The Big One: la minaccia silenziosa che si annida in California

Attraversata dalla faglia di San Andreas, la California vive sotto la minaccia del “Big One”, un grande terremoto dagli effetti devastanti che potrebbe colpire in qualsiasi momento.

Molly Malcolm

Speaker (American accent)

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The Big One terremoto California

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California doesn’t have more recorded earthquakes than any other US state; it has the third most, after Alaska and Oklahoma. But it does have the most destructive earthquakes, because it is so populous. The 1906 earthquake in San Francisco had a 7.8 Richter magnitude and was the most destructive earthquake in US history. It caused fires that destroyed much of the city and killed about three thousand people. In 1994, another earthquake with a magnitude of 6.7 struck a suburb of Los Angeles. It caused billions of dollars of property damage and killed more than sixty people.

Terremoto San Francesco 1906


Most earthquakes occur along cracks in the crust of the Earth called faults, and there are hundreds of active faults in California. But the one that people worry about the most is the San Andreas Fault, which is about 750 miles long and ten miles deep. It forms the tectonic boundary between the Pacific Plate and North American Plate, and is about thirty miles from Downtown Los Angeles, which is at the centre of the most populous city in California, home to about four million people.


The San Andreas Fault is capable of causing an earthquake reaching 8.2 in magnitude, which experts say would cost the state billions of dollars in damage and potentially kill thousands of people. Unfortunately, there is no way to predict or prevent earthquakes, but there are ways to minimize the damage and number of people killed. This is one of the objectives of scientists at Caltech Seismological Laboratory, which was established in 1921 and is an international focal point for the study of earthquakes.


Between thirty and sixty earthquakes a day are recorded at the Caltech Seismological Laboratory in Southern California, although most go unfelt. Earthquakes detectable by people, of a magnitude of 3.5 and up, take place about once or twice a week, while larger ones, of up to five in magnitude, occur from three to five times a year, and can cause damage to property. Jennifer Andrews is a seismologist working on the Earthquake Early Warning project at the Laboratory. When she met with Speak Up, we began by asking her what factors determine whether you can feel an earthquake or not.  
Jennifer Andrews  (English accent): One of the big factors is ... are you in a building, are you stood on soft soil or are you on hard rock? So hard rock tends to shake less. Here in Southern California, unfortunately, we have a lot of soft soil basins. Los Angeles is a very good example. It’s got something like five kilometers of soft soil, so it shakes like a bowl of jelly.


According to Andrews, while science has advanced, it is still very difficult to predict an earthquake. 

Jennifer Andrews: We don’t think we’ll ever be able to predict them. And scientific experiments have been done on everything from the atmosphere, gases being released, electronic signals, magnetic signals, animals – they’ve put animals in boxes waiting to see if they respond before earthquakes. So everything’s been studied but nothing’s reliable to the extent that you can say, “Right, tomorrow at this time we’re going to have an event that’s this big”, which is what’s useful. That’s a useful prediction. But we can’t do anything like that and it’s unlikely we ever will. 


The best we can do, she said, is to be prepared for when a major earthquake does happen. 

Jennifer Andrews: Preparedness is a hugely important program here in Southern California. It’s encouraging people to have water and food, encouraging them to have a plan with their families, especially for communication in the immediate aftermath, certainly of the Big One. We will have cell towers down, so a lot of infrastructure missing. So encouraging people to know what their plan is, where will they meet, what will their family do. That’s really important.


Andrews went on to talk about the science behind the Early Warning project. 

Jennifer Andrews: We have a very dense and extensive network of seismometers here in Southern California and in fact across the west coast. And so those instruments pick up the ground moving. They’re located close to all of our active faults and they pick up the ground moving very, very early. And if we can process that information fast enough, we can send that information electronically faster than the seismic waves themselves are moving out away from the earthquake. So we can tell people that the shaking is about to come before it happens, before it reaches them.

But it’s all a game of speed, because we only have a few seconds, basically. But a few seconds are very, very important, so the advice to everybody is drop, cover and hold on. Get under a table, get under a desk, so that nothing can fall on you. In terms of automated actions, the program is working with transportation systems, things like slowing down high-speed trains. Things like hospitals have systems with flashing lights, so the surgeon is taking things like scalpels out of bodies before the strong shaking arises. There’s an awful lot that can be done in that time, both to reduce injury and to reduce damage.

scala di magnitudo terremoto


In comparison to most earthquake-affected zones in the world, wealthy California is well prepared. Andrews was confident that in the event of a large quake, Los Angeles would still remain standing.

Jennifer Andrews: The duration of shaking here in Los Angeles, on our bowl full of jelly, could be up to two minutes, so that’s two minutes of strong shaking. Most buildings are built to save life, so that means that in theory, unlike in the movies, we shouldn’t have complete collapse, we shouldn’t have huge skyscrapers coming down, but they will be damaged. So, when the Big One comes, we’re talking about utilities, power and water being offline for some days to weeks. And then of course there’s the huge rebuild effort.


But the greatest danger is not from nature, says Andrews, but from manufactured structures. 

Jennifer Andrews: There is a saying around here that earthquakes don’t kill people, buildings do. So it’s very much the built environment that’s our danger. You see some of the devastating reports that come out of, say, Nepal, from the recent Kathmandu earthquake, where you have complete building collapse. We’re not in the same position. Our biggest hazard here is usually secondary structures in buildings, so ceiling panels coming down, glass shattering, masonry falling off of walls ... So things hitting people, and that’s why we tell people to drop, cover and hold on.

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