So let’s begin with some numbers. In the week of 10-16 April, 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) included a total of 1,523 seismic events.
The total is (more or less) what we might expect but, like last week, there were no large events and so (again like last week) we have the opportunity to look at tectonic settings and earthquakes which might otherwise slip beneath the radar.
(No weekly round-up can really afford to ignore the larger events and these mainly occur in the same settings — typically subduction zones/in around the Pacific.)
This week, the largest tremors came in at just M5.5 when, in a typical week, we’d reasonably expect to see at least one event of at least M6 (which, for the record, is over three times the size on a seismograph and over five times larger in terms of energy released). So let’s have a look at some small quakes, and see what they tell us.
The Week’s Biggest Earthquake: M5.5, Cyprus
Now here’s something really worth noting. The largest earthquake this week was in the Mediterranean and, at M5.5, was the largest to occur in the area in several months, since the M5.8 which occurred just off Greece in August 2014.
The Mediterranean is the last hurrah of a dying ocean, being squeezed out as Africa closes in on Eurasia; but it isn’t going quietly. As the crust of the former Tethys Ocean is broken into slivers of crust, contorted, fractured, subducted or uplifted (depending on the speed and direction of movement) earthquakes are the inevitable result.
In general they aren’t large, although we can expect an M6 or two every year (there have been nine since the beginning of 2010) and the eastern Mediterranean is the location of most of them.
This week’s champ came in just off the south west coast of Cyprus, a tectonic setting where subduction dominates. The shallow depth of the ‘quake and its location in the overriding plate, suggest that crustal deformation is the probable cause.
The Puerto Rico Trench
Subduction comes into play in the second of our featured earthquakes — or rather, earthquake series. One of just two (possibly three) short subduction zones on the edge of the (expanding) Atlantic Ocean, the Puerto Rico Trench regularly experiences small earthquakes as the North American plate collides with the Caribbean plate.
This is by no means uncommon. In this area the tectonics are, as in the Mediterranean, dominated by subduction but complicated by thrusting and rifting. Most of those on this week’s map lie to the north of Puerto Rico itself and are of shallow to intermediate depth.
It’s probable that these quakes are also the result of deformation, though one or two of the deeper ones may be directly associated with movement at or near the plate interface.
This week’s earthquakes may be small but, like most subduction zones, the Puerto Rico Trench is capable of producing much larger tremors — including three of M7.0 or more in the last century.
US Earthquakes: California
California produced the largest US earthquake this week, with an M4.7 off Ferndale.
In this area a so-called triple junction between the North American, Pacific and Juan de Fuca plates is the location of regular small-to-medium earthquakes, and even some large ones.
These occur at shallow depths along the Mendocino Fracture Zone (between the Juan de Fuca and Pacific plates) but because of the lateral movement even the larger tremors rarely cause any damage — and this week’s was hardly felt at all by those onshore.
Oceans Opening, Oceans Closing
They say that when one door closes another one opens. So it is with oceans. As the continents collide, the oceans between them disappear and the collision causes earthquakes (as in the Mediterranean). As they open, the leading edges collide with continents on either edge and cause earthquakes (as with the Atlantic and the Puerto Rico Trench). The cycle (it’s called the Wilson Cycle) goes on; and one day, the Atlantic edges will close and the ocean will become a remnant… just like the Mediterranean.