The variation in the pattern of earthquake activity continued in the week of 30 October-5 November 2014.
One earthquake of at least magnitude 7 (≥M7.0), one of ≥M6.0 and no fewer than 25 of ≥M5.0 appeared on the United States Geological Survey’s real time earthquake map, which includes all earthquakes in the US and its territories and those of ≥M4.0 elsewhere.
This real-time earthquake map is continuously updated, and showed a total of 1,424 tremors at the time of writing.
As usual, the distribution of the larger earthquakes largely followed the boundaries of the Earth’s tectonic plates with one or two outliers.
Smaller earthquakes were more widely distributed and included an M4.6 in central Siberia and even an M3.2 in New Brunswick.
The Week’s Biggest Earthquake: M7.1, Fiji
Although the USGS describes the largest earthquake of the week (which registered as M7.1) as being in Fiji it would be more accurate to say that the epicentre (the point on the earth’s surface immediately overlong the actual breaking of the crust) lies roughly midway between the main islands of Fiji and Tonga.
The location of the epicentre is close to an aseismic ridge, the Lau Ridge, and at first glance there seems to be an association between the two. The depth of the earthquake indicates that this is in fact misleading. At almost 450km deep, this earthquake is almost certainly the result of faulting at the interface between the Pacific and Australian tectonic plates.
The latter is descending below the former and earthquake depth increases with distance from the plate margin (indicated in this case by the Tonga Trench).
If an earthquake is associated with subduction, depth will increase with distance: shallower earthquakes at a great distance from the trench are more likely to be caused by crustal deformation.
As a footnote, it’s worth observing that M7 is generally regarded as the point at which a submarine earthquake might be capable of generating a tsunami. But magnitude is just one factor in tsunami genesis and the depth of the Fiji-Tonga tremor meant that much of the energy which it generated would have been dissipated before it reached the surface.
East Africa: Rift Valley Earthquakes
This week Africa saw earthquakes of M5.3 (Zambia), M5.1 (Tanzania) and M4.5 (Uganda). Although the African continent has no active plate margins, the East African Rift Valley is a zone of extension where continental breakup is expected to occur at some point within the geological future.
The two larger earthquakes occurred in the highlands to the east and west of the rift and only the smaller M4.5 tremor occurred within the rift itself. It’s likely, however, that all three resulted from movement along normal faults as the incipient continental margin develops.
US Earthquakes: Nevada
Just for a change, this week’s US earthquake news isn’t focussed on California, Alaska or Oklahoma. Instead the largest earthquake in the mainland US was in the very north-west of Nevada, over 307 km from Carson City.
Nevada is part of the tectonically-active Basin and Range tectonic province and, as such, is seismically active at a low to medium level. This week’s earthquake is interesting because it occurred in an area of the state where seismic hazard is considered relatively low.
Extensional Tectonics and Earthquakes
The week’s largest earthquake ran true to type; the larger earthquakes tend to be the product of subduction. But the earthquakes in Africa and Nevada both illustrate that within continents, extensional tectonics can also cause significant seismic activity — and that in Africa, at least, may be associated with possible development of new plate margins.