June 12-18 proved to be another seismically quiet week – though no less interesting for that.
First, the numbers: the United States Geological Survey’s real time earthquake map, which includes all tremors of at least magnitude 4 (≥M4.0) worldwide and all tremors in the US and its territories, showed nothing ≥M7.0 and just one event of ≥M6.0.
But that one, as we shall see, is a subject of fascination for seismologists.
Elsewhere the distribution of tremors of ≥M5.0 showed the expected concentration around the margins (especially the western margins) of the Pacific, where 15 out of 23 events of this size occurred, while the collision zone between India and the Himalayas was the source area for three others and the mid-Atlantic ridge for two more.
The Week’s Largest Earthquake: M6.4, South Indian Ocean
At M6.4, the largest earthquake of this week may not cause many raised eyebrows; but it draws attention to an interesting situation – a sizeable earthquake associated with an aseismic ridge. These ridges are significant raised linear features on the sea bed but are not active spreading zones. The term aseismic implies an absence of earthquake activity – but the Ninetyeast Ridge, where this week’s tremor occurred, has experienced three tremors of M7 or more in the past century.
An earthquake digest is no place to discuss the complex details; but in general scientists consider that the ridge is the trace of a large hotspot (an upwelling of magma which reached the surface beneath the ocean).
Hotspots remain static but the plates move, leaving a physical trace over millions of years. The Kerguelen Islands lie, which are the present day surface expression of the hotpot, lie at the southern end of the ridge.
The source of this week’s earthquake, however, remains unclear. It’s worth noting that the compressive forces between the Indo-Australian and Eurasian plates have generated stresses responsible for other earth tremors – and which may be the early stages of continental breakup.
M5.7 Quake in Japan
The Japanese archipelago is the product of tectonic movement and the coming together of no fewer than four of the earth’s tectonic plates – the Pacific to the west, Eurasian to the east, Philippine Sea to the south and the Okhotsk microplate to the north.
With their varying rates and directions of movement, these plates create huge crustal stresses and are one of the most seismically active areas on the planet.
This week’s M5.7, off Iwaki, is geographically close to the location of the massive Tohoku-Oki earthquake of 2011. This subduction-generated tremor was one of the largest on record and, along with its associated tsunami, killed thousands.
This week’s tremor was very much smaller, and the location and depth suggest that in fact, although it’s almost certainly associated with the subduction zone, it’s more likely to be the result of movement within the overriding plate than movement at the plate boundary.
US Earthquakes: Alaska
With Alaska the most seismically active part of the US, it isn’t surprising that this week’s largest tremor (and jointly the second largest of the week in the world) occurred there.
Alaska’s earthquakes are mainly at the subduction zone or one of the many crustal faults which run broadly parallel to it. This week’s event, however, was further north and its location (at the western end of the Brooks range) and depth (24km) imply that its cause was more local.
Earth Tremors: Continuing Mysteries
For simplicity, we tend to divide earthquakes into simple subcategories, but the fact that this week’s largest earthquake occurred at a nominally aseismic ridge is an illustration of the complexity of the earth’s physical system – and that we don’t fully understand it yet.