Fiji, Crete and Oklahoma Yet Again: Earthquakes 17-23 April 2015


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Earthquakes in the week 17-23 April 2015

This image depicts the earthquakes that took place in the week 17-23 April 2015. Image by USGS.

When it comes to earthquakes, normal service, it seems, has resumed, at least for the time being.

After a couple of weeks of relative quiet, the United States Geological Survey’s real time earthquake map for the week 16-22 April recorded six earthquakes of at least magnitude 6 (≥M6.0) with five of them pretty much where we’d expect, in the western Pacific. The sixth, in Greece, is discussed below.

The map, which includes earthquakes of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere, included 25 tremors of ≥M5.0 — again largely among the tectonic confusion of the western Pacific, but also including tremors along subduction zones off Sumatra and in the South Atlantic. Overall, the numbers of smaller tremors were, as they have been all along, broadly what might be expected with 224 of ≥M2.5 and 1,585 overall.

The Week’s Biggest Earthquake: M6.4, Fiji/Wallis and Fortuna Islands

The diffuse boundary between the Pacific and Australian plates was the location of this week's largest earthquake.

The diffuse boundary between the Pacific and Australian plates was the location of this week’s largest earthquake. Image by USGS.

One of the most fascinating features on a tectonic map of the world is the gap, thousands of miles long, between the northern end of the Tonga Trench (which curves round in a right angle and subducts from east to west) and the southern end of the New Hebrides Trench (which curves in the opposite direction and subducts from west to east).

Tectonic maps show no marked boundary here, though such a boundary must exist; it isn’t marked because it’s diffuse and poorly understood, with seismologist Robert Yeats devoting just a couple of sentences to it in his 500-page book on the world’s fault zones.

Clearly, however, there must be complex things going on, even if the actual boundary is poorly defined, if only because the motions at the two trenches are opposed (and there are plenty of other things going on too). Between them, these forces produced the week’s largest earthquake, an M6.5 between Fiji and the Wallis and Fortuna Islands.

This tremor, at a depth of just 10km, is almost certainly the result of deformation resulting from the conflicting forces at play in this area of the crust, and is likely to be the result of extensional faulting along spreading ridges within the Lau Basin.

M6.0 Earthquake, Greece

The closure of the Mediterranean is the source of many earthquakes.

The closure of the Mediterranean is the source of many earthquakes. Image by USGS.

Last week’s digest included some famous last words on Mediterranean. “In general aren’t large, although we can expect an M6 or two every year,” we wrote, and almost before the editor had hit the ‘publish’ button, last week’s biggest earthquake, an M5.5 off Cyprus, was outmuscled by an M6.0 further to the west, just off the island of Crete.

The detail of the tectonic setting remains the same, with this larger earthquake and its nine aftershocks (two ≥M5.0 and the others ≥M4.0) also occurring at shallow depths along the Pliny Trench and also probably the result of crustal deformation. They provide further evidence that the death of the former Tethys Ocean involves much contortion of the crust — and much seismic activity.

US Earthquakes: Oklahoma

The earthquake swarm in Oklahoma continues

The earthquake swarm in Oklahoma continues. Image by USGS.

With reluctance, this digest returns to Oklahoma — not because there’s anything earth-shattering to reveal (research is ongoing but nothing new has come to our attention this week) but simply because we can’t ignore it. The human-induced earthquake swarm in the central area of the state just won’t go away — this week there were 31 recorded tremors of ≥M2.5, the largest of M4.1. Once upon a time such tremors would have been worth an article but now they’re ten a penny. But don’t worry — we’ll keep you updated.

Mother Earth is a Contortionist

There are many areas on the planet where conflicting forces apply and this week’s featured earthquakes in Greece and the western Pacific occurred in just two of them. New crust is created, the direction of movement changes, slivers of crust move in different directions.

With all the stresses and strains it’s no wonder the Earth cracks. I would, too.

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