The Bay Area’s San Andreas Fault system featuring the Hayward Fault (middle to bottom right) and Calaveras Fault (bottom right)
The new tunnel is located in a seismically active part of California between the Calaveras and Hayward faults. Between these two major faults are several small faults that cross the path of the tunnel. These faults consist of very short sections of intensely fractured sandstone and fault gouge (fault rock rich in clay, characterized by a mixture of fine grain-like rock fragments). Due to their incohesive nature, these formations are extremely weak, a stark contrast to other materials encountered while digging through this section.
A sample cross section view of the new tunnel alignment. The red markings indicate fault zones that intersect the tunnel.
Briones formation contains fossilized shell fragments (circled in yellow)
During excavation, a majority of the ground encountered is part of the Briones Formation, which includes massive sandstone characterized by coarse, grain-like quartz sandstone and shell fragments. These rocks are extremely old, dating back to the Late Miocene Epoch nearly 12 million years ago! These rocks aren't just old, but they are so strong that drill and blast methods were utilized to mine through the tough ground.
This is only a glimpse of the geology hidden beneath Sunol Valley. So the next time you look down and ask, “What’s underground?”, keep in mind that that there are ancient rocks, ever-shifting faults, and whole lot more going on just underneath your feet.