After the major earthquake which struck Pakistan on 24 September, the rumblings of the Earth continued in the week of 26 September-2 October.
Six earthquakes of at least magnitude 6 (≥M6.0) occurred, one of them an M6.8 aftershock to the Pakistan earthquake and the remaining three around the margins of the Pacific.
Overall the United States Geological Survey’s real time earthquake map (undergoing a shakeup of its own due to the government shutdown) recorded a total of 1,438 earthquakes (all magnitudes in the US and its territories and ≥M4.0 elsewhere) during the week
The Week’s Largest Earthquake: M7.0, Peru
The convergence of the dense basaltic Nazca plate with the more buoyant continental crust of the largely static South American plate has created one of the planet’s classic earthquake zones, with the former being forced beneath the latter, resulting in a band of both earthquakes and volcanoes. Such zones involve complex crustal fractures, with thrust faulting dominating at the interface between the two plates and normal, reverse and lateral faulting all at play in the over-riding plate.
The M7.0 which occurred on 25 September was a fairly straightforward example of a subduction earthquake: United States Geological Survey data relating to the earthquake indicate that it resulted from thrust faulting ‘on or near the interface’ between the plates. Although strong shaking was experienced in the immediate vicinity and the tremor caused some damage to buildings, there were no reported deaths or injuries.
M6.7, Sea of Okhotsk
There was further subduction-related seismic activity on the other side of the Pacific Ocean, where and M6.7 earthquake struck in the shallow Sea of Okhotsk, east of the Kamchatka peninsula. Tectonically, this region is also a subduction zone (though slightly more complex than that off South America). In this case the Pacific plate is subducting beneath a southward extension of the North American plate (the Okhotsk microplate) along the Kuril Arc. The depth and location of the tremor suggest that the rupture took place at or close to the plate interface, although data aren’t yet available to confirm this.
Though large in magnitude, the depth of the tremor reduced the chance of damage (much energy from deep earthquakes dissipates before reaching the surface) and although it occurred offshore, no tsunami was generated. Large-magnitude earthquakes are by no means uncommon in the region: in May 2013 an M8.3, thought to be the largest deep earthquake recorded, occurred in the same area.
U.S. Earthquakes: California
While the Pacific rocked under three major quakes, the United States enjoyed a relatively quiet week, seismically speaking – at least in terms of major tremors. The USGS map, as always, shows a high level of minor tremors each week in seismically active areas. This week in California a swarm of minor earthquakes shows up in the Coastal Ranges to the north of San Francisco.
Although these tremors didn’t occur directly along any major faults, they are associated with the continuing ‘docking’ of a number of slivers of crust which have come together over millions of years (as a result of plate movements) to form the state of California.
Three out of the week’s four largest earthquakes – in Peru, the Sea of Okhotsk and the southern Pacific off New Zealand – are the result of subduction (all, incidentally, involving the Pacific plate). Typically, major earthquakes are associated with subduction – but as Pakistan’s M7.8 of last week, and its M6.8 aftershock of this week, demonstrate subduction zones are by no means the only source for large and damaging earthquakes.
USGS. Real-time earthquake map. (2013). Accessed October 2, 2013.
USGS. M7.0 – 50km S of Acari, Peru. (2013). Accessed October 2, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.