Marianas Islands, Indian Ocean and California: Earthquakes 28 July-3 August 2016

After a few weeks of relative inaction, the week of 28 July-3 August 2016 came up with something which has to be one of my favourite things. Right up there with raindrops on roses and whiskers on kittens, you’ll find a vey big earthquake in the middle of nowhere that does no-one any harm.

The M7.7 tremor in the Marianas Islands dominated the United States Geological Survey’s real time earthquake map for this week. The map, which includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, didn’t actually have a huge amount more than in previous weeks. Of the 1600 or so shown in total, there was our giant friend in the Pacific (M7.7) — but there was just one other earthquake ≥M6.0 and 23 ≥M5.0.

Distribution-wise there was nothing out of the ordinary, except for our second featured earthquake. Most larger tremors occur along the margin of the Earth’s tectonic plates and this is what we saw — except for one in the middle of the Indian Ocean.

The Week’s Biggest Earthquake: M7.7, Marianas Islands

Make no mistake: M7.7 is a big earthquake, one that you might expect to be destructive. If on land, it’s big enough to generate a lot of shaking; if underwater it’s more than big enough to generate a tsunami.

Not this time. In so remote an area, there’s hardly anyone to feel a tremor, even one of this size. The USGS Pager earthquake summary lists the numbers exposed to different levels of shaking — and it’s a list of big fat zeros.

And yet it was a big earthquake, the result of the subduction of the Pacific plate beneath the margin of the Philippine sea plate. The rate of convergence is around 40mm per year and large earthquakes in this area and in a subduction setting are by no means uncommon. In technical mood, the USGS notes that: “indicate oblique rupture occurred on either a south-southwest or northwest striking reverse fault.”
In layman’s language, it explains that: “The earthquake likely represents the release of stress resulting from the distortion of the Pacific plate at depth.”

But here’s a thing. We have here three of the key components (magnitude ≥M7.0, reverse faulting and submarine location) for tsunamigenesis, yet no tsunami occurred. The answer lies in another feature of the earthquake — its depth. At 212km, it’s considered intermediate depth. Tsunamis are much more likely to occur if an earthquake is shallow and the energy generated has no chance to dissipate.

M6.1 Earthquake, Indian Ocean

Another of my favourite things, though something which doesn’t fit snappily into a song, is the existence of scientific detail. This can shed a little light on some very deep, dark places — in this case, the M6.1 which occurred in the middle of the Indian Ocean, a very long way from anywhere, another earthquake felt by nobody.

Mid-ocean earthquakes aren’t unusual but most are located near to mid-ocean ridges and are the result of extensional tectonics.

Not this one. It’s a bit of a conundrum; but there’s one piece of information which we do have. For larger earthquakes — not for all — the USGS publishes a piece of information called the moment tensor solution (more on this later). That suggests that the earthquake was the result of compressional tectonics.

I don’t know, because the location is so remote and information is so scarce, but that suggests to me that something interesting is happening deep down under the Indian Ocean. Unfortunately I don’t know what it might be, because the information just doesn’t exist. And that’s not one of my favourite things.

US Earthquakes: California

There’s nothing especially noteworthy in the US this week, so I can go back to another thing that pleases my eye — the way the earthquakes on the US map for California follow the lines of the San Andreas fault zones.

Of course there are plenty of other earthquakes in the area but these many mini-tremblors, most of them also too small to be heeded, are an elegant illustration of the fact that, just sometimes, thing happen as you expect them to.

Last Thoughts: Moment Tensor Solutions

I try not to get technical, but when the USGS publishes a moment tensor solution (represented on a so-called beachball diagram) I’m a little bit happier. This piece of information tells you (in short) whether you’re looking at a compressional, extensional or lateral movement as the origin for the earthquake.

If a forest of mathematical equations is among your favourite things, you can find any number of papers outlining how these are calculated. Or you can follow a simple rule of thumb. If the beachball is dark in the middle, the source is compressional. If it’s pale in the middle, it’s extensional. and if it’s in quadrants, it’s lateral.

Simple, perhaps. But it works.