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At least 829 records · Page 46Linked to original sources
USGS2000

Fracture process zone in granite

In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement-controlled tests to 2 μm/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is ∼9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be ∼2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.

Journal of Geophysical Research B: Solid Earth↗
USGS2001

Earthquake stress drop and laboratory-inferred interseismic strength recovery

We determine the scaling relationships between earthquake stress drop and recurrence interval t r that are implied by laboratory-measured fault strength. We assume that repeating earthquakes can be simulated by stick-slip sliding using a spring and slider block model. Simulations with static/kinetic strength, time-dependent strength, and rate- and state-variable-dependent strength indicate that the relationship between loading velocity and recurrence interval can be adequately described by the power law V L ∝ t r n where n≈−1. Deviations from n=−1 arise from second order effects on strength, with n>−1 corresponding to apparent time-dependent strengthening and n<−1 corresponding to weakening. Simulations with rate and state-variable equations show that dynamic shear stress drop Δτ d scales with recurrence as d Δτ d / d ln t r ≤σ e ( b-a ), where σ e is the effective normal stress, μ=τ/σ e , and ( a-b )= d μ ss / d ln V is the steady-state slip rate dependence of strength. In addition, accounting for seismic energy radiation, we suggest that the static shear stress drop Δτ s scales as d Δτ s / d ln t r ≤σ e (1 +ζ)( b-a ), where ζ is the fractional overshoot. The variation of Δτ s with ln t r for earthquake stress drops is somewhat larger than implied by room temperature laboratory values of ζ and b-a . However, the uncertainty associated with the seismic data is large and the discrepancy between the seismic observations and the rate of strengthening predicted by room temperature experiments is less than an order of magnitude.

Journal of Geophysical Research B: Solid Earth↗
USGS2000

Acoustic properties of a crack containing magmatic or hydrothermal fluids

We estimate the acoustic properties of a crack containing magmatic or hydrothermal fluids to quantify the source properties of long-period (LP) events observed in volcanic areas assuming that a crack-like structure is the source of LP events. The tails of synthetic waveforms obtained from a model of a fluid-driven crack are analyzed by the Sompi method to determine the complex frequencies of one of the modes of crack resonance over a wide range of the model parameters α/ a and ρ f /ρ s , where αis the P wave velocity of the rock matrix, a is the sound speed of the fluid, and ρ f and p s are the densities of the fluid and rock matrix, respectively. The quality factor due to radiation loss ( Q r ) for the selected mode almost monotonically increases with increasing α/ a , while the dimensionless frequency ( v ) of the mode decreases with increasing α/ a and ρ f /ρ s . These results are used to estimate Q and v for a crack containing various types of fluids (gas-gas mixtures, liquid-gas mixtures, and dusty and misty gases) for values of a , ρ f , and quality factor due to intrinsic losses ( Q i ) appropriate for these types of fluids, in which Q is given by Q −l = Q r −l + Q i −1 . For a crack containing such fluids, we obtain Q ranging from almost unity to several hundred, which consistently explains the wide variety of quality factors measured in LP events observed at various volcanoes. We underscore the importance of dusty and misty gases containing small-size particles with radii around 1 μm to explain long-lasting oscillations with Q significantly larger than 100. Our results may provide a basis for the interpretation of spatial and temporal variations in the observed complex frequencies of LP events in terms of fluid compositions beneath volcanoes.

Journal of Geophysical Research B: Solid Earth↗
USGS2001

Source parameters for the 1952 Kern County earthquake, California: A joint inversion of leveling and triangulation observations

Coseismic leveling and triangulation observations are used to determine the faulting geometry and slip distribution of the July 21, 1952, M w 7.3 Kern County earthquake on the White Wolf fault. A singular value decomposition inversion is used to assess the ability of the geodetic network to resolve slip along a multisegment fault and shows that the network is sufficient to resolve slip along the surface rupture to a depth of 10 km. Below 10 km, the network can only resolve dip slip near the fault ends. The preferred source model is a two-segment right-stepping fault with a strike of 51° and a dip of 75° SW. The epicentral patch has deep (6–27 km) left-lateral oblique slip, while the northeastern patch has shallow (1–12.5 km) reverse slip. There is nearly uniform reverse slip (epicentral, 1.6 m; northeast, 1.9 m), with 3.6 m of left-lateral strike slip limited to the epicentral patch. The seismic moment is M 0 = 9.2 ± 0.5 × 10 19 N m ( M w = 7.2). The signal-to-noise ratio of the leveling and triangulation data is reduced by 96% and 49%, respectively. The slip distribution from the preferred model matches regional geomorphic features and may provide a driving mechanism for regional shortening across the Comanche thrust and structural continuity with the Scodie seismic lineament to the northeast.

California↗
USGS2001

Rapid fluid disruption: A source for self-potential anomalies on volcanoes

Self-potential (SP) anomalies observed above suspected magma reservoirs, dikes, etc., on various volcanoes (Kilauea, Hawaii; Mount Unzen, Japan; Piton de la Fournaise, Reunion Island, Miyake Jima, Japan) result from transient surface electric fields of tens of millivolts per kilometer and generally have a positive polarity. These SP anomalies are usually attributed to electrokinetic effects where properties controlling this process are poorly constrained. We propose an alternate explanation that contributions to electric fields of correct polarity should be expected from charge generation by fluid vaporization/disruption. As liquids are vaporized or removed as droplets by gas transport away from hot dike intrusions, both charge generation and local increase in electrical resistivity by removal of fluids should occur. We report laboratory observations of electric fields in hot rock samples generated by pulses of fluid (water) through the rock at atmospheric pressure. These indicate the relative amplitudes of rapid fluid disruption (RFD) potentials and electrokinetic potentials to be dramatically different and the signals are opposite in sign. Above vaporization temperatures, RFD effects of positive sign in the direction of gas flow dominate, whereas below these temperatures, effects of negative sign dominate. This suggests that the primary contribution to observed self-potential anomalies arises from gas-related charge transport processes at temperatures high enough to produce vigorous boiling and vapor transport. At lower temperatures, the primary contribution is from electrokinetic effects modulated perhaps by changing electrical resistivity and RFD effects from high-pressure but low-temperature CO2 and SO2 gas flow ripping water molecules from saturated crustal rocks. If charge generation is continuous, as could well occur above a newly emplaced dike, positive static potentials will be set up that could be sustained for many years, and the simplest method for identifying these hot, active regions would be to identify the SP anomalies they generate.

Hawai'i↗
USGS2001

Modeling consolidation and dewatering near the toe of the northern Barbados accretionary complex

At the toe of the northern Barbados accretionary complex, temperature and pore water chemistry data indicate that fluid flow is channeled along the décollement and other shallow thrust faults. We examine mechanisms that may prevent consolidation and maintain high permeability over large sections of the décollement. High-resolution bulk density data from five boreholes show that the décollement is well consolidated at some sites while other sites remain underconsolidated. Underconsolidated décollement behavior is associated with kilometer-scale negative-polarity seismic reflections from the décollement plane that have been interpreted to be fluid conduits. We use a coupled fluid flow/consolidation model to simulate the loading response of a 10-km-long by 680-m-thick slice of sediment as it enters the accretionary complex. The simulations capture 185 ka (5 km) of subduction, with a load function representing the estimated effective stress of the overriding accretionary prism (3.8° taper angle). Simulation results of bulk density in the décollement 3.2 km arcward of the deformation front are compared with observations. The results show that persistent high pore pressures at the arcward edge of the simulation domain can explain underconsolidated behavior. The scenario is consistent with previous modeling results showing that high pore pressures can propagate intermittently along the décollement from deeper in the complex. Simulated seaward fluxes in the décollement (1–14 cm yr −1 ) lie between previous estimates from modeling studies of steady state (<1 cm yr −1 ) and transient (>1 m yr −1 ) flow. Maximum simulated instantaneous fluid sources (2.5×10 −13 s −1 ) are comparable to previous estimates. The simulations show minor swelling of incoming sediments (fluid sources ∼−3×10 −15 s −1 ) up to 3 km before subduction that may help to explain small-scale shearing and normal faulting proximal to the protodécollement.

