“[The 1906 San Francisco Earthquake] does not seem to have affected any one with a sense of final destruction, with any foreboding of irreparable disaster. Every one is talking of it this afternoon, and no one is in the least degree dismayed.” 
– H. G. Wells, The Future in America: A Search after Realities

Within 30 minutes of the opening credits of San Andreas, all of Los Angeles, San Francisco, and the laws of physics are reduced to rubble.

Sitting there dumbfounded—a gaggle of geology graduate students around me, providing commentary in the style of Mystery Science Theater 3000—I wonder what the rest of the movie could possibly be about. The film’s trump card, a colossal magnitude 9.1 earthquake, has already been played. What next?

Two of our main characters contemplate the destruction of the "big one."

Two of our main characters contemplate the destruction caused by the “big one.”

I won’t spoil more than I already have, but I’ll give you a hint: San Andreas is a typical disaster movie. By the umpteenth explosion, jaded movie-goers are blinking lazily and reaching for their phones.

But that’s OK. Both San Andreas and its audience are under no illusion as to what the film is: another adrenaline-steeped summer blockbuster. And sometimes, that’s exactly the kind of fun movie-goers want.

So I won’t criticize San Andreas for not being the two-hour-long IMAX PhD dissertation all geologists secretly desire. (If you’re into that kind of thing, see San Andreas: Hollywood Fantasy Is Not California Reality or  5 Things the San Andreas Movie Got Wrong.) However, the film raises some interesting, real-world questions: (1) What’s the actual risk of living near the San Andreas Fault? And (2) Why is the human race so fond of building in diaster-prone areas in the first place?

At highway speeds, it would take over 12 hours to travel the trace of the San Andreas Fault. Stretching 810 miles from the Salton Sea, southeast of Los Angeles, to offshore of Cape Mendocino, five hours north of San Francisco, the fault separates two tectonic plates: the Pacific Plate, which is moving northwest, and the North American plate, which is headed southeast. Because they’re travelling in opposite directions, coastal California and the rest of the continent slip past each other at an average rate of one to two inches a year. Keep this up, and in about 20 million years, California’s western sliver will become a Pacific island, with San Francisco residing at roughly the same latitude as Portland.

That’s pretty slow pacing for a disaster movie.

Satellite image of California and Nevada with overlaid fault lines. Red traces are modern faults, while orange and yellow faults have been active in the past 15,000 and 130,000 years, respectively. The thick red line is the San Andreas Fault. Landsat image via Google Earth. Fault data from USGS.

This relatively gentle slipping motion is called fault creep, and it doesn’t produce large earthquakes. It’s like two cars inching past each other in a narrow lot, scraping off a bit of paint as they go.

Some parts of the San Andreas experience consistent creep, such as the central portion of the fault, while others hardly slip at all.

That lack of movement, ironically, is when things get risky.

To illustrate, let’s imagine our two cars are trying to inch past each other, but something catches—perhaps their side-view mirrors, or a door handle. They’re stuck.

But our drivers are determined to get out of the lot and into California traffic. They keep on the gas, their cars groaning at the point of contact, stress building up. Eventually, this stress is released—maybe a side-view mirror breaks off—and the cars lurch past each other.

That’s our earthquake.

Like our banged up cars, sections of a fault can become stuck too. The idea is called elastic-rebound theory.  A stuck fault builds up energy and is at high risk for an earthquake. When the fault finally slips, it does so dramatically, an entire landmass grinding past another, several feet in an instant. Thus, contrary to what one might expect, locations along the fault that have experienced a large quake in recent years are considered low risk—they’re unstuck.

A fence near Bolinas, CA, originally straight and continuous, was displaced 8.5 feet by the 1906 quake. Photo by G. K. Gilbert.

A fence near Bolinas, CA, originally straight and continuous, was displaced a modest 8.5 feet by the 1906 San Francisco earthquake. Photo by G. K. Gilbert.

The 1906 San Francisco earthquake created a lateral displacement of over 20 feet across the San Andreas Fault in places. Over 80% of the city crumbled and burned, and 3,000 people were killed.

Since then smaller earthquakes have occurred, such as the 1989 Loma Prieta quake and the 1994 Northridge quake, but could the “big one” portrayed in San Andreas really happen?

As our movie star professor puts it, yes—the “big one” is really a matter of “when, not if.”

Though earthquakes are notoriously difficult to predict, they’re not random. By examining deposits produced by earthquakes, geologists can get a general idea of how often they strike. The San Andreas requires roughly 200 years to build up enough stress to rupture, slip, and cause a major earthquake. But it’s tough to calculate exactly “when.”

