have played out on the silver screen for decades,

terrifying viewers with quakes that can collapse skyscrapers

or topple entire cities.

Here's what will happen if the big one hits the West Coast.

On July 4, 2019, Ridgecrest, California,

was hit with a 6.4 magnitude earthquake

and then a 7.1 just one day later.

But neither of these compare to the long-awaited big one,

which scientists predict will eventually

rattle the golden coast.

But when it hits, what will that actually look like?

Here's what experts say could happen

in the seconds, hours, and days after the big one.

While experts can't know exactly when a quake will occur,

they have a pretty good idea of where.

California is located in a hot zone of fault lines,

the most notorious of them the San Andreas Fault.

John Vidale: You know, here in California you have dangers

from a number of different kinds of earthquakes.

The major danger is from the earthquakes

on the San Andreas Fault system.

Narrator: On average, the San Andreas Fault

ruptures every 150 years.

The southern parts of the fault have remained inactive

for over 200 years.

Vidale: We haven't had a big earthquake in

Southern California really since 1857.

Narrator: In other words, we're overdue for a major shake.

According to a 2008 federal report,

the most likely scenario is a 7.8 magnitude quake

that would rupture a 200-mile stretch

along the southernmost part of the fault.

Vidale: It's basically moving the ground several yards

over an area of 50 square miles.

So the power of a magnitude 7.8 earthquake

is probably close to the power

used in the whole state for a year.

Basically something that we as a civilization

have trouble creating, short of, like, a nuclear explosion.

Narrator: If you are near the epicenter of the earthquake,

it will be nearly impossible to stand.

Vidale: People have this idea of running out of bed,

out of their buildings, and that's a terrible idea,

because a lot of what we see in earthquakes is people

with broken legs and people who've run through glass.

The best thing to do, like we always say,

is duck, cover, and hold.

Get under some piece of furniture.

The main point is to protect your head and chest.

Narrator: During and immediately following the shaking,

buildings could collapse.

John Wallace: The number of buildings that were constructed

before about 1980 is really significant,

and most of these buildings

are very vulnerable to damage and collapse.

Narrator: In this time-lapse video,

you can see how building components would hold up

in a high-magnitude earthquake.

Wallace: 'Cause the San Andreas will produce

the kind of long-period shaking

which would be very damaging to very tall buildings,

say, in downtown LA, and Century City,

and Long Beach, and so forth.

Older steel buildings, the connections in them

have not necessarily been designed

to withstand the maximum forces

that actually can be generated.

Narrator: Unreinforced structures are the least stable,

but even buildings up to code could crumble.

John Stewart: The building code, with its

minimum requirements, does not ensure that the building

will be serviceable after an earthquake.

It's intended to not kill anybody.

There's a sense that if it's modern, code-designed,

it's earthquake-proof and everything should be great,

but that's not the reality.

Narrator: Five steel high-rises

could collapse completely, while 10 others

will be red-tagged, or unsafe to enter.

And, no, the quake would not cause a tsunami,

despite what movies would have you believe.

Vidale: To trigger a tsunami, it takes an earthquake

that moves the ocean floor,

and most of the San Andreas is on land,

so there would be a little bit of waves generated

from a San Andreas earthquake,

but nothing that would be dangerous.

Narrator: The quake could kill about 1,800 people

and leave 50,000 or more with injuries.

While people could die from falling debris

and collapsed structures,

the highest death toll would be from fires.

Vidale: Historically, they biggest hazard

from earthquakes has been fire.

In the 1906 earthquake there were 3,000 or 4,000 people

who were just caught in that wave of fire

that swept through the city.

Narrator: The aftermath of the big one

will wreak havoc on infrastructure and the economy.

Scott Brandenberg: Below our streets and our buildings

is this really complicated network of infrastructure

that could be damaged, and a lot of the things

we take for granted every day

won't be available anymore, right?

Like water, electricity,

being able to drive where you need to drive.

Narrator: Parts of the San Andreas Fault

intersect with 39 gas and oil pipelines.

This could rupture high-pressure gas lines,

releasing gas into the air

and igniting potentially deadly explosions.

Stewart: So, if you have natural-gas lines that rupture,

that's how you can get fire and explosions.

Narrator: And after the fires burn out,

one of the biggest concerns in a major earthquake

is access to fresh water.

The major aqueduct networks

that pump water into Southern California

all cross the San Andreas Fault

and could be seriously damaged.

Stewart: So we would be without the lifelines

that bring in imported water to the region.

They cross through tunnels,

cross through aqueducts near the surface.

All of these would be ruptured,

and so we would be losing 60% of our water supply.

Many of these distribution lines for water

are near sewer lines, which would also be broken,

so now you have a situation where contaminants

are potentially getting into the water supply.

Narrator: Experts say you should keep

at least a two-week supply of water in your home.

As the ground shakes and sediments shift,

there will be landslides throughout Ventura

and Western Los Angeles County.

Brandenberg: There could be thousands of landslides.

There have been earthquakes that have produced thousands.

Landslides definitely can cause fatalities, property damage.

We have a lot of people who live up in the hills. Right?

So that's the location where you would be likely

to see landslides affecting people.

Narrator: And finally, the big one

will severely impact the economy.

Major transportation networks, like highways and railways,

could be unusable for weeks and even months.

Brandenberg: Some bridges may not be passable

after an earthquake.

We've had bridges collapse during past earthquakes.

Stewart: You might start seeing

key industries leave, population loss,

and this could have, you know,

devastating long-term impacts for the region.

Narrator: The estimated financial cost

of the big one is a whopping $200 billion,

with $33 billion in building damages

and $50 billion in lost economic activity.

This all sounds pretty bad, but keep in mind

that this is based off of a worst-case scenario.

The true impact of a major earthquake

is based on a range of unknowable factors.

Also, smaller earthquakes on faults

directly beneath major population centers

are a serious concern.

Vidale: But the worst-case earthquakes

are hard to predict.

You know, that earthquake in Japan in 2011,

their cost almost entirely came

because their nuclear power plant melted down.

It's very hard to predict what's gonna fail

in a big earthquake.

Narrator: So, how can Californians

prepare for the big one?

Brandenberg: Really have a plan in place.

You know, where are you going to meet?

What are you going to do?

Have water ready.

I have a 55-gallon drum full of water.

There's some chemical additive I put in it

so it's potable for five years.

Fifty-five gallons is the right amount for my...

I have a family of four.

That'll last us for two weeks.

Canned food.

You know, you have to be ready.

I would say it's best just to plan

to stay sort of where you are.

Getting out of LA is bad enough

without an earthquake, right?

Traffic's already terrible.

If roads are closed and people are all trying to leave,

it's gonna really be bad.