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But government officials have told Indian media that engineers were aware the tunnel traversed a “shear zone” of brittle rock, and that it had experienced numerous smaller rock falls over the past few years.

Preliminary findings indicated the collapse may have been caused by a geological fault, known as a "shear zone", a member of the panel told Reuters, speaking on condition of anonymity as he is not authorised to talk to the media.

There is a line of potentially ripening fault zones along the so-called Eastern California Shear Zone, one of the state’s most significant seismic zones, which carries a good chunk of the earthquake burden needed to accommodate tectonic plate movement as the Pacific plate slides northwest past the North American plate.

“The Garlock acts as a mechanical bridge between the northern Eastern California Shear Zone and the San Andreas.”

The Ridgecrest quakes occurred in a vast seismic region called the Eastern California Shear Zone, representing a broad swath of the state that includes Palm Springs and the Owens Valley.