Some weeks there’s not a lot of earthquake activity — some weeks there is. The week of 28 March-3 April 2015 was one of the latter, with the United States Geological Survey’s real time earthquake map (which shows tremors of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) showing a total of just over 1,550 earthquakes, four of them larger than M6.0 and one of M7.5.
On average we can expect an earthquake of at least M7 somewhere in the world once a month or so (in the whole of 2014 there were 14) so this wasn’t an unexpected event; nor, given its location in Papua New Guinea, in the complicated tectonic setting of the western Pacific, was it in an unusual location.
Elsewhere, the larger tremors (at least M5.0) showed the expected distribution along the margins of the Pacific, both east and west (although the Pacific coast of North America remained quiet) and Indonesia, with other events in the Mediterranean, China and along the Southeast Indian Ocean ridge.
The Week’s Biggest Earthquake: M7.5, Papua New Guinea
M7.5 is certainly large enough to be not merely noteworthy but also destructive. As a general rule of thumb, seismologists consider a submarine tremor of M7 large enough to generate a tsunami and this tremor did just that, though media reports suggest that the wave was only small (around half a metre) and caused no damage.
Papua New Guinea is no stranger to large earthquakes and two of the fourteen 2014 earthquakes mentioned above were in this area of the Pacific.
The map shows the cause: Two microplates, the South Bismarck and Solomon microplates, are caught between the converging Pacific and Australian plates. There are different motions at play but the key on here is that the Solomon plate, in the south, is subducting northwards beneath the South Bismarck and Pacific plates.
During the process of subduction, one plate is forced beneath another and the evidence of location and depth (along with the element of vertical movement which would have triggered the tsunami) suggests that this week’s earthquakes was indeed the result of the downward movement of the Solomon Sea plate and was caused by movement along, or close to, the interface between it and the South Bismarck plate.
M6.5 and M6.4 Earthquakes; Tonga
As if to emphasise the high levels of seismic activity in the western Pacific, the week’s second and third largest tremors were also in the region, though in this case thousands of kilometres further along the boundary, close to the Tongan archipelago.
In this area the boundary between the Australian and Pacific plates is much simpler — a length of thousands of kilometres from New Zealand to Tonga along which the Pacific plate subducts beneath the Australian plate.
This week’s tremors were at the northern end of this zone, where it bends round to the west. Both were at relatively shallow depths (15.5km and 11.5km) suggesting that they were the result of deformation within the overriding Pacific plate rather than at or near the boundary between the two.
US Earthquakes: The New Madrid Seismic Zone
The deeply buried ancient rift which underlies the margin between Tennessee, Missouri, Arkansas and Kentucky sprang into life in the 1820s with a series of three major earthquakes which devastated the town of New Madrid.
Since then, there has been intermittent activity, possibly as part of a centuries-long series of aftershocks. With this in mind the M3.6 tremor which struck near the town of Steele, is nothing unusual.
Earthquakes: Variation is a Normal Thing
It’s always tempting to try and sensationalise; but in all of the week’s seismic activity was more or less to be expected. The fact is that earthquakes can occur in lots of different tectonic settings and normal activity in one area can be the abnormal in another. An M7 in the western Pacific raises no eyebrows but if that were to occur in Missouri (and it has done in the past) we really would be taking about it.