This week no-one seems to care about poor old Earth; most people with an interest in planetary (and extra-planetary) science are concentrating their attention on NASA’s New Horizon spacecraft as it closes on on the dwarf planet, Pluto.
But just because we aren’t looking at the Earth beneath our feet doesn’t mean nothing’s happening — it’s business as usual in the week of 8-14 July, even if that business is a little on the slow side.
In total the United States Geological Survey’s real time earthquake map, which includes tremors of all sizes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, recorded 1,677 tremors: 109 of them were ≥M4.0; 25 ≥M5.0; and just one larger than M6.0.
These numbers are by no means unusual and the distribution was also free of any surprises, with all but one of those ≥M4.5 occurring at or near the main tectonic boundaries.
The exception, an M4.9 in central Russia, occurred in an area (the Baikal Rift) where extensional tectonics mean that medium-sized tremors are not unusual.
The Week’s Largest Earthquake: M6.7, Solomon Islands
There’s nothing unusual about the week’s largest earthquake, either. The congested margin between the Pacific and Australian plates in the west Pacific, marked by jostling slivers of crust called microplates and complicated by differing directions and speeds of plate movement, is a regular occupant of the largest earthquake slot.
This week we’re looking at an M6.7 just to the south-west of the Solomon Islands, where the Australian plate is subducting beneath the Pacific plate along the South Solomon Trench.
But that’s a very generalised explanation; a closer look at tectonic maps shows that the earthquake occurred close to an area of extensional tectonics, in the Woodlark Basin.
With limited detail available and so many different forces at play, it’s impossible to pinpoint the cause of the earthquake; but its epicentre (in the over-riding plate) and depth (10km) imply that it is more likely to be associated with the subduction process rather than with extension, although it isn’t clear whether it was the direct result of movement at or near the plate interface or deformation of the crustal rocks in the overlying plate.
Earthquakes in the Scotia Arc
Most large earthquakes are concentrated in the Pacific and eastern Indian Oceans. There’s a reason for that — it’s because that’s where the major subduction zones occur and those are the areas associated most strongly with the larger tremors. The Atlantic, by contrast, has only two short subduction zones, and one of those was notably active this week.
Shaking in the Scotia Arc (where the South American plate subducts westwards beneath the Scotia microplate, creating the volcanic arc of the South Sandwich Islands) this week could perhaps be best descried as ‘little and often’. The largest tremor was just M5.3 but there were six of at least M4.8 along the arc and at some distance from it.
The Scotia Arc is remote and little is known in detail of its tectonics. But the geographical spread of the earthquakes and their depths don’t suggest a single event and aftershocks, but rather a more general movement. This isn’t really that unusual: in the past decade there have been almost 400 earthquakes of at least M5.0 in the area, but if they pass without comment it’s because the region is so remote that no-one really pays it much attention.
US Earthquakes: M3.8, California
Like the Scotia Arc, California shakes a lot. This week we’re looking at an M3.8, midway between (though some way to the west of) Sacramento and San Francisco.
As you might expect, this tremor is associated with the San Andreas Fault Zone (SAFZ), though some way from the main fault itself. The earthquake occurred close to the Collyami fault zone, which is broadly aligned (north-west to south east) with the SAFZ and is part of the south western boundary of an extensional basin occupied by Clear Lake.
Although there was just one significant earthquake in the area this week, the Clear Lake basin has experienced 174 earthquakes of at least M3.0 in the past decade, the vast majority of them along the south western margin.
Longer-Term Earth Science Perspectives
In a weekly digest we look tend to look at earthquakes individually. In reality, frequent small earthquakes (such as those covered this week in both California and the South Atlantic) are often part of a much longer term pattern of shaking, even when there is no obvious series of foreshocks, mainshock and aftershocks.