The United States Geological Survey’s real time earthquake map, which includes earthquakes of all magnitudes in the US and its territories and those of greater than magnitude 4 (≥M4.0) elsewhere, showed just over 1400 in total in the week of 4-10 December 2014.
Again, the map showed all the larger-magnitude earthquakes at or very near the Earth’s major plate boundaries.
Four tremors were ≥M6.0: two of them in the eastern Pacific (an M6.6 and its M6.0 aftershock off Panama); one in Indonesia; and one off Papua New Guinea.
The latter, which was the largest tremor of the week, produced 10 aftershocks which, along with the mainshock, accounted for over a quarter of the total earthquakes ≥M5.0 (41).
The Week’s Largest Earthquake: M6.8, Papua New Guinea
As is so often the case, the largest earthquake of the week (M6.8) occurred among the jumble of microplates which mark the tectonically-congested boundary between the Pacific and Australian plates from Fiji to Papua New Guinea.
In this area the Pacific plate and Solomon microplate are converging, with the result that the latter is subducting beneath the the former to the north-east (along the Solomon Trench) and beneath the South Bismarck Plate (along the New Britain Trench) to the north west.
The earthquake is associated with the former margin and may have been caused either by deformation within the overriding plate or rupture at to near the plate interface itself.
In this case, however, the usual clues of depth (10km) and location relative to the plate boundary (a few km into the over-riding place) might support either interpretation.
M6.6 Earthquake: Panama
The second largest tremor of the week, an M6.6 off the cost of Panama, is also the product of a complex tectonic situation within an overall convergent margin. Although the tectonic situation both north and south is fairly straightforward, this week’s tremor is equally difficult to decipher on the basis of available data.
The epicentre of the quake was around 50km from the southern end of the subduction zone between the Cocos plate and central America; and its depth was 20km.
Just to the south, however, lies the Panama Fracture Zone which marks the boundary between the Nazca and Cocos plates and which is characterised by shallow, moderate earthquakes such as that observed this week.
US Earthquakes: More on Nevada
In Nevada, the earthquake swarm continues, with 80 tremors recorded in the past week and literally thousands since the sequence began in July.
Not all are shown because, as the Nevada Seismological Laboratory notes, “thousands more that are taking place just can’t be located because of the small number of seismic stations in that part of the state.”
The cause of the swarm remains unclear, with authorities considering deep volcanism or tectonic stretching as possible candidates.
Earthquakes: Complexity and Mystery
Apparently straightforward earthquake settings (such as the subduction zone of the Middle Americas) can get very complicated with the existence of microplates and contrasting directions and speed of movement.
Both of this week’s larger earthquakes clearly illustrate this; and the ongoing swarm in Nevada provides further evidence of how little we know, in some settings, about earthquakes and precisely why they occur.
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