The week of 13-19 October produced pretty much what we’d expect in terms of earthquake activity, as represented on the United States Geological Survey’s real time earthquake map. The map, which includes, broadly speaking, earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included three larger tremors, all well in excess of M6.0, along with 25 of at least M5.0 in its total of almost 1300.
The larger tremors, as usual, were mostly associated with the margins of the planet’s tectonic plates, with the main concentration in the west of the Pacific Ocean (which is where a number of minor plates and crustal slivers make things particularly complicated). But there are smaller earthquakes well away from plate margins, too — some of them very small — which are of interest.
The Week’s Biggest Earthquake: M6.9, Papua New Guinea
I mentioned above that things are, to say the least, structurally complicated in the western Pacific. Here, the broad boundary where the Pacific plate marches with the Australian plate (in the south) and the Philippine Sea plate (further north) is compressional, but it’s blurred both by changes in direction of movement from segment to segment.
The section of the margin north of Australia, from Indonesia to the Tonga Trench, is particularly complicated. Here, the crust is split into a series of microplates which sit between the Pacific and Australian plates. There’s a lot going on, and it’s unsurprising, as these smaller plates jostle for position between the jaws of the two larger ones, that this area regularly experiences large earthquakes — and this week’s M6.9 was no exception.
Despite the complex setting, the earthquake itself appears to have fairly straightforward origins. It occurred at the southern margin of the South Bismarck microplate, where the Solomon microplate is subducting beneath it. The scientific data indicate compressional movement and its location, in the overriding plate, suggests that it was probably caused by deformation at relatively shallow depths (35km) as as result.
Interestingly, a second large earthquake (M6.4) also occurred in the area, around 300km to the north. This smaller tremor, around 400km from the subduction zone and at a depth of around 450km, occurred two days before the week’s largest earthquake and bears the hallmarks of a more classic subduction earthquake (caused by movement at or near the plate interface).
In this particular case, I venture to hazard a guess that the second, larger earthquake may have resulted from stresses and strains caused by movement of the first. They’re certainly close enough in time and space.
M6.6 Tremor, Indonesia
Further to the west, the northern margin of the Australian plate, where it marches with the Sunda plate, is much more straightforward and, in fact, offers us a classic example of a subduction zone. This zone, thousands of km long, regularly produces some of the world’s larger (and unfortunately deadly) earthquakes.
This week’s tremor, though large, was effectively neutralised by its depth. Like the second of the Papua New Guinea earthquakes discussed above, it occurred at depth (this time over 600km) and at roughly equivalent distance from the margin, indicating that it also probably results from movement at or near the interface between the two plates.
Deep earthquakes, however, tend to lose much of the energy they release before it reaches the surface, with the result that they tend to be less damaging. It’s the large, shallow earthquakes that are the problem. And when they occur offshore they can cause tsunamis resulting in extensive damage and severe loss of life.
US Earthquakes: The Mississippi Valley
My eye was caught on the earthquake map this week by a cluster of tiny tremors, probably barely felt. They spanned three states — Arkansas, Tennessee and Missouri — and the largest of them was just M2.0. What made these, so small they’ll barely have been felt, so interesting? It’s what lies beneath.
In 1821, a series of major earthquakes ripped along a deeply buried, ancient rift zone which underlies part of the central United States. This area, known as the New Madrid Seismic Zone, has been trembling ever since. These latest earthquakes are a reminder that it’s still there — though it isn’t yet clear what potential this rift has for larger, more damaging tremors in the future.
Last Thoughts: Deadly Potential
To the best of my knowledge, none of the earthquakes — even those larger than M6.0 — featured this week caused any significant damage or any injuries. But it’s worth bearing in mind that all of them occurred in areas where previous earthquakes have had catastrophic impacts.
Most noteworthy, the Boxing Day earthquake of 2004 and its resulting tsunami killed an estimated 225,000 people. A local tsunami generated by an earthquake in Papua New Guinea in 1998 killed an estimated 2,000 people. And while the New Madrid earthquakes in 1821-2 caused much damage but few casualties because of the sparse population, an earthquakes series — or single event — of comparable magnitude in the same area would have very significant impact today.
Most of the time earthquakes, even large ones, cause few casualties. We should be grateful for that.