Sierra LaMar vanished about a month ago on her way to catch a school bus.
A helicopter, off-road teams and search and rescue crews with rock climbing experience are focusing on remote areas between Morgan Hill and Gilroy.
Agencies across the Bay Area are assisting the Santa Clara County Sheriff's Department. A K-9 unit is also involved.
The Santa Clara County Sheriff's Office said they've looked there before but the goal of Saturday's search is to be more thorough.
There were no new leads that triggered this search which was planned in advance. It will continue until night fall.
Meanwhile, Sierra LaMar posters will also be posted on BART cars, advising riders of the teen's disappearance.