As rule of thumb, geologists expect a larg(ish) earthquake — let’s say magnitude 6 or larger (≥M6.0) — on a pretty regular basis.
This week things were, in that sense at least, quiet. The United States Geological Survey’s real time earthquake map shows the largest tremors of the week 28 August-3 September 2014 as M5.8, in Japan and Vanuatu.
That said, there were plenty of medium-sized tremors with 29 measuring ≥M5.0 and 21 ≥M2.5. The latter is an underestimate, given that the map shows only tremors of ≥M4.0 outside the US and its territories.
And if the magnitude of the week’s earthquakes is unusual, the distribution is not: the typical locations were the areas on the margin of the planet’s tectonic plates, in particular the western Pacific.
The Week’s Largest Earthquake: M5.8 Vanuatu
The western Pacific, with its complex array of microplates and slivers of crust, is an area where plates converge overall; but within this context there are areas where crust is created as well as destroyed, where the directions of convergence change along the length of the same boundary and, as a consequence, where earthquake activity is a regular occurrence.
This week the joint largest earthquake was at a section of the margin between the Pacific and Australian plates where the latter is descending beneath the former at a subduction zone. Vanuatu is an island chain resulting from this process.
The epicentre of the earthquake was in the overriding plate, some way to the east of the margin itself. This, along with its depth (118km), implies that the event was caused by movement along the plate interface rather than by deformation in the Pacific plate, making it a classic, if small, subduction zone earthquake.
Iceland: Divergent Boundaries
Much has been written, not least here on Decoded Science, about the current eruption near to Iceland’s Bárðarbunga volcano. At present no explosive eruption has occurred — but the rifting of the earth’s surface has produced a spectacular fissure event.
The eruption has been accompanied by significant and continuing earthquake activity. Iceland lies on the mid-Atlantic ridge and above a mantle hotspot, both of which contribute to volcanism. The USGS map of earthquakes in Iceland this week shows 12 tremors exceeding M4.5, although there have been many more smaller events.
The map of these larger tremors is instructive because it illustrates their relationship to the divergent boundary between the Eurasian and North American plates. The plates move apart, typically very slowly — but recent weeks have seen an increased rate of rifting, and increased earthquake activity, as magma moves upwards.
US Earthquakes; Aftershocks in California
Last week’s Napa Valley earthquake was a significant event in economic and human terms. Such events typically generate aftershock sequences; in this case the ensuing aftershocks, though numerous (35 in the past seven days), were relatively small (the largest of them was just M3.2).
Although not located along a surface fault, the location and orientation of these quakes forms a line broadly parallel to the fault zones to the west and east (Rogers Creek and West Napa respectively), indicating the likely existence of associated deformation.
Earthquakes and Ocean Ridges
Quiet weeks can sometimes be the most interesting. Iceland, where an ocean ridge finds surface expression, is fascinating enough; but the planet’s other ocean ridges are also moving as tectonic motion forces the plates apart.
This week there were tremors of at least M4.5 along the ridges in the central and southern Pacific oceans, the Indian Ocean and the north Atlantic. These submarine earthquakes, too small and shallow to cause tsunamis and in remote locations, pass largely un-noticed. But they are as much part of the dynamic earth processes as those in California or the subduction zones of the western Pacific.