After several weeks of turbulence, the Earth seems to be taking a breather. Levels of seismic activity in the week 25 April – 1 May were lower than they have been for some time as the aftershocks following a number of large tremors decreased in size and frequency.
In total, the United States Geological Survey’s real time earthquake map showed 1,756 tremors of all sizes in the US and its territories and at least magnitude 4 (≥M4.0) elsewhere; but the number of larger tremors was dramatically lower than it has been.
There were just two tremors of ≥M6.0, both in the western Pacific, where the majority of those ≥M5.0 also occurred. The map also shows a scattering of tremors in continental interiors, along with several at mid-ocean ridges.
The Week’s Largest Earthquake: M6.6, New Caledonia
Although a mere infant by comparison with some of the tremors which have shaken the western Pacific in recent weeks (there have been seven larger ones in the past 30 days) the week’s largest earthquake, at M6.6, is significant. This week’s largest quake occurred where the Australian plate sinks beneath the Pacific plate and was the biggest to occur in the eastern section of the plate margin; the larger tremors have been further to the west.
The epicentre (the point on the surface immediately above the break in the rock) was west of New Caledonia and the focus (the place where the faulting actually occurred) at a depth of around 105km. Together these suggest that the earthquake occurred at or close to the interface between the two plates. Although such offshore subduction earthquakes can be associated with tsunamis, the magnitude and depth of the tremor both offset the risk and no tsunami was generated.
M4.7, Australia Quake
The most remarkable seismic event to occur this week is the M4.7 which struck near Yunta in South Australia. Australia, as Robert Yeats notes, is “far from any plate boundaries” but he goes on to remark on several medium-sized shallow earthquakes.
Generally speaking, however, seismic hazard in the Australian continent is extremely low, especially when compared to the continental margin to the north – a fact clearly illustrated by seismicity maps.
The Flinders Range, where the earthquake occurred, is one of the more seismically active in Australia. The recent earthquake, however, is only minor in the great scheme of things with Geoscience Australia noting that it could have been felt up to 125km away but was likely to have caused damage only within a radius of around 10km.
Like most of the rest of the planet, the US was calm this week. The whole of the US and Alaska mustered just four tremors of ≥M4.5, three of them in Alaska; and again the most noteworthy feature is the ongoing swarm of minor earthquakes which continues to trouble central Oklahoma and southern Kansas – and which is still the subject of monitoring by the USGS.
Quiet? Yes, But Interesting
Big earthquakes are usually (though not always) news; but small ones can be newsworthy too, especially where they are unusual. The Oklahoma earthquakes, a regular feature of this digest, are of interest because they may be man-made, while the Australian earthquake catches the attention because it is unexpected. Size isn’t everything!