Earthquake reality check: Facts behind "San Andreas"

LOS ANGELES -- The San Andreas Fault awakens, unleashing back-to-back jolts that leave a trail of misery from Los Angeles to San Francisco. Skyscrapers crumble. Fires erupt. The letters of the Hollywood sign topple. Tsunami waves swamp the Golden Gate Bridge.

Hollywood's favorite geologic bad guy is back in "San Andreas" -- a fantastical look at one of the world's real seismic threats.

The San Andreas has long been considered one of the most dangerous earthquake faults because of its length. At nearly 800 miles long, it cuts through California like a scar and is responsible for some of the largest shakers in state history.

In the film, opening this Friday, a previously unknown fault near the Hoover Dam in Nevada ruptures and jiggles the San Andreas. Southern California is rocked by a powerful magnitude-9.1 quake followed by an even stronger magnitude-9.6 in Northern California.

U.S. Geological Survey seismologist Susan Hough accompanied The Associated Press to an advance screening of the film. Despite the implausible plot, she said the San Andreas will indeed break again, and without warning.

"We are at some point going to face a big earthquake," she said.

JUST HOW BIG?

The San Andreas is notorious for producing big ones, but a magnitude-9 or larger is virtually impossible because the fault is not long or deep enough, Hough noted.

The most powerful temblors in recorded history have struck along offshore subduction zones where one massive tectonic plate dives beneath another. The 1960 magnitude-9.5 quake off Chile is the current world record holder.

The San Andreas has revealed its awesome power before. In 1906, a magnitude-7.8 reduced parts of San Francisco to fiery rubble. Nearly five decades earlier, a similar-sized quake rattled the southern end of the fault.

In 2008, the USGS led a team of 300 experts that wrote a script detailing what would happen if a magnitude-7.8 hit the southern San Andreas. They wanted to create a science-based crisis scenario that can be used for preparedness drills.

The lesson: It doesn't take a magnitude-9 or greater to wreak havoc. Researchers calculated a magnitude-7.8 would cause 1,800 deaths and 50,000 injuries. Hundreds of old 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, but anything larger is dubious.

WILL THERE BE A WARNING?

In the film, Lawrence Hayes, a fictional seismologist at Caltech (a real university), notices spikes in "magnetic pulses" that light up California like a Christmas tree, heralding a monster quake.

Despite a century of research, earthquake prediction remains elusive. Scientists can't predict when a jolt is coming and are generally pessimistic about ever having that ability.

Every warning sign scrutinized -- animal behavior, 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. Researchers have scoured every imaginable signal trying to find reliable precursors, but nothing has panned out," Hough said.

The latest focus has been on creating early warning systems that give residents and businesses a few seconds heads up after a quake hits, but before strong shaking is felt.

Japan has the most advanced seismic alert system in the world while the U.S. is currently testing a prototype.

A TSUNAMI IN SAN FRANCISCO?

Unlike the film, the San Andreas can't spawn tsunamis.

Most tsunamis are triggered by underwater quakes, but they can also be caused by landslides, volcanoes and even meteor impacts.

Giant tsunami waves are formed when the Earth's crust violently shifts, displacing huge amounts of seawater. The larger the magnitude, the more these waves can race across the ocean without losing energy.

The San Andreas is 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.

Hough said the movie got one aspect right: The tide suddenly ebbing out signals a tsunami is coming.

More than 80 -- mostly small -- tsunamis have been observed along California's coast in the past, triggered mainly by faraway quakes.

COULD THE EAST COAST FEEL IT?

In the movie, the scientist warned that shaking would be felt on the East Coast.

Even the largest possible San Andreas quake won't rattle the East Coast. (Sorry, New York).

While seismic waves from great quakes can make the Earth reverberate like a bell, the ringing can only be detected by sensitive instruments because it's so low.

Historical accounts show shaking from the 1906 San Andreas quake was barely felt in western Nevada and southern Oregon, Hough said.

DROP, COVER AND HOLD ON!

When the ground starts to shake, the seismologist played by Paul Giamatti 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 practice diving under a table and learn other preparedness 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 Dwayne "The Rock" Johnson, told 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 said.