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USGS2005

Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause

The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. I argue that these features are manifestations of large vertical movements of a segment of the Puerto Rico trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. ago over a time period as short as 14-40 kyr. I explain these vertical movements by a sudden increase in the slab's descent angle that caused the trench to subside and the island to rise. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. The rapid (14-40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5??W and trend southwestwardly toward eastern Puerto Rico. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount.

Journal of Geophysical Research B: Solid Earth↗
USGS2005

An efficient algorithm for double-difference tomography and location in heterogeneous media, with an application to the Kilauea volcano

Improving our understanding of crustal processes requires a better knowledge of the geometry and the position of geological bodies. In this study we have designed a method based upon double-difference relocation and tomography to image, as accurately as possible, a heterogeneous medium containing seismogenic objects. Our approach consisted not only of incorporating double difference in tomography but also partly in revisiting tomographic schemes for choosing accurate and stable numerical strategies, adapted to the use of cross-spectral time delays. We used a finite difference solution to the eikonal equation for travel time computation and a Tarantola-Valette approach for both the classical and double-difference three-dimensional tomographic inversion to find accurate earthquake locations and seismic velocity estimates. We estimated efficiently the square root of the inverse model's covariance matrix in the case of a Gaussian correlation function. It allows the use of correlation length and a priori model variance criteria to determine the optimal solution. Double-difference relocation of similar earthquakes is performed in the optimal velocity model, making absolute and relative locations less biased by the velocity model. Double-difference tomography is achieved by using high-accuracy time delay measurements. These algorithms have been applied to earthquake data recorded in the vicinity of Kilauea and Mauna Loa volcanoes for imaging the volcanic structures. Stable and detailed velocity models are obtained: the regional tomography unambiguously highlights the structure of the island of Hawaii and the double-difference tomography shows a detailed image of the southern Kilauea caldera-upper east rift zone magmatic complex. Copyright 2005 by the American Geophysical Union.

Hawaii↗
USGS2005

Slicing up the San Francisco Bay Area: Block kinematics and fault slip rates from GPS-derived surface velocities

Observations of surface deformation allow us to determine the kinematics of faults in the San Francisco Bay Area. We present the Bay Area velocity unification (BA??VU??, "bay view"), a compilation of over 200 horizontal surface velocities computed from campaign-style and continuous Global Positioning System (GPS) observations from 1993 to 2003. We interpret this interseismic velocity field using a three-dimensional block model to determine the relative contributions of block motion, elastic strain accumulation, and shallow aseismic creep. The total relative motion between the Pacific plate and the rigid Sierra Nevada/Great Valley (SNGV) microplate is 37.9 ?? 0.6 mm yr-1 directed toward N30.4??W ?? 0.8?? at San Francisco (??2??). Fault slip rates from our preferred model are typically within the error bounds of geologic estimates but provide a better fit to geodetic data (notable right-lateral slip rates in mm yr-1: San Gregorio fault, 2.4 ?? 1.0; West Napa fault, 4.0 ?? 3.0; zone of faulting along the eastern margin of the Coast Range, 5.4 ?? 1.0; and Mount Diablo thrust, 3.9 ?? 1.0 of reverse slip and 4.0 ?? 0.2 of right-lateral strike slip). Slip on the northern Calaveras is partitioned between both the West Napa and Concord/ Green Valley fault systems. The total convergence across the Bay Area is negligible. Poles of rotation for Bay Area blocks progress systematically from the North America-Pacific to North America-SNGV poles. The resulting present-day relative motion cannot explain the strike of most Bay Area faults, but fault strike does loosely correlate with inferred plate motions at the time each fault initiated. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS2005

Models of lithosphere and asthenosphere anisotropic structure of the Yellowstone hot spot from shear wave splitting

Teleseismic shear wave splitting measured at 56 continuous and temporary seismographs deployed in a 500 km by 600 km area around the Yellowstone hot spot indicates that fast anisotropy in the mantle is parallel to the direction of plate motion under most of the array. The average split time from all stations of 0.9 s is typical of continental stations. There is little evidence for plume-induced radial strain, suggesting that any contribution of gravitationally spreading plume material is undetectably small with respect to the plate motion velocity. Two stations within Yellowstone have splitting measurements indicating the apparent fast anisotropy direction (ϕ) is nearly perpendicular to plate motion. These stations are ∼30 km from stations with ϕ parallel to plate motion. The 70° rotation over 30 km suggests a shallow source of anisotropy; however, split times for these stations are more than 2 s. We suggest melt-filled, stress-oriented cracks in the lithosphere are responsible for the anomalous ϕ orientations within Yellowstone. Stations southeast of Yellowstone have measurements of ϕ oriented NNW to WNW at high angles to the plate motion direction. The Archean lithosphere beneath these stations may have significant anisotropy capable of producing the observed splitting.

