Here at Decoded we are always wary of sweeping generalisations. So what, if anything, should we make of the fact that all but three of the earthquakes of at least M5.0 (≥M5.0) recorded on the United States Geological Survey’s real time map this week were around the Pacific margin? (The map records tremors of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere.)
Well, the answer (an uninspiring one) is ‘not much.’
In most weeks a significant number of the larger earthquakes are in and around the Pacific and the predominance of them this week is almost certainly just statistical variability. If we include those ≥M4.0 (112 in total) and ≥M2.5 (276) we can see that there was plenty of activity elsewhere– that’s normal, too.
The Week’s Largest Earthquakes: Chile
This week there were two earthquakes of ≥M6.0, both in a broadly similar tectonic setting, though at (roughly) 3,000 km apart, they are sufficiently distant from one another to be considered separate occurrences. Both result from the convergence of the Nazca and South American tectonic plates along the Peru-Chile Trench, with the latter descending beneath the former along the entire Andean margin.
There’s some evidence that the two quakes are different. The larger, to the north, came in at M6.4 and was at a depth of 124km and had its epicentre around 200km inland from the trench itself. The more southerly earthquake registered M6.2, was around 30km from the trench, and occurred at depth of 10km.
This information, in particular the relative depths, suggests that the first occurred at or near the interface between the two plates while the second is more likely to have resulted from internal deformation in the overriding plate — demonstrating that there can be more than one direct cause of an earthquake even within one tectonic setting.
M5.9 Earthquake in Taiwan
Over on the other side of the Pacific, a much more complicated tectonic setting, with more interacting plate motions, produced an earthquake of M5.9 in Taiwan.
The island is in a tricky position, affected by the northward movement of the Philippine Sea plate where it subducts beneath the Eurasian plate and the eastward subduction of the Eurasian plate beneath the Philippine Sea plate.
This complicated setup produces an area of deformation which runs along the length of Taiwan in a series of normal, reverse and lateral faults.
The M5.9 earthquake occurred close to the end of the first subduction margin, at depth of around 30km, and is most likely to have resulted from internal deformation as a result of the differing plate motions.
US Earthquakes: Alaska
Up in the north of the Pacific, the Pacific plate is again subducting, this time beneath Eurasia. Whereas the Andean margin produced a mountain range, the subduction has produced the island arcs of the Aleutian and other island chains. The two earthquakes of M5.0 and M5.1 here, the largest in the USA this week, were the product of this motion. Again, the depths of the two suggest that one may be the result of deformation and the second of movement at the plate interface.
Last Words: All Around the Ring of Fire
It wasn’t our intention to produce a Pacific edition of the earthquake digest, but that was the way the Earth moved this week. Realistically, the levels of seismic activity at the tectonic margins around and within the ocean are so high that it’s not really surprising.
After all, the ring of tectonically-generated volcanoes around the ocean isn’t know as the Ring of Fire for nothing.