Effects of local geological conditions in the San Francisco Bay region on ground motions and the intensities of the 1906 earthquake

1976 ◽  
Vol 66 (2) ◽  
pp. 467-500 ◽  
Author(s):  
Roger D. Borcherdt ◽  
James F. Gibbs
Keyword(s):  
San Francisco ◽  
San Francisco Bay ◽  
Ground Motions ◽  
Empirical Relation ◽  
Good Evidence ◽  
Average Intensity ◽  
Seismic Zonation ◽  
Nuclear Explosions ◽  
Geological Conditions ◽  
Geological Units

abstract Measurements of ground motion generated by nuclear explosions in Nevada have been completed for 99 locations in the San Francisco Bay region, California. The recordings show marked amplitude variations in the frequency band 0.25 to 3.0 Hz that are consistently related to the local geological conditions of the recording site. The average spectral amplifications observed for vertical and horizontal ground motions are, respectively: (1, 1) for granite, (1.5, 1.6) for the Franciscan Formation, (3.0, 2.7) for the Santa Clara Formation, (3.3, 4.4) for alluvium, and (3.7, 11.3) for bay mud. Spectral amplification curves define predominant ground frequencies in the band 0.25 to 3.0 E for bay mud sites and for some alluvial sites. Amplitude spectra computed from recordings of seismic background noise at 50 sites do not generally define predominant ground frequencies. The intensities ascribed to various sites in the San Francisco Bay region for the California earthquake of April 18, 1906, are strongly dependent on distance from the zone of surface faulting and the geological character of the ground. Considering only those sites (approximately one square city block in size) for which there is good evidence for the degree of ascribed intensity, the intensities for 917 sites on Franciscan rocks generally decrease with the logarithm of distance as Intensity = 2 . 6 9 - 1 . 9 0 log ( Distance in kilometers ) . ( 1 ) For sites on other geological units, intensity increments, derived from this empirical relation, correlate strongly with the Average Horizontal Spectral Amplifications (AHSA) according to the empirical relation Intensity Increment = 0 . 2 7 + 2 . 7 0 log ( AHSA ) . ( 2 ) Average intensity increments predicted for the various geological units are −0.3 for granite, 0.2 for the Franciscan Formation, 0.6 for the Great Valley sequence, 0.8 for the Santa Clara Formation, 1.3 for alluvium, and 2.4 for bay mud. The maximum intensity map predicted on the basis of these data delineates areas in the San Francisco Bay region of potentially high intensity for large earthquakes on either the San Andreas fault or the Hayward fault. The map provides a crude form of seismic zonation for the region and may be useful for certain general types of land-use zonation.

On the characteristics of local geology and their influence on ground motions generated by the Loma Prieta earthquake in the San Francisco Bay region, California

1992 ◽  
Vol 82 (2) ◽  
pp. 603-641 ◽  
Author(s):  
Roger D. Borcherdt ◽  
Gary Glassmoyer
Keyword(s):  
Ground Motion ◽  
San Francisco ◽  
San Francisco Bay ◽  
Ground Motions ◽  
Loma Prieta Earthquake ◽  
Franciscan Complex ◽  
Horizontal Acceleration ◽  
Geological Conditions ◽  
Peak Horizontal Acceleration ◽  
Loma Prieta

Abstract Strong ground motions recorded at 34 sites in the San Francisco Bay region from the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions. Peak horizontal acceleration and velocity inferred for sites underlain by “rock” generally occur on the transverse component of motion. They are consistently greater with lower attenuation rates than the corresponding mean value predicted by empirical curves based on previous strong-motion data. Theoretical amplitude distributions and synthetic seismograms calculated for 10-layer models suggest that “bedrock” motions were elevated due in part to the wide-angle reflection of S energy from the base of a relatively thin (25 km) continental crust in the region. Characteristics of geologic and geotechnical units as currently mapped for the San Francisco Bay region show that average ratios of peak horizontal acceleration, velocity and displacement increase with decreasing mean shear-wave velocity. Ratios of peak acceleration for sites on “soil” (alluvium, fill/Bay mud) are statistically larger than those for sites on “hard rock” (sandstone, shale, Franciscan Complex). Spectral ratios establish the existence of predominant site periods with peak amplifications near 15 for potentially damaging levels of ground motion at some sites underlain by alluvium and fill/bay mud. Average spectral amplifications inferred for vertical and the mean horizontal motion are, respectively, (1,1) for sites on the Franciscan Complex (KJf), (1.4, 1.5) for sites on Mesozoic and Tertiary rocks (TMzs), (2.1, 2.0) for sites on the Santa Clara Formation (QTs), (2.3, 2.9) for sites on alluvium (Qal), and (2.1, 4.0) for sites on fill/Bay mud (Qaf/Qhbm). These mean values are not statistically different at the 5% significance level from those inferred from previous low-strain data. Analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 km in the cities of San Francisco and Oakland.

