Some weeks are quiet and some are not so quiet. Last week, in terms of earthquakes at least, was definitely one of the quiet ones.
Normally we’d expect at least one earthquake of at least magnitude 6 (≥M6.0) somewhere in the world, usually in the Pacific. This week, the biggest the planet could muster crept in at M5.9.
The United States Geological Survey’s real time earthquake map records (broadly speaking) tremors of all magnitudes in the US and its territories and those of at least M4.0 elsewhere.
Although the total was, if anything, higher than usual at over 1700, there were just 18 in excess of M5.0 and 94 ≥M4.0.
There was nothing unusual about the distribution. All of the larger tremors (those of ≥M4.5) occurred at or near the margins of the planet’s tectonic plates.
The Week’s Largest Earthquake: M5.9, South of Australia
We are in remote territory here, as the USGS map for this event suggests. Map? It looks more like a submission to an art exam by a student who’s trying too hard to make a point.
When I say this earthquake occurred south of Australia, I really mean in the middle of nowhere: the nearest land (Tasmania) is over 1100 km distant. What we’re dealing with is a mid-ocean ridge earthquake, caused by the divergence of two of the earth’s tectonic plates (in this case the Antarctic and Australian plates).
The plates move apart and magma moves up to fill the gap and create new sea floor, or the magma wells up and forces the plate apart: we can argue endlessly about which is cause and which is the effect. Either way the result is the same: movement of the crust causes earthquakes.
Such earthquakes are anything but uncommon. This week, nine out of the 70 ≥M4.5 occurred at these margins. It’s just that such earthquakes are not usually significant in size (though there are rare exceptions) and, being so far away from land, are rarely felt by anything other than seismometers.
Earthquakes along the Kermadec Trench: A Wadati-Benioff Zone
Science has an elegance to it. In its textbook form, an ocean trench, along which one plate subducts beneath another, can produce a classic pattern in which earthquakes at or near the plate boundary have their epicentres further into the over-riding plate as the depth of the interface increases. This is called the Wadati-Benioff Zone.
We rarely get a chance to see this pattern but this week the earthquakes on the map of the Kermadec trench-Tonga Trench, a subduction zone which stretches from north of New Zealand up towards Fiji and Tonga, are a classic example. The two earthquakes almost on the trench itself occurred at a depth of 10km. The northernmost, a little further away, was at 56km. The two just west of that were 95 and 98km; and those much further west were at 443 and 553 km.
Of course there’s some variation — there’s deformation around the interface at depth, where the crust is hot and relatively, the interface is more diffuse. Nevertheless, this series of subduction earthquakes offers us clues about what’s going in deep within the Earth.
US Earthquakes: Wyoming
So it was quiet in the US, as well as everywhere else. Even Alaska couldn’t muster anything more than an M4.3. So this week we can have a quick peek into the Wyoming wilderness — the Wind River Range, to be precise.
This week the area produced an M4.0 — not the largest on record, although it’s worth noting that the USGS site doesn’t even have a page for the largest earthquake in Wyoming.
Areas of uplift such as this are inevitably faulted and faults are where earthquakes occur, even if they aren’t large ones. The USGS map shows no faults in the area — but there must be one. We just haven’t yet found it.
Last Thoughts: Remote Areas
Faults in advanced, dense-populated places such as California are mapped in enormous detail. Lives depend upon it and if a foolproof way of predicting an earthquake is ever found it’s bound to involve knowing where, as well as when, a tremor will occur.
Where there are fewer people, and where the terrain is less forgiving, it’s harder and – arguably less worthwhile – investing scarce financial, human and technological resources. That’s why areas such as Wyoming have earthquakes where no fault is mapped, and why no-one really takes much notice of slight shaking in the deep seas.