In earthquake terms, life is never dull. Well, not often. The week of the 18-24 February 2016 was, admittedly, one of the quieter weeks with just a single earthquake managing to tiptoe up to magnitude 6 (M6.0) — pretty much the least we’d expect to see in any given week.
You might expect that the United States Geological Survey’s real time earthquake map would be reasonably blank, too, bereft as it is of any significant earthquakes. As it turns out, that isn’t the case. Just because there aren’t any large earthquakes doesn’t mean that the number of smaller ones is reduced.
This week the map, which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included 24 tremors of at least M5.0 (about average). But there were more than usual of ≥M4.0 (130, compared to a typical figure of around 100) and 1650 in total (usually around 1500). Such is the variability of seismicity.
The Week’s Largest Earthquake: M6.0, Indonesia
We’ve been here before: several times. Indonesia, where several of the Earth’s tectonic plates interact, is a regular location of the week’s largest earthquake. This week the tremor in question weighed in (so to speak) at a featherweight M6.0.
In this area there’s a lot going on, tectonically. The westwards motion of the Pacific plate culminates in its subduction beneath the Eurasian plate; but Eurasia, in this area, is crushed and fragmented into many smaller plates and crustal slivers.
The USGS map doesn’t show all these. There are, in any case, too many for cartographic simplicity. But if you look at detailed fault maps, such as those produced by seismologist Robert Yeats, it’s clear that theres’ a lot going on. The earthquake occurred to the south of the Philippine Trench (subduction); to the east of the Halmahera Thrust Zone (convergence); and to the north of the Sorong Fault (lateral movement).
In all likelihood the actual movement wasn’t a response to any one of these but just the result of deformation caused by the enormous conflicting forces at play within the Earth’s crust in this region.
M5.8 Earthquake, South Sandwich Islands
In contrast to the complicated plate interactions of the western Pacific, the tectonic setting of the South Atlantic is pleasantly straightforward. The Atlantic as a whole is pretty quiet in earthquake terms, certainly when compared with its neighbour in the eastern hemisphere. Most of its earthquakes occur along the Mid-Atlantic Ridge and they tend to be significantly smaller than those in the Pacific.
Part of the reason for this is that large earthquakes are usually associated with subduction zones and the Atlantic, an ocean still growing, has just two of these, both short. One is in the Caribbean and the other in the remote South Atlantic, east of the island of South Georgia. It’s the latter that catches our attention this week, with an earthquake of M5.8.
With little in the way of conflicting movement, the subduction zone beneath the South Sandwich Islands is easy to interpret. The South American plate is subducting beneath the Scotia microplate along a short volcanic island arc. This week’s earthquake, at a depth of 120km and at roughly the same distance from the plate boundary on the over-riding plate, is almost certainly the direct result of movement at or near the plate interface.
US Earthquakes: California
Usually the largest earthquake in the US occurs in tremor-prone Alaska, with its major subduction zone. California, though at least as notorious and quite capable of producing large earthquakes (though not quite as large as those of the Last Frontier) comes a close second.
This week California carried off the honours with a tremor of M4.9 in the San Joaquin Valley. The epicentre was some way east of the San Andreas Fault Zone and not associated with any mapped faults. In fact, it was in an area where the seismic hazard is relatively low — for California, at least. That doesn’t mean that such a tremor is either unusual or unexpected.
Last Thoughts: Earthquake Zones
Sometimes earthquakes are so simple. If you asked me for an example of a straightforward subduction earthquake, I’d send you straight to the South Sandwich Islands. But in most cases the tectonic setting is more complicated.
Plate margins may be clearly delineated (none better than the San Andreas) but even these cause deformation — and earthquakes — over a wide area. And some, such as our third example in the western Pacific, involve so many competing plate motions and types of tectonic activity that it’s hard to see from the map exactly what the trigger for any single earthquake event might be.