I will try to do a better read on your proposed route - it sounds interesting and promising. I am on the other side of the State so it takes me a while to grasp locales in Socal.

Clem Tillier on the HSR blog site reported that PB was proposing tunneling thru the San Gabriel fault, presumably with a BART type slip gallery, but it was presumably ok in their professional opinion because it is considered an inactive fault. Of course if geologists knew of the White Wolf fault in Tehachapi in 1952 they also considered it inactive. That 1952 quake was the second largest in California in the 20th century.

BTW I am pretty sure the BART slip chamber is "used up" and they are going to have to address that.