After several weeks of quiescence the Earth is stirring again. In the week of 30 January-5 February, the United States Geological Survey’s real time earthquake map recorded four earthquakes of at least magnitude 6 (≥M6.0) and 21 of ≥M5.0.
Overall there were 1,638 recorded earthquakes – an underestimate of total activity, since the map shows only those of ≥M4.0 for areas outside the United States and its territories.
As usual, most of the larger tremors occurred in the Pacific, many in the western region; but aftershocks from last week’s earthquake in Greece’s Ionian sea also contributed.
The Week’s Biggest Earthquake: M6.5, off New Zealand
The Kermadec Trench, which extends northwards from New Zealand to Tonga, is possibly the simplest section of the otherwise complex boundary between the Pacific and Australian plates. This steep and narrow subduction zone is characterised by numerous earthquakes whose foci deepen from east to west; that which struck on 2 February was close to the subduction zone and its depth, at just 40km, suggests that it was either at or close to the plate interface.
The remoteness of the area (the epicentre was south of the island of L’Esperance, the southernmost in the Kermadec island chain) means that even large tremors pass largely unremarked. The only realistic sources of potential damage are tsunamis; and although this week’s tremor was shallow and underwater, both factors in generating a tsunami, its magnitude was small and no tsunami occurred.
M6.1 Banda Sea, Indonesia
One of three tremors of M6.1 this week, the earthquake that struck in southern Indonesia was also located along the margin of the Australian plate, but further west where it is adjacent to the Philippine Sea plate. This is, however, an over-simplification as much of the western Pacific consists of smaller micro plates caught between the major plates. In this area the setting is further complicated by the close proximity of the westward moving Pacific plate.
The earthquake occurred where the Australian continent collides with the island arcs of the Banda Sea and there’s no information at present about the dominate mechanism which caused the tremor, although existing fault maps suggest that subduction is most likely to be the cause. At the time of writing there were no reports of damage or injuries.
Earthquake in the US: New Madrid
Possibly the largest sequence of earthquakes to have struck the United States in historic times is the sequence of three tremors, their magnitudes estimated at as much as M8.4-M8.7, which hit the town New Madrid of southern Missouri in late 1811 and early 1812. The occurrence of a very small tremor this week – just M2.6 – is in itself unremarkable but is a reminder that even in apparently stable tectonic zones major earthquakes can occur.
But in reality the New Madrid area is not such a stable area. It lies above a deeply-buried ancient rift system which formed some hundreds of millions of years ago and was possibly reactivated in the Cretaceous and Tertiary periods (as recently as 1.6 million years ago). The triple New Madrid earthquake of 1811-12 can be attributed to activity along this rift and, in contrast to other minor seismic events in the continental US (which tend to have alternative causes) local minor earth movement is also associated with it.
As recently as January 2014 the USGS issued a press release on the subject following an investigation of the long-held theory that seismic activity in the area is the result of an ongoing series of aftershocks. The USGS scientists argued that the rift is still very much active and that further activity can’t be ruled out. “In other words,” they conclude, “the New Madrid Seismic Zone is not dead.”
Tectonic Settings – Simple, Complex or Downright Mysterious?
This week shows the rich spread of plate tectonics and its implications. Classic settings such as the Kermadec Trench contrast with the messy jumble of crust in the western Pacific – and sleeping giants, tectonic margins which formed many millions of years ago and which no longer have a surface expression, can wake and cause damage even where no obvious danger exists.