In the first five months of this year there were five quakes of more than magnitude 4.0, the first time that has happened since 1994, sending jitters through the city.

“A big earthquake is inevitable,” Los Angeles’ recently appointed “quake tsar” Dr Lucy Jones, told The Telegraph. “Each earthquake that happens increases the probability. We know a lot about earthquakes, we know it will happen. We know everything but the time.”

It is not unlikely that Dr Jones, 59, a US Geological Survey seismologist, may herself feature as a character in San Andreas, the movie.

^{Dwayne Johnson, whilst filiming the San Andreas blockbuster (Rex)}

Universally known as the “Earthquake Lady” she rose to fame 20 years ago following the 6.7-magnitude Northridge earthquake , which killed 57 people and caused $20 billion in damage.

In the 1990s she once appeared before TV cameras holding her one-year-old son and successfully calmed a panicking city. Her informative tweets are now avidly followed by fretting Angelinos.

After persuading her to take on the lead role in preparing for the “Big One” the city’s Mayor Eric Garcetti said: “If I had to create an expert out of clay, I couldn’t have done better.”

But Dr Jones is worried. And when she worries, everyone else should. What is particularly keeping her up at night is the number of old concrete buildings that have not been retrofitted and could topple, or collapse inwardly into a pancake. The expensive retrofitting process involves reinforcing buildings, for example with steel braces.

“LA was built in the 50s and 60s and all of the buildings from that time have some real problems,” she said. Dr Jones herself would never live in one.

According to a recent study by the University of California there are 1,451 concrete buildings that have not been retrofitted, including about 50 hotels, 50 churches, and 25 nursing homes.

It has been estimated that 5 per cent of these, about 75 in total, would collapse in a large earthquake, but it cannot be predicted which ones.

^{A model of Southern California showing the motion of the Pacific and North American plates, and the big bend of the San Andreas fault where the plates squeeze together (www.earthquakecountry.info)}

“We are looking at what would happen in an earthquake and taking it all the way through to evacuation,” said Dr Jones. “The problem with Southern California is we have 23 million people.

“Even if we were able to predict an earthquake in LA and evacuate, how many people would be killed on the freeways trying to get out?”

Despite that an enormous decades-long prediction effort has been going on.

Three hours north of Los Angeles the San Andreas fault passes through the tiny town of Parkfield, population 18, which has bestowed on itself the title “earthquake capital of the world”. Visitors are greeted with a sign that reads “Be here when it happens”. The town’s bridge is cracked and bent, showing the slipping of the fault.

Parkfield is the most closely monitored earthquake zone in the world because of the regularity and consistency of its shaking. Between 1857 and 1966 there were six successive 6.0-magnitude earthquakes, averaging one every 22 years.

^{The San Andreas fault, Parkfield, California (Getty)}

Since the 1980s a battery of sensors has been installed, including in a hole drilled a mile and three quarters down into the San Andreas fault.

According to the USGS the aim is to achieve a “better understanding of what happens on and near a fault during the earthquake cycle” and to “aid in predicting the time and severity of future quakes”.

But as yet scientists are still waiting for a revelation.

“There’s a lot of people scratching their heads and trying to come up with a method (of prediction). It’s a learning process,” said Tim McCrink, a senior geologist with the California Geological Survey.

“If you had a view down to the eight-to-10 mile depth of rock, and had some way of measuring the stress in the rocks at those depths, prediction would be a more easy thing to do, but we can’t. We just can’t see it. It’s beyond our ability to measure at this point.”

There are hundreds of faults beneath California, with previously unknown ones still being found. It recently emerged that a $200 million shopping centre and a skyscraper in Hollywood are being built on top of newly discovered faults.

California Governor Jerry Brown wants to throw millions of dollars at mapping all the faults, and he puts the chance of a catastrophic earthquake in the foreseeable future at “50-50”.

^{Jerry Brown, California Governor (Rex)}

In March a 5.1-magnitude quake, Los Angeles’ biggest of the year so far, occurred on the Puente Hills fault, which was only discovered in 1999. The shaking shut down rides at Disneyland.

Seismologists now say a 7.5-magnitude event on the Puente Hills would be “the quake from hell” because it runs right under downtown Los Angeles.

They have estimated that would kill up to 18,000 people, make several million homeless, and cause up to $250 billion in damage.

According to Dr Jones it would “hit all of downtown” which is littered with concrete buildings that haven’t been retrofitted. Such a quake on that fault is only likely once every 2,500 years, but no-one has any idea when the last one was.

The San Andreas, by contrast, is 30 miles away from downtown Los Angeles and even an 8.0-magnitude quake on it would be expected to kill far fewer people. Hollywood might more accurately have called its film “Puente Hills”, but it probably didn’t have the same ring to it.