This is a very stupid idea from the part of Elon Musk. He should rather concentrate on space transport. Everyone knows that California is a very seismically active state. If a major earthquake strikes, then these tunnels are going to amplify the casualties.
Well, if there will be an earthquake in California, nothing will be saved, and people who do not use tunnels will not be able to save anything either. Already many times talked about what dangers lie in wait for California and really good is not enough.
Since it is a seismically active area, most of the buildings are protected against earthquakes, and therefore the damages might be minimal. But it is much more difficult to protect tunnels from these earthquakes.