Although the film “San Andreas” has been largely panned by seismologists for its accuracy in describing the “Big One” expected to hit Los Angeles, debate is ongoing over whether the catastrophe shown in the movie could actually happen. The latest disaster flick shows a massive earthquake caused by a shift in the San Andreas Fault, which forms the tectonic boundary between the Pacific Plate and the North American Plate. Angelenos have witnessed a series of small temblors this year along the Inglewood and Newport Beach fault lines which have rattled a few nerves.
A new study, published this week in the Journal of Geophysical Research: Earth Science, has scientists warning that while the extent of the damage shown in the movie is unlikely to happen in real life, there are several long faults with the potential for tsunami-generating earthquakes within 90 miles of the west coast. Mark Legg runs a Southern California consulting firm called Legg Geophysical and was the lead author of the study. He said the many active faults offshore could produce greater than magnitude 7 earthquakes and tsunamis. He added that, unlike in the movie, such disasters would probably be caused by the little known “California Continental Borderland” which is an undersea landscape off the coast of Southern California and northern Baja California.
Legg’s team has conducted surveys of the region and has found a “complicated logjam” of faults produced by the movement of the Pacific Plate, which is steadily sliding northwest relative to the North American Plate. Legg and his colleagues collected data about seafloor depth to work out the lengths of two of the largest faults in the logjam—the Santa Cruz-Catalina Ridge Fault and the Ferrelo Fault, the later situated farther out to sea.
“What (Legg’s team) was searching for are signs—like those seen along the San Andreas—that indicate how much the faults have slipped over time and whether some of that slippage could cause some of the seafloor to thrust upwards,” according to the American Geophysical Union, which publishes the Journal of Geophysical Research. While analyzing the data, researchers found that the blocks of crust were being subjected to vertical movements in the region because of a phenomenon known as “transpression” which happens when faults slip horizontally relative to each other. Such a process is believed by scientists to have created the Transverse Ranges in southern California and has the potential to cause the seafloor to rise and send a tsunami-generating pulse toward the shore. If a tsunami were to hit southern California, experts believe that low lying areas such as Huntington Beach would be the first to witness catastrophic flooding.
“Hollywood usually exaggerates for effect, and the new movie is no exception,” said Dr. Lucy Jones, seismologist at CalTech in Pasadena and a familiar face to Angelinos following any sizeable temblor. “Magnitude 9 earthquakes—as seen in the movie—occur only in subduction zones which are places where tectonic plates collide, pushing one plate under another and deforming the sea floor to create a tsunami.” Jones said it has been millions of years since there was an active subduction zone under Los Angeles or San Francisco. The modern San Andreas Fault, she explained, “maxes out” at about magnitude 8.3 and it’s mostly on land. It will never produce a big tsunami.
“The gapping chasm viewers see rupturing the San Andreas in central California belongs to the realm of the completely impossible,” Jones said. “If the fault could open like that, there would be no friction—and without friction there would be no earthquake.”
Jones said that the portrayal of emotions in the film is highly plausible. “Loss of communication brings fear to families, while the knowledge of how to protect yourself and those around you reduces that fear and boosts the chances of survival. It’s good that the movie shows that knowing the fundamentals of first, how to ‘drop, cover, hold on’ and that tsunamis are preceded by a draw-down of the ocean provides valuable information during an emergency. Knowing that [telephone] landlines will still work when cell phones are down, and having a ‘plan B’ all can make a difference in making life easier and safer after a big earthquake.”