Japan, Bolivia and Alaska: Earthquakes 14-20 January 2016

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The map shows earthquakes of at least M4.5 in the week of 14-20 January 2016

The map shows earthquakes of at least M4.5 in the week of 14-20 January 2016. Image by USGS.

So, what’s happening in the world in earthquake terms? The answer is a typical January one — not a lot. The largest earthquake recorded on the United States Geological Survey’s real time earthquake map was magnitude 6.7 (M6.7) which is large but not exceptionally so. The map, which records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed just one other of ≥M6.0.

Overall, the map showed a lot of intermediate earthquakes but nothing standing out. There were 30 tremors of ≥M5.0 and 112 of ≥M4.0, out of a total of just over 1700. These figures are perhaps slightly larger than an average week but not exceptionally so.

And, as usual, almost all of those of any size (let’s say ≥M5.0) occurred pretty much where we’d expect to see them — around the margins of the Earth’s major tectonic plates.

The Week’s Biggest Earthquake: M6.7, Japan

The week's largest earthquake was an M6.7 off Japan

The week’s largest earthquake was an M6.7 off Japan. Image by USGS.

Just last week I was discussing an earthquake in Japan — not the largest, but large enough at M6.2. You may remember that I quoted the seismologist Robert Yeats and his observations on what he called the ‘diffuse plate boundary’ and the differing interpretations of what lies beneath. We’re back in that territory again, both literally and metaphorically.

This week the earthquake under discussion occurred on the opposite side of the island of Honshu, to the east. At several hundred kilometres from its predecessor, this tremor is probably too far away for the two to be linked. The tectonic setting here is one of subduction, with the Pacific plate descending beneath the Okhotsk microplate (a southern extension of the North American plate).

In his book Active Faults of the World, Yeats reproduces maps of the angle of descent and these suggest that in the approximate region of the earthquake the boundary between the two plates is around 40-50km below the surface. This ties in neatly with the depth of this week’s M6.7, at 51.4km. This implies that the earthquake is the result of movement at or near the plate interface.

M6.1 Tremor, Bolivia

A deep earthquake in Bolivia is associated with subduction in the Pacific

A deep earthquake in Bolivia is associated with subduction in the Pacific. Image by USGS.

Depth is also a key feature of the second-largest earthquake of the week, and if I ran a weekly feature on the deepest earthquake of the week, this would be it, registering at 582km depth. This is significant.

Looking at the map, it’s obvious that most South American tremors occur along or close to the west coast and are associated with the subduction of the Nazca plate beneath the South American continent along the Peru-Chile Trench. This week’s M6.2, on the eastern side of the Rockies, is an apparent outlier.

Or is it? As we saw with the Japanese tremor, there’s an identifiable relationship between the angle of descent of a plate and the depth of earthquakes along the interface. In the central part of South America, the angle of subduction is relatively flat before steepening to the east. As with the Japanese earthquake, comparing the depth and location of the earthquake with maps of the descending slab indicates a direct relationship between the earthquake and the subducting plate.

US Earthquakes: Alaska

The US's largest earthquakes were in Alaska

The US’s largest earthquakes were in Alaska. Image by USGS.

Alaska is constantly active in seismological terms, so much so that its low-level earthquakes tend to get overlooked. This week the rest of the US was relatively quiet, but Alaska kept on shaking, producing the four largest earthquakes in the US. The largest of these was only M4.8, so nothing significant. But the background noise is constant, with almost 400 tremors (all magnitudes) recorded on the USGS map.

Last Thoughts: Earthquakes Run Deep

When we look at earthquakes on a map, we’re looking in two dimensions. This week’s two featured earthquake are a reminder that the processes of plate tectonics operate in three dimensions. What we see on the map is not the location of the earthquake (the focus) but the point on the Earth’s surface above which it occurred (the epicentre). To gain further clues as to the origin of the earthquake we can add what we know about its depth.

Even then, of course, we may not reach a conclusion. But it certainly helps.

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