Not a lot happened in seismic terms in the week of 22-28 January 2015 – but the United States Geological Survey’s real time earthquake map nevertheless offers points worthy of note.
The USGS map shows tremors of all magnitudes in the US and its territories – and tremors of at least magnitude 4 (≥M4.0) elsewhere. This week, the map recorded a total of 1,465 tremors with just two of them ≥M6.0 and 17 ≥M5.0.
While most of the larger earthquakes of the week were, as usual, around the margins of the planet’s major tectonic plates (the six largest, and 14 of those ≥M5.0, were all in the western Pacific) but there were several quakes in central Asia, around the northern margins of the Indo-Eurasian convergence zone.
M6.8 Vanuatu: The Week’s Biggest Earthquake
The complex and difficult plate boundary between the Pacific and Australian tectonic plates in the western Pacific was yet again the source of the week’s largest earthquake — an M6.8 which occurred at a depth of almost 220km around 80km north of Port Vila.
At this particular point the Pacific-Australian margin is relatively tectonically straightforward, with the Australian plate subducting westwards beneath the Pacific plate along the length of the New Hebrides Trench at a rate of up to 12.4 centimetres annually.
In this setting the depth of the earthquake and the location of its epicentre (on the overriding plate) suggests that the cause was fracturing at or near the interface between the two plates.
Despite the offshore location of the tremor, the was no tsunami. This was because of a combination of the magnitude (which was on the small side for tsunamigenesis, or making a tsunami) and the depth (energy from deeper earthquakes dissipates further before it reaches the surface).
The Andean Margin
In recent months there has been relatively little in the way of major earth tremors along the Andean margin in the eastern Pacific, despite the existence of a subduction zone that stretches along almost the entire Pacific coast of South America and is capable of generating so-called ‘megathrust’ earthquakes of at least M8.0 — and is the source of the largest earthquake of the twentieth century, the M9.5 which struck near Valdivia in Chile in 1960.
This week’s earthquakes saw nothing remotely approaching that magnitude but they do illustrate that the margin is very much active (two, at M4.7 and M4.6, were just a few hundred km north of the epicentre of the 1960 ‘quake).
The collision of ocean and continent has led to the uplift of the Andean mountain chain, and the location and depth of the western South America’s earthquakes implies that faulting is occurring not just along the subduction zone itself but also within the mountains as a result of uplift.
US Earthquakes: Cascadia
One of the more interesting things about this week (and most other weeks) is the blank section of the offshore subduction zone which runs from northern California to Vancouver Island. Although this, the Cascadia subduction zone, has been quiet for many hundreds of years it’s interesting to note that January 26th was the 115th anniversary of the largest known earthquake in the history of the lower 48 states.
Seismologists have dated the earthquake from written records of a tsunami in Japan and (broadly) identified its location through tsunami deposits on the coasts of Washington and Oregon, though its exact epicentre is unknown. The Cascadia subduction zone has been quiet ever since but there’s evidence that the fault may rupture spectacularly every few hundred years. Cascadia, rather than the San Andreas, may be the source of the next ‘Big One’ in the US.
Earth Science: Expect the Unexpected
Some places shake little and often. Some shake little and often with the capability of occasional major earth tremors. And some lie quiet over hundreds of years, only to spring a surprise when it’s least expected. With different types of margins, different stresses and different timescales, a quiet margin on the map isn’t necessarily a safe one.