Earth Quackery: San Andreas
There’s no better time to experience epic catastrophes on the big screen than summertime. This season’s first major disaster picture, San Andreas, promises all that we’ve come to expect from the genre: crashing high-rises, massive tidal waves, panicky scientific experts, and The Rock coming to the rescue on a gravity-defying helicopter. San Andreas also envisions what many Californians have feared for decades: “The Big One,” a giant earthquake along the San Andreas fault that would cause massive damage and destruction to the region. But as is typical of Hollywood event movies, San Andreas goes to extremes, imagining scenarios and calamities that don’t reflect the geological science of the actual fault.
Sloan Science and Film spoke with Dr. Kate Scharer, a geologist from the Earthquake Science Center at the U.S. Geological Survey, about the historical record and the projected future of the San Andreas fault, and what we know and don’t know about large ground-rupturing earthquakes.
Sloan Science and Film: First off, what are your specific areas of research?
Kate Scharer: A lot of my research is in paleoseismology. I try to understand the history of earthquakes on a fault, by using the geologic record to see earthquakes that happened before the written historic record. In California, that’s basically anything before the early-to-mid 1800s. The last earthquake on the southern San Andreas fault was in 1857. The question I’m trying to answer is: We know there was an earthquake 158 years ago, but how often do they happen? That’s my fundamental research question: The timing and size of large ground-rupturing earthquakes.
SSF: You called them ground-rupturing earthquakes. But they don’t really crack open the earth as we see in the movies, right?
KS: Right, the better way to put it is that they tear the ground. An earthquake doesn’t create a giant chasm; instead it produces a big tear—like tearing a piece of paper in half, it creates a jagged edge on the ground surface. In my work, I dig trenches across the fault, and we look into the layers of soil and sand that were laid down over time. When one of these damaging earthquakes happens, it rips or tears apart the ground, and then that is covered by new layers that are not ripped up.
SSF: So there’s this statistic that may be worrying to some that another large earthquake has a 60% chance of happening in Los Angeles within the next thirty years. How do you know that?
KS: There are several paths that lead to that USGS forecast, which is for Los Angeles as a region. For San Andreas fault, we know from paleoseismology that, on average, it ruptures every 100 years, based on records we have that span over 3,000 years. But the timing not perfectly periodic, like a clock. Rather, the pattern is what is called “quasiperiodic”—it has a jazzy beat. Sometimes, it’s fifty years; sometimes, it’s 250 years, But on average, it’s about 100. Overall, the data shows there are not huge clusters, like eight in a very short period, nor are there long gaps with no earthquake. And the last large rupture on the southern portion of the San Andreas was in 1857.
SSF: What we do know about magnitudes?
KS: The magnitude is proportional to the length of the fault that is ruptured. If the fault ruptures along a thirty mile patch, that’s equivalent to about a magnitude of 7.0. A 180 mile rupture would be about a 7.7. We use paleoseismic data from sites along the fault to see when neighboring sites ruptured at the same time. We find the past southern San Andreas ruptures are typically around magnitude 7.5.
SSF: So if the whole San Andreas fault is roughly 800 miles, could we have a massive mega-earthquake along the entire fault?
KS: It’s an intriguing question. What’s interesting about the San Andreas is that in the middle, there is the creeping section around Parkfield, California. And instead of the strain building over time and then releasing in big earthquakes like 1906 or 1857, it slips constantly in this creeping section. But in the 2011 Japanese earthquake, the rupture itself penetrated into a creeping section. So the question of big earthquakes rupturing through creeping sections is on the radar of researchers. But that’s not the expectation for the San Andreas fault. Historically, the 1906 San Francisco earthquake ruptured down and ended before getting to the creeping section, and the 1857 earthquake it started south of the creeping section, but did not rupture into it. So in the historic record, the creeping section is keeping up with the tectonic plate rate.
SSF: So just to clarify, there isn’t much of a possibility of a quake along the entire San Andreas fault?
KS: In the models, the odds of a big long rupture are miniscule. But it is still an area that people are exploring. When you look at the 2011 Japanese earthquake, it did things that we didn’t expect, so it makes you reassess the behavior of creeping sections. But in theory and historically, if the strain is released slowly by creeping, it isn’t building up to a massive rupture.
SSF: What about these giant tidal waves that we see in earthquake movies? Is that commensurate with San Andreas?
KS: To make a tsunami, you have to displace the ocean floor up or down. There has to be a fault that comes out on the surface of the ocean floor, so when the fault moves up, you push the water up. Or they can be produced by large submarine landslides, so if a quake happens and it triggers a landslide in the ocean, you can get a tsunami. But the San Andreas doesn’t intersect with the ocean for much of its path, and it moves the plates side to side, not up and down, so it’s not a tsunami-generating fault. The 2011 Japanese earthquake produced a tsunami, because the land under the ocean moved up, vertically.
SSF: How good are we at predicting earthquakes?
KS: We don’t. The number you mentioned earlier, a 60% chance of earthquake in the next thirty years—that’s a "forecast." Forecasts give you your odds over a long time, which is more valuable than predicting quakes. Because if you’re an engineer designing a bridge or a building, you want to know how many earthquakes that piece of infrastructure is going to experience over the next thirty years. That’s what engineers need to know. They don’t care exactly when it happens, say on a Tuesday at 11 pm. Of course, we would like to predict earthquakes, but actually for a lot of the built environment, you want to know the experience of a structure’s lifetime. We are working towards an earthquake early warning system, which would help on the short term to reduce damages from an earthquake. For example, so you can stop trains or industrial manufacturing processes. Earthquake early warning uses the difference in speed between P-waves, which happen first, and S-waves, which happen second during an earthquake. The P-wave travels faster than a S-wave, so if you’re 15 miles away, and you feel the P-wave, then a few seconds later you’ll feel the S-wave. By having a network of seismic stations, we can calculate the time difference between the waves and then triangulate where the earthquake is, and how long before the waves will travel out to other locations.
SSF: Are there any other public misconceptions about earthquakes that you’d like to dispel?
KS: In recent conversations with friends about the Nepal earthquake, it seems a lot of people are startled by the aftershocks. But these are actually expected; we know quite a bit about aftershock patterns, their size and the timing. So overall, it is important for people to know in terms of the period after an earthquake, that large aftershocks can continue to affect an area years after a main shock and that can affect rebuilding activities. So being prepared for the short-term effects of the main shock and making your communities more resilient are valuable! Good advice on planning can be found at www.earthquakecountry.org.