A new US Geological Survey report from Menlo Park scientists led by Suzanne Hecker breaks new ground on mapping a formerly undocumented fault in the heart of Santa Rosa. The Chronicle reports that the team was the first to map, thanks to airborne laser beams known as Lidar, this particular section of the
Rodgers Creek Fault. And, oh goodie, it's "broader" and larger than heretofore thought. From the abstract:

The zone of faulting at the surface is broader and covers an area that extends farther east than previously assumed, which means the surface-rupture hazard is more widespread than previously thought. There are subtle fault “scarps” (the vertical scars left on the landscape from fault motion) on the gently-sloping floodplain of Santa Rosa Creek, on which the city is built, that define a surface depression that is one-quarter-mile wide by one-mile long.

Further, according to Hecker's report, “There is a significant probability of a major earthquake on the Rodgers Creek Fault in the coming decades,” a probability put at 31 percent when it comes to a magnitude 6.7 or great quake on the Rodgers Creek and Hayward faults within 30 years.

Santa Rosa, with its 174,000 souls, might also be negatively affected in the event of a quake due to aftershocks. That, the report found, could be due to a dense magnetic body of rock that the team also discovered.

Related: New Theory Holds San Andreas Might Be A 'Zipper Fault'