I should begin with a technical note. I run the digest for a seven day period and so, occasionally, a noteworthy earthquake occurs on the day of publication but after the weekly post goes live. That happened this week, and the earthquake was one that can’t be ignored. So in fact this week we’ll be making reference to at least one earthquake from the 26 of October, 2016.
So, to the numbers. The week of 27 October-2 November 2016 saw a total of just under 1700 earthquakes recorded on the United States Geological Survey’s real time earthquake map. The map, which includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) showed two events ≥M6.0; 34 ≥M5.0; and 123 ≥M4.0.
These numbers were slightly higher than usual, though not exceptionally so. This was largely explained by the significant number of aftershocks — 20 of at least M4.0 — associated with the week’s largest earthquake, in central Italy.
Most of the larger tremors (≥M4.5) were, as usual, associated with active seismic zones around the margins of the earth’s tectonic plates, though there were occasional outliers, which included shocks in Siberia, South Africa and Oklahoma (of which, more later).
The Week’s Biggest Earthquake: M6.6, Central Italy
Perhaps the most significant point to emerge from this week’s devastating series of earthquakes in central Italy was that, despite considerable damage, there have at the time of writing been no reported deaths. This comes after the smaller earthquake (M6.2) in the same area which cost around 300 lives.
Earthquakes in central Italy are nothing new. The coming together of Africa and Eurasia and the compression of the Mediterranean Basin produces complex regional tectonics, and even on a national scale, Italy is being subjected to conflicting earth movements. The Apennine mountain range, which runs the length of central and southern Italy, is by no means unused to earthquake activity.
Although the overall motion is one of compression, central Italy’s earthquakes tend to be caused by extension, and are triggered by normal faulting (when one side of a fault slips downwards relative to another). This was the case with this week’s M6.5 and its associated shocks, as well as with the tremors from earlier this year (August onwards) in and around the town of Norcia.
Italy can expect more. This earthquake isn’t the largest the country has experienced, although it is the third largest in the Apennines since 1900, one of nine in excess of M6.0. More worryingly, three have occurred in the current earthquake series — which may not be over.
M5.9 Tremor, Central Chile
Elsewhere in the world, there are other active margins with regular tectonic activity — so regular, that they tend to get overlooked. One such area is the Pacific coast of South America, where an active subduction margin (where the Nazca plate descends beneath the South American continent along the Peru-Chile Trench) generates regular intermediate earthquakes and occasional large — sometimes very large — tremors.
This week the largest earthquake outside Italy was an M5.9 which occurred just off the Chilean coast, around 120km from the port city of Santiago. In some areas — indeed, in Italy — this would have raised headlines. Not so here.
To place it in context, just this one part of the Chilean plate margin from Ovalle in the north to Conception in the south (a distance of around 600 km) has experienced over 110 tremors of at least the same magnitude as this week’s M5.9 in the last half century. Three of them were at least M8.0 — which is over 100 times the size of this week’s earthquake.
US Earthquakes: Oklahoma
We haven’t checked in on Oklahoma for a while. The reason isn’t because there’s nothing happening here but because there’s nothing new happening. The anthropogenic earthquake swarm (related to the injection of wastewater used in the fracking process) continues; and seismologists keep studying it.
It is worth noting when a larger-than-normal tremor shows up. I’ve always taken this to be one which reaches M4.5 and that’s what we saw this week. I should perhaps increase my threshold for what’s noteworthy, because the number of earthquakes breaching that M4.5 threshold does seem to be increasing. I haven’t kept detailed statistics, but somebody, somewhere, will be doing that. I’ll let you know when I find out.
Last Thoughts: Is Surviving an Earthquake the Luck of the Draw?
Residents of the central Apennines have been luckier this month than they were back in August: a larger earthquake cost no lives when a smaller one killed hundreds.
There are two probable reasons for this. The first is related to the condition of buildings (vulnerable properties may have been either destroyed in the first shock or left empty because of damage caused by it). The second is that this week an earlier shock of M5.5 alerted the population to the possibility of further damage and gave them the chance to get to safety.
It’s worth noting that not all earthquakes have that warning foreshock. It’s buildings that kill people, not earthquakes — but the element of luck is in how much, if any, chance the population has to escape.