The United States Geological Survey’s real time earthquake map shows a slightly unusual pattern of tremors this week.
Although the total number of tremors was unexceptional – there were 1,331 overall, with 219 of at least magnitude 2.5 (≥M2.5), 103 ≥M4 and 26 ≥M5.0 – it’s remarkable that none of the top three occurred in or around the Pacific.
Even more unusually, this week’s largest tremor shook Europe; slightly less surprisingly, the next two, in India and China, are associated with the continental collision between India and Eurasia.
The Pacific Ring of Fire was anything but calm, however; a dozen earthquakes of at least M5.0 (almost half of the total) were around the Pacific, reflecting an expected distribution of seismic activity.
M6.9 Earthquake In Greece
A look at the map shows that the tremor which struck in the northern Aegean Sea on 24 May is associated with the northwards movement of Africa against Eurasia and the closure of the Mediterranean (the remnant of a former ocean). Reuters reported that residents felt the ‘quake, which had a magnitude of M6.9, across Greece, Turkey and Bulgaria, but it caused no deaths or injuries and only minor damage.
Tectonically, the eastern Mediterranean is complicated; the convergence of the two continents not only involves a number of different types and directions of plate motion but sweeps up slivers of crust along the way.
South of Greece a short subduction zone separates Africa and Europe. Behind it, the northern Aegean is dominated by lateral movement, in a probable extension of the North Anatolian Fault.
Information from the USGS, supported by the shallow depth of the tremor (10km) at so great a distance from the subduction zone, indicates that this tremor was indeed a product of movement along the North Anatolian Fault, accommodating the westward movement of the Anatolian block rather than northward movement of Africa.
The Azores Triple Junction
Meanwhile, in the mid-Atlantic, the earth also trembled. An M5.2 quake just to the north of the Azores draws attention to the Azores triple junction where the African, North American and Eurasian plates meet – or, more accurately, diverge. All three boundaries here are constructive margins, where the plates are moving apart, and new ocean crust is being produced at mid-ocean ridges.
Although constructive margins aren’t associated with earthquakes to the extent that other boundaries are, tremors are inevitable because the injection of magma along the ridge and the associated spreading involves the fracturing and relocation of the existing crust.
As a result, ocean ridges are subject to regular earthquakes (though these are often relatively minor). This week’s USGS map shows five such events of ≥M5.0, four of them in the Atlantic and one in the Indian Ocean.
US Earthquakes: Revisiting the San Andreas Fault
The US was almost suspiciously quiet this week – not even the usually-active Alaskan subduction zone could muster a tremor any larger than M4.3. But smaller earthquakes tell their own tale.
The lateral faults of the San Andreas zone, where the Pacific and North American plates slide past one another, generate regular small earthquakes along its length. Although along this line the largest recorded were just M3.0, the state of California experiences literally hundreds of small tremors in a typical week.
Earthquakes: Convergent, Divergent, Conservative
This week’s quake activity indicates that plate margins, of all kinds, are seismologically active almost all the time, though sometimes at a low level – and also that the association of large earthquakes with subduction zones, though generally true, doesn’t always hold. Not only were this week’s three largest tremors not in the Pacific but not one of them was subduction-related except (in the case of Greece) in the most general sense.