Yellowstone↗
USGS2005

Waveform tomography of crustal structure in the south San Francisco Bay region

We utilize a scattering-based seismic tomography technique to constrain crustal tructure around the southern San Francisco Bay region (SFBR). This technique is based on coupled traveling wave scattering theory, which has usually been applied to the interpretation of surface waves in large regional-scale studies. Using fully three-dimensional kernels, this technique is here applied to observed P, S, and surface waves of intermediate period (3-4 s dominant period) observed following eight selected regional events. We use a total of 73 seismograms recorded by a U.S. Geological Survey short-period seismic array in the western Santa Clara Valley, the Berkeley Digital Seismic Network, and the Northern California Seismic Network. Modifications of observed waveforms due to scattering from crustal structure include (positive or negative) amplification, delay, and generation of coda waves. The derived crustal structure explains many of the observed signals which cannot be explained with a simple layered structure. There is sufficient sensitivity to both deep and shallow crustal structure that even with the few sources employed in the present study, we obtain shallow velocity structure which is reasonably consistent with previous P wave tomography results. We find a depth-dependent lateral velocity contrast across the San Andreas fault (SAF), with higher velocities southwest of the SAF in the shallow crust and higher velocities northeast of the SAF in the midcrust. The method does not have the resolution to identify very slow sediment velocities in the upper approximately 3 km since the tomographic models are smooth at a vertical scale of about 5 km. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS2005

Source mechanism of Vulcanian degassing at Popocatépetl Volcano, Mexico, determined from waveform inversions of very long period signals

The source mechanism of very long period (VLP) signals accompanying volcanic degassing bursts at Popocatépetl is analyzed in the 15–70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two eruptions (23 April and 23 May 2000) representative of mild Vulcanian activity are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500 m below the western perimeter of the summit crater, and the modeled source is composed of a shallow dipping crack (sill with easterly dip of 10°) intersecting a steeply dipping crack (northeast striking dike dipping 83° northwest), whose surface extension bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation, reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of 3–5 min. The largest moment release occurs in the sill, showing a maximum volume change of 500–1000 m 3 , pressure drop of 3–5 MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is less (200–300 m 3 ), with a corresponding pressure drop of 1–2 MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources are single-force components with magnitudes of 10 8 N, consistent with melt advection in response to pressure transients. The source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds. Such source behavior is consistent with the opening of a pathway for escape of pent-up gases from slow pressurization of the sill driven by magma crystallization. The opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40 MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius (100 m), crack aperture (5 m), bubble number density (10 10 –10 12 m −3 ), initial bubble radius (10 −6 m), final bubble radius (∼10 −5 m), and net decrease of gas concentration in the melt (0.01 wt %).

Popocatépetl Volcano↗
USGS2005

Applying petrophysical models to radar travel time and electrical resistivity tomograms: Resolution-dependent limitations

[1] Geophysical imaging has traditionally provided qualitative information about geologic structure; however, there is increasing interest in using petrophysical models to convert tomograms to quantitative estimates of hydrogeologic, mechanical, or geochemical parameters of interest (e.g., permeability, porosity, water content, and salinity). Unfortunately, petrophysical estimation based on tomograms is complicated by limited and variable image resolution, which depends on (1) measurement physics (e.g., electrical conduction or electromagnetic wave propagation), (2) parameterization and regularization, (3) measurement error, and (4) spatial variability. We present a framework to predict how core‐scale relations between geophysical properties and hydrologic parameters are altered by the inversion, which produces smoothly varying pixel‐scale estimates. We refer to this loss of information as “correlation loss.” Our approach upscales the core‐scale relation to the pixel scale using the model resolution matrix from the inversion, random field averaging, and spatial statistics of the geophysical property. Synthetic examples evaluate the utility of radar travel time tomography (RTT) and electrical‐resistivity tomography (ERT) for estimating water content. This work provides (1) a framework to assess tomograms for geologic parameter estimation and (2) insights into the different patterns of correlation loss for ERT and RTT. Whereas ERT generally performs better near boreholes, RTT performs better in the interwell region. Application of petrophysical models to the tomograms in our examples would yield misleading estimates of water content. Although the examples presented illustrate the problem of correlation loss in the context of near‐surface geophysical imaging, our results have clear implications for quantitative analysis of tomograms for diverse geoscience applications.

