If there’s such a thing as ‘normal’ in earthquake parlance, then the week of 16-22 February, 2017 was probably close to it.
There’s been a noticeable quietening on earthquake activity over the past few weeks, and at some point things will escalate; but this week is as average as it gets. (Sorry, statisticians.)
The United States Geological Survey’s real time map included two earthquakes in excess of magnitude 6 (M6.0) in its total of almost 1250 recorded tremors.
Note that the map is definitely not comprehensive and a great many tremors go unrecorded, but in general, it does record most of the larger tremors (of at least M4.5) worldwide and tremors of all magnitudes (not the same as all tremors) in the US and its territories.
Bearing these caveats in mind, the earthquake totals for the week show 22 of M5.0 or larger and 104 of at least M4.0. And not only are the numbers normal — the distribution is, too, with all the larger ‘quakes (bar one, an M4.7 off Hawaii which we’ll talk about below) associated with the margins of the earth’s tectonic plates.
The Week’s Biggest Earthquake: M6.5, Bolivia
This week’s biggest earthquake is an interesting one. It’s the better part of 700km from the nearest tectonic boundary — the margin between the Nazca and South American plates — but the fact that it occurred at an unusual depth (almost 600km) indicates that there’s an association.
The subduction of the Nazca plate beneath South America takes place along the Peru-Chile Trench, and this margin is responsible for the uplift of the Andean mountain range. A look at a topographic map indicates that the earthquake occurred at the eastern edge of the Andes.
Fault maps of the area indicate significant thrust faults in this area — but the earthquake occurred very, very deep down within the Earth, far below the actual point of subduction (which, in this area, is relatively shallow at a few hundred km). This suggests that the most likely trigger for the M6.5 is deformation in the underlying plate.
It’s worth noting that, while large, shallow earthquakes can be extremely damaging, deeper earthquakes are less likely to be so. That’s because the deeper the earthquake, the more energy is lost as the seismic waves travel towards the surface. The USGS summary of the event indicates that over 6.5 million people will have felt something after this tremor — but none of them will have felt very much.
M5.6 Earthquake, Indonesia
Almost an order of magnitude smaller than the week’s largest, an earthquake of M5.6 struck at the northern tip of the island of Sumatra. Like that in the Andes, this tremor occurred some way from the subduction zone with which it’s associated (in this case the Java Trench, along which the Australian plate depends beneath Eurasia).
This tremor, however, is shallow — its focus (the point at which the rupture actually occurred) was at 20km below the surface, where the plate interface itself is much deeper.
A look at maps which show more than just the plate boundaries gives an idea of where the faults associated with this convergence actually lie — and the most likely culprit for triggering this earthquake is not movement along the plate boundary itself, but along the major strike slip fault which runs along the spine of Sumatra, accommodating some of the convergent movement.
According to the USGS, fewer people felt this tremor — but even though it was a tenth the size, the intensity of the shaking was greater. Again, that’s a function of depth, with the energy released having less far to travel to reach the surface.
US Earthquakes: Hawaii
Hawaii is in a constant state of trembling as lava rises from a deep magma plume to feed its volcanoes. It’s obvious: if rock is rising, it has to find space to reach the surface and so forces other rock aside (and, conversely, when an eruption is over, an empty magma chamber may deflate, with the same effect).
Most of the earthquakes are small, but this week, just north of the Big Island, it did muster a sightly larger event of M4.7.
Looking at earthquakes on Hawaii this week, there does seem to be a pattern — shallow earthquakes around the active volcanoes of Mauna Loa and Kilauea, and more widely-spread tremors at slightly greater depth (30-40km) which are associated with the movement of magma in the deep chamber below the islands.
The Third Dimension: What Depth Tells Us
We look at maps of earthquakes in two dimensions — but seeing where they occur on the planet’s surface (the epicentre) is only half the story. Faults that occur at the surface overlie deeper activity, especially in collision zones where layers upon layers of rock have been stacked on top of one another.
Earthquakes often have a shallow focus, and shallower earthquakes, as we’ve seen above, tend to have a significant impact on humans. But most earthquakes — certainly the major ones — are intimately the result of deep Earth processes: they don’t just occur at the surface.
The depth of an earthquake is a crucial clue to its origins.