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Research about San Francisco, California

Source-linked reports with geographic coverage including San Francisco, California.

At least 109 records · Page 6Linked to original sources
USGS1991

On the observation, characterisation, and predictive GIS mapping of strong ground shaking for seismic zonation

Tragic earthquakes of the last decade in Mexico, Armenia, and the United States have re-emphasised the importance of local geologic site conditions in determining amounts of damage and consequent loss of life. Extensive data sets in the San Francisco Bay region on strong earthquake ground motions, the damage distributions from past earthquakes, and geologic materials provide the basis to quantify site condition effects for purposes of earthquake hazard mitigation. These observational data are reviewed and analysed to provide methodologies for the characterisation and predictive mapping of potential variations in strong ground shaking for seismic zonation. The methodologies are based on existing geologic maps. They provide a method for seismic zonation applicable to many urbanised seismic regions of the world.

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USGS1991

The Marina District, San Francisco, California: Geology, history, and earthquake effects

A northwest-trending valley in the bedrock surface is buried by firm Pleistocene bay clay, a dense Pleistocene sand layer, soft Holocene bay sediments, loose to dense Holocene beach and dune sands, and artificial fill that have an aggregate maximum thickness of about 90 m (300 ft). Artificial filling of a cove at the site of The Marina District proceeded gradually from the late 1860s to 1912, when major hydraulic filling was done for the Panama-Pacific International Exposition. The remains of thousands of piles driven for the Exposition very probably still exist and have had unknown effects on long-term ground settlement and earthquake-related ground displacements. Intensity maps of the 1906 earthquake, and seismic recordings and severe building damage in 1989, reported by others, indicate that ground motion was amplified on both natural and artificial ground. This suggests that the configuration of the bedrock surface and the location and thickness of various clay and sand deposits underlying the fill had an important effect on the shaking. However, most of the settlement and liquefaction and the damage to pipelines, building foundations, streets, side-walks, and curbs occurred in areas of artificial fill consisting mainly of loose sand.

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USGS1990

Surface displacements in the 1906 San Francisco and 1989 Loma Prieta earthquakes

The horizontal displacements accompanying the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake are computed from geodetic survey measurements. The 1906 earthquake displacement field is entirely consistent with right-lateral strike slip on the San Andreas fault. In contrast, the 1989 Loma Prieta earthquake exhibited subequal components of strike slip and reverse faulting. This result, together with other seismic and geologic data, may indicate that the two earthquakes occurred on two different fault planes.

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USGS1988

Suspended sediment transport under estuarine tidal channel conditions

A modified version of the GEOPROBE tripod has been used to monitor flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within San Francisco Bay, California. Measurements were made every 15 minutes over three successive tidal cycles. They included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed; mean suspended sediment concentration profiles from seven miniature nephelometers operated within 1 m of the seabed; near-bottom pressure fluctuations; vertical temperature gradient; and bottom photographs. Additionally, suspended sediment was sampled from four levels within 1 m of the seabed three times during each successive flood and ebb cycle. While the instrument was deployed, STD-nephelometer measurements were made throughout the water column, water samples were collected each 1–2 hours, and bottom sediment was sampled at the deployment site. From these measurements, estimates were made of particle settling velocity ( w s ) from size distributions of the suspended sediment, friction velocity (U * U ∗ "> ) from the velocity profiles, and reference concentration ( C a ) was measured at z = 20 cm. These parameters were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles. Three suspended sediment particle conditions were evaluated: (1) individual particle size in the 4–11 φ (62.5-0.5 μm) range with the reference concentration C a at z = 20 cm ( C φ ), (2) individual particle size in the 4–6 φ size range, flocs representing the 7–11 φ size range with the reference concentration C a at z = 20 cm ( C f ), and (3) individual particle size in the 4–6 φ size range, flocs representing the 7–11 φ size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress. In addition, computations of particle flux were made in order to show vertical variations in horizontal mass flux for varying flow conditions.