Journal of Geophysical Research B: Solid Earth↗
USGS2004

Spatial and temporal multiyear sea ice distributions in the Arctic: A neural network analysis of SSM/I data, 1988-2001

Arctic multiyear sea ice concentration maps for January 1988-2001 were generated from SSM/I brightness temperatures (19H, 19V, and 37V) using modified multiple layer perceptron neural networks. Learning data for the neural networks were extracted from ice maps derived from Okean and ERS satellite imagery to capitalize on the stability of active radar multiyear ice signatures. Evaluations of three learning algorithms and several topologies indicated that networks constructed with error back propagation learning and 3-20-1 topology produced the most consistent and physically plausible results. Operational neural networks were developed specifically with January learning data, and then used to estimate daily multiyear ice concentrations from daily-averaged SSM/I brightness temperatures during January. Monthly mean maps were produced for analysis by averaging the respective daily estimates. The 14-year series of January multiyear ice distributions revealed dense and persistent cover in the central Arctic surrounded by expansive regions of highly fluctuating interannual cover. Estimates of total multiyear ice area by the neural network were intermediate to those of other passive microwave algorithms, but annual fluctuations and trends were similar among all algorithms. When compared to Radarsat estimates of multiyear ice concentration in the Beaufort and Chukchi Seas (1997-1999), average discrepancies were small (0.9-2.5%) and spatial coherency was reasonable, indicating the neural network's Okean and ERS learning data facilitated passive microwave inversion that emulated backscatter signatures. During 1988-2001, total January multiyear ice area declined at a significant linear rate of -54.3 x 10 3 km 2 /yr -1 (-1.4%/yr -1 ). The most persistent and extensive decline in multiyear ice concentration (-3.3%/yr -1 ) occurred in the southern Beaufort and Chukchi Seas. In autumn 1996, a large multiyear ice recruitment of over 106 km 2 (mostly in the Siberian Arctic) fully replenished the previous 8-year decline in total area, but it was followed by an accelerated and compensatory decline during the subsequent 4 years. Seventy-five percent of the interannual variation in January multiyear sea ice area was explained by linear regression on two atmospheric parameters: the previous inter's (JFM) Arctic Oscillation index as a proxy to melt duration and the previous year's average sea level pressure gradient across the Fram Strait as a proxy to annual ice export. Consecutive year changes (1994-2001) in January multiyear ice volume were significantly correlated with duration of the intervening melt season (R 2 = 0.73, -80.0 km 3 /d -1 ), emphasizing a large thermodynamic influence on the Arctic's mass sea ice balance during summers with anomalous melt durations.

Journal of Geophysical Research↗
USGS2005

Wave spectral energy variability in the northeast Pacific

The dominant characteristics of wave energy variability in the eastern North Pacific are described from NOAA National Data Buoy Center (NDBC) buoy data collected from 1981 to 2003. Ten buoys at distributed locations were selected for comparison based on record duration and data continuity. Long‐period (LP) [ T > 12] s, intermediate‐period [6 ≤ T ≤ 12] s, and short‐period [ T < 6] s wave spectral energy components are considered separately. Empirical orthogonal function (EOF) analyses of monthly wave energy anomalies reveal that all three wave energy components exhibit similar patterns of spatial variability. The dominant mode represents coherent heightened (or diminished) wave energy along the West Coast from Alaska to southern California, as indicated by composites of the 700 hPa height field. The second EOF mode reveals a distinct El Niño‐Southern Oscillation (ENSO)‐associated spatial distribution of wave energy, which occurs when the North Pacific storm track is extended unusually far south or has receded to the north. Monthly means and principal components (PCs) of wave energy levels indicate that the 1997–1998 El Niño winter had the highest basin‐wide wave energy within this record, substantially higher than the 1982–1983 El Niño. An increasing trend in the dominant PC of LP wave energy suggests that storminess has increased in the northeast Pacific since 1980. This trend is emphasized at central eastern North Pacific locations. Patterns of storminess variability are consistent with increasing activity in the central North Pacific as well as the tendency for more extreme waves in the south during El Niño episodes and in the north during La Niña.

Journal of Geophysical Research C: Oceans↗
USGS2005

A comparison of hydrographically and optically derived mixed layer depths

Efforts to understand and model the dynamics of the upper ocean would be significantly advanced given the ability to rapidly determine mixed layer depths (MLDs) over large regions. Remote sensing technologies are an ideal choice for achieving this goal. This study addresses the feasibility of estimating MLDs from optical properties. These properties are strongly influenced by suspended particle concentrations, which generally reach a maximum at pycnoclines. The premise therefore is to use a gradient in beam attenuation at 660 nm (c660) as a proxy for the depth of a particle-scattering layer. Using a global data set collected during World Ocean Circulation Experiment cruises from 1988-1997, six algorithms were employed to compute MLDs from either density or temperature profiles. Given the absence of published optically based MLD algorithms, two new methods were developed that use c660 profiles to estimate the MLD. Intercomparison of the six hydrographically based algorithms revealed some significant disparities among the resulting MLD values. Comparisons between the hydrographical and optical approaches indicated a first-order agreement between the MLDs based on the depths of gradient maxima for density and c660. When comparing various hydrographically based algorithms, other investigators reported that inherent fluctuations of the mixed layer depth limit the accuracy of its determination to 20 m. Using this benchmark, we found a ???70% agreement between the best hydrographical-optical algorithm pairings. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗
USGS2005

