It’s been quiet, but the week of 3-9 September 2015 saw things revving up just a little bit, seismically speaking. Not too much, but a bit.
The United States Geological Survey’s real time earthquake map, which shows events of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included 31 tremors in excess of M5.0, although just two of those were larger than M6.0.
As usual, the distribution of the larger earthquakes (≥M4.5) largely follows the margin’s of the Earth’s tectonic plates, with the expected concentration along the subduction zones in the Pacific and the Indian Ocean.
This week there are one or two intra-continental earthquakes worthy of note, the largest of which, in Siberia, is considered below.
The Week’s Biggest Earthquake: M6.4, North of New Zealand
All the evidence suggests that this week’s largest tremor, an M6.4 some 450 kilometres north of New Zealand, is that thing of elegance, a classic subduction earthquake. The tremor occurred a few kilometres west of the Kermadec Trench, which, along with Tonga Trench, forms the boundary along which the Pacific plate subducts beneath the Australian plate in the south western Pacific Ocean.
Seismologist Robert Yeats notes that the angle of subduction in the Kermadec Trench is relatively steep. Taking this into account, the location of the epicentre (around 20km west of the trench) and its depth (34km) together suggest that the cause of the earthquake was in fact movement either at or near the plate interface rather than deformation within either the subducting or the overriding plate.
The largest tremor was followed by a series of smaller aftershocks, mostly to the west and south, all of which have shallow focuses, suggesting that these in their turn are the result of deformation.
At M6.4 the mainshock is large but not excessively so: the USGS earthquake archive indicates that the Kermadec Trench has experienced 19 earthquakes of at least this magnitude in the last half century, of which five were ≥M5.0.
M4.9 Earthquake in Russia
Away from the subduction zones, there is movement afoot. In the past few weeks we’ve considered earthquakes in Africa resulting from the extensional tectonics of the East African Rift Valley, and this week we’ve seen an earthquake of M4.9 occurring in a less well-known example, the Baikal Rift.
The rift is largely occupied by Lake Baikal and forms a broad ’S’ shape from the Hamar-Daban mountains in the south to the Barguzun Mountains in the north. (If these names mean nothing, have a look at the map, where the topography of the rift is clear.)
Like other rift valleys, it is the product of extensional tectonics and characterised by normal faulting on either side. This week’s earthquake occurred at the northern part of the rift system.
Though it’s remote, the Baikal Rift is, as Yeats observes, “the most seismically active of the world’s rift zones” and is capable of generating significant earthquakes. In the past fifty years there have been 51 earthquakes in the area in excess of M5.0. The largest known in the region occurred in 1957 and had a magnitude of M7.8.
US Earthquakes: Alaska
The Aleutian Islands are the surface expression of the major subduction zone which spans the northern Pacific Ocean, where the Pacific plate descends beneath the north American plate.
Characterised by a volcanic island chain and regular earthquakes, this week the zone saw an earthquake of M5.6, along with a series of smaller shocks, close to the Andreanof Islands.
In contrast to the Kermadec Trench earthquake, however, the depth and location suggest that this tremor was probably not the result of movement directly between the plates but of deformation within the overriding plate.
‘Quakes Could Be a Whole Lot Bigger
The Kermadec subduction zone can generate earthquakes much larger than M7.0. So, more surprisingly, can the Baikal Rift. Alaska can boast the second largest earthquake on record, at M9.2 back in 1964. It may have looked like a busy week in terms of earthquakes — but be assured, the planet can do a whole lot more.