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USGS1988

The ecology of the soft-bottom benthos of San Francisco Bay: a community profile

This profile, part of a series of profiles concerning coastal habitats of the United States, is a detailed examination of the soft-bottom benthos of San Francisco Bay. A U.S. Fish and Wildlife Service and California Department of Fish and Game report (1979) entitled "Protection and Restoration of San Francisco Bay Fish and Wildlife Habitat" provides clear recognition of the importance of intertidal and subtidal soft-bottom habitats and their associated organisms to the bay's birds and fishes and to the overall functioning of the estuary. The purpose of this profile is to provide a description of the structure and functioning of the benthic community in San Francisco Bay (exclusive of its tidal marshes, which are discussed by M. Josselyn [1983] in another profile). The habitats covered in this volume include all nonvegetated soft-bottom intertidal and subtidal areas of the bay between the Golden Gate and the mouths of the Sacramento and San Joaquin Rivers to the northeast, and to the southern extremity of the bay. The profile provides a reference to the scientific information concerning the animals and plants of the bay's benthic communities, their importance to the bay ecosystem, and their value as a resource measured in human terms. Because there have been few process-oriented studies of the benthos of San Francisco Bay (e.g., field and laboratory rate-measurement experiments), the material presented herein is largely descriptive. Nonetheless, we have described the processes that interconnect the various physical, chemical, and biological components of the benthic environment, and the important couplings between this environment and the water column above, with reference to research results from other estuaries where necessary. We consider the role of the benthic community as a food source for fish, aquatic birds, and humans; as a consumer or degrader of organic materials including wastes; as a recycler of minerals and nutrients; and as an accumulator of pollutants.

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USGS1988

Phytoplankton growth rates in a light-limited environment, San Francisco Bay

Phytoplankton cells reside in a turbulent medium partitioned into an upper photic zone that sustains photosynthesis, and a lower aphotic zone that does not. In estuaries, vertical mixing rates between these 2 zones can be rapid (< 1 generation time) because of tidal stirring and because the mixing depth is generally shallow. Moreover, the photic depth is characteristically shallow in estuaries because of the high seston concentrations that typify these systems (e.g. Cloern 1987). Hence the mean light exposure of phytoplankton cells, and their rates of photosynthesis and growth in estuaries, should be related to the ratio of photic depth Z p to mixing Z m , (defined as either the water column depth, or the surface layer depth in a stratified estuary). Grobbelar (1985) has measured phytoplankton production in turbid waters and used the ratio Z p :Z m as a simple index to quantify the degree of light limitation. Harris and his colleagues have examined in detail the importance of the Z p :Z m ratio in lakes for influencing variability of phytoplankton production and photosynthetic parameters (Harris et al. 1980), and variability of species composition both temporally (Harris & Piccinin 1980) and spatially (Haffner et al. 1980). Phytoplankton growth rates have only rarely been measured in estuaries (Malone 1977, Furnas 1982, Harding et al. 1986) where cycling rates between the photic and aphotic zones can be much faster than in lakes or the open ocean. This study was motivated by the need for quantitative measures of phytoplankton population growth rate in an estuarine environment, and was designed around the presumption that growth rates can be related empirically to light exposure. We conducted the study in San Francisco Bay (California, USA), which has large horizontal gradients in light availability (Z p :Z m ) typical of many coastal plain estuaries, and nutrient concentrations that often exceed those presumed to limit phytoplankton growth (Cloern et al. 1985). We tested the hypothesis that light availability is the primary control of phytoplankton growth, and that previous estimates of growth rate based on the ratio of productivity to biomass (Cloern et al. 1985) are realistic. Specifically, we wanted to verify that growth rate varies spatially along horizontal gradients of light availability indexed as Z p :Z m , such that phytoplankton turnover rate is rapid in shallow clear areas (high Z p :Z m ) and slow in deep turbid areas (low Z p :Z m ). We used an in situ incubation technique which simulated vertical mixing, and measured both changes in cell number and carbon production as independent estimates of growth rate across a range of Z p :Z m ratios.

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USGS1987

Modeling of estuarne chlorophyll a from an airborne scanner

Near simultaneous collection of 34 surface water samples and airborne multispectral scanner data provided input for regression models developed to predict surface concentrations of estuarine chlorophyll a. Two wavelength ratios were employed in model development. The ratios werechosen to capitalize on the spectral characteristics of chlorophyll a, while minimizing atmospheric influences. Models were then applied to data previously acquired over the study area thre years earlier. Results are in the form of color-coded displays of predicted chlorophyll a concentrations and comparisons of the agreement among measured surface samples and predictions basedon coincident remotely sensed data. The influence of large variations in fresh-water inflow to the estuary are clearly apparent in the results. The synoptic view provided by remote sensing is another method of examining important estuarine dynamics difficult to observe from in situ sampling alone.