Numerical modeling of an estuary: A comprehensive skill assessment

Numerical simulations of the Hudson River estuary using a terrain-following, three-dimensional model (Regional Ocean Modeling System (ROMS)) are compared with an extensive set of time series and spatially resolved measurements over a 43 day period with large variations in tidal forcing and river discharge. The model is particularly effective at reproducing the observed temporal variations in both the salinity and current structure, including tidal, spring neap, and river discharge-induced variability. Large observed variations in stratification between neap and spring tides are captured qualitatively and quantitatively by the model. The observed structure and variations of the longitudinal salinity gradient are also well reproduced. The most notable discrepancy between the model and the data is in the vertical salinity structure. While the surface-to-bottom salinity difference is well reproduced, the stratification in the model tends to extend all the way to the water surface, whereas the observations indicate a distinct pycnocline and a surface mixed layer. Because the southern boundary coindition is located near the mouth the estuary, the salinity within the domain is particularly sensitive to the specification of salinity at the boundary. A boundary condition for the horizontal salinity gradient, based on the local value of salinity, is developed to incorporate physical processes beyond the open boundary not resolved by the model. Model results are sensitive to the specification of the bottom roughness length and vertical stability functions, insofar as they influence the intensity of vertical mixing. The results only varied slightly between different turbulence closure methods of k-??, k-??, and k-kl. Copyright 2005 by the American Geophysical Union.

New York↗
USGS2005

Pitted cones and domes on Mars: Observations in Acidalia Planitia and Cydonia Mensae using MOC, THEMIS, and TES data

Domes and cones with summit pits located in Acidalia Planitia and Cydonia Mensae were studied using MOC and THEMIS images and a TES‐derived thermal inertia map. North of 40.5°N latitude, the features have a dome‐like morphology, and south of that latitude, the morphology is more cone‐like. Layering is apparent in the summit craters of fresher looking southern cones, and asymmetric aprons were observed in some instances. Some of the northern domes also display layering in their summit craters, but asymmetric aprons were not observed. The northern domes can also display multiple summit pits or no summit pits at all and can occur in association with higher‐albedo “pancake” features. The northern domes are higher in albedo but have apparent thermal inertias that are lower than the surrounding plains. The apparent thermal inertia values of the southern cones range from values comparable to the surrounding plains to slightly lower. From the TES thermal inertia map, we infer that the thermal inertia values of the pitted cones are between those of basaltic fine dust and sand, while those of the surrounding plains are closer to that of basaltic sand. While a unique interpretation of the origin of the pitted cones is not possible with the available data, we do not find compelling evidence to suggest an origin related to either basaltic volcanism or ground‐ice. Instead, an origin for these features through some combination of mud volcanism and evaporite deposition around geysers and/or springs is most consistent with the observations.

Journal of Geophysical Research E: Planets↗
USGS2005

Three decades of Martian surface changes

The surface of Mars has changed dramatically during the three decades spanned by spacecraft exploration. Comparisons of Mars Global Surveyor images with Viking and Mariner 9 pictures suggest that more than one third of Mars' surface area has brightened or darkened by at least 10%. Such albedo changes could produce significant effects on solar heating and the global circulation of winds across the planet. All of the major changes took place in areas of moderate to high thermal inertia and rock abundance, consistent with burial of rocky surfaces by thin dust layers deposited during dust storms and subsequent exposure of the rocky surfaces by aeolian erosion. Several distinct mechanisms contribute to aeolian erosion on Mars. Prevailing winds dominate erosion at low latitudes, producing diffuse albedo boundaries and elongated wind streaks generally oriented in the direction of southern summer winds. Dust devils darken the mid to high latitudes from 45 to 70 degrees during the summer seasons, forming irregular albedo patterns consisting of dark linear tracks. Dust storms produce regional albedo variations with distinct but irregular margins. Dark sand duties in southern high latitudes appear to be associated with regional darkening that displays diffuse albedo boundaries. No surface changes were observed to repeat regularly on an annual basis, but many of the changes took place in areas that alternate episodically between high- and low-albedