It was quiet — goodness, it was quiet — this week. From 18-25 May, 2017, the United States Geological Survey’s real-time earthquake map didn’t include any earthquakes of magnitude 6 or more (≥M6.0) and only showed 15 of at least M5.0. That’s almost as low as I can remember.
The map isn’t infallible and doesn’t include all earthquakes (it includes those of all magnitudes in the US and its territories — which is not the same as all of them — and those of ≥M4.0 worldwide) but at the larger magnitudes it is accurate. The map showed almost 1650 tremors in total, with 1893 of them of at least M2.5.
As usual, the distribution was almost exactly as we’d expect, with the larger earthquakes associated wit the major tectonic plate boundaries. There was one outlier — an earthquake of M4.6 between Greenland and Baffin Island.
The Week’s Biggest Earthquake: M5.9, Philippines
The largest earthquake to feature on the USGS map this week was an M5.9 in the Philippines. The epicentre (the point on the Earth’s crust immediately above the earthquake) was in the Bohol Sea, around 100km from the island of Mindanao at a depth of about 520km.
This is an intriguing earthquake at several levels. The depth suggests that it might be associated with subduction, but of what and where? Subduction occurs when one plate is forced downwards beneath another and in this case there are a couple of immediately obvious options. To the east of the quake, the Philippine Sea plate subducts westwards along the Philippine Trough. To the west of the quake, the subduction is in the opposite direction, along the Negros Trench. Much of the archipelago is caught between these two and there are active faults along an axis between them.
There are two problems here. The first is the depth. If the earthquake directly related to subduction then one of the plates must be dipping exceptionally steeply — typically, dip is around 45 degrees and, though commonly shallower, can reach depths of around 70 degrees. I don’t know (and couldn’t find) the angle of dip of the plate at this location, but the relationship between the depth and the location of the plate margin suggests that, if it is a subduction earthquake, then the angle of dip must be one of the steepest known.
The issue is further complicated by the direction of movement. We might expect a subduction earthquake to have been caused by compressional movement: this one was caused by lateral movement. On the basis of the available data (which may, of course, later be corrected) I’m tempted to conclude that the earthquake was caused by some kind of movement deep in the Earth in what’s a very complicated tectonic setting.
M4.3 Earthquake in Iceland
It’s much less complicated in Iceland, even when data are limited.
Regular readers of this digest will know that I have a particular interest in what’s going on up in the North Atlantic. In Iceland, the divergent margin between the North American and Eurasian plates overlies a mantle plume, with the result that there’s a multitude of volcanic activity.
Earthquakes here can be associated either with the movement of these plates, moving apart on either side of Iceland’s central rift valley, or with the inflation and deflation of magma chamber beneath the island’s volcanoes. Without detailed information on this week’s M4.3 earthquake, it’s impossible to be certain what caused it — but, given that it’s just the largest in a series of tremors that are notable in a local, if not a worldwide, context, and the fact that it occurred close to the relatively recently-active Bárðarbunga volcano, which erupted in 2014-15, I would venture to suggest that it’s the latter.
Whether this suggests an imminent eruption is a different question: but it’s a reminder, if we needed it, that earthquakes and volcanoes are closely linked in some areas of the planet.
US Earthquakes: The Juan de Fuca Plate
Meanwhile, over off the coast of Oregon, we have another earthquake for which there’s no detailed summary. At one level it’s actually pretty clear — the M4.1 a couple of hundred km from Oregon itself is most likely to be caused by shallow deformation within the Juan de Fuca plate, which is descending beneath the western coast of the US along the Cascadia subduction zone.
If the earthquake itself is reasonably straightforward, the tectonic setting is less so. The Juan de Fuca plate is a remnant of the much larger Farallon plate, most of which has been long subsumed beneath the North American continent. In time, the jagged trailing end of the plate (characterised by lateral movement) will subduct and even smaller remnants be left.
In the meantime, what’s left of this larger plate draws ever closer to its destruction. It’s no surprise it shivers a bit in the middle from time to time.
Last Thoughts: We Are Detectives
Sometimes the USGS produces event summaries that do my work for me, analysing all the data they publish (and some they don’t) to indicate the source and nature of an earthquake, along with specific information on local faults and so on.
This week, the earthquakes are too small for that, so I’ve been left to pick apart the available data and draw my own conclusions. It may not be as easy as repeating the wisdom of the USGS and I may not be completely correct — but it’s very much more satisfying.