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USGS1987

Real-time landslide warning during heavy rainfall

A real-time system for issuing warnings of landslides during major storms is being developed for the San Francisco Bay region, California. The system is based on empirical and theoretical relations between rainfall and landslide initiation, geologic determination of areas susceptible to landslides, real-time monitoring of a regional network of telemetering rain gages, and National Weather Service precipitation forecasts. This system was used to issue warnings during the storms of 12 to 21 February 1986, which produced 800 millimeters of rainfall in the region. Although analysis after the storms suggests that modifications and additional developments are needed, the system successfully predicted the times of major landslide events. It could be used as a prototype for systems in other landslide-prone regions.

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USGS1986

Metabolism of reduced methylated sulfur compounds in anaerobic sediments and by a pure culture of an estuarine methanogen

Addition of dimethylsulfide (DMS), dimethyldisulfide (DMDS), or methane thiol (MSH) to a diversity of anoxic aquatic sediments (e.g., fresh water, estuarine, alkaline/hypersaline) stimulated methane production. The yield of methane recovered from DMS was often 52 to 63%, although high concentrations of DMS (as well as MSH and DMDS) inhibited methanogenesis in some types of sediments. Production of methane from these reduced methylated sulfur compounds was blocked by 2-bromoethanesulfonic acid. Sulfate did not influence the metabolism of millimolar levels of DMS, DMDS, or MSH added to sediments. However, when DMS was added at &sim;2-&mu;M levels as [ 14 C]DMS, metabolism by sediments resulted in a 14 CH 4 / 14 CO 2 ratio of only 0.06. Addition of molybdate increased the ratio to 1.8, while 2-bromoethanesulfonic acid decreased it to 0, but did not block 14 CO 2 production. These results indicate the methanogens and sulfate reducers compete for DMS when it is present at low concentrations; however, at high concentrations, DMS is a &ldquo;noncompetitive&rdquo; substrate for methanogens. Metabolism of DMS by sediments resulted in the appearance of MSH as a transient intermediate. A pure culture of an obligately methylotrophic estuarine methanogen was isolated which was capable of growth on DMS. Metabolism of DMS by the culture also resulted in the transient appearance of MSH, but the organism could grow on neither MSH nor DMDS. The culture metabolized [ 14 C]-DMS to yield a 14 CH 4 / 14 CO 2 ratio of &sim;2.8. Reduced methylated sulfur compounds represent a new class of substrates for methanogens and may be potential precursors of methane in a variety of aquatic habitats.

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USGS1986

Variability in the fractionation of Cu, Ag, and Zn among cytosolic proteins in the bivalve Macoma balthica

Gel filtration chromatographs of cytosols from the clam Macorna balthica analysed from both field and laboratory treated specimens showed that uptake of Cu, Ag, and Zn in the metallothionein-like protein (MLP) pool follows exposure both in nature and in the laboratory. Specimens collected from San Francisco Bay over 18 mo showed strong temporal variability in the fractionation of the metals among cytosolic proteins. A marked increase in Cu, Ag, and Zn in a very low molecular weight pool occurred when concentrations were highest In the MLP pool. The correlation between total cytosollc metal and MLP-metal also appeared to approach a hyperbolic character at the highest concentrations.

