I don’t log the numbers of earthquakes I write about every week. I often think I should, because that would be a useful source of comparative data for quiet weeks when not a lot happens. Weeks like this week.
Off the top of my head, I think this is one of the quietest weeks in terms of earthquake activity that I can remember. Not the quietest (I have a desperate memory of trying to produce a digest with just a dozen earthquakes over magnitude 5, the largest of which didn’t get anywhere near M6.0) — but quiet nevertheless.
The week of 24-30 March 2016 produced a little more than that, but not much. The United States Geological Survey’s real time earthquake map, which (broadly speaking) records tremors of all magnitudes in the US and its territories and those of at least M4.0 elsewhere, rustled up a total of just 18 of at least M5 (≥M5.0), the largest of which was M5.7. Just 204 were ≥M2.5 and only 86 ≥M4.0.
Am I downhearted? Not a bit of it. There’s always something new and always something interesting to say, or an observation to make, about earthquakes.
The Week’s Biggest Earthquake: M5.7, Alaska
M5.7 isn’t, in the great scheme of things, that large an earthquake — certainly not in the Aleutian Islands. Anyone who’s been paying attention to the digest over the past few weeks might remember that I’ve discussed the M6.3 of the 12 March, also in the Aleutian Islands. You might wonder if this week’s event is an aftershock of that mainshock.
The tectonic setting is the same (the subduction of the Pacific plate beneath the North American plate along the Aleutian Trench) but the distance between the two (this week’s was about 300km or so to the east) indicates otherwise. What it does show is that the Aleutian margin is highly, and regular active.
Subduction zones are active volcanically as well as seismically. This week also saw an eruption of the Pavlov volcano, even further along to the east. Volcanoes are associated with subduction zones just as earthquakes are, but the earthquakes and the eruption aren’t linked. They just indicate that some parts of the earth’s surface are very active indeed.
M5.3 Tremor, Gulf of California
One thing we can be sure of: the Pacific is always active. Surrounded by the so-called ‘Ring of Fire’, a seismically and volcanically-active belt following the margins of a number of major plates, its major earthquakes are usually along subduction zones.
The plate margins that surround the Pacific are not just subduction zones. There are significant areas of lateral movement (most notoriously the San Andreas Fault Zone) but it was one of the few areas of constructive margin that generated our second featured earthquake, an M5.3 in the Gulf of California.
South of the San Andreas, the nature of the boundary between North America and the Pacific changes from lateral to constructive: as the map shows, it follows a stepped pattern, with active spreading centres offset by transform faults.
The tension generated causes earthquakes, some of them significant. Since the beginning of the century there have been many larger than this week’s along (or close to) the margin — the largest of them reaching M7.0.
US Earthquakes: Oklahoma
In the quietest week, there’s something to say about Oklahoma. Yet again we saw a scattering of earthquakes in this supposedly-stable continental interior, and yet again we can attribute them to human activity.
We’re starting, perhaps, to understand the implications of what’s happening in Oklahoma. This year, for the first time, the USGS has produced seismic hazard maps for anthropogenic, as well as natural, earthquakes, and have considered short-term, rather than long-terms risks.
If you’re one of the 7 million or so who live in this central area of the US, you might want to pay attention to these forecasts.
“Within a few portions of the CEUS , the chance of damage from all types of earthquakes is similar to that of natural earthquakes in high-hazard areas of California”, notes the USGS.
Food for thought, there.
Last Thoughts: Fracking or Wastewater Injection?
Scientific knowledge evolves, piece by piece, study by study. In Oklahoma, the evidence suggests that the cause of the earthquakes is not fracking itself but that: “wastewater disposal [is] the primary cause for recent events in many areas of the CEUS”.
This week, however, a study of anthropogenic earthquakes in Canada suggests that the reverse may be true in some areas: “There are more earthquakes in Western Canada that are more related to hydraulic fracturing than waste-water injection by a factor of about two,” according to David Eaton, a geophysicist quoted in HuffPost Alberta.
Even when we think we know, we still have much to learn.