The week of 31 October-6 November 2013 saw a flurry of earthquake activity, much of it in the western Pacific. Although there were no earthquakes of magnitude 7 or greater (≥M7.0) there were four of ≥M6.0 and 29 ≥M5.0.
This week’s largest tremor occurred off the coast of Chile but most of the larger earthquakes occurred along the western Pacific margins, from Tonga to Japan, with noteworthy activity, albeit at a relatively low level, also occurring in the Sunda Arc south of Sumatra.
The Week’s Largest Earthquake: M6.6, Chile
Lying as it does just above one of the Earth’s major subduction zones, it’s no surprise that the west coast of South America regularly experiences significant earthquakes. This week the coast of Chile saw the largest tremor – a ‘quake of M6.6 which had its epicentre exactly on the coast, 43km from the town of Coquimbo. According to news media, the tremor shook buildings but there were no damage or injuries.
In this area, the Pacific plate is converging with the South American plate at a rate of around 8cm per year and is being forced beneath it. Such subduction zones generate major earthquakes as tension builds up across large areas of an interface between the two plates, thousands of kilometres long and hundreds deep. The potential for significant rupturing of the crust means that the Chilean subduction zone has in the past generated major earthquakes of ≥M8.0. In this case, however, the depth of the earthquake (29km) and its location some 100 km from the plate interface suggest, though they do not confirm, that the earthquake was caused by faulting within the overriding plate rather than at the interface between the two.
Earthquakes in Fiji and Tonga
Another earthquake-prone area is the western central Pacific Ocean. The complex configuration between the Pacific and Australian plates is characterised by changes in the nature of the margin and in the direction of subduction (where it occurs).
Changes in the direction of the boundary mean that there is a u-shaped bend between the two, with the Pacific plate subducting beneath the Australian plate north of New Zealand and the reverse occurring beneath the New Hebrides. Between the two the margin is diffuse and characterised by spreading ridges and fracture zones.
With such complexity it’s no surprise that earthquakes are frequent and sometimes significant. This week, the focus of earthquake activity was along the Tonga Trench, where a series of shallow tremors occurred in the over-riding Pacific plate. These are distinct from the much deeper tremors which occurred to the west, almost certainly directly associated with subduction.
Earthquakes in the USA: The Juan de Fuca plate
The largest earthquake to occur in the lower 48 states this week occurred offshore. Although a major subduction zone, the Cascadia subduction zone, runs along the west coast off Washington and Oregon, most of the earthquake activity here is associated with faulting along fracture zones rather than with the subduction zone itself, as spreading from the western edge of the remnant Juan de Fuca plate is accommodated by lateral movement – generating frequent shallow earthquakes. This week’s tremor, at M4.5, was typical for the setting.
Non-Subduction Earthquakes in Subduction Zones
The Chilean and Tongan tremors we examine this week are directly associated with subduction zones but are not, strictly speaking, subduction earthquakes. In each case (subject to confirmation from more detailed study) the initiating mechanism appears to be faulting in the over-riding plate associated with subduction rather than rupture and the actual interface between the two plates.
Fox News. Strong magnitude-6.6 earthquake rocks north Chile. (2013). Accessed November 6, 2013.
USGS. Real time earthquake map. (2013). Accessed November 6, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.