As if to demonstrate the variability of seismic activity across the globe, the number of larger earthquakes recorded on the United States Geological Survey’s real-time earthquake map this week was unusually high, following last week’s anomalously low figure.
Although the map, which records all earthquakes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed fewer in total, there were 310 of ≥M2.5 (compared to 196 the week before); 140 of ≥M4.0 (96); and 38 of ≥M5.0 (18). At the very top end of the week’s magnitude scale there were seven tremors of at least M6.0.
Much of this increase is accounted for by aftershocks following the largest earthquake of the week, the M7.0 which struck in the northern Pacific (see below).
This apart, the major concentration of tremors was as expected, with most activity in the western Pacific and a scattering along the convergence zone between India and Asia and on the planet’s mid-ocean ridges.
The Week’s Largest Earthquake: M7.0 Alaska
The largest earthquake of the week was the M7.0 which struck off Alaska’s Aleutian Islands on 30 August. The Aleutian island chain is the product of volcanic activity resulting from melting of subducted crust and is characterised by very high levels of seismic activity as the Pacific plate moves beneath the North American plate.
This week’s earthquake activity resulted from thrust faulting at the interface between the two plates.
The earthquake was followed by an extensive series of aftershocks which, as noted above, account for a significant proportion of the week’s total tremors. At the time of writing the region has experienced over 250 aftershocks of all magnitudes, around half of them ≥M2.5 and 15 ≥M5.0.
M6.5, Izu Islands, West Pacific
The western Pacific is the regular site of frequent and significant seismic activity – so much so that earthquakes of M50 or greater often barely rate a mention. This week, an earthquake of M6.5 occurred within the Philippine plate at a depth of 407km. The epicentre was on the sea floor close to the northern apex of the plate: its depth and tectonic setting suggest that, like the Alaskan earthquake, this tremor was also a product of subduction of the Pacific plate, this time along the Ryuku Trench.
Earthquakes in the US
The largest US earthquake, in Alaska, is covered above; but a glance at the earthquake map of the lower States shows an unusually large event. At M4.3, the tremor which shook parts of eastern Texas on 2 September was a minnow, 1/500th the size of the larger event and releasing a tiny fraction (just 1/11220th) of its energy. Nevertheless the Texas quake is noteworthy because it occurred within a largely stable continental setting and in a state where the largest recorded earthquake – in 1931) had a magnitude of just 5.8.
With Quakes: Context Counts
Large earthquakes occur with an almost monotonous regularity in some parts of the world. It’s rare, for example, that a week passes without at least an M6 in the western Pacific. That’s because some tectonic settings – most notably subduction zones – are prone to much larger tremors than others.
So the Texas tremor, probably resulting from slippage along an existing, old fault in a region far distant from active plate boundaries, is in its way at least as worthy of comment as the largest of the week.
USGS M7.0 – 94km ESE of Adak, Alaska. (2013). Accessed 4 September 2013.
USGS Historic earthquakes list. (2013). Accessed 4 September 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.