Fiji, Yellowstone and (Maybe) Greenland: Earthquakes 15-21 June 2017

There may not have been that many earthquakes recorded on the United States Geological Survey’s real-time earthquake map this week, but there’s plenty to choose from in terms of interesting — and occasionally unusual — seismic events.

The map includes (broadly speaking) earthquakes of all magnitudes in the US and its territories and those of all magnitudes elsewhere, though it does underestimate the total number, which this week was just under 2000, quite considerably. (There are a whole lot of reasons for this, far too many to reiterate here.) It does, however, pick up the major events, so we can be reasonably certain that there’s nothing significant missing.

This week there were just two events of M6.0 or larger, both in the southwest of the Pacific Ocean, along with 23 of at least M5.0 and 72 exceeding M4.0. These are at the lower end of the range we might normally expect to see, though remaining well within it.

The Week’s Biggest Earthquake: M6.1, South of the Fiji Islands

We have a couple of curiosities in the digest this week, but the largest earthquake of the week is as classic as they get. It occurred around 620km south of the Fiji Islands and its epicentre (the point on the earth’s surface immediately above the earthquake itself) lies midway between two sections of the boundary between the Pacific and Australian plate boundaries — the diffuse Hunter Fracture Zone to the North and the subduction zone along the Tonga-Kermadec Trench to the east.

The position on a two-dimensional map may be ambiguous, but we don’t have to look far to find the source. The earthquake depth is key — it occurred at a depth of 512km. This is (roughly) the same as the distance between it and the Tonga-Kermadec subduction zone. Given that the angle of descent here is (again roughly) 45 degrees, it’s a reasonably safe bet that the earthquake occurred as a result of movement at or near the interface between the two plates.

If we needed any confirmation of that, it’s in the USGS data page, which indicates that compressional movement was the cause. So, with this one earthquake at least, there are no surprises and no mysteries to unravel.

The Orphan Tsunami: What’s Up In Greenland?

There’s an alarming video circulating on YouTube this week — video footage of the aftermath of a tsunami in, of all places, Greenland. The event is reported to have left four people missing.

Tsunamis are generally associated with earthquakes — and large earthquakes, at that. But this one leaves a lot of questions unanswered and a lot of seismologists scratching their heads. Some of my trusted sources have different answers.

According to the BBC: “The authorities believe a magnitude four earthquake caused the tsunami.”

The American Geoscience Union blog thinks the matter through a little more thoroughly (as you might expect from a science blog rather than a news outlet): “This, combined with the localised nature of the tsunami, suggest that the cause is most likely to have been a very large landslide.”

And my go-to source, the USGS, shows the event on the map as a landslide but refuses to commit as to its cause: “More analysis is required to determine if this landslide was triggered by a small earthquake. The current location and magnitude are poorly constrained by the data.”

This, in a sense, links back to the comment in the opening section about underestimates. We can be sure that the tsunami wasn’t triggered directly by an earthquake: Any earthquake large enough to displace enough water to generate a tsunami would have been picked up by the network of sensors worldwide. But smaller earthquakes might go unrecorded in remote and largely unsettled areas such as Greenland — so may have been a trigger for the landslide and, thus, the tsunami.

US Earthquakes: Yellowstone

There’s a lot of nonsense talked about Yellowstone and its supervolcano (and I mean a lot). You’ll have seen it yourself: Seasonal migration of bison means they’re fleeing the park for fear of an imminent eruption (not that that would help them much in the event of one); every minor earthquake presages an apocalyptic mega-eruption; and so on.

This week there has been more seismic activity than usual in the Yellowstone area — specifically, to quote the USGS: “464 events with the largest magnitude of ML 4.4 (MW 4.4)… The swarm consists of one earthquake in the magnitude 4 range, 5 earthquakes in the magnitude 3 range, 57 earthquakes in the magnitude 2 range, 238 earthquakes in the magnitude 1 range, 157 earthquakes in the magnitude 0 range, and 6 earthquakes with magnitudes of less than zero.” (NB: Only a few of these appear on the USGS map.)

But there’s no need to panic. “Earthquake swarms are common in Yellowstone and, on average, comprise about 50% of the total seismicity in the Yellowstone region,” says the USGS, reminding us that this number: “is fewer than weekly counts during similar earthquakes swarms in 2002, 2004, 2008 and 2010.”

And we’re still here.

Last Thoughts: A Complicated System

Earthquakes are just one part of the activity of a very dynamic Earth. We can’t always separate one part of our planet’s system from another, and both the Yellowstone earthquake swarm and the Greenland tsunami demonstrate this.

The Yellowstone earthquakes are a natural side-effect of the swelling and subsiding of the underlying magma chamber (I like to think of this as the Earth breathing, though one shouldn’t really anthropomorphise science). Internal heat is the engine that drives the planet and finds its expression elsewhere — in Hawaii and other volcanic islands, and in the deep rifts at the heart of every ocean.

Greenland’s disaster looks to have been the immediate result of a landslide rather than an earthquake, but we can’t say for certain that an earthquake wasn’t the cause. Even if it wasn’t (the melting of ice releasing pressure is another possibility) we’re still left with some movement of land as a result of the whole complex system.

© Copyright 2017 Jennifer Young, All rights Reserved. Written For: Decoded Science