Atlantic Ocean, Northern Barbados Accretionary Com↗
USGS2001

Experimental observations of pressure oscillations and flow regimes in an analogue volcanic system

Gas-liquid flows, designed to be analogous to those in volcanic conduits, are generated in the laboratory using organic gas-gum rosin mixtures expanding in a vertically mounted tube. The expanding fluid shows a range of both flow and pressure oscillation behaviors. Weakly supersaturated source liquids produce a low Reynolds number flow with foam expanding from the top surface of a liquid that exhibits zero fluid velocity at the tube wall; i.e., the conventional "no-slip" boundary condition. Pressure oscillations, often with strong long-period characteristics and consistent with longitudinal and radial resonant oscillation modes, are detected in these fluids. Strongly supersaturated source liquids generate more energetic flows that display a number of flow regimes. These regimes include a static liquid source, viscous flow, detached flow (comprising gas-pockets-at-wall and foam-in-gas annular flow, therefore demonstrating strong radial heterogeneity), and a fully turbulent transonic fragmented or mist flow. Each of these flow regimes displays characteristic pressure oscillations that can be related to resonance of flow features or wall impact phenomena. The pressure oscillations are produced by the degassing processes without the need of elastic coupling to the confining medium or flow restrictors and valvelike features. The oscillatory behavior of the experimental flows is compared to seismoacoustic data from a range of volcanoes where resonant oscillation of the fluid within the conduit is also often invoked as controlling the observed oscillation frequencies. On the basis of the experimental data we postulate on the nature of seismic signals that may be measured during large-scale explosive activity.

Journal of Geophysical Research B: Solid Earth↗
USGS2001

Travel times of P and S from the global digital seismic networks: Implications for the relative variation of P and S velocity in the mantle

We present new data sets of P and S arrival times which have been handpicked from long-period vertical and transverse component recordings of the various global seismic networks. Using events which occurred from 1976 to 1994 results in ∼38,000 globally well-distributed measurements of teleseismic P and ∼41,000 measurements of S . These data are particularly useful for looking at the relative variation of S and P velocities in the lower mantle. We describe both the measurement techniques and the gross characteristics of the data sets. The size of our data sets allows us to exploit the internal consistency of the data to identify outliers using a summary ray analysis. Since the polarity of each arrival is also known, we can construct fault plane solutions and/or compare with polarities predicted by the Harvard centroid moment tensor solutions to further diagnose phase misidentification. This analysis results in ∼5% of the data being identified as outliers. An analysis of variance indicates that the S residual travel times are dominated by the effects of three-dimensional structure but the P data have comparable contributions from noise and source mislocation effects. The summary ray analysis reveals the basic character of lower mantle structure, and there are large-scale patterns in both the S and P data sets that correlate quite well with each other. This analysis suggests that on average, d ln v s / d ln v p is an increasing function of depth in the mantle going from a value of ∼1.7 at the top of the lower mantle to an apparent value of 4 near the base of the mantle. This latter extreme value of R seems to result mainly from data which sample one region in the lowermost mantle under the central Pacific, where large positive S residuals are associated with very small P residuals. Such an anomaly cannot be thermal in origin.

Journal of Geophysical Research B: Solid Earth↗
USGS2001

Deformation across the Pacific-North America plate boundary near San Francisco, California

We have detected a narrow zone of compression between the Coast Ranges and the Great Valley, and we have estimated slip rates for the San Andreas, Rodgers Creek, and Green Valley faults just north of San Francisco. These results are based on an analysis of campaign and continuous Global Positioning System (GPS) data collected between 1992 and 2000 in central California. The zone of compression between the Coast Ranges and the Great Valley is 25 km wide. The observations clearly show 3.8±1.5 mm yr −1 of shortening over this narrow zone. The strike slip components are best fit by a model with 20.8±1.9 mm yr −1 slip on the San Andreas fault, 10.3±2.6 mm yr −1 on the Rodgers Creek fault, and 8.1±2.1 mm yr −1 on the Green Valley fault. The Pacific-Sierra Nevada-Great Valley motion totals 39.2±3.8 mm yr −1 across a zone that is 120 km wide (at the latitude of San Francisco). Standard deviations are one σ. The geodetic results suggest a higher than geologic rate for the Green Valley fault. The geodetic results also suggest an inconsistency between geologic estimates of the San Andreas rate and seismologic estimates of the depth of locking on the San Andreas fault. The only convergence observed is in the narrow zone along the border between the Great Valley and the Coast Ranges.

