A recent study concludes that small- to moderate-sized earthquakes are greater contributors to earthquake hazard and risk than large earthquakes.
By D’Maia Curry, freelance science writer
Citation: Curry, D., 2020, When preparing for earthquakes, don’t forget the small ones, Temblor, http://doi.org/10.32858/temblor.146
A Twitter discussion of a recent earthquake hazards paper has raised some eyebrows. It went something like this: Just as cows kill 10 times as many people a year as sharks, small earthquakes cause more damage than large ones. Does that mean that cows are more dangerous than sharks? And by extension, small earthquakes are more dangerous than large ones? That is the central takeaway of the new study, published in Seismological Research Letters. It’s a matter of statistics and communication.
“Big earthquakes cause big shaking, but they don’t happen very often,” says Sarah Minson, a seismologist at the U.S. Geological Survey (USGS) in Moffett Field, Calif., and lead author of the new study. “Little earthquakes only cause little shaking but shaking is variable and every once in a while, a little earthquake causes greater-than-expected shaking, possibly even a lot of shaking,” Minson says. “And little earthquakes happen a lot.”
The team’s findings do not change the seismic hazard for a region, Minson notes, but do offer guidance for how seismic hazards are communicated.
Minson and her colleagues compared two datasets tracking earthquakes from magnitude-0.5 to -7.9. One dataset included about 10,000 magnitude-0.5 to -4.5 earthquakes in Southern California, measured in one year. The other dataset included about 600 magnitude-3 to -7.9 earthquakes from active shallow, crustal regions around the planet. The team also looked at a shaking hazard forecast for Los Angeles.
With these datasets, the team compared the predicted and observed magnitudes of earthquakes that produced that level of shaking. They then developed an analytical model to determine if the resulting behavior is universal for all shaking levels and to quantify the extent to which seismic hazard is skewed toward smaller-magnitude earthquakes.
They found that the earthquakes that cause most strong shaking were smaller in magnitude than expected. This was true for all distances from the epicenter. Small earthquakes seem to therefore be significant contributors to seismic hazard.
Many earthquakes have produced surprisingly large ground shaking for their magnitude. Three such California quakes struck over the past few decades: the 1994 magnitude-6.7 Northridge earthquake, the 2004 magnitude-6.0 Parkfield earthquake and the 2009 magnitude-5.2 Olancha earthquake. Each of these earthquakes caused significant ground shaking that was uncharacteristic for a smaller-magnitude earthquake, the team reported.
Minson and her team wanted to know if strong shaking is more likely to be due to a large-magnitude earthquake or an oddball among the multitude of common small-magnitude earthquakes.
“Overwhelmingly,” Minson says, the research shows that “if you feel ground shaking, it is most likely to be, as my grandmother put it, a little earthquake with ambition.”
But not shocking
These findings are not surprising to the seismology community, says Ronnie Kamai, a seismic studies researcher at Ben-Gurion University of the Negev in Israel who was not involved in the new research. Many of our calculations are driven by the variability of ground motion, she says. Very often the ground motion observed during an actual earthquake event is higher than the estimated ground motion derived from the calculations, Kamai notes.
The findings also do not change the overall seismic hazard because current “building codes built on existing understandings of seismic hazard [already] include all possible shaking from all magnitude earthquakes,” Minson says. So knowing that smaller earthquakes contribute much more to overall seismic hazard does not change the way we build and structurally prepare for earthquakes.
However, the authors say, these results might change how we communicate earthquake hazards. Given that localized shaking from smaller earthquakes is just as threatening as larger magnitude earthquakes, and perhaps more so given frequency, public outreach should focus on preparing for small and moderate earthquakes rather than telling stories based on a future “Big One” event like a magnitude-8 earthquake on the San Andreas Fault.
“I think a lot of people assume the shaking they should be worried about is the shaking from a magnitude-8 earthquake,” says Elizabeth Cochran, a seismologist at USGS in Pasadena, Calif., and co-author of the paper. “While you are waiting for a magnitude-8 on the San Andreas, you can feel greater shaking from a smaller earthquake that is right underneath you,” Cochran says.
“Talking about the ‘Big One’ is not a very effective communication tool as it creates a sense of fatalism,” Minson adds. “It doesn’t really feel survivable,” she says. “If you talk about the smaller Northridge earthquake, people are like ‘Oh yeah, I prepared for that. I can survive that.” So changing public outreach strategies, she says, could have many positive effects.
Minson, S. E., Baltay, A. S., Cochran, E. S., McBride, S. K., & Milner, K. R. (2018). Shaking is Almost Always a Surprise: The Earthquakes That Produce Significant Ground Motion. Seismological Research Letters.