No matter what you try and build in California people will say "but what about earthquakes?"  The bigger the project, the louder their dismissals.  

This is of course based on pure ignorance.  The bigger a project the more likely it will have the most state-of-the-art earthquake safeguards, which are tremendous.  Earthquakes are simply not a concern for a project like this.  If a quake occurs and the tracks spread there will be immediate breaks put on all running trains.  After the quake you fix the tracks.  It's not really a huge deal.

Tall buildings, for the record, are built with counter-weighting and shock absorption that makes them far safer in an earthquake than your new McMansion.