Last week offered us a major earthquake to talk about. The week of 10-16 March 2016 was a whole lot quieter, with the largest event — an M6.3 in Alaska — coming it at a full order and a half of magnitude smaller. And, on a logarithmic scale like the one used to measure earthquakes, that’s a huge difference. In energy terms, it’s less than 177th of the size.
In the light of this, what we deem to be larger earthquakes aren’t really that large; but here are the numbers. The United States Geological Survey’s real time earthquake map, which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed a total of just over 1500 earthquakes for the week. Just one was ≥M6.0 and 28 ≥M5.0.
Earthquakes occur in particular settings so the distribution was, as it almost always is, unsurprising. Most were at or near the margins of the Earth’s tectonic plates. The two which look like anomalies on the map, in China and Kazakhstan, are both associated with the (very) broad zone of deformation associated with the collision of india with Eurasia and the uplift of the Himalayas and Tibet.
The Week’s Largest Earthquake: M6.3, Alaska
It’s fortuitous that Alaska is so remote and sparsely populated: otherwise the regular occurrence of (often major) earthquakes would be a whole lot more noteworthy than it actually is. (Imagine if an M6.3 were to occur in LA.)
Alaska’s high levels of seismic activity are the result of the subduction of the Pacific plate beneath the North American plate. Where rocks come together, strain builds up and is eventually released as an earthquake. The Aleutian Trench is one of the planet’s major subduction zones, so this week’s M6.3 and its associated aftershocks, three of them at least M5.0, are hardly a surprise.
Although there’s no detailed information on the tremor, the location of the earthquakes (around 60km from the trench) and their depth (mostly 10-20km) suggests that the source was deformation in the overriding North American plate rather than movement along the plate margin itself.
It’s worth remembering, as we try to keep the week’s largest earthquake in perspective, that the Aleutian subduction zone is capable of producing some very large earthquakes indeed. The USGS archive shows 162 earthquakes of at least M6.3 in the past 50 years, and six larger than M80 in the last century.
The largest of these had a magnitude of M9.2. For interest, that’s 22,387 times larger than this week’s.
Earthquakes in the South Atlantic: The Scotia Plate
Time to move from the North Pacific to the South Atlantic. Here, the Scotia microplane is caught between the South American and Antarctic plates. Its northern and southern margins are moving laterally, while at its eastern edge the South American plate subdues beneath it.
Because the area is so remote, information on earthquakes in this very deep south corner of the planet is limited. We can reasonably deduce that this week’s earthquakes are of two kinds — subduction earthquakes in the Scotia arc to the east and those caused by lateral (strike slip) faulting along the northern margin.
US Earthquakes: Oklahoma
So Oklahoma is still shaking, as it has done increasingly since 2010. I don’t want to repeat myself on the causes of these tremors, other than to reiterate the near-certainty of their human origin. This week the USGS map shows 25 of at least M2.5 in the northern and central parts of the state and creeping across the border into Kansas.
I’m not the only one who’s been watching them. The New York Times reported that “…the state Corporation Commission asked well operators in a Connecticut-size patch of central Oklahoma to reduce by 40 percent the amount of oil and gas wastes they are injecting deep into the earth” in an attempt to reduce the number of tremors.
The NYT went on to add that “the directives were phrased as requests” though it also noted that these requests were “effectively order[s]”.
It’ll be interesting to see whether this happens, how long it takes and whether it makes a difference.
Last Thoughts: Subduction in the Atlantic Ocean
We looked at the Atlantic Ocean above. Compared to the Pacific the Atlantic has virtually no subduction zones — just the Scotia Arc and, a similar tectonic setting, the volcanic arcs of the Caribbean. There’s also some evidence of a third (developing) subduction zone off the Straits of Gibraltar. Meanwhile the Pacific is surrounded by the things — Alaska, South America, Japan and others.
Why the difference? The Pacific is a mature ocean and is moving towards its decline. Eventually (please forgive the over-simplification) the subduction zones will consume the ocean floor faster than it can be produced and the ocean will close. Meanwhile the Atlantic is still growing — and in a hundred million years or so those short subduction zones will develop into larger ones.
The Atlantic is the new Pacific; but we won’t be around to see it happen.