2016: New maps detail wider Rodgers Creek fault through Santa Rosa

This story was originally published in 2016.

Federal geologists have released the most detailed maps to date showing just where the Rodgers Creek fault slices through Santa Rosa, detailing where it runs beneath churches, medical offices, a shopping center and three elementary schools.

The new maps, produced using aerial lasers able to make precise topographical measurements, show the fault zone is wider and stretches farther east than previous assumed, said Suzanne Hecker, a geologist with the U. S. Geological Survey and the lead author of the study.

The fault, which was responsible for two 1969 quakes that killed one person and damaged several buildings in Santa Rosa, has long been thought to run north-south along the city's east side - from Taylor Mountain, through Doyle Park, near Santa Rosa Memorial Hospital and past the city's Rural Cemetery.

But because the area has long been developed, geologists have not been able to use aerial photography to precisely locate the tell-tale topographical features identifying where the fault reaches the surface - areas known as traces.

“The reason we didn't know exactly where the fault went through Santa Rosa before was because it was obscured by urban development,” Hecker said. “Now we're able to exactly delineate where the traces go through the city.”

Now, instead of a single dotted line previously used to identify the fault through the city, new USGS maps show a network of fault lines and intersecting fragments. One passes close to the Santa Rosa French-American Charter School on Sonoma Avenue. Another passes nearby, just east of the Community Baptist Church. Others run beneath Brook Hill and Proctor Terrace elementary schools, as well as the Town and Country Shopping Center.

The new measurements were taken in 2007 using a technology called LIDAR, which stands for Light Detection and Ranging. Lasers mounted on an airplane were able to measure the topography with enough precision to allow researchers to observe slight changes in elevation. Hecker likened the technology to the machines used in stores to scan barcodes.

The detailed data has been used in other studies, such as one that helped the Sonoma County Water Agency know where to reinforce its water aqueduct as it passes across the fault zone. But Hecker said this is the first time the data was integrated with other information to create a detailed portrait of the urban fault zone.

The greater width of the fault zone is one key finding, Hecker said. The new maps show fault lines that now stretch east almost to Farmer's Lane.

The information is useful because it can help residents understand how close their homes and businesses are to the surface faults.

While all structures in the area are at risk of shaking from a major seismic event, those located on top of fault traces are exposed to additional hazards as the ground shifts, Hecker said.

The location of the surface faults were identified in part by taking note of subtle inclines that are evidence of past fault motion. These “scarps” are like vertical scars on the landscape. Some, like the incline of a few feet on Sonoma Avenue east of French-American Charter School, are too gradual for most people to notice. Others, like a section of Leonard Avenue east of Alderbrook Drive, results in a more noticeable incline, Hecker said.

Another key finding is that a highly dense area of magnetic rock at least 1,000 feet below the surface may be having an effect on the fault. Hecker said the area might be responsible for pressure building up on the fault, which may contribute to more intense shaking during a large earthquake.

The Rodgers Creek fault, known as a strike-slip fault, runs from San Pablo Bay at the southern tip of Sonoma County north through Santa Rosa to Healdsburg.

Likely a northern extension of the Hayward fault, the fault occurs as the Pacific tectonic plate moves north and the North America plate slides south. The large area of dense magnetic rock on the eastern side of the fault may be grabbing the softer rock on the western side, creating a “stuck patch” that is building up pressure in the fault, Hecker said.

“The suggestion is that if the stuck patch releases in a strong earthquake, that it could cause a relatively strong shaking,” Hecker said.

The area of dense, magnetic rock deep beneath the surface has been identified by previous studies.

But Hecker's analysis suggests that the mass - which may extend several miles beneath the surface - could be responsible not only for the buildup of pressure, but also the existence of multiple fault lines on the surface above and quake activity to the north, notably the 1969 quakes, which had magnitudes of 5.6 and 5.7.

The new study does not give new insightsinto when the next quake might strike along the fault. The USGS calls the Hayward-Rodgers Creek lines a “fault system” with a 31 percent likelihood of a magnitude-6.7 quake or greater in the next 30 years, the highest probability in the Bay Area.

Any major earthquake on the Rodgers Creek fault could prove to be a “double whammy” for Santa Rosa because the city sits in a basin of thick sediment that easily transmits shockwaves that can make shaking worse during large quakes, geoligists said.

“It's an opportunity to really remind people that they can do things to remain earthquake safe,” she said.

The study, titled “Detailed Mapping and Rupture Implications of the 1 km Releasing Bend in the Rodgers Creek Fault at Santa Rosa, Northern California,” was published in the Bulletin of the Seismological Society of America.

You can reach Staff Writer Kevin McCallum at 521-5207 or kevin.mccallum@pressdemocrat.com. On Twitter @srcitybeat.