Big earthquakes like the Sendai quake that devastated Japan in March don’t cause similar disasters on the other side of the globe, a new study suggests.

Like ranks of falling dominoes, tremors on the scale of the Sendai quake can trigger other earthquakes, say geophysicists at the U.S. Geological Survey in Menlo Park, California But, based on analyses of about 30 years of seismic data, those shocks are all very small or sit close to the original fault break, the group reports online March 27 in Nature Geoscience.

“If California is ready to go, it’s because California is ready to go,” says Jian Lin, a geophysicist at the Woods Hole Oceanographic Institute in Massachusetts. “Not because an earthquake in California would be triggered by Japan.”

There was previously some room for doubt, says study co-author Tom Parsons. Fault lines aren’t islands. Like tossing a rock into a pond, breaking chunks of earth send out waves that can circle the globe several times over. In a 2008 study, Parsons and his colleagues discovered that as those waves pass, far-flung corners of the planet start to buzz with lots of tiny temblors. That worldwide rumbling raised some eyebrows, he says: “If the whole world becomes an aftershock zone, should we worry about big ones?”

In the current study, the team narrowed its focus to just that: the big ones. Parsons and his colleagues traced the seismic wave aftermath of every magnitude 7 or larger earthquake — more than 200 in all — during the past 30 years. Following the initial earthquake, faults within 1,000 kilometers [621 miles] frequently ruptured domino-style. On the far scale, however — think Japan to California — the group couldn’t find a second domino larger than magnitude 5.

“It might be a surprising result to the public,” Lin says. “But it’s not a surprising result to an earthquake scientist like myself.”

How shaking could trigger tiny faults but not big ones remains unclear, Lin says. But he likes to think of faults as picket fences: Your neighbor can knock down your fence by pushing on it directly — just as earthquakes often shift the forces on nearby faults. Your neighbor could also knock down your rickety old fence with vibrations, say by turning up the stereo way too high. No matter how loud that music gets, however, it won’t knock a sturdy fence down in the next town over, Lin says.

The study does, however, include a number of uncertainties, says Emily Brodsky, a geophysicist at the University of California, Santa Cruz. It can be difficult to determine whether a large quake was triggered independently or by a previous smaller quake elsewhere, she says, and the aftershock record is far from complete. The scarcity of seismograms in, say, the open ocean leaves holes in the data. “It’s actually a little embarrassing how bad our ability to detect earthquakes under certain circumstances are.”

For Parsons, resolving the physics behind these questions is goal No. 1. Knowing how earthquakes work makes a difference when drawing quake-hazard maps for places like Japan and California. “Ultimately, we’d like to understand the physics behind it,” he says. “That makes us a lot more comfortable when we’re forecasting things.”

Image: USGS global seismicity map 1990-2009 (M5.5+ 1990-2002, M3.5+ U.S. lower 48 and Hawaii, M4.5+ Alaska)

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