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.
We estimate site response at 24 sites in this region by inverting seismograms of local events (radius < 100km) for source, site, and propagation characteristics. Regional seismicity was recorded from January, 2004, to September, 2006. We inverted spectra of 33 earthquakes for P-waves and 25 earthquakes for S-waves, covering a range of hypocentral distances and azimuths. The inversion scheme fits the logarithms of seismic body-wave spectra to the Brune source model with an ωˆ-2 slope at high frequencies conditioned by geometric spreading and anelastic attenuation with frequency independent crustal attenuation. Residuals from this step are projected onto the set of sources and sites. These two steps are iterated until the square of the residuals is minimized. Finally, geotechnical data is used to estimate site amplification and determine the absolute amplitudes of the source and site spectra. The events used in this study were all recorded by at least four stations and cover a range of hypocentral distances and azimuths, giving good constraint on both source spectra and wave propagation characteristics.
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.
The resulting paper from this work is available.
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".