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Quakes may bring nearby rocks closer to rupture

Lab studies could explain how a seemingly stable geologic fault can fail

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6:16pm, January 9, 2013
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Some geologic faults suffer from a Jekyll-and-Hyde personality: Sections considered resistant to powerful earthquakes can sometimes produce enormous temblors. New research shows how a quake on one fault segment can weaken a neighboring section, allowing a once-steady segment to suddenly slip.

The findings could explain why Japan saw the devastating March 2011 Tohoku quake on the fault segment it did. Seismic hazards in many other fault zones may also need to be reassessed, scientists report online January 9 in Nature.

“For Tohoku, it’s a fairly convincing scenario,” says Kelin Wang, a geophysicist at the Geological Survey of Canada in Sidney, British Columbia. “But it’s not the only scenario.”

Seismologists have traditionally expected faults to act in one of two ways: A fault can creep along at a steady pace or it can be stuck, slowly building up strain until the energy is suddenly unleashed in an earthquake. Creeping faults shouldn’t generate giant quakes because they don’t accumulate much strain. That’s why the magnitude 9 Tohoku earthquake was a surprise: It largely occurred on a fault segment assumed to be creeping.

That segment — which slipped by as much as 50 meters during the quake — ruptured roughly seven to 15 kilometers beneath the seafloor near the Japan Trench. The trench marks the beginning of a subduction zone, where the west-moving Pacific plate of Earth’s crust dives beneath the plate that carries northern Japan. Large seismic events weren’t expected in such a shallow portion of a subduction zone. With less overlying material, the shallow end should experience less compression and therefore be stronger than deeper segments, says study coauthor Nadia Lapusta, a solid mechanics specialist at Caltech. And although earthquakes of magnitudes 7 and 8 had occurred in the deeper parts of this subduction zone, they hadn’t been recorded in the shallow region.

In the new study, Lapusta and Hiroyuki Noda  of the Japan Agency for Marine-Earth Science and Technology in Yokohama simulated how nearby seismic activity can weaken a stable, creeping fault segment. If a quake on one segment generates heat fast enough, it causes water to expand in tiny pore spaces in the rocks of an adjacent segment. As water pressure builds, the fault weakens, the friction is reduced and the quake can rupture the normally resistant fault segment.

The simulation is based on the properties of rocks collected from boreholes in the Chelungpu fault, the site of a magnitude 7.6 earthquake that struck Taiwan in 1999. Because the results re-create observations from the Tohoku quake, the study probably explains what happened in Japan in 2011, Lapusta says.

But some scientists say it’s too soon to make that conclusion. “A specific connection to Japan is a stretch,” says Thorne Lay, a seismologist at the University of California, Santa Cruz. Little is known about the actual frictional properties of the fault segment that slipped during the Tohoku event, he says. And scientists don’t really know whether the fault was creeping or locked prior to the earthquake because the region’s global-positioning system network for monitoring ground movement is based on land. Japan is now working to build such a network on the seafloor near the Japan Trench.

The United States is doing something similar for the Cascadia subduction zone in the Pacific Northwest, says Ross Stein, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif. “We should be thinking of what this may mean for subduction zones thought to be creeping,” he says. “What really is the hazard?”

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