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USGS1985

Benthic fluxes in San Francisco Bay

Measurements of benthic fluxes have been made on four occasions between February 1980 and February 1981 at a channel station and a shoal station in South San Francisco Bay, using in situ flux chambers. On each occasion replicate measurements of easily measured substances such as radon, oxygen, ammonia, and silica showed a variability (??1??) of 30% or more over distances of a few meters to tens of meters, presumably due to spatial heterogeneity in the benthic community. Fluxes of radon were greater at the shoal station than at the channel station because of greater macrofaunal irrigation at the former, but showed little seasonal variability at either station. At both stations fluxes of oxygen, carbon dioxide, ammonia, and silica were largest following the spring bloom. Fluxes measured during different seasons ranged over factors of 2-3, 3, 4-5, and 3-10 (respectively), due to variations in phytoplankton productivity and temperature. Fluxes of oxygen and carbon dioxide were greater at the shoal station than at the channel station because the net phytoplankton productivity is greater there and the organic matter produced must be rapidly incorporated in the sediment column. Fluxes of silica were greater at the shoal station, probably because of the greater irrigation rates there. N + N (nitrate + nitrite) fluxes were variable in magnitude and in sign. Phosphate fluxes were too small to measure accurately. Alkalinity fluxes were similar at the two stations and are attributed primarily to carbonate dissolution at the shoal station and to sulfate reduction at the channel station. The estimated average fluxes into South Bay, based on results from these two stations over the course of a year, are (in mmol m-2 d-1): O2 = -27 ?? 6; TCO2 = 23 ?? 6; Alkalinity = 9 ?? 2; N + N = -0.3 ?? 0.5; NH3 = 1.4 ?? 0.2; PO4 = 0.1 ?? 0.4; Si = 5.6 ?? 1.1. These fluxes are comparable in magnitude to those in other temperate estuaries with similar productivity, although the seasonal variability is smaller, probably because the annual temperature range in San Francisco Bay is smaller. Budgets constructed for South San Francisco Bay show that large fractions of the net annual productivity of carbon (about 90%) and silica (about 65%) are recycled by the benthos. Substantial rates of simultaneous nitrification and denitrification must occur in shoal areas, apparently resulting in conversion to N2 of 55% of the particulate nitrogen reaching the sediments. In shoal areas, benthic fluxes can replace the water column standing stocks of ammonia in 2-6 days and silica in 17-34 days, indicating the importance of benthic fluxes in the maintenance of productivity. Pore water profiles of nutrients and Rn-222 show that macrofaunal irrigation is extremely important in transport of silica, ammonia, and alkalinity. Calculations of benthic fluxes from these profiles are less accurate, but yield results consistent with chamber measurements and indicate that most of the NH3, SiO2, and alkalinity fluxes are sustained by reactions occurring throughout the upper 20-40 cm of the sediment column. In contrast, O2, CO2, and N + N fluxes must be dominated by reactions occurring within the upper one cm of the sediment-water interface. While most data support the statements made above, a few flux measurements are contradictory and demonstrate the complexity of benthic exchange. ?? 1985 Dr W. Junk Publishers.

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USGS1984

Chemical determination of particulate nitrogen in San Francisco Bay. Nitrogen: chlorophyll a rations in plankton

Particulate nitrogen (PN) and chlorophyll a (Chl a ) were measured in the northern reach of San Francisco Bay throughout 1980. The PN values were calculated as the differences between unfiltered and filtered (0&middot;4 &mu;m) samples analyzed using the UV-catalyzed peroxide digestion method. The Chl a values were measured spectrophotometrically, with corrections made for phaeopigments. The plot of all PNChla data was found to be non-linear, and the concentration of suspended particulate matter (SPM) was found to be the best selector for linear subsets of the data. The best-fit slopes of PNChla plots, as determined by linear regression (model II), were interpreted to be the N: Chl a ratios of phytoplankton. The Y -intercepts of the regression lines were considered to represent easily-oxidizable detrital nitrogen (EDN). In clear water ( < 10 mg l &minus;1 SPM), the N: Chl a ratio was 1&middot;07 &mu;g-at N per &mu;g Chl a . It decreased to 0&middot;60 in the 10&ndash;18 mg l &minus;1 range and averaged 0&middot;31 in the remaining four ranges (18&ndash;35, 35&ndash;65, 65&ndash;155, and 155&ndash;470 mg l &minus;1 ). The EDN values were less than 1 &mu;g-at N l &minus;1 in the clear water and increased monotonically to almost 12 &mu;g-at N l &minus;1 in the highest SPM range. The N: Chl a ratios for the four highest SPM ranges agree well with data for phytoplankton in light-limited cultures. In these ranges, phytoplankton-N averaged only 20% of the PN, while EDN averaged 39% and refractory-N 41%.

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