The week of 18-24 September was a quiet one, seismologically.
Unusually, no earthquakes of magnitude 6 or greater (≥M6.0) were recorded this week on the United States Geological Survey’s real time earthquake map, which includes all tremors in the US and its territories and those of ≥M4.5 elsewhere.
The largest had a magnitude of just M5.7.
As usual, tremors were concentrated on the western margin of the Pacific Ocean.
This week, the region was the location for over half of the 81 tremors of ≥M4.5, although there was no significant event, or even any obvious cluster of events, in this area with tremors scattered from New Zealand to the Sea of Okhotsk.
The Week’s Largest Earthquake: M5.7, South Atlantic
The Atlantic is an unusual focus for the week’s largest tremor. Large earthquakes are usually associated with subduction zones, and the Atlantic has very little in the way of destructive margins; but an earthquake of M5.7 occurred at one of its two short sections of subducted crust, the South Sandwich Islands, on 22 September. (The second section is in the Caribbean.)
The tectonic setting is one in which a small crustal plate, the Scotia microplate, is caught between two larger ones (the Antarctic and South American plates).
The area is remote and the exact details of the plate boundaries are unclear. In the north and south they are probably transverse; in the west the margin is diffuse and unidentified (it isn’t shown on maps); and in the west there is a short subduction zone.
This area, evident at the surface in the volcanic arc of the South Sandwich Islands (volcanic arcs are a feature of subduction zones), is where the South American plate descends beneath the Scotia microplate.
The epicentre of the earthquake, to the west of the plate boundary, taken along with its depth (110km) implies that the source for the tremor was movement at or near the interface between the two plates.
North Atlantic: M5.6, Reykjanes Ridge
The media have (rightly) been entranced by recent events in Iceland, where there’s a clear association between the spectacular eruption of the ever-growing Holuhraun lava field and a cluster of earthquakes. Very much further south (around 2,000 km) on the mid-Atlantic Ridge, the week’s second largest earthquake registered at M5.6.
Although, like the South Atlantic earthquake outlined above, the earthquake occurred in a remote area and at great depth, some parallels can be drawn with the ongoing earthquake activity in Iceland. Both are part of the mid-Atlantic ridge system, where the North American and Eurasian plates re moving apart, though relatively slowly.
Earthquakes in such settings are associated with this movement, either by fracturing or because of the movement of upwelling magma at the ridges.
As in Iceland, these earthquakes may be associated with ongoing sea floor eruptions, although in the case of the Reykjanes Ridge there’s no available evidence to suggest whether or not such an event is occurring.
US Earthquakes: Keeping Up With Oklahoma
The earthquakes in Oklahoma (associated with wastewater injection as part of drilling for oil) just keep on coming.
This week the USGS map recorded 28 tremors of at least M2.5, the largest of them and M4.0 to the south east of Medford.
In the past 30 days the total number of tremors of ≥M2.5 recorded by the USGS in the state was 111 – and there’s no sign of any let-up.
Earthquakes: Things We Don’t Know
With two of the largest earthquakes this week taking place below deep oceans in remote parts of the globe, the limits of our knowledge are exposed.
While some areas (such as the San Andreas fault zone or parts of the East Pacific Rise) are the subject of extensive study, there are many other zones of earthquake activity which have never been considered in detail – and we can only speculate about the detailed causes and settings of this week’s two largest earthquakes.