California↗
USGS2001

Resolution analysis of finite fault source inversion using one- and three-dimensional Green's functions 2. Combining seismic and geodetic data

Using numerical tests for a prescribed heterogeneous earthquake slip distribution, we examine the importance of accurate Green's functions (GF) for finite fault source inversions which rely on coseismic GPS displacements and leveling line uplift alone and in combination with near-source strong ground motions. The static displacements, while sensitive to the three-dimensional (3-D) structure, are less so than seismic waveforms and thus are an important contribution, particularly when used in conjunction with waveform inversions. For numerical tests of an earthquake source and data distribution modeled after the 1994 Northridge earthquake, a joint geodetic and seismic inversion allows for reasonable recovery of the heterogeneous slip distribution on the fault. In contrast, inaccurate 3-D GFs or multiple 1-D GFs allow only partial recovery of the slip distribution given strong motion data alone. Likewise, using just the GPS and leveling line data requires significant smoothing for inversion stability, and hence, only a blurred vision of the prescribed slip is recovered. Although the half-space approximation for computing the surface static deformation field is no longer justifiable based on the high level of accuracy for current GPS data acquisition and the computed differences between 3-D and half-space surface displacements, a layered 1-D approximation to 3-D Earth structure provides adequate representation of the surface displacement field. However, even with the half-space approximation, geodetic data can provide additional slip resolution in the joint seismic and geodetic inversion provided a priori fault location and geometry are correct. Nevertheless, the sensitivity of the static displacements to the Earth structure begs caution for interpretation of surface displacements, particularly those recorded at monuments located in or near basin environments.

Journal of Geophysical Research B: Solid Earth↗
USGS2001

Resolution analysis of finite fault source inversion using one- and three-dimensional Green's functions 1. Strong motions

We develop a methodology to perform finite fault source inversions from strong motion data using Green's functions (GFs) calculated for a three-dimensional (3-D) velocity structure. The 3-D GFs are calculated numerically by inserting body forces at each of the strong motion sites and then recording the resulting strains along the target fault surface. Using reciprocity, these GFs can be recombined to represent the ground motion at each site for any (heterogeneous) slip distribution on the fault. The reciprocal formulation significantly reduces the required number of 3-D finite difference computations to at most 3NS, where NS is the number of strong motion sites used in the inversion. Using controlled numerical resolution tests, we have examined the relative importance of accurate GFs for finite fault source inversions which rely on near-source ground motions. These experiments use both 1-D and 3-D GFs in inversions for hypothetical rupture models in order (1) to analyze the ability of the 3-D methodology to resolve trade-offs between complex source phenomena and 3-D path effects, (2) to address the sensitivity of the inversion results to uncertainties in the 3-D velocity structure, and (3) to test the adequacy of the 1-D GF method when propagation effects are known to be three-dimensional. We find that given "data" from a prescribed 3-D Earth structure, the use of well-calibrated 3-D GFs in the inversion provides very good resolution of the assumed slip distribution, thus adequately separating source and 3-D propagation effects. In contrast, using a set of inexact 3-D GFs or a set of hybrid 1-D GFs allows only partial recovery of the slip distribution. These findings suggest that in regions of complex geology the use of well-calibrated 3-D GFs has the potential for increased resolution of the rupture process relative to 1-D GFs. However, realizing this full potential requires that the 3-D velocity model and associated GFs should be carefully validated against the true 3-D Earth structure before performing the inverse problem with actual data.

Journal of Geophysical Research B: Solid Earth↗
USGS2001

Thermal thickness and evolution of Precambrian lithosphere: A global study

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat‐producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

Journal of Geophysical Research B: Solid Earth↗
USGS2001

Rheology of ice I at low stress and elevated confining pressure

Triaxial compression testing of pure, polycrystalline water ice I at conditions relevant to planetary interiors and near-surface environments (differential stresses 0.45 to 10 MPa, temperatures 200 to 250 K, confining pressure 50 MPa) reveals that a complex variety of rheologies and grain structures may exist for ice and that rheology of ice appears to depend strongly on the grain structures. The creep of polycrystalline ice I with average grain size of 0.25 mm and larger is consistent with previously published dislocation creep laws, which are now extended to strain rates as low as 2×10 −8 s −1 . When ice I is reduced to very fine and uniform grain size by rapid pressure release from the ice II stability field, the rheology changes dramatically. At 200 and 220 K the rheology matches the grain-size-sensitive rheology measured by Goldsby and Kohlstedt [1997, this issue] at 1 atm. This finding dispels concerns that the Goldsby and Kohlstedt results were influenced by mechanisms such as microfracturing and cavitation, processes not expected to operate at elevated pressures in planetary interiors. At 233 K and above, grain growth causes the fine-grained ice to become more creep resistant. Scanning electron microscopy investigation of some of these deformed samples shows that grains have markedly coarsened and the strain hardening can be modeled by normal grain growth and the Goldsby and Kohlstedt rheology. Several samples also displayed very heterogeneous grain sizes and high aspect ratio grain shapes. Grain-size-sensitive creep and dislocation creep coincidentally contribute roughly equal amounts of strain rate at conditions of stress, temperature, and grain size that are typical of terrestrial and planetary settings, so modeling ice dynamics in these settings must include both mechanisms.

