There is no exaggeration in stating that historic heritage is one of the most outstanding and valuable assets of Italy. The smallest villages or the largest cities, all boast hundred- (sometimes thousand-) year old buildings of great cultural, architectural, or artistic interest. Amazingly enough, a vast majority of these ancient palaces and houses are still in use as administration, commercial, religious, cultural or even residential buildings. It is not uncommon to visit a friend who lives in a 19th-century house or have a business meeting in a room that was covered in paintings 350 years ago. My grandparents’ family apartment is part of an architectural ensemble that was originally built in the 1480’s as a church, and the two buildings across the street are 300- and 950-year old.

However, this cultural marvel has a flip side: the most valuable asset of the country is also its heaviest (and perhaps also deadliest) liability. Not one of these buildings meets any kind of seismic design principle. In Italy it usually takes a small earthquake only to lead to a big disaster. Italians know it well, and have to live –or sometimes die– with it.

Many Italians will therefore remember 2016 as a terrifying year, the “earthquake annus horribilis” of the country[1]. Three main reasons to that:

1. More earthquakes hit Italy in 2016 than in most of the previous years.
2. Several earthquakes were of a quite high magnitude.
3. The concentration of earthquakes on a limited area made it even more stressful for the local populations and the infrastructures.

Caveat: Only earthquakes of a magnitude equal to or higher than 2.5 on the Richter scale are considered here.

Number of earthquakes

In 2016, the total number of earthquakes was significantly higher than in most of the previous years: 3,315 earthquakes in 2016 against 1,077 per annum on average (median = 1,052).

Although the destructive power of earthquakes is defined by their magnitude, the psychological effect on local population is also heavily impacted by the total number of tremors. That is most likely the reason why 2016 is considered a very bad year: endless series of earthquakes kept the affected population under a high level of stress for months. The 2 big earthquakes that hit Italy in August and October came along with tens, if not hundreds, of lighter quakes daily; and it took several months after the bigger ones for the daily number of quakes to go back somehow closer to the usual daily average.

The long-term evolution of the average number of daily earthquakes shed more light on the way 2016 is perceived, from 2-to-3 quakes per day on average in years 2013, 2014, 2015 to more than 11 daily earthquakes in 2016 overall. The transition seems even more brutal if we focus on 2016 only, with less than 2 quakes daily on average before August 24th to more than 23 from August 24th onwards. The cumulative number of earthquakes shows in an even crisper way that after almost 9 months of stillness: the end of August and first half of September turned quite shaky all of a sudden. Afterwards, while the situation seemed normalizing over the first half of October, a second, and much stronger, earthquake swarm hit the country.

It is also well-known by psychologists who have patients suffering from post-seismic stress that earthquakes that happened during the night are more stressful for the victims. Being abruptly woken up in the middle of the night, often time in complete darkness because of grid damage, surrounded by a dense cloud of dust in an unrecognizable house is a perturbing and fearful experience. Perhaps this is the reason why populations that are regularly hit by destructive earthquakes tend to believe that the strongest and most violent quakes usually happen during night time. Looking at the data confirms that there is no correlation whastoever between the time of the day/night and the intensity of quakes –it is all a matter of psychological impact on the victims.

Earthquakes magnitude

The Richter scale of earthquakes magnitude is logarithmic –with an increment of 1 on the scale representing an approximate thirtyonefold increase in magnitude. The devastating impact of earthquakes therefore depends much more on a few outliers than on a large number of smaller quakes. Unfortunately, even magnitude-wise, 2016 was not a good year at all.

In a single year, Italy was hit by 5 earthquakes of a magnitude of 5.25 or above, 2 of which had a magnitude equal to or higher than 6, comprising a 6.5 quake on October 30. Nothing similar had happened in ages (at least not since 1985 –which is the oldest year in the dataset).

Earthquake magnitude translates the ground wave amplitude into a number, and ultimately summarises the energy released. It is a common practice to also express the earthquakes energy as if it was caused by the explosion of TNT, although this is not very precise. As we know that a magnitude 1 earthquake releases 1.995262e+6 Joules of energy and that the explosion of 1 ton of TNT releases 4.184e+9 Joules, it is not hard to convert magnitude, energy and TNT equivalent.[2]

In 2016, the cumulative intensity of energy realeased by quakes in Italy was massive compared to previous years:
119,567 cumulated tons of TNT-equivalent in 2016 against 4,402 per annum on average, i.e. approx. 27.5 times more !

Infrastructural damage caused by an earthquake is a function of the energy released (amongst others), which is also affected by the focal depth it takes place at: the deeper, the smoother. No chance, in 2016, the average depth of earthquakes was lower.

Earthquakes geographic distribution

In 2016, the concentration of earthquakes on a limited area of the territory (Central Italy) had 3 major consequences, 2 negative and 1 “positive”:

• the released energy hit the same areas again and again
• each new quake increased the level of stress of the same populations
• many families had their house devastated or damaged by the first big eqrthquake on August 24 and had no other choice than leaving them. They moved either outside the area or to tent cities, making them safer when the 6.5 earthquake hit the same area a few weeks later (October 30) crushing more buildings.

Notes

[1] While I was polishing this paper before posting, 3 bigger quakes (magnitude >= 5) hit the same area of Central Italy on 18 January, causing additional damage.

[2] See this page.

To download the R code and dataset, click here (1.4 MB).