After the above-average numbers of earthquakes recorded last week (mostly accounted for by the aftershocks of Alaska’s M7.0 tremor) the planet returned to a state of relative quiescence.
The United States Geological Survey’s real time earthquake map recorded a total of 1543 tremors, of all magnitudes in the US and at least magnitude 4 (≥M4.0) elsewhere. Of these, 23 were ≥M5.0 and four ≥M6.0 – three of them on the Pacific margins and the fourth on the Mid-Atlantic Ridge.
The Week’s Largest Earthquake: M6.6, Guatemala
September 7 saw a tremor of M6.6 strike at a depth of 67km with an epicentre on Guatemala’s coastal strip, close to the coast. Although the tremor was not as strong as last week’s largest, the M7.0 in Alaska, its proximity to population meant that it was locally damaging: although early news reports suggested that the damage was limited, some sources reported at least one death and several injuries.
Convergence of the Cocos plate with the more stable north American and Caribbean plates along the line of the Middle America subduction zone caused this ‘quake.
Rates of movement vary along the plate boundary but off Guatemala convergence takes place at around 76mm per year and results in subduction of the Cocos plate, with earthquakes occurring in association with subduction. Information from the USGS suggests that the cause of this earthquake is likely to have been faulting within the Cocos plate rather than at the interface between the two.
Afghanistan: A Continental Collision Zone
Four minor earthquakes in north-eastern Afghanistan a few hundred kilometers from the capital, Kabul, illustrate the continuing impact of the collision between the Indian subcontinent and Eurasia. The main contact is marked by the arc of the Himalayas but both eastern and western boundaries of this arc are characterised by complex, broadly north-south trending, fault zones hundreds of kilometres in length. This week’s earthquakes appear to have been associated with movements along these faults.
These earthquakes are small but they illustrate the continuing seismic activity in the area. Historically, the region has experienced major earthquakes, including one estimated at M7.3 in Kabul as long ago as 1505. Yeats reports research suggesting that a major earthquake of a similar magnitude in this area is overdue, but speculates that slow creep along the fault system may have reduced the likelihood of such an event.
Earthquakes in the US
The resonance of the M7.0 in Alaska on 30 August continues to be felt, with over 200 aftershocks in the past seven days. The magnitude of these shocks diminishes over time, the largest being M5.6.
Elsewhere in the US there was nothing larger than M4.5 with the usual patterns of small earthquakes along the San Andreas fault zone and along the margins of tectonic blocks which form the Basin and Range provinces of the Rockies. Yellowstone also experienced some seismic activity, in this case associated with the magmatic hot spot (a rising plume of magma below the continent) which supplies energy for the geothermal activity of Yellowstone National Park.
Seismic Activity This Week: All Kinds of Everything
One week there are many earthquakes: the next there are few. The variation in magnitude from one week to the next, and the variation in tectonic origins displayed by this week’s earthquakes, demonstrate not just the unpredictability of the Earth’s movement but also the range of different types of tectonic setting in which those tremors can occur.
United States Geological Survey. Real time earthquake map. Accessed 11 September 2013.
United States Geological Survey. M6.6 – 5km ESE of Ciudad Tecun Uman, Guatemala. Accessed 11 September 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.