Q & A: Great San Francisco Earthquake continues to educate
Gregory Beroza
Most scholars trace the beginning of modern seismology in the United States to the Great San Francisco Earthquake of 1906. When it struck, scientists throughout California—including Stanford geologist John Casper Branner—began making observations and collecting data along the San Andreas Fault. Recently, Stanford geophysics Professors Gregory Beroza and Paul Segall, with graduate student Seok Goo Song, conducted a major re-analysis of the infamous temblor. In an interview with Stanford Report science writer Mark Shwartz, Beroza explains how scientists studying the San Andreas Fault continue to learn lessons from the Great Earthquake of 1906.
Q: What was the significance of the 1906 earthquake, in terms of geophysics and our understanding of earthquakes in general?
GB: The 1906 earthquake is thought of as a watershed event in the history of earthquake science. It was the event that really clinched the connection between earthquakes and faulting. Before that, it was thought that earthquakes and faulting were related, but the one-to-one correspondence between the two was not understood.
It led to, as well, the elastic rebound theory, which details how energy is accumulated in the Earth's crust as elastic potential energy for hundreds of years and then suddenly released as kinetic energy in the forms of seismic waves and faulting.
Q: What were the results of your recent analysis of the 1906 quake?
GB: What we found in studying the old data from the 1906 earthquake is that we resolved a long discrepancy in the size of the earthquake between magnitude 7.7 and 7.9. Our study suggests that it was M7.9. The previous two models [published in 1993 and 1997] disagreed with each other, not just in the size but also the length of the earthquake. We were able to show that both data sets were consistent with the earthquake rupturing a full 300 miles—from San Juan Bautista in the south up to the very northern end of the San Andreas Fault in Cape Mendocino—and that the faulting was massive over the entire distance of the quake.
Q: What would the difference have been between a M7.7 and M7.9 earthquake?
GB: A 7.9 earthquake would have been 50 percent bigger, so instead of being a 200-mile-long rupture, it was 300 miles long, so it's a significant difference.
The 7.7 scenario had less slip on the fault, and hence the waves from it would have been a lot smaller. Also, the northernmost 100 miles of the fault did not slip in the M7.7 scenario.
Q: What do you think might happen today if an earthquake of similar magnitude struck in the same place?
GB: Of course, there's uncertainty in exactly what happened in 1906, despite our study. The strength of shaking in close to these really big earthquakes is also uncertain, because we have very little data on scale of how the ground moves in these big events. And there are uncertainties in how buildings respond.
Given those uncertainties, what seems to be a consensus is that a repeat of that earthquake would kill more than 1,000 people in the Bay Area. It would cause at least $100 billion in direct losses, so it would be a catastrophe. Of course, things would be worse if we had not done all this earthquake-resistant design.
It's thought that at least 3,000 people died in San Francisco itself in the 1906 earthquake. San Francisco is bigger than it was in 1906, and of course the whole urban conglomeration of the Bay Area is gigantic. Many, many times more people will be exposed to this strong shaking.
To some extent it's inevitable when you shake a big urban area that strongly that bad things will happen. We just want to minimize them, if possible.
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