This week’s digest comes with a health warning related to the shutdown of some government services. The data for the digest are drawn from the United States Geological Survey’s real-time earthquake map which currently carries the notification that:
“Due to a lapse in Federal funding, the USGS Earthquake Hazards Program has suspended most of its operations. While the USGS will continue to monitor and report on earthquake activity, the accuracy or timeliness of some earthquake information products, as well as the availability or functionality of some web pages, could be affected by our reduced level of operation.”
Although it’s unlikely that anything major is missing, it’s possible that some seismic events may have slipped through the net.
Even bearing this in mind, the week’s record is not startlingly different, numerically speaking, from previous weeks: There were six tremors of at least magnitude 6 (≥M6.0) worldwide, though the interesting feature is that the two largest, at M6.4 and M6.2, both took place at ocean ridges rather than the subduction zones which are typically the source of the largest earthquakes. Not that subduction zones were inactive: Anything but. A glance at the map shows the usual distribution of intermediate tremors around the many subduction zones of the Pacific.
The Week’s Largest Earthquake: M6.4, South Indian Ocean
With molten rock rising upwards to fracture already-thinning crust, the Earth’s mid-ocean ridges are naturally vulnerable to earthquakes. Typically, however, these are by not major tremors because the heat in the rock means that energy dissipates rapidly. The largest ocean ridge earthquake on record occurred in 1942 and is estimated to have had a magnitude of around M7.9. Though large in its own terms, this released just one two hundred and fiftieth the energy of the largest earthquake on record (M9.5, Chile, 1960).
Earthquake in Europe: M5.3, Romania
The convergence of the African and Eurasian plates, which raised the Alps and Carpathian mountains, is the source of earthquakes within southern Europe – although these are not typically as large as those experienced at subduction zones.
This movement is likely associated with the M5.3 which struck in Romania; taking place in a region where crustal microplates (the West European, East European and Moesian plates) are in close juxtaposition and where both thrust faulting and lateral faulting are in play. A look at the location of the tremor compared to maps of local faulting suggests that the latter is the dominant mechanism, although there are not currently data available to confirm this.
The tremor is worthy of comment as it is one of the largest to have struck in Romania in recent years. The largest on record is an M7.2 in 1940 which cost around 1,000 lives: The USGS historic earthquakes list suggests that it may have been the largest to strike in the country since 2004.
U.S. Earthquakes: Offshore East Coast
An unusual tremor took place off the Atlantic coast, some 550 km from the coast of South Carolina. At M4.5 this is noteworthy not just for its size but also for its location. Earthquakes in stable continental interiors are relatively common as movement occurs along old faults: but this earthquake occurred out in the ocean.
Although there’s no available information, the location appears to suggest that it took place at the edge of the continental shelf- that is, at a transition between continental and oceanic crust. The implication is that the tensions between dense ocean crust and more buoyant continental crust caused slippage, probably along a normal fault. Such tremors aren’t without precedent: in 1929, an earthquake of M7.2 occurred on the continental margins off the Canadian Grand Banks.
There was an earthquake in the Atlantic, far from any active plate margins. We saw an earthquake in Romania as well. This week also saw two tremors of >M6.o along ocean ridges. In themselves, none of these is terribly unusual – but they come to our attention in the absence of any large subduction zone earthquakes. This demonstrates not only that the Earth moves in many different ways and for many different reasons, but also that few places are truly without any seismic hazard.
Okal, E.A. and Stein, S. The 1942 Southwest Indian Ocean Ridge earthquake: largest ever recorded on an oceanic transform. (1987). Geophysical Research Letters. Accessed October 9, 2013.
USGS. Real time earthquake map. (2013). Accessed October 9, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.