The week of 7-13 April 2016 was a strange one, in some ways. For the first time I found myself restricted by the self-imposed structure of the weekly digest, debating whether or not to stick to it (really, there was nothing much of note in the US this week, for example); and as for this week’s largest earthquake…well.
In the end I decided to stick with the old, so bear with me (as the actress said to the bishop). As always, I’ll start with the numbers. The United States Geological Survey’s real time earthquake map, which includes (broadly speaking) tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) included a total of just over 1600 events — pretty much par for the course.
Of these, there were three of at least M6.0 (Vanuatu, Afghanistan and Indonesia); 34 of ≥M5.0; and 117 of ≥M4.5. These, which are the ones which show up on the default settings of the map (presumably because they are considered the most notable of the week) were mostly associated with the planet’s tectonic plates — though there were a few outliers.
The Week’s Biggest Earthquake: M6.7, Vanuatu
It is, as they say, deja vu all over again.
If you are an alert reader with phenomenal recall, you may think you’ve read this before. To make my life a little easier, I could just refer you to last week’s digest, or copy and paste. Because just as last week, this week’s largest earthquake is just off Vanuatu. And in fact, it’s almost a carbon copy of last week’s second-largest earthquake, which as in itself almost a carbon copy of the largest (sorry if this sounds convoluted).
Okay, this week’s M6.7 occurred on the 7 April rather than last week’s on the 6 April; it was 106 km west of the town of Sola rather than 105 west-south-wet of it; and it was 26km deep instead of 24km. That apart, everything else I said still stands.
I commented last week about the similarity in size and location between the then-largest earthquake and another of similar size. It does seem that the earthquake series off Vanuatu, which has included 24 tremors of at least M4.5 to date, is either not a classic earthquake series with an obvious mainshock, or may not be over.
That isn’t a prediction: just an observation.
M6.6 Tremor, Afghanistan
The collision between the Indian subcontinent and Eurasia has produced a diffuse zone of uplift within which earthquakes are common. You can see it on the map: it includes, but is by no means confined to, the Himalayas, the Tibetan plateau, the mountains of Myanmar to the east and the Hindu Kush and other ranges to the west.
This week saw an earthquake in Afghanistan, some 170km from the capital, Kabul. The BBC’s online news service reported that: “The tremor was felt in Kabul, Islamabad, Lahore and Delhi, forcing residents to leave their homes”.
Earthquakes in this part of the world can be deadly, for a number of reasons — type of building, density of population, vulnerability of the topography to earthquake-induced landslides and so on. At the time of writing there were no reports of fatalities or injuries, although a smaller earthquake (M6.1) in 2002 in broadly the same area killed an estimated 1200 people.
The clue is almost certainly in the depth, rather than magnitude. The energy released by an earthquake is lost as it travels through the Earth; the deeper the earthquake, the more of it is lost. The 2002 ‘quake was at a depth of just 8km; this week’s occurred 210km below the surface. It illustrates that depth is a key factor in the effects of an earthquake, though by no means the only one.
US Earthquakes: Hawaii
There hasn’t been a lot of excitement in the US, at least as far as earthquakes are concerned (and perhaps we should be thankful for that). That’s fine: it gives us an opportunity to look at Hawaii, somewhere that often gets ignored.
We associate Hawaii more with volcanoes than earthquakes, and rightly so; but the two are interlinked. Movement of magma causes rocks to move; so active volcanic systems are characterised by regular, though usually small, earthquakes. So it is with Hawaii, where the USGS map shows 32 small tremors, mostly around the volcanoes of Mauna Loa and Kilauea.
Last Thoughts: More on Volcanic Earthquakes
In a way it’s a shame that the USGS map is limited in the earthquake it lists. It would have shown us a tremor of M3.3 in Iceland. This is rather larger than any in Hawaii; and it’s particularly interesting because it’s one of a cluster beneath Vatnajökull — the area where, in 2014-15, the Holuhraun volcanic eruption occurred.
Of course, movement of magma doesn’t in itself presage an eruption — earthquakes can occur when magma moves out of a system as well as into it. But earthquakes can give us lots of clues about what’s going on beneath us; and I’m sure I won’t be the only one who’s watching what’s going on in Iceland.