It’s an interesting week for the earthquake geek; but I’ll do the numbers first. The United States Geological Survey’s real time earthquake map, which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed a total of almost 1650 earthquakes for the week of 19-25 November 2015.
Of these, there were three in excess of >7.0; five of at least M6.0 and 27 ≥M5.0. Most, as usual, were located close to the margins of the earth’s major tectonic plates.
So far so standard. So what is it that stands out in this week’s earthquake map? Well, instead of the week’s largest earthquake we have two, an apparent matching pair, twins at M7.6, both located on the western side of the Andes, hundreds of km from the Peru-Chile Trench.
The Week’s Largest Earthquakes: M7.6 (x2) in Peru
Seismologists have a term for what happened this week in eastern Peru: they call it a doublet.
Although as Gibowicz and Lasocki remark: “There is no clear definition of a doublet,” they go on to note that: “Usually a doublet is defined, somewhat arbitrarily, as a pair of events with a magnitude difference of no more than 0.2 units, spatial separation smaller than100 km, and temporal separation of a few years.”
Ladies and gentlemen, let me present to you this week’s twin earthquakes in Peru. Of the same magnitude (M7.6), within a few tens of kilometres apart on the surface and the second just 11km deeper than the first, both occurring within less than six minutes… It could be a long time before we see a clearer example of a doublet.
Tectonically speaking the two were associated with subduction of the Nazca plate beneath the South American plate. Even if they had not occurred as a pair they would have been noteworthy for their depth — they occurred at more than 600km depth, whereas the majority of larger earthquakes are shallow.
The USGS notes that: “Over the past century, 91 earthquakes with a magnitude of M 7 or more have occurred at depths greater than 300 km globally”. By comparison, the USGS earthquake archive for the same period includes a total of 1274 tremors of at least M7.0. To further illustrate how unusual this week’s event is, only 22 earthquakes of at least M7.0 have been recorded at depths greater than 600km in the last hundred years.
M7.0 Earthquake, Solomon Islands
Serendipitously, the next-largest earthquake of the week occurred in the Solomon Islands, an area where doublet earthquakes are not uncommon, the most recent having occurred in April 2014. The regularity with which such large earthquakes strike in the Solomon Islands and the looseness of the definition, however, mean that the occurrence of such a doublet might easily pass unremarked.
This week’s M7.0 is in fact an illustration of just how active the western part of the Pacific is, in a tectonic sense. Occurring along the San Cristobal Trench, where the Australian plate subducts beneath the Pacific plate, it was the third earthquake of ≥M7.0 to strike in the Solomon Islands so far in 2015.
The previous two occurred within a couple of days of one another in May — but they don’t constitute a doublet, being too far apart and too different in magnitude.
US Earthquakes: Oklahoma
There’s no real doubt that induced seismicity is the cause of most earthquakes in Oklahoma and this week’s M4.7 is almost certainly yet another in the ongoing earthquake swarm associated with wastewater injection, albeit one of the larger ones to date.
It’s always worth remembering, that Nature can do the job by itself, and that the state has experienced larger tremors in the days before petrochemical exploration. But not much larger, and not many. Food for thought, there…
Last Thoughts: One Earthquake Triggers Another
Sometime, just sometimes, I get really excited by an earthquake event and the Peruvian doublet is one of them. The USGS is a little less excited about it than I am, though it does, in the earthquake report, note that “The latter earthquake was almost certainly triggered by the earlier event”.
It’s a question that often gets asked and in most cases the answer is that a large earthquake in one area does not trigger a large earthquake elsewhere (unless it’s an aftershock). There are exceptions to every rule and this week’s Peruvian earthquakes illustrate that fact, and the concept of the doublet, beautifully.