In the disaster movie San Andreas, the titular Californian fault line ruptures with, literally, Earth-shattering consequences on a scale never before seen in the earthquake-prone area.
It leaves a trail of misery and destruction that stretches from Los Angeles to San Francisco – and causes the film’s stars, Dwayne Johnson, Carla Gugino and Alexandra Daddario, to have a very bad day.
Skyscrapers crumble, fires erupt, tsunami waves swamp the Golden Gate Bridge and, horror of horrors, the letters of the Hollywood sign fall down.
The film offers a somewhat fantastical look at an all-too-real seismic threat. San Andreas has long been considered one of the most dangerous earthquake faults because of its length. At nearly 1,287 kilometres long, it cuts through California like a scar and has been responsible for some of the largest quakes in state history.
In the film, which is out today, another, previously unknown, fault line near the Hoover Dam in Nevada ruptures and triggers San Andreas. Southern California is rocked by an incredibly powerful magnitude-9.1 quake, followed by an even stronger magnitude-9.6 in the north.
Obviously, Hollywood rarely lets the facts get in the way of a good story – or stunning special effects. So US Geological Survey seismologist Susan Hough took a look at the film to judge whether the disaster it depicts is pure fantasy – or a realistic depiction of what might happen when San Andreas rumbles again for real.
Her conclusion is not particularly comforting for those living along the fault line. While some of the wilder excesses of the film are pure Hollywood fiction, including an implausible plot, she says that San Andreas will indeed break again – and without warning.
“We are, at some point, going to face a big earthquake,” she says.
How big?
San Andreas is notorious for producing big ones, but a magnitude-9 or larger, as is the case in the film, is virtually impossible because the fault is not long or deep enough, Hough says.
The most powerful quakes in recorded history happened offshore, when one massive tectonic plate slips beneath another. The magnitude-9.5 quake off Chile in 1960 is the world-record holder.
San Andreas has revealed its own awesome power. In 1906, a magnitude-7.8 reduced parts of San Francisco to fiery rubble. Nearly 50 years earlier, a similar-sized quake rattled the southern end of the fault. In 2008, the USGS led a team of 300 experts who wrote a document detailing what would happen if a magnitude-7.8 hit the southern San Andreas, to help prepare for such an emergency.
The lesson: it doesn’t take a magnitude-9 to wreak havoc. Researchers calculated 1,800 people would be killed and 50,000 injured by a magnitude-7.8. Hundreds of brick buildings and concrete structures, and a few high-rise steel buildings, would collapse.
Computer models show the San Andreas is capable of producing a magnitude-8.3 quake.
Will there be a warning?
In the film, seismologist Lawrence Hayes (played by Paul Giamatti) notices spikes in “magnetic pulses” that herald a coming monster quake.
After a century of research, earthquake prediction remains elusive. Scientists can’t predict when a jolt is coming and are pessimistic about ever having the ability. Every potential warning sign – animal behaviour, weather patterns, electromagnetic signals, atmospheric observations, levels of radon gas in soil or groundwater – has failed.
“We wish it were as simple as the movie portrays – it isn’t,” Hough says. “Researchers have scoured every imaginable signal trying to find reliable precursors. Nothing has panned out.”
The latest focus is on creating early-warning systems that give people a few seconds warning after a quake hits, but before the strong shaking starts.
Tsunami in San Francisco?
Unlike the events in the film, the San Andreas Fault cannot cause a tsunami.
They are triggered by underwater quakes. Giant tsunami waves are formed when the Earth’s crust violently shifts, displacing huge amounts of seawater.
The San Andreas is a strike-slip fault, in which opposing blocks of rocks slide past each other horizontally. A big San Andreas quake can spark fires and other mayhem, but it can’t displace water and flood San Francisco.
More than 80 – mostly small – tsunamis have been observed along California’s coast in the past, triggered mostly by faraway quakes.
Will East-Coast US feel it?
In the movie, the scientist warns that the coming quake will be so strong, the shaking would be felt on the East Coast.
In reality, even the largest possible San Andreas quake could not rattle the East Coast. While seismic waves can make the Earth reverberate like a bell, the ringing can only be detected by sensitive instruments because it is so low.
Drop, cover and hold on?
When the ground starts to shake, the seismologist makes the ideal public service announcement: “Drop, cover and hold on.”
Since 2008, millions of people in California and elsewhere have participated in yearly disaster drills in which they practise diving under a table and learn other survival tips.
If you’re outdoors when the ground moves, experts recommend bracing against a wall, similar to what search-and-rescue helicopter pilot Ray Gaines, played by Johnson, tells scared survivors in the movie.
“Having Paul Giamatti shouting, ‘Drop, cover and hold on!’ and The Rock telling people to crouch against a wall if they can is one heck of a PSA,” Hough says.
