Seismologists surprised by deep California quakes

Mysterious earthquakes rattle deep beneath Southern California.

The tiny quakes originate from tens of kilometers below ground along the Newport-Inglewood Fault near Long Beach, Calif., seismologists report in the Oct. 7 Science. Rocks at that depth should be too hot and malleable to snag and break — a key mechanism behind earthquakes. Some unknown process must be at play in the roots of the fault that makes this new type of earthquake possible, says study coauthor Asaf Inbal, a seismologist at Caltech.

“This is a wide open question that we don’t have a good answer to,” he says.

The quakes puzzle scientists, but they don’t pose a direct threat, says Yuri Fialko, a geophysicist at the Scripps Institution of Oceanography in La Jolla, Calif. “Luckily for us, they remain small and don’t grow into bigger, potentially damaging events,” he says. Still, small earthquakes “may eventually lead to the triggering of larger earthquakes” and warrant further study.

The Newport-Inglewood Fault sits along a boundary where two chunks of Earth’s crust grind past one another. Seismometers monitor the fault, but the unusually deep quakes are usually too weak to be heard. Seismic noise from sources such as passing trucks and landing planes conceals the quakes, which register around magnitude 2.0 at most, with weaker temblors dipping into negative magnitudes.

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Inbal and colleagues analyzed data from a six-month survey in 2011 that deployed about 5,300 seismometers in the area, one of the densest amounts ever arrayed over such a fault. Combining the data with information from another, smaller survey in 2013 amplified faint earthquake signals above the background noise. The deepest newfound quakes come from Earth’s upper mantle at depths of more than 30 kilometers — far below the conventional seismically active zone for the fault, the researchers found. At these depths, rocks should flow when under stress, rather than accumulating strain and breaking.

One possible explanation is that upper mantle fluids flow into the fault and provide a source of high pressure that extends the depth of seismic activity, the researchers propose. More observations should reveal whether this explanation holds water.