San Andreas — The Movie: Facts and Fallacies
Let’s get to the main point right away: There is no chance of a magnitude-9 earthquake devastating San Francisco or Los Angeles.
Earthquakes are caused by the sliding of crustal blocks: The more area involved in the sliding, the larger the earthquake.
The area that slid during the catastrophic earthquake in the Tohoku region of Japan in 2011 was 300 miles long and extended as deep as 120 miles into the Earth. That produced a mega-earthquake—a magnitude-9.0 event—that generated a giant sea wave that swept over the nearby coast and drowned 16,000 people. The wave also crippled the Fukushima Daiichi Nuclear Power Plant, causing the release of radioactive material. The San Andreas fault in California has ruptured for 300 miles—most recently in 1906—but the fault extends only 6 miles into the Earth. And so the largest earthquake possible along the San Andreas fault is a magnitude-8.2 event, a major catastrophe, but not the mega-earthquake that recently struck Japan.
Also, unlike one of the scenes depicted in the movie “San Andreas,” there is no splitting open of the ground as great chasms. Again, earthquakes are caused by a sliding of crustal blocks, and so there can be a great shifting of the ground—during the 1906 San Francisco earthquake, which was a magnitude-7.9 event, the ground shifted as much as 20 feet—but there will be no great gashes or ripping open of the ground surface.
And earthquakes do not produce 1000-foot sea waves. The sea waves produced by the 2011 Tohoku earthquake were as high was 130 feet. In comparison, the largest wave produced by the 1906 San Francisco earthquake—which was a rupture of the San Andreas fault—was four inches.
And now onto what the movie “San Andreas” got right.
Early in the movie Dwayne “The Rock” Johnson is piloting a helicopter over Los Angeles and sees seismic waves sweeping across and undulating the cityscape. That could be real. There are many credible reports of people seeing seismic waves race across the ground. In 1906 Police Sergeant Jesse Cook was standing in front of the Levy Produce Company talking to the proprietor’s son when he heard a low rumble. He looked up Washington Street and saw the street rise, later describing it “as if the waves of the ocean were coming towards me.” The waves swept under him, and the surrounding buildings began to shake. It took almost a minute for the shaking to stop.
The possibility of multiple earthquakes is also real. (For example, the 1971 San Fernando earthquake north of Los Angeles consisted of three distinct shocks that occurred over ten minutes. And in 1987 near the Salton Sea in southern California two magnitude-6 earthquakes occurred 12 hours apart.)
One of the things seismologists have learned recently about earthquakes is that they do not occur randomly and they do not reoccur like clockwork. Instead, earthquakes—even large ones—cluster in space and time. And when a cluster of large earthquake occurs—whether over days or many years—it is an earthquake storm.
This idea was first proposed in 1992 by Professor Amos Nur of Stanford University. The best example of an earthquake storm occurred between 1939 and 1999 along the North Anatolian fault in northern Turkey when 13 major earthquakes occur. And there are many similarities between the North Anatolian fault and the San Andreas fault.
Both are boundaries where tectonic plates are sliding passed each other horizontally. In Turkey the boundary is between the Anatolian plate, which includes most of the country of Turkey, and the Eurasian plate. In California the boundary is between the North American and Pacific plates. And, in Turkey and California, the rate the plates are sliding passed each other is the same, about 1.5 inches a year. This may not seem like much, but after 100 years, that’s 150 inches—twelve-and-a-half feet—that has to bee accommodated by sliding during a large earthquake.
But is there evidence that an earthquake storm has ever happened along the San Andreas fault?
Recent research shows that between the years 1720 and 1760—just before the first Spanish missions were established in California—there were at least six major earthquakes in the San Francisco Bay area. These six earthquakes ruptured a variety faults, including the San Andreas fault, the Hayward fault, the San Gregorio fault, the Rogers Creek fault and the Calaveras fault. And each earthquake was equal to to greater than the 1989 Loma Prieta earthquake (also known, informally, as the World Series earthquake because it occurred just before the third game of the 1989 World Series at Candlestick Park south of San Francisco) that damaged the Bay Bridge, collapsed the two-level Interstate 880/Cypress Viaduct in Oakland, broke gas mains in the Marina District of San Francisco, causing fires, and devastated much of Santa Cruz.
But what of the severity of the ground shaking shown in the movie “San Andreas”? Is that possible?
Yes, it is.
During a large earthquake the ground surface can accelerate at more than 1 g, that is, at more than the gravitational acceleration we always feel as we walked around sleep or stand. That means, during a large earthquake, you might feel more than twice your body weight one moment, then be in free-fall the next moment. It is the accelerations that you feel during an earthquake. And it is the accelerations that cause buildings to crumble and bridges to collapse.
And that brings us to a major question.
As any seismologist will tell you: Every major earthquake has a surprise. And the surprise that is waiting during the next rupture of the San Andreas fault is this: Will the tall buildings in Los Angeles withstand the violent accelerations?
Los Angeles and its surroundings pose a special challenge to engineers who try to design buildings and bridges to withstand earthquake shaking because Los Angeles and its surroundings stand on more than a mile of ocean sediments. And the question is whether these sediments will amplify shaking from a large earthquake.
Furthermore, the amount of acceleration and the duration of shaking will depend not only on which segment of the San Andreas fault ruptures—or whether one of the many faults that crisscross the Los Angeles basin ruptures—but also the direction of the rupture.
Think of it this way. You are in a speedboat heading straight across a quiet lake. If you look over the side of the boat, you see identical bow waves on each side. Now turn the speedboat to the left. What happens? The bow wave on the left side of the boat increases greatly in height, while the bow wave on the right side diminishes. The same happens with earthquake waves.
An earthquake begins with the ground rupturing at a spot, then the rupture grows by running down the length of the fault. The rupture is the speedboat. The earthquake waves behave like the bow waves. if the rupture makes a turn, then the earthquake wave on the side of the turn increases in height.
And so the amount of ground acceleration—and, hence, destruction—expected in Los Angeles by a rupture of the San Andreas fault depends on whether the rupture originates in the north and proceeds south or starts in the south and runs norths. It is the latter that is of greater concern.
Running from south to north—from, say, Palm Springs toward Lancaster and the Mojave Desert—the San Andreas makes a sweeping left-hand turn. That will cause earthquake waves to intensify in the Los Angeles basin. (In addition, they will also reflect off the solid mountains, such as, the San Gabriel Mountains and the Santa Monica Mountains, as the waves pass through the ocean sediments to cause the basin to shake, as one noted seismologist has said, “like a bowl of jelly.”) The shaking could go on for as long as two minutes, causing the tall buildings to sway back-and-forth a dozen or more times.
Which brings us to the punchline: Even though a tremendous amount of power is released, earthquakes are survivable. It requires planning and knowledge.
And remember, as a promo for San Andreas says: We always knew this day would come.