When preparing for earthquakes, don’t forget the small ones

By D’Maia Curry

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.

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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.

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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.

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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.

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