Journal of Geophysical Research B: Solid Earth↗
USGS2000

New 40Ar/39Ar age of the Bishop Tuff from multiple sites and sediment rate calibration for the Matuyama-Brunhes boundary

Precise dating of sanidine from proximal ash flow Bishop Tuff and air fall Bishop pumice and ash, California, can be used to derive an absolute age of the Matuyama Reversed-Brunhes Normal (M-B) paleomagnetic transition, identified stratigraphically close beneath the Bishop Tuff and ash at many sites in the western United States. An average age of 758.9±1.8 ka, standard error of the mean (SEM), was obtained for individual sanidine crystals or groups of several crystals, determined from ∼70 individual analyses of sanidine separates from 11 sample groups obtained at five localities. The basal air fall pumice (757.7±1.8 ka) and overlying ash flow tuff (762.2±4.7 ka) from near the source yield essentially the same dates within errors of analysis, suggesting that the two units were emplaced close in time. A date on distal Bishop air fall ash bed at Priant, California, ∼100 km to the west of the source area, is younger, 750.1±4.3 ka, but not significantly different within analytical error (±1 standard deviation). Previous dates of the Bishop Tuff, obtained by others using conventional K-Ar and the fission track method on zircons, ranged from ∼650 ka to ∼1.0 Ma. The most recent, generally accepted date by the K-Ar method on sanidine was 738±3 ka. We infer, as others before, that many K-Ar dates on sanidine feldspar are too young owing to incomplete degassing of radiogenic Ar during fusion in the K-Ar technique and that many older K-Ar dates are too old owing to detrital or xenocrystic contamination in the larger samples that are necessary for the technique. The new dates are similar to recent 40Ar/39Ar ages of the Bishop Tuff determined on individual samples by others but are derived from a larger proximal sample population and from multiple analysis of each sample. The results provide a definitive and precise age calibration of this widespread chronostratigraphic marker in the western United States and northeastern Pacific Ocean. We calculated the age of the M-B transition at five sites, assuming constant sedimentation rates, the age of the Bishop ash bed and one or more well-dated chronostratigraphic horizons above and below the Bishop Tuff ash bed and M-B transition, and stratigraphic separations between these datum levels. The age of the M-B transition is 774.2±2.8 ka, based on the average of eight such calculations, close to other recent determinations, and similar to that determined from the astronomically tuned polarity timescale. Our approach provides an alternative and surprisingly precise method for determining the age of the M-B and other chronostratigraphic levels. The above dates, calculated using U.S. Geological Survey values of 27.92 Ma for the Taylor Creek (TC) sanidine can be recalculated to other widely used values for these monitors. For example, using recently published values of 28.34 Ma (TC) and 523.1 Ma (McLure Mountain hornblende, MMhb-1), the resulting ages are ∼774 ka for the Bishop Tuff and ash bed and ∼789 ka for the M-B transition.

California↗
USGS2001

Creation of residual flows in a partially stratified estuary

The creation of residual flows in estuaries is examined using acoustic Doppler current profiler data sets from northern San Francisco Bay. The data sets are analyzed using principal component analysis to examine the temporal variability of the flows which create the residual circulation. It is seen that in this periodically and partially stratified estuary the residual flows are created through a series of pulses with strong variability at the 24‐hour timescale, through the interaction of shear, stratification and mixing. This interaction is captured through the use of a dimensionless number, the horizontal Richardson number ( Ri x ), which is developed to examine the local balance between the stratifying and destratifying forces at the tidal timescale. It is seen that Ri x is a valuable parameter in predicting the onset of the residual‐creating events, with a threshold value of ≈3 on ebb tides. This critical value is argued to be a threshold, above which the stratification and shear flow create a feedback effect, each further intensifying the other. This feedback results in a highly variable exchange flow which creates the estuarine residual in intermittent pulses rather than as a steady flow. Although typically attributed to baroclinic forcing, an argument is made that these pulses of residual‐creating exchange flow could be created by barotropic forcing in the presence of variable stratification which is asymmetric between flood and ebb tides. This result poses a great challenge for turbulence modeling, as the timing and magnitude of stratification and shear must be correctly simulated on the tidal timescale in order to reproduce the effects seen in the data sets presented.

