins, they can still move.
Editor’s Note: The original version of this article incorrectly identified the Eastern coast of Australia as the Western coast of Australia – many thanks to our readers for pointing out the typo.