It’s gone suspiciously quiet this week, relatively speaking at any rate, as far as earthquakes go.
In the period from 24-30 June 2015, the United States Geological Survey’s real time earthquake map, which includes all earthquakes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, recorded just two earthquakes of M6.0, in Papua New Guinea and in the Pacific north of New Zealand.
It’s true that there was respectable number of earthquakes of at least M5.0 —31 of them, in fact — but even of these, the significant majority (25) were smaller than M5.5 so it was hardly an earth-shattering week by any standards. And the numbers of other earthquakes remained pretty much as normal too, with 100 ≥M4.0 out of a total of 1,413.
With the distribution of the larger tremors pretty much as expected, around the margins of the Earth’s tectonic plates (with one exception, which we’ll look at later) you might be forgiven for thinking that this week’s digest might also be a bit thin. But earthquakes and tectonic activity don’t have to be large to be interesting.
The Week’s Largest Earthquake: M6.0, North of New Zealand
Though the same size as another tremor, in Papua New Guinea, the M6.0 to the north of New Zealand justifies its billing as the main featured earthquake this week because it – and its aftershocks – accounted for over a fifth of those tremors in excess of M5.0.
The earthquake occurred along what is in fact a fairly straightforward subduction zone, where the Pacific plate moves westward against the Australian plate and descends beneath it. Movement along the plate interface, or faulting as a result of deformation of either the descending or overriding plate, causes earthquakes.
In fact a quick look at the USGS’s earthquake archive shows that this week’s earthquake is nether the the largest, nor even the second largest, to have occurred in the immediate area over the past 12 months. In fact there have been four others in roughly the same area, two of them very close to the epicentre of the most recent and the largest of which measured M6.9.
The depths and locations of all of these tremors (all were shallow and their epicentres occurred on both sides of the plate boundary) suggest that they, and indeed the most recent, are the result not of movement at the plate boundary but of deformation resulting from the great stresses this plate convergence generates.
European Earthquakes: Greece
Earth-shattering things are happening in Greece right now and are nothing to do with seismology. But Greece’s position in Europe is marginal in a tectonic, as well as an economic, sense.
The collision of northwards-moving Africa with the Eurasian continent takes place along a fractured, complex zone extending the length of the Mediterranean.
One area of seismic activity is the short (in global terms) subduction zone which curves from Greece to Turkey, south of the island of Crete.
This week saw two shallow earthquakes, an M4.6 at the western end of the trench and an M4.7 some 200km or so north of the subduction zone itself at the Turkish end.
Like the New Zealand tremor, these occurred at shallow depths and are also probably the result of deformation as the continents move together. Such tremors are by no means unusual in the area and probably scarcely raised an eyebrow locally — even if the Greeks didn’t have other things to worry about right now.
US Earthquakes: Hawaii Five-Two
The one larger tremor of the week which wasn’t at a plate boundary was the M5.2 which shook Hawaii on 28 June. But because it was anomalous in terms of all those plate-boundary earthquakes, that doesn’t mean it was either unusual or unexpected. Hawaii overlies a volcanic hotspot and is the result of rising magma. As it moves upwards to create the island chain’s volcanoes, it inevitably fractures the overlying rock. The result? Earthquakes.
The location of the tremor, south of the currently-erupting Kilauea, adds weight to the conclusion that it is the result of rising magma. It’s worth noting, however, that even this large (for Hawaii) earthquake isn’t the biggest the island can produce — the largest on record, in 1868, had a magnitude of M7.9
Last Thoughts: The Music of the Mountains
Decoded’s Earth Science correspondent is on holiday at the moment, and as such likes to seize the opportunity to get deep inside the Earth more regularly than others might think strictly necessary. It brings its rewards.
I’ve already mentioned the ongoing collision between Africa and Eurasia, one which continues to crumple Europe across its southern third — the Pyrenees, Alps, Carpathians and other mountain ranges all result from this convergence.
If you have access to a piece of technical equipment called a geophone, you can hear the sound the mountains make. In a tunnel in the solid rock of the Austrian Alps, you can listen to a recording (much amplified) of the sound made by the mountain within which you stand – as it groans its way upwards under the pressures of continental collision.
Mountains talking. How cool is that?