Tunnels are quite safe in California. As long as the tunnel is stronger than the surrounding rock, high stresses cannot get to it. Most earthquakes are low-stress events (low PGV). I once did a study on this.
I find this interesting, since they are excavating the want I would have wanted for the Niagara tunnel. They are using two stages, and grinding the rock out. With Niagara, I would have finished with a more elliptical section, to cope with the high stresses. Here, they have soft rock and low stresses. With the existing new Niagara tunnel, we can expect the tunnel lining to start cracking in a few years.
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