The news of another strong and deadly earthquake in Italy on Wednesday morning sadly comes as no surprise to seismologists.
Likewise the rising death toll, which was 10 with the first reports, rising to 73 by early afternoon local time, as civil defense workers began to reach the smaller settlements in the worst affected areas.
Photographs of villages like Pescara del Tronto, one of the closest villages to the fault, show the odd semi-intact house sticking up like a bad tooth from piles of rubble.
Yet this was an earthquake of magnitude 6.2 — a strong event, but by no means a great one in global terms. Comparing it to the Tohoku earthquake in Japan five years ago, this is smaller in energy terms by about 27,000 times. This is why the familiar magnitude scale (conventionally miscalled “the Richter Scale,” which is not a scientific term) has no units — the numbers get astronomical too quickly. In fact, on average, earthquakes this size happen somewhere in the world about every three days.
But when, as is most common, they happen in remote areas or far out at sea, they go unnoticed by everyone bar the professional seismologists. Even a great earthquake will not reach the news if it happens far into the Southern Ocean. It’s not just size that makes a quake a killer — location is key.
It is interesting to compare world maps of the largest earthquakes and the deadliest: they look quite different. The largest are mostly strung around the Pacific Rim; the deadliest occur in a belt stretching from the Mediterranean to China. This is where you have both earthquakes and densely populated areas, with people living in houses made of stone. These buildings, often not in the best repair, collapse easily and bury the inhabitants under heavy rubble. Dust is also a killer; victims not crushed may simply suffocate.
One also has to consider an earthquake source in three dimensions: if the causative fault is deep, the shock waves will spread out, to a degree, before they reach the surface. When it is shallow, it delivers a much more concentrated punch. In the case of Wednesday’s earthquake in the Rieti region, the focal depth was only 4 kilometers, according to the Italian National Earthquake Center — a very shallow event.
Central Italy has a history of strong earthquakes. Many will remember well the L’Aquila earthquake, similar in size, of 2009, which occurred to the southeast of the Rieti event. But also, in 1997, there was the Umbria-Marche earthquake sequence, further to the northwest. Both were complex sequences; in the case of the 1997 earthquakes, there was no clear main shock, unlike the normal sequence where the strongest shock comes first, with aftershocks tailing off with time. Interestingly, the aftershocks of the Rieti earthquake, of which there have been around 200 at the time of writing, show a marked NNW-SSE trend, from L’Aquila to Norcia, undoubtedly marking out the responsible fault.
What will not be so well remembered is the earthquake of January 14, 1703, with an estimated magnitude of 6.7, in almost exactly the same place, with the same trending fault, to judge from the distribution of damage. Italy has fewer earthquakes than, say, California, but a lot more in the way of historical documentation, so it is possible to construct an excellent earthquake history stretching back hundreds of years. With that in mind, professional historians are working in Italy alongside seismologists to unravel the picture of the country’s seismicity over time.
In 1703, the January 14 earthquake was followed by another, equally strong, further south at L’Aquila. Hopefully, that will not be repeated this time, given that L’Aquila has had a strong event just recently, but it underscores the complex relationships between earthquakes in this part of Italy.
The ultimate cause is the collision between Africa and Europe, which sets up a complex pattern of stresses and crustal movement along the length of the Italian peninsula. The geological forces are inexorable, and what we see traced in the historical record will doubtless continue for thousands of years to come.
