Periods of seismic quiet follow periods of seismic activity as sunshine follows rain. There’s no hard and fast scientific law on that, but it’s certainly the way it seems to pan out. After a few weeks of frantic activity (remember all those earthquake in excess of M7.0?) the week of 15-21 September, 2016 was a quiet one.
The largest tremor of the week to appear on the United States Geological Survey’s real time earthquake map had a magnitude of just 6.1 (M6.1). The map, which broadly includes earthquakes of all magnitudes in the US and its territories and those of at least M4 (≥M4.0) elsewhere, includes just under 1800 events in total — but only two were ≥M6.0, and 25 came in at ≥M5.0.
Nor was there much to get excited about in terms of location. All but one of the larger tremors occurred pretty much where you’d expect them to, with just one away from the main areas of activity along the boundaries of the planet’s tectonic plates. More on that later.
The Week’s Biggest Earthquake: M6.1, Izu Islands
The largest earthquake of the week, at M6.1, occurred in the Pacific Ocean. The USGS gives its location as ‘Japan region’ which illustrates rather neatly how remote it was — over 500 km from Japan itself, to be more specific.
A large part of the western margin of the Pacific Ocean overlies the tectonic plate known as the Philippine Sea plate. This slab of old, dense ocean crust borders the main Pacific plate to the west; the two are coming together and the Pacific plate is descending beneath its neighbour.
Subduction (as the process is called) results in earthquakes. Simply put, the descending plate drags the overriding plate down with it, forming a deep ocean trench. At some stage, part of the upper plate will rebound, causing earthquakes. The available evidence from the USGS — including the location of the earthquake relative to the trench, its depth and the direction of movement — suggests that this week’s earthquake is a direct result of movement at or near the plate boundary.
The earthquake was, however, too small and too remote to make any headlines — and the USGS estimates that only a couple of thousand people all have felt any shaking at all.
M5.1 Tremor, Comoros Islands
I mentioned an outlier in the introduction — an earthquake of intermediate size where we might not normally expect it. This one, an M5.1, occurred well away from plate margins, just south of the Comoros Islands, between Africa and Madagascar.
There’s no detailed data relating to this tremor so I can only speculate as to its exact cause from looking at the little we do know. The most important thing we know is that the tectonic setting is one of extension — Madagascar was formerly joined to Africa and has split from it over a period of millions of years. Most of the movement, however, seems to have been associated with a feature called the Davie Ridge which lies some way to the west of the earthquake’s epicentre and which is, in any case, described as “currently inactive” (Miska).
So the origins of this earthquake are a little bit of a mystery. From the information that is available I would hazard that it is indeed the result of a wider extension process resulting from the separation of Madagascar from Africa and is probably caused by normal faulting. But I stand to be corrected.
US Earthquakes, California
I was out of commission last week, so didn’t deliver the weekly digest. That means I missed something I would have liked to comment on, so in the absence of anything exotic in the US this week, I’d like to go back and pick up on it.
Not that this — an earthquake of M3.5 in California — is exotic, by any manner of means. But because it’s in so closely-studied a fault zone, the available evidence tells us pretty nearly everything about it, despite its magnitude being so much smaller than the others featured above.
It occurred along a defined and very well-known fault (the Hayward Fault) as a result of lateral movement. It was small — so small that the USGS hasn’t published an impact assessment for it — but we do know the it was felt throughout much of the San Francisco Bay area.
Last Thoughts: All Kinds of Everything
We had a bit of everything this week — or I think we did. I know we had some lateral movement in San Francisco and some compressional movement under the sea, way south of Japan. I’m pretty sure, despite the degree of speculation, that we had some extensional movement in the Mozambique Strait.
I’ve said it before, but it bears repeating. Seismic zones in developed countries are (of course) much better studied than those in remote areas where no-one is likely to be affected by them. There’s a reason for it — because the main significance of earthquake study is in the impacts of the humans likely to be affected by seismic events.