Sumatra, Iceland and California: Earthquakes 9-15 March 2017

The week of 9-16 March 2016 was a quiet one once again, with just one earthquake registering at least magnitude 6 (≥M6.0) on the United States Geological Survey’s real time earthquake map. Usually we’d expect at least one of this magnitude and that’s the absolute minimum that we got — a single earthquake of M6.0.

In total the map, which includes (broadly speaking) earthquakes of all magnitudes in the US and its territories and those of at least M4.5 in the rest of the world, recorded just under 1500 tremors, of which 29 were at least M5.0 and 70 at least M4.0. These numbers are broadly consistent with a typical week (in so far as that exists); it’s just that there’s usually more in the way of larger earthquakes.

In terms of distribution, again it was pretty much as usual, with most of the larger tremors occurring at or in association with the margins of the earth’s tectonic plates. This week, however, there was less activity than usual in the Pacific, which is commonly the location of the largest earthquake in any given week: this time the honours were stolen by and M6.0 in the Indian Ocean (of which, more below).

The Week’s Biggest Earthquake: M6.0, off Sumatra

The USGS describes the largest tremor of this week as being “SSW of Mohean, India” which is technically correct as the closest land is in India’s Nicobar Islands. I prefer tectonics to politics any day, and so I’ve identified it as being off Sumatra, justifying that on the basis that it’s closely associated with the Sunda Trench, which is part of the major subduction zone that runs along the north-eastern margin of the Indian Ocean and where the Indo-Australian plate descends beneath Eurasia.

Location is the key to determining the cause of this earthquake. It lies in the descending plate (the Indo-Australian) close to the boundary — but it lies 10km to the seaward side of the trench, at a location where the descent has (probably) not begun. Further clues come from its shallow depth (also 10km) and from the USGS data on the nature of the faulting, which suggests lateral movement.

Taken together, this available information suggests that the earthquake was the result of deformation within the Indo-Australian plate rather than movement at or near the plate interface — good news, since this margin is capable of producing very large earthquakes indeed, some of them producing large and devastating tsunamis.

Iceland: Volcanic Earthquakes

The USGS map is the most comprehensive source of earthquake data but it isn’t the only one, and sometimes looking at regional sources yields interesting information on earthquakes and earthquake patterns too small to be included on a worldwide map. I make no excuses, this week, for looking at one of my particular areas of interest — Iceland.

Most of the earthquakes reported worldwide are tectonic in nature — the result of movement of major tectonic plates. In other areas they may be volcanic, caused by inflation and deflation of the Earth’s crust as magma chambers empty and refill. Volcanic earthquakes give clues to magma movement but don’t necessarily presage an eruption, though it’s always useful to keep an eye on them.

The Iceland Meteorological Office’s earthquake maps show regular low-level earthquake activity, and this week my eye was caught by something a little larger than usual — an earthquake of M4.1 beneath the Bárðarbunga caldera (which erupted spectacularly in 2014-15).

As I note above, this doesn’t necessarily mean there’s an eruption coming, but it, along with the rest of Iceland’s earthquake activity, is an indicator that the mantle beneath the country is very much active.

US Earthquakes: California

Like Iceland, California constantly trembles – though, given its reputation, large earthquakes (by which I mean those of M6.0 or more) are surprisingly scarce. Even smaller ones do, however, tend to make an impact — partly because they occur in densely-settled places where people are very much connected and likely to talk about them. That’s why the first thing I saw about a (smallish) earthquake in California was on my Facebook feed.

The earthquake itself, at M3.1, was a pretty straightforward event, caused by lateral movement along one of many parallel faults in the San Jacinto Fault Zone (part of the San Andreas) where the Pacific and North American plates slide past one another.

There wasn’t a lot of movement and shaking was, at its greatest, only light. But the location of the earthquake, close to San Bernardino, meant that it was felt across significant areas of greater Los Angeles. The USGS doesn’t give an impact summary so there’s no clear estimate of how many people noticed it. But you can bet your bottom dollar that it was millions.

Last Thoughts: Something of Everything

Some weeks the featured earthquakes follow a theme — all subduction, for example, or all parts of earthquake swarms, or whatever. This week, there’s no theme. They’re all different.

We have an offshore earthquake at a subduction zone, though not one caused directly by subduction. We have an earthquake near LA caused by two plates sliding past one another, and we have a pattern of earthquakes probably associated with volcanism.

If I had more words to play with, I could probably find you other earthquakes from different tectonic settings to look at. Earthquakes are anything but uniform — they come in different locations, and different magnitudes, and they affect different areas in different ways.