Another week, and the Earth is still trembling.
The United States Geological Survey’s real-time earthquake map records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere. The map showed 1,365 earthquakes worldwide in the week of 27 November through 3 December, 2014.
Just two of these earthquakes were M6 or greater, but there were 33 of ≥M4.0, nine of them associated with the M7.1 event in Indonesia on 15 November. There were a further eight tremors in a cluster on the Carlsberg Ridge in the north-western Indian Ocean.
This week all the larger tremors occurred at or close to plate boundaries, with no tremors of ≥M4.5 away from these margins.
Week’s Second Largest Earthquake: The Philippine Sea
The largest tremor, for the third consecutive week, was part of the aftershock sequence from the 15 November Indonesian earthquake. Not far behind was the M6.3 tremor which occurred to the west of the Philippine island of Mindanao.
The whole of the western Pacific is tectonically far more complex that it appears on the (inevitably simplified) maps of major plate boundaries: Yeats describes it as “one of the most complex active subduction zones on Earth.” Varying types and directions of plate movement have generated many microplates and blocks of crust which mean that seismic activity is frequent and often significant.
A look at slightly more detailed fault maps of the area seems to suggest that this week’s M6.3 occurred as a result of subduction along the Cotobato Trench, where a small sliver of crust is being forced downwards and westwards beneath the floor of the Celebes Sea.
The depth of the tremor, however, is inconsistent with this. At almost 620 km depth, the implication is that it in fact originates at a different and deeper subduction zone, probably along the Philippine Trench itself. In other words, it is what it appears to be from the simplified maps.
The Andean Margin
By contrast, the Andean margin is a much simpler subduction zone (though not without its anomalies). Formed by the subduction of the Nazca plate beneath the South American continent, it provides a long, narrow subduction zone off the western coast of the continent.
Unsurprisingly this is a zone of frequent earthquakes and the scale of the subduction zone renders it capable of generating so-called ‘megathrust’ earthquakes, including the largest on record — the M9.5 earthquake of 1960.
This week’s activity was relatively minor but instructive, with an almost classical pattern of small, shallow earthquakes close to the boundary and the the occasional deeper one further away from the trench. The occurrence of a shallower earthquake at a distance, in Ecuador, is probably associated with deformation in the upper crust as a result of the uplift of the Andean mountain chain.
US Earthquakes: Arizona
The collision of the Pacific and North American plates, and the uplift of the Western Cordillera, give rise to regular small to medium-sized earthquakes, and the occasional larger one, along a network of normal and reverse faults which trend broadly north-south.
The largest earthquake in the lower 48 states this week was just such an example. The M4.7 in central Arizona was elegantly summarised to news media by Dr David Brumbaugh of the Arizona Earthquake Centre as “a large enough earthquake to be felt, but not quite large enough to really get too concerned about.”
Nothing truly out of the ordinary, in other words. As a footnote, the largest earthquake recorded in the state was in 1959 and was almost ten times larger, at M5.6.
Jumbles of smaller crustal pieces often don’t reach the great depths of the major subduction zones. This means there’s effectively a layer of thin, shallow pieces superimposed on the pattern of the major plates.
An earthquake on a deep fault or plate interface may appear on the surface to be associated with a shallower one elsewhere — as is the case with this week’s tremor in the Philippine Sea. Who’d be a seismologist?