Much more certain is “where.”

The northern portion of the San Andreas has only a 6.4% chance of a major (greater than magnitude 6.7) earthquake in the next 30 years: Much of the stress in this part of the fault was released in 1906. However, a similar quake on the nearby Hayward Fault is almost three times more probable, though anything greater than a magnitude 7.5 is unlikely.

The southern portion of the San Andreas, however, is long overdue. It hasn’t seen a major quake for over 300 years, and the stress has been building up. Even with improved building codes and the retrofitting of old structures, when the next “big one” happens—a 19.0% chance of a 6.7 in the next 30 years, and a 17.3% chance for a 7.5—it will devastate the Los Angeles area.

Despite all the exaggerations of Hollywood, the threat of disaster in California is real.

Risk of large (>6.7) earthquakes in California. The Hayward, Calaveras, and Southern San Andreas are the likely suspects for future large quakes. From UCERF3 2014 Report.

Risk map of large (>6.7) earthquakes along California faults. The Hayward, Calaveras, and Southern San Andreas are the likely suspects for future quakes. From UCERF3 Report.

But this isn’t one of those things that happens only in California. Almost every place in the U.S. is vulnerable to some sort of natural disaster. Floods. Volcanoes. Hurricanes. Tornadoes.

All along the east coast of the U.S., we build resort towns on barrier islands, so named because they protect the mainland from storms and hurricanes. We build cities in the floodplains of rivers, which are, as the name suggests, created by large floods.

Behind the Seattle skyline, Mt. Rainer looms, waiting to bury nearby Tacoma with lahars.

Pick a point on the map at random, and chances are it has all the makings of a disaster movie.

Disaster map for the United States. Produced by crisishq.com using data from Redcross.org and Noaa.gov.

Natural disaster map of the United States. Produced by crisishq.com using data from Redcross.org and Noaa.gov.

So what compels us to build on the fault-line, in the tsunami zone, in the floodplain, in the shadow of the volcano?

Perhaps I’m biased, but it’s the same reason these areas are so susceptible to disaster: geology.

At first glance, the locations of large cities seem logical: by a river, lake, railroad, or coast, for ease of transportation and trade. But why doesn’t every little river town and railroad stop become a metropolis?

Los Angeles in the 1800s was one of these small towns. Though it had the railroad by 1885, it mostly just had farms. But in 1892, oil was discovered near present-day Dodger Stadium. By the 1920s, LA was producing 25% of the world’s oil and had surpassed San Francisco as the largest U.S. city west of the Mississippi River.

The extent of the old LA oil field. Dodger Stadium is visible near the upper right of the field. Modifed from "LosAngelesFieldDetail" by Antandrus at en.wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:LosAngelesFieldDetail.jpg#/media/File:LosAngelesFieldDetail.jpg.

The extent of the old LA oil field. Dodger Stadium is visible near the upper right of the field. Modified from “LosAngelesFieldDetail” by Antandrus at en.wikipedia.

San Francisco itself has a similar story. Despite boasting a natural harbor, the result of downwarping between the San Andreas and the Hayward Faults, San Francisco remained a small town until the 1848 gold rush. In just one year, its population increased from 1,000 to 25,000.

But again, this isn’t one of those things that happens only in California:

Oklahoma City, oil. Indianapolis, natural gas. Denver, gold. Houston, oil. Charlotte, gold. Pittsburgh, coal.

Towns may spring up because of railroads or rivers, but it’s often geology that turns them into cities.

So, Los Angeles’ and San Francisco’s locations don’t reflect some innate, masochistic fondness for natural disasters, despite the public’s fascination with the film genre. Rather, the places we build reflect another of our obsessions: natural resources.

But the Bay Area, now a hub for tech companies, mines data, not gold. The Los Angeles of today produces movies, not oil. And the San Andreas Fault bides its time.

How do Californians feel about all of this? Results of a 2011 survey indicate that CA residents actually overestimate the earthquake risk compared to the official UCERF3 report: 56% of participants expect a major earthquake within the next 10 years.

However, these same residents don’t think this major quake would be all that big of a deal: Over 68% indicated the danger posed by earthquakes in California is on par with that posed by other natural disasters elsewhere. Twenty percent claim the rest of the U.S., with its hurricanes and tornadoes and floods, actually has it worse.

Does the general public not comprehend the danger? Or are they just playing the odds?

At the end of San Andreas, our survivors huddle together on the Marin Headlands, surveying the ruins of San Francisco below. Amidst the unfurling of an American flag, Dwayne “The Rock” Johnson’s character has the last word: “Now we rebuild.”

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