The world seemed to wake up this week, seismically speaking.
Although just two earthquakes of at least magnitude 6 (≥M6.0) appear on the United States Geological Survey’s real time earthquake map for the 23-29 January, the number of tremors of ≥M5.0 was higher than in recent weeks, at 32.
Unusually, the relative high number wasn’t accounted for by a single large tremor and its aftershocks, but the earthquakes were scattered across the globe, with two thirds in and around the Pacific Ocean and the remainder along the margins of the Eurasian plate.
Utah and Nevada Earthquakes
The complex motions associated with the uplift of the Rocky Mountains have led to extensive faulting throughout much of western north America. Fault maps show a web of broadly parallel roughly north-south trending fault which are the source of many small shallow earthquakes. This week the cluster of earthquakes on the Utah-Nevada border, with magnitudes of up to 5.1, proved typical of the setting.
The Week’s Major Earthquake: M6.1, Greece
One of two tremors of M6.1 to occur this week, (the other was in Indonesia) the M6.1 which struck near Lixourion in Greece qualifies as the most noteworthy – both because it is relatively unusual for its location, and because it produced a significant cluster of aftershocks – a total of 37 measuring at least M4.0 were recorded on the USGS map. The earthquake caused minor damage and some injuries, according to news reports.
The island of Cephalonia, off which the tremor occurred, lies at the northern end of the subduction zone which curves south of Greece and Anatolia. Here the northward movement of the African continent is forcing the Mediterranean Basin – itself the remnants of a much larger ocean – beneath the Eurasian continent.
The exact initiating mechanism for the earthquake and its aftershocks isn’t clear, but flat maps show that local extensional movement occurs in the region alongside the overall convergence and the shallow nature of the earthquakes, together with their location to the north of the active subduction zone, implies that this may be the dominant motion.
Mid-Ocean Ridge Earthquakes: The Reykjanes Ridge
A further cluster of earthquakes ≥M4.0 also occurred along the European margin, this time in the mid-Atlantic, along the margin between the North American and European plates where eight tremors of appear on the USGS map. Here the two plates are moving apart at a relatively slow rate of around 20mm per year.
Earthquakes occur at constructive margins such as this when the divergent motion forces rocks apart and causes fracturing. As a result such tremors are not uncommon, although the magnitude of events in such a tectonic setting is generally much smaller than those in zones of convergence.
Quakes This Week: Not Just the Pacific
Typically, the Pacific produces most of the earthquakes, in terms of number and magnitude. While the distribution of tremors for this weeks holds true, the clusters in Greece and in the mid-Atlantic – and even in the United States, between Nevada and Utah, are a reminder that other parts of the world are just as vulnerable to earth movements.