Site Response in the Northern San Francisco Bay Area, California

Santa Rosa, California sustained unexpectedly high damage from the 1906 San Francisco earthquake (M7.8) and the 1969 Santa Rosa Earthquake sequence (M5.6 and M5.7). At the nearby city of Napa, ground motion was also unexpectedly high during the 2000 Yountville earthquake (M5.2). This history of unexpectedly strong ground motion, in combination with high metropolitan populations in Santa Rosa (over 450,000) and Napa (over 100,000), and their close proximities to Holocene active faults such as the Rodgers-Healdsburg fault system, West Napa Fault, and Green Valley fault, make estimation of site response in these regions particularly important.

It is well known that near surface geology can have large impacts on the amplification of seismic waves, and therefore on site response. The determination of site response characteristics is thus an important component of estimating the seismic hazard for these cities as well as the entire San Francisco Bay area. Knowledge of site response characteristics enables better spectral analysis of small and moderate-sized earthquakes as well as better models of wave propagation characteristics in the North Bay region. This project utilizes an inversion scheme for source, site, and propagation characteristics to determine site response at seismic recording stations in the northern San Francisco Bay Area around Santa Rosa, Napa, and Sonoma.

Menlo Park office

We have been recording since 2003 with as many as 14 stations. We now have ten stations distributed between Santa Rosa and Napa Valley. Preliminary analyzes demonstrate that amplifications can be expected in both regions from local severe earthquakes. While small magnitude earthquakes have been recorded and analyzed, the array now consists of accelerometers so that if severe shaking occurs, it will be recorded on scale and will lead to a better understanding of ground motion to expect in this region.

Seismic cone penetration testing (SCPT) was done at three stations in Santa Rosa and three stations in Napa to determine near surface (< 20 m) S-wave velocities and constrain absolute site amplification. High site response is estimated from the inversion in the city of Napa, in agreement with slower near-surface S-wave velocities from SCPT. Response is estimated to be lower at adjacent sites outside of the valley. Site response is also high in the city of Santa Rosa, although lower than Napa. High response in Santa Rosa is in agreement with faster near-surface S-wave velocities from SCPT.

Data from this array may be obtained by contacting IRIS. To browse which stations are available, click "data holdings", then click "by station." You will first need to select a network. The North Bay Network is "YK." You will then be asked to select a station. For example, if you wanted station Santa Rosa you would click "SAR." Next select the date you're interested in. You can now see what data is available for that date. To request data, fill out the form. Remember, this North Bay data is from network "YK".