50th Anniversary of the 1969 Santa Rosa, California, Earthquakes: A Mitigation Retrospective and Earthquake Hazard Outlook

Santa Rosa, the largest Bay Area city north of San Francisco, has been twice strongly shaken by earthquakes. The downtown area was destroyed by the 1906 earthquake (generated on the San Andreas fault more than 30 km away) and was damaged again in 1969 by a pair of nearby moderate-sized (magnitude 5.6 and 5.7) earthquakes. Following the 1969 earthquakes, the City of Santa Rosa established what was at the time some of the strongest policies in the United States to address seismically vulnerable buildings. The city reduced the hazard using two approaches: 1) expanding an existing urban renewal project to include the heavily damaged central business district and 2) creating requirements to rehabilitate older, mostly unreinforced masonry buildings elsewhere in the city. Santa Rosa's commitment to mitigating structural hazards was remarkable in light of the moderate size of the 1969 earthquakes and the absence of building collapses or fatalities. Earthquake mitigation has continued in Santa Rosa and surrounding Sonoma County, but has been outpaced by investment in more densely urbanized regions of the Bay Area. In the 30 years since the 1989 Loma Prieta earthquake jolted the South Bay region, only about 2% of reported mitigation investment in the nine-county region has been in Sonoma County. During this same period, earthquake science has elucidated the severity of the hazard. The highest likelihood of a large, regionally damaging earthquake (32% of at least one M>6.7 by 2043; Field et al., 2015) is on the combined Hayward and Rodgers Creek faults, the latter of which crosses beneath Santa Rosa. The significant shaking damage that occurred in Santa Rosa in the 1906 and 1969 earthquakes appears to be partly due to the configuration of an underlying sedimentary basin, in particular, a basin-edge geometry that enhances ground motions. Moreover, several lines of evidence, including potential-field anomalies, fault-trace geometry, and the distribution of seismicity and shallow creep, suggest that a large locked patch of the fault lies below and south of Santa Rosa. Rupture of this asperity could potentially release strong seismic energy beneath the city. In view of the high hazard, additional investment in earthquake resilience is vital; Santa Rosa's history of taking effective action should serve the city well in rising to the challenge.