Everything has quietened down a little in terms of earthquakes.
In the week of 30 October – 5 November 2015, the United States Geological Survey’s real time earthquake map, which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed over 1400 tremors worldwide.
Breaking these down a little further shows just one ≥M6.0 and 30 of ≥M5.0.
As usual, these earthquakes occurred either at or very close to the planet’s major plate boundaries, with most of them occurring around the tectonically-congested areas of the western Pacific and the Philippine Sea.
This part of the world regularly produces the largest earthquake in a given week — and has done so again.
The Week’s Largest Earthquake: M6.3, East Timor
The northern margin of the Australian plate is tectonically complex, running from west of the Fijian Islands, across the western Pacific, through the Banda Sea and along the subduction zone of the Java Trench. Little of it is textbook and most is characterised by a highly complex arrangement of plates, microplates and even buried plates. The nature and direction changes several times along the thousands of kilometres of its length.
Bearing this in mind, it’s hardly surprising that the region regularly produces significant earthquakes and, once again, the largest tremor of the week (on 4 November) occurred here — an M6.3 north of the island of East Timor.
To understand its cause, we have to look more closely at the tectonics than the (necessarily) simplified USGS map.
South of Timor, the northern margin of Australia is termed ‘passive’ i.e. it changes from oceanic to continental crust with no obvious plate margin. Northwards, however, the collision of the oceanic crust with the rest of Eurasia is characterised be a series of thrust belts.
In the absence of detailed information, it’s hard to be clear as to the origin of the tremor. Available evidence on location and depth (14km) suggests that the most likely cause was faulting caused by compressional forces, with the earthquake occurring along one of those thrust belts, the Wetar Thrust.
The Deep, Deep South: Earthquakes in the Scotia Sea
The bottom of the world has plate tectonics too. It’s just that it’s so remote and inhospitable (not to mention barely inhabited) that we know very little about it, and that the earthquakes barely trouble anyone.
This week there was a cluster of three earthquakes along the margin between the Antarctic plate and the Scotia microplate, over 1200 km from the southern tip of South America.
In his otherwise exhaustive book on tectonic zones, seismologist Robert Yeats passes it without comment; but there is information on the Smithsonian Institution’s interactive map, This Dynamic Planet. The map suggests that the boundary changes in nature from divergent to convergent and back again a number of times along its length.
At the location of this week’s shallow (10km) earthquakes which ranged between M4.8 and M5.1, the nature of motion is convergent, though not a distinct subduction zone. The tremors occurred some way to the south of the actual margin which, together with their shallow depth, implies the were the result of crustal deformation.
US Earthquakes: Arizona
We’re used to reading about the human-induced earthquake swarm in Oklahoma and Kansas, which this week produced a tremor of M4.1. A couple of states further across, a tremor of the same size, along with one slightly smaller, struck in central Arizona.
These, we can say for certain, are NOT related to fracking or wastewater injection (the Arizona Geological Survey states emphatically that: “In fact no wells have ever been drilled in Arizona for shale-gas”). They are more likely to be caused by low-level movement along existing faults associated with the uplift of the Rocky Mountains.
Last Thoughts: A Bit of Everything
This week’s earthquakes are a confusing bunch. Plate margins aren’t like the simplified versions in textbooks. They change direction. They switch from convergent to divergent and back again. and, to cap it all, supposedly stable continental interiors produce same-size earthquakes, some of them human-induced and some clearly not.
Some might think earthquakes dull — but every one is different, and every one is interesting, if you only look closely enough.