Rodgers Creek Fault Traced through Santa Rosa

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For the first time, scientists at the U.S. Geological Survey have mapped the active surface trace of the Rodgers Creek Fault through the central part of the northern California city of Santa Rosa. Urban development has, until now, obscured its exact location.

For the first time, scientists at the U.S. Geological Survey have mapped the active surface trace of the Rodgers Creek Fault through the central part of the northern California city of Santa Rosa. Urban development has, until now, obscured its exact location. Knowing where the fault reaches the ground surface and the total width of the zone of faulting are important for assessing the hazard from surface rupture in a large earthquake.

“We used detailed topographic imagery from airborne lidar and subsurface imaging from high-resolution geophysical surveys to trace the fault through Santa Rosa,” said USGS geologist Suzanne Hecker, lead author of the study.

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.

Scientists also discovered, through analysis of gravity and magnetic data, an especially dense, magnetic body of rock below the ground surface on the east side of the scarp-defined depression. This dense, magnetic body of rock appears to be in contact with the east side of the Rodgers Creek Fault, and its northern end is associated with concentrations of small earthquakes, mainly aftershocks from the 1969 magnitude 5.6 and 5.7 Santa Rosa earthquakes. The pattern of surface faulting and earthquake activity may be an expression of deformation resulting from intensified frictional resistance where the fault abuts the dense, magnetic body of rock at depth. 

On the basis of the lidar and geophysical observations, and preliminary data from previous studies suggesting a change in the rate of surface creep on the fault in the vicinity of Santa Rosa, USGS scientists speculate that the dense, magnetic body of rock may be a stuck patch, or “asperity,” on the fault, where stress concentrates. A large earthquake on the Rodgers Creek Fault that ruptures through the proposed asperity has the potential to intensely shake the sedimentary basin beneath Santa Rosa. This basin is already known, from an earlier study of the damaging 1969 Santa Rosa and 1906 San Francisco earthquakes, to amplify shaking at the ground surface. The Rodgers Creek Fault is thought to be among the most likely faults to produce the next large earthquake in the Bay Area. 

A pre-publication version of the full report “Detailed Mapping And Rupture Implications Of The 1-Km Releasing Bend In The Rodgers Creek Fault At Santa Rosa, Northern California,” is available online in the Bulletin of the Seismological Society of America.

Shaded relief image of the Santa Rosa area showing active faults
Shaded relief image of the Santa Rosa area showing active faults (black lines) and the detailed rupture pattern of the Rodgers Creek Fault where it crosses central Santa Rosa (in red). The orange, bean-shaped area represents the dense, magnetic body of rock on the east side of the fault beneath Santa Rosa. This body of rock may be largely responsible for the pattern of surface faulting and may influence the distribution of small earthquakes (white and blue circles, blue for 1969 aftershocks) and the occurrence of the damaging 1969 earthquakes (approximately located by blue stars).
Google Earth image of central Santa Rosa showing the surface trace of the Rodgers Creek Fault
Google Earth image of central Santa Rosa (dated 24 October 2009) showing the surface trace of the Rodgers Creek Fault (red lines) and the inferred location of the fault prior to this study (orange dotted line). Yellow lines on either side of the fault are the boundaries of the lidar survey.