You probably haven’t been paying much attention to the Earth this week. It’s hardly surprising, when most of the media attention, not just in the US but way beyond it, has been on the heavens. But our dynamic planet keeps moving, and the ground keeps shaking.
In the week of 17-23 August 2017, there was nothing outstanding in terms of either the number of the scale of seismic activity, though one or two details are worthy of note. In total, the United States Geological Survey’s real time earthquake map recorded just over 1600 earthquakes, of which just two were larger than magnitude six, 17 at least M5.0 and 99 at least M4.0.
The map doesn’t give a complete picture, but it’s certainly reliable in recording the larger earth tremors. In number as well as distribution, this week’s earthbound events threw up no surprises.
The Week’s Biggest Earthquake: M6.6, Mid-Atlantic
I’ve mentioned before that earthquakes on ocean ridges are common but tend not to be the largest on the planet. I could add that there’s a part of me that dreads one of them registering, as this week, as the largest on the planet — they are remote, and they don’t attract a whole lot of attention so that there isn’t usually a significant amount of available information.
So here we are, looking at the largest earthquake of the week — an M6.6 which took place in the middle of the Atlantic, 850 km from the coast of Africa. Ocean ridges from the longest mountain range on Earth, running through every ocean: they’re places where upwelling molten rock drives the planet’s tectonic plate apart. With this kind of movement, it’s hardly surprising we see earthquakes.
The ridges are regularly offset by fracture zones, which is where most of the associated earthquakes occur. This week’s event (at the Romanche Fracture Zone, almost on the Equator) is a classic of its type, with lateral movement along one of many crustal fractures in the area.
Such earthquakes can be very large — an event of M7.1 occurred in the North Atlantic as recently as 2015 — but because the displacement is lateral not vertical, they tend not to be tsunamigenic and therefore even large events pass unremarked upon by anyone other than seismologists.
M4.2 Earthquake, Italy
This week’s earthquake in Italy was small, at just M4.2, but it was significant. Striking in a popular tourist area at the height of the summer, it killed two people and injured many more, producing drama that played out in front of the watching media as three young children were rescued from the rubble of their home.
Earthquakes of this magnitude and larger aren’t uncommon, though they are largely focussed in the Apennine Mountains. Some of them are extremely damaging and loss of life is unfortunately not uncommon. They result from the coming-together of Africa and Eurasia, which produces a complex tectonic setting: in the immediate area around the earthquake there are normal, strike-slip and reverse faults, and Yeats indicates that subduction earthquakes occur beneath the Tyrrhenian Sea.
This earthquake was too small to warrant any significant detail so we don’t know what type of movement caused it. At such a shallow depth (9km), however, and with reference to fault maps, it seems most likely that it resulted from movement along a normal fault extending beneath the Bay of Naples.
US Earthquakes: East or Central?
This week’s US map shows that the east and central regions of the US are, as usual, not heavily troubled by seismic activity. But such activity does occur, and this week an article flitting through my Twitter feed caught my eye.
While it’s long been known that earthquakes in the east are generally more damaging than those further west (the result of seismic waves moving more quickly in older, colder, more rigid rock) new research has looked more closely at earthquake mechanisms in the eastern and central US and come up with more detailed information.
The suggestion is that eastern earthquakes are more likely to involve reverse faulting (i.e. with a component of vertical movement) whereas those in the central US are more likely to involve lateral movement. The vertical component, while not as significant on land as under water, is nevertheless important in determining the damage caused (though of course there are many other factors).
Last Thoughts: Size Isn’t Everything
The first of our featured earthquakes was a mind-boggling 250 times larger than the second, releasing almost 4000 times as much energy — but it’s the impacts of the smaller one that made the news. In the US, eastern earthquakes cause more damage than those further west, even when the magnitudes are the same.
What matters in earthquake management isn’t just the magnitude of the earthquake. It’s a far more complicated thing than that. Assessing hazard involves many components. For example, a large strike-slip earthquake underwater in a remote area will be less of a hazard than a smaller, reverse-fault earthquake in shallow water near an inhabited coast.
There are many other factors, too. The density of population, the nature of the environment, the type of buildings in place and the level of preparedness for a major seismic event must all be considered.
Earthquake hazard assessment is complicated — and size isn’t everything.