Some weeks are busier than others in earthquake terms. The week of 21-27 January 2016 may not have produced an excessive number of recorded earthquakes, but there was certainly plenty to talk about.
The United States Geological Survey’s real time earthquake map, which includes all tremors in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of around 1550 events, which is pretty much par for the course.
One of these tremors exceeded M7.0 and two others were at least M6.0. The higher-than-usual total of those ≥M4.0 (40 of them) can be traced down to a handful of aftershocks to one earthquake in the Mediterranean (see below) and a collection of 16 widely spread across the always-active western Pacific.
In another week I might be tempted to discuss the latter in more detail, or indeed to look at the M6.6 in Mexico or the M4.5 in Ethiopia. But there are plenty of other things to discuss.
The Week’s Biggest Earthquake: M7.1, Alaska
Last week I observed that: “Alaska is constantly active in seismological terms, so much so that its low-level earthquakes tend to get overlooked”. Famous last words. No-one was overlooking Alaska this week. It produced the week’s biggest tremor, at M7.1, cut the power to thousands and caused a flurry on social media.
The earthquake was the result of the collision of the Pacific and North American tectonic plates, with the former, moving northwards, descending beneath the latter as the two come together.
Alaska is tectonically complicated and not all its earthquakes are directly associated with the narrow subduction zone. The depth (almost 130km) and location (several hundred kilometres north of the actual boundary) imply that this week’s tremor is more likely to be associated with a major thrust fault which passes along the Shelikof Strait.
The earthquake was, as I’ve noted, the largest of the week. In truth, though, Alaska is capable of much bigger than that. It has the dubious distinction of being the source of the second largest earthquake on record, an M9.2 which generated a Pacific-wide tsunami. This week there was no tsunami: the earthquake was too deep and located only just offshore.
M6.3 Earthquake, Western Mediterranean
This week saw an earthquake of M6.3 in the Mediterranean, just off the coast of Morocco. The USGS map, inevitably simplified, shows a tectonic margin between Africa and Eurasia, running along the North African coast, curving northwards across the Straits of Gibraltar and looping back upon itself through the mountains of southern Spain. But if the seismic setting of the Mediterranean — any of it — were to share a status update, it would be one word. Complicated.
When contents converge across an ocean, something has to give. Eventually there will be major uplift and possibly a mountain range on the scale of the Himalayas. Just now, there are the very messy remains of an ancient ocean, the Iapetus Ocean, trapped between the two. They form the Mediterranean.
There’s a lot going on. Continental collision generates thrusting and uplift rather than subduction (though there is a hint of a developing subduction zone west of Gibraltar) and more detailed tectonic maps show that the boundaries on the map represent broad zones of parallel thrust faults, where the collision of continents least to upward movement. This week’s M6.3 lies on one of these.
US Earthquakes: Oregon
It isn’t just Alaska. Much of the US west coastal region is trembling quietly away on a daily basis. Off Oregon, the last remnants of an ancient tectonic plate, the Juan de Fuca plate, are subducting beneath the US.
This forms a subduction zone (Cascadia) capable of a potentially devastating earthquake; but almost all of the seismic activity observed there is in fact the result of movement at or near the ocean ridge which forms its western edge.
This week’s largest earthquake in the southern US, an M4.9 off the coast of Oregon, was a classic example of the case.
Last Thoughts: No Tsunamis This Week
None of the earthquakes discussed above generated a tsunami, though all were in locations where major tsunamis have occurred in the past and may well occur again. The reason? It’s always worth remembering that tsunamigenesis is not just a function of magnitude, or of location, or of depth, or of the type of movement. To generate a tsunami of any size, a whole host of variables must come into play. That’s why major tsunamis are, thankfully, not always the product of large earthquakes.