Observation of local site effects at a downhole-and-surface station in the Marina District of San Francisco

1992 ◽  
Vol 82 (4) ◽  
pp. 1563-1591 ◽  
Author(s):  
Hsi-Ping Liu ◽  
Richard E. Warrick ◽  
Robert E. Westerlund ◽  
Eugene D. Sembera ◽  
Leif Wennerberg
Keyword(s):  
Ground Motion ◽  
San Francisco ◽  
Particle Motion ◽  
San Francisco Bay ◽  
Ground Motions ◽  
Three Dimensional ◽  
Spectral Ratio ◽  
Body Waves ◽  
P Wave ◽  
Earthquake Location

Abstract The Marina District of San Francisco, California, with its artificial fill and a thick section of sand and clay covering a northwest-trending valley in the bedrock, suffered extensive damage during the 18 October 1989 Loma Prieta earthquake. Following the earthquake, the USGS drilled a hole at Winfield Scott School at Beach and Divisadero Streets; the borehole intersects bedrock surface at a 79.5-m depth. Two three-component seismometers, one in bedrock at a 88-m depth and one located at the surface, have been installed at the site; each seismometer consists of one vertical and two orthogonally oriented horizontal geophones having a natural period of 0.5 sec. Between August 1990 and January 1991, more than 50 earthquakes have been recorded digitally. Eight among these, ranging in magnitude between 2.8 and 3.6 and originating on the Calaveras, Franklin, Greenville, and Hayward faults and on faults parallel and close to the San Andreas fault, generated seismograms with high signal-to-noise ratio. Horizontal ground-motion amplification, expressed as spectral ratio between ground motions at the surface and those in the bedrock, has been calculated for motions in two orthogonal directions (along Divisadero and Beach Street); each ground-motion spectrum has been calculated using an entire seismogram consisting of body waves, surface waves, multiply reflected and scattered coda waves, and a short section (∼ 2 sec) of pre-event ambient noise. Before calculating spectral ratio, each spectrum has been smoothed using a truncated Gaussian window 0.61-Hz wide. Except for the lowest-frequency spectral-ratio peak at ∼ 1 Hz, frequency of other peaks depends on earthquake location. Amplitude of spectral-ratio peaks also show variation depending on ground-motion direction and earthquake location. For example, amplitude of the 1-Hz spectral-ratio peak varies from 7.2 to 12.7. The surface-downhole spectral ratio therefore provides only partial information on how ground motions are amplified by sediment deposits. If we choose to use this ratio for earthquake engineering applications, the ratios from the eight earthquakes give an indication of the variation in spectral ratio to be expected from earthquakes with similar magnitudes and epicentral distances on various Bay area faults. Also noteworthy are the observations that the two horizontal-component seismograms recorded by each seismometer have similar coda amplitude and duration regardless of earthquake location and that particle-motion polarization becomes complex shortly after the P-wave and S-wave onset. The complex particle-motion polarization indicates that wave fields in the bedrock and at the surface are three-dimensional; the bedrock topography underlying the site has been delineated previously to be three-dimensional from drill-hole information. We suggest from these observations that three-dimensional effects need to be considered when modeling site amplification in the Marina District. Finally, the eight earthquakes are divided into two groups, comprising those whose epicenters are located east of San Francisco Bay and those whose epicenters are located south of San Francisco Bay. Within each group, spectral-ratio peaks from different earthquakes line up with each other, thus showing consistency in spectral-ratio peaks as a function of earthquake location.

scholarly journals ACOUSTIC DOPPLER VELOCIMETER BACKSCATTER FOR QUANTIFICATION OF SUSPENDED SEDIMENT CONCENTRATION IN SOUTH SAN FRANCISCO BAY

2017 ◽  
pp. 34
Author(s):  
Mehmet Öztürk ◽  
Paul Work
Keyword(s):  
Suspended Sediment ◽  
San Francisco ◽  
San Francisco Bay ◽  
Empirical Relation ◽  
Acoustic Method ◽  
Sediment Concentration ◽  
Acoustic Doppler Velocimeter ◽  
Data Set ◽  
Suspended Sediment Concentrations ◽  
Optical Turbidity

A data set was acquired on a shallow mudflat in south San Francisco Bay that featured simultaneous, co-located optical and acoustic sensors for subsequent estimation of suspended sediment concentrations (SSC). The optical turbidity sensor output was converted to SSC via an empirical relation derived at a nearby site using bottle sample estimates of SSC. The acoustic data was obtained using an acoustic Doppler velocimeter. Backscatter and noise were combined to develop another empirical relation between the optical estimates of SSC and the relative backscatter from the acoustic velocimeter. The optical and acoustic approaches both reproduced similar general trends in the data and have merit. Some seasonal variation in the dataset was evident, with the two methods differing by greater or lesser amounts depending on which portion of the record was examined. It is hypothesized that this is the result of flocculation, affecting the two signals by different degrees, and that the significance or mechanism of the flocculation has some seasonal variability. In the earlier portion of the record (March), there is a clear difference that appears in the acoustic approach between ebb and flood periods, and this is not evident later in the record (May). The acoustic method has promise but it appears that characteristics of flocs that form and break apart may need to be accounted for to improve the power of the method. This may also be true of the optical method: both methods involve assuming that the sediment characteristics (size, size distribution, and shape) are constant

A Qualitative Study of Ecstasy Sellers in the San Francisco Bay Area

2004 ◽  
Author(s):  
Sheigla Murphy ◽  
Paloma Sales ◽  
Micheline Duterte ◽  
Camille Jacinto
Keyword(s):  
Qualitative Study ◽  
San Francisco ◽  
San Francisco Bay ◽  
San Francisco Bay Area ◽  
Bay Area
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