Journal of Geophysical Research C: Oceans↗
USGS2001

Secondary currents in a curved, stratified, estuarine channel

This paper presents a study of secondary circulation in a curved stratified channel in northern San Francisco Bay over a 12.5-hour tidal cycle. Secondary currents were strong at times (varying by up to 35 cm/s from top to bottom) but relatively transient, as the balance between centrifugal and lateral baroclinic forcing changed over time. The short travel time around the bend did not allow a steady state balance to develop between centrifugal and lateral baroclinic forcing. During the flood tide the confluence of two streams with different velocities produced a strong lateral gradient in streamwise velocity. As a result, lateral advection was a significant term in the streamwise momentum balance, having the same order of magnitude as the barotropic and baroclinic pressure gradients, and the frictional terms. During the first part of the ebb, secondary currents were induced by lateral baroclinic forcing. The direction of the secondary circulation reversed later in the ebb, as the baroclinic forcing became weaker than the centrifugal acceleration. The gradient Richardson number showed that stratification was stable over most of the tidal cycle, decreasing the importance of friction and allowing secondary currents to persist.

California↗
USGS2002

Across-shelf sediment transport: Interactions between suspended and bed sediment

We use a two-dimensional, time-dependent sediment-transport model to quantify across-shelf transport, deposition, and sorting during wave-driven resuspension events characteristic of those that dominate sediment transport on many continental shelves. Decreases in wave-orbital velocities as water depth increases, and the resulting cross-shelf gradient in bed shear stress favor a net offshore transport of sediment. On wide, flat shelves (slopes ∼0.1%percnt;), these gradients are low, and the depth to which the seabed is reworked depends mainly on bottom shear stress and local sediment availability. On narrow, steep shelves (slopes ∼0.5%percnt;), however, the gradient in bottom stress generates significant cross-shelf suspended sediment flux gradients that create regions of net erosion and deposition. While the magnitude of waves generally determines the water depth to which sediment can be resuspended, erosional and depositional patterns on narrow shelves are sensitive to cross-shelf gradients in wave energy, nonlocal sediment availability, and the direction and magnitude of the cross-shelf current. During energetic waves, cross-shelf divergence of suspended sediment flux can create a coarsened, erosional area on the inner shelf that abuts a region of fine-grained sediment deposition on the mid-to-outer shelf. If currents are strongly shoreward, however, flux divergence leads to erosion over the entire shelf.

Journal of Geophysical Research C: Oceans↗
USGS2000

Numerical model of frazil ice and suspended sediment concentrations and formation of sediment laden ice in the Kara Sea

A one-dimensional (vertical) numerical model of currents, mixing, frazil ice concentration, and suspended sediment concentration has been developed and applied in the shallow southeastern Kara Sea. The objective of the calculations is to determine whether conditions suitable for turbid ice formation can occur during times of rapid cooling and wind- and wave-induced sediment resuspension. Although the model uses a simplistic approach to ice particles and neglects ice-sediment interactions, the results for low-stratification, shallow (∼20-m) freeze-up conditions indicate that the coconcentrations of frazil ice and suspended sediment in the water column are similar to observed concentrations of sediment in turbid ice. This suggests that wave-induced sediment resuspension is a viable mechanism for turbid ice formation, and enrichment mechanisms proposed to explain the high concentrations of sediment in turbid ice relative to sediment concentrations in underlying water may not be necessary in energetic conditions. However, salinity stratification found near the Ob' and Yenisey Rivers damps mixing between ice-laden surface water and sediment-laden bottom water and probably limits incorporation of resuspended sediment into turbid ice until prolonged or repeated wind events mix away the stratification. Sensitivity analyses indicate that shallow (≤20 m), unstratified waters with fine bottom sediment (settling speeds of ∼1 mm s −1 or less) and long open water fetches (>25 km) are ideal conditions for resuspension.

Kara Sea↗