Arguably the biggest talking point of the week, in seismological terms at least, is the occurrence of an earthquake of magnitude 5.2 (M5.2) in the state of Arizona.
In total, the United States Geological Survey’s real time earthquake map, which shows tremors of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere, recorded six tremors of ≥M6.0 in the week of 26 June-2 July, five of them in the west Pacific and the sixth, which was the largest, in the south Atlantic.
With the exception of the Arizona tremor, the pattern of distribution was much as expected, with a concentration around the subduction zones of the Pacific and the eastern Indian Ocean.
A lesser number (four out of 36 registering ≥M6.0) occurred along the planet’s mid-ocean ridges. In all the USGS map shows a total of almost 1600 tremors, 250 of them of at least M2.5.
The Week’s Largest Earthquake: M6.9, South Sandwich Islands
Given the location of the planet’s subduction zones, it’s relatively unusual to find the week’s largest tremor in the Atlantic. This still-opening ocean does, however, have two short subduction zones, one just west of the Caribbean and one west of the South Sandwich Islands. It was this which generated the M6.9, which occurred on 29 June, and its cluster of aftershocks.
The area is remote and so the tectonic setting has been relatively little studied. Broadly speaking, the earthquake occurred where the Scotia microplate (a westward extension of the Nazca plate) is caught between the Antarctic and South American plates, the latter of which is moving westwards and subducting beneath it.
The magnitude, shallow depth and offshore location of the tremor are all contributory factors to a possible tsunami. Despite this, however, no widespread tsunami appears to have occurred (although there may have been some local waves which were not recorded) – probably because the magnitude was not quite large enough.
The Western Pacific: M6.4, Tonga
The Western Pacific, with is complex tectonic setting, slivers of microplates between the Australian and Pacific plates and plate boundaries which vary in nature and in direction of movement, is extremely seismically active and regularly produces earthquakes of at least M6. This week’s M6.7 and its M6.4 and M5.6 aftershocks occurred in between the island groups of Fiji and Samoa.
Tectonically speaking, these were located at the northern end of the Tonga Trench, just south of the point where it curves westward and the subduction zone gives way to a diverse and poorly-defined boundary between the plates, with fracture zones and spreading centres all playing their part.
The location and depth of these tremors (all around 10km) implies that they aren’t directly associated with the subduction zone itself but with fracturing caused by the competing stresses in this particular area of crust.
US Earthquakes: Arizona
One of the largest earthquakes to occur in Arizona struck close to the border with New Mexico in an area more usually considered to be relatively stable. Earthquakes of this size are unusual here and the USGS report on the tremor notes that: “In the 40 years prior to this earthquake, the USGS recorded 11 earthquakes larger than magnitude 3.0 within 150 km of this earthquake, the largest of which was a magnitude 3.9 about 50 km to the northeast.”
In the event there was nothing sinister about the tremor, which turned out to be caused by normal faulting in an area which, again according to the USGS, “may mark a transition between Basin and Range extension and the more tectonically stable Colorado Plateau.”
The Mysteries of Seismology
Although the mechanisms of earthquakes are fairly straightforward, there are always anomalies. Some areas are poorly understood (the South Sandwich Islands); some are highly complex (the diffuse boundary between Fiji and Samoa); while others spring unusually large surprises (Arizona). This illustrates the complexity of the Earth’s dynamics – especially where opposing tectonic forces interact.