After the destructive earthquake that leveled Amatrice and the surrounding area this week, we’ve been asked many times if the quake was near our property. Well, it depends on what you’d call “near.”
Amatrice (indicated by the pin) is about 100 km south and a little west of Colmurano (indicated by the star), but the quake was felt throughout Marche. Kevin said his house, near San Ginesio, “rocked several times during that night,” but sustained no damage. (It was built about 10 years ago, following the current building regulations designed to reduce earthquake damage.) He also said that 150 houses in the province of Macerata had already been condemned due to the damage.
Since our property has nothing but a ruin on it right now, I’m assuming we had no damage that will affect us. But since the local authorities are now occupied with property inspections and safety evaluations, we’ll probably be facing some delay in getting our approval to build. (The design itself should be fine, as it was made to meet the latest earthquake safety requirements.)
Having well-constructed buildings is important. As I read, “Earthquakes don’t kill people. Falling buildings kill people.” And tsunamis.
But will we be experiencing future quakes that might affect us? What sort of probabilities are we talking about?
The Fault in Our Ground
A major fault line between the large Eurasian plate and the small Adriatic plate runs down the center of Italy through the Appenini mountain range. There’s also pressure from the large Africa plate in the area. This yields over 1000 earthquakes a year in Italy of various magnitudes, all but a handful small.
Looking at Italy as a whole, the highest risk for dangerous quakes follows that fault. This map shows the area in the mountains near Amatrice in the very dark red, indicating high danger. Our area of Marche is less risky, being in the lightest pink color, but still about the middle for risk.
This closeup map is very interesting, but lacking some key information. It shows a past quake near Macerata big enough to include but of some magnitude less than 5.5, but it doesn’t say when. Also, it estimates the annual risk for an event in our area as about 0.20%, but doesn’t say how big the event needs to be to “count.” That translates to a quake of some significant size (>5.0??) every 500 years.
Or it translates to a probability of 5% in the next 25 years. Compare that to this estimate from the US Geological Survey (USGS): “They concluded that there is a 72 percent probability (or likelihood) of at least one earthquake of magnitude 6.7 or greater striking somewhere in the San Francisco Bay region before 2043.”
I wanted a refresher on both how the scale works and how to translate a magnitude into some meaningful terms.
I was surprised to learn that the USGS hasn’t used the Richter scale since the 1970s. They now use the “moment magnitude scale.” (MMS) It is similar to the Richter scale, both in calculation and in damage assessments.
MMS is measured not on a linear scale but a logarithmic one. In practical terms, this means the differences between the intensity of quakes of similar numbers is a lot more than it seems. For example, a 6.0 quake has 32 times the intensity (energy released) of a 5.0, while a 7.0 has 1000 times the intensity of that 5.0.
Interestingly, the amount of time an earthquake lasts gives a good estimate of its intensity. The quake that struck San Francisco during the 1989 World Series lasted about 15 seconds and measured 6.9. The huge earthquake in Japan in 2011 lasted 4 minutes and was a 9.0. (The highest recorded MMS was in 1960 in Chile: 9.5) Anything over a minute or so is going to be real bad.
The intensity is also linked to the length of the fault line that is displaced. It’s thought that the maximum for a San Andreas quake is about 8.2. That’s big, but small compared to the potential of the Cascadia subduction zone near Seattle: 9.2.
The Amatrice earthquake was measured at 6.2. Here’s a sense of what happens in a 6.0-6.9 quake. It matches this quake fairly well: “Damage to a moderate number of well-built structures in populated areas. Earthquake-resistant structures survive with slight to moderate damage. Poorly designed structures receive moderate to severe damage. Felt in wider areas; up to hundreds of miles/kilometers from the epicenter. Strong to violent shaking in epicentral area.”
Small earthquakes happen all the time. Fortunately. the big ones are rare.
First image: Google Maps
Second image: http://www.geopod.it
Third image: http://zonesismiche.mi.ingv.it/mappa_ps_apr04/italia.html
Fourth image: https://en.m.wikipedia.org/wiki/Moment_magnitude_scale
Fifth image: http://redsismica.uprm.edu/English/education/earthquakes/size.php