I went to the workshop and press conference (last winter? 150th anniversary of the 1857. They are working one some worst cases. For the southern San Andreas to be a worst case, the rupture would have to start in the SE and propagate NW...through some complex areas..."structural knots" (?). If that happened, and the 3D simulations of ground motion for LA basin and gthe Valley are correct, earthquake engineers expect that some percent of 20 story steel buildings will collapse: those with brittle welds will collapse over a wider area than those with ductile welds. But, some of the people saying they will collapse given such ground motions doubt that those ground motions will occur. That is much of the reason why there is a focused effort to study the southern San Andreas (SOSAFE).
Chris