The week of 6-12 November 2014 threw up a few interesting things, seismologically speaking. In terms of numbers and locations, though, the larger earthquakes, at least, fell pretty much into an expected pattern.
There were two earthquakes of at least magnitude 6 (≥M6.0) recorded on the United States Geological Survey’s real time earthquake map which shows all tremors in the US and its territories and those of ≥M4.0 elsewhere.
Both of these were in the highly seismically active areas of the western Pacific.
Elsewhere most larger tremors occurred at or near plate boundaries with the occasional anomaly, such as earthquakes of between M4 and M5 in Africa and the for north east of Canada.
There were 28 tremors ≥M5.0 and 113 ≥M4.0 out of a total of 1,484 shown on the map.
The Week’s Biggest Earthquake: M6.6, Papua New Guinea
This week, the largest recorded tremor was one of M6.6 which occurred in the Solomon Sea, between Papua New Guinea and New Britain.
Though the tectonic setting is shown on most maps as the margin between the Australian and Pacific plate this is (necessarily) simplified.
In fact this margin, with complex changes in both the nature and type of plate boundary, is characterised by many smaller plates, termed microplates, jostling for position in the archipelagos of the western Pacific.
The most recent earthquake occurred at the margin between two of these plates; the Solomon plate, to the south, is being forced beneath the South Bismarck plate, to the north. The available evidence on location and depth suggests that the tremor is associated with this convergence, although whether the cause is movement at the plate boundary or deformation within the over-riding plate is unclear.
Indian Ocean Earthquakes: The Sunda Trench
In contrast to the Western Pacific, the seismic belt of the eastern Indian Ocean is relatively simple. In the past this margin has been the focus for some of the planet’s larger earthquakes — the last decade has seen three in excess of M8.0, including the M9.1 Boxing Day earthquake of 2004.
This week a line of medium-sized tremors (M4.5-M5.4) provides a neat illustration of activity along the subduction zone between the Indo-Australian plate and Philippine Sea plate, where the former subducts beneath the latter.
Although (as in the western Pacific) much of this area is broken into microplates, the nine tremors which occurred along the length of this margin between northern Sumatra and south-central Java occurred at roughly similar depth (most at 50-60km) and distance from the margin.
Although there is a significant zone of thrust faulting parallel to the trench, this is largely on land and the tremors this week appear likely to be directly linked to movement at or near the plate interface.
US Earthquakes: The Nevada Swarm
And so to Nevada. Last week, the state produced three earthquakes in excess of M4.0 and this week the activity continued; in the past seven days an area of around 10km by 20km in north west Nevada has recorded 117 tremors, the largest of them registering M4.7.
At the time of writing the rate of seismic activity appears to be slowing.
Although earthquake swarms such as this are not uncommon, they are often associated with known faults or volcanic activity.
The Nevada swarm is unusual in that it is in an area identified as being of low seismic hazard; and there’s little available information to suggest an obvious cause (the latest comment from the Nevada Seismological Laboratory came on 5 November).
The Uncertain Earth
Earthquakes are predictable — up to a point. We know broadly where they are likely to occur and can calculate areas of high seismic hazard. Beyond that, we can search for clues but little else. Not all large earthquakes are preceded by identifiable signals and not all identifiable signal are followed by earthquakes. The Nevada swarm is just one illustration of the puzzles that seismologists have to face.
Bearing the uncertainties of our dynamic planet in mind, it’s reassuring that six Italian scientists jailed for failing to predict a deadly earthquake in the town of L’Aquila in 2009 have been cleared of charges of manslaughter.
Perhaps one day scientists may be able to predict earthquakes with a degree of accuracy — but that day is too far off for us to hold them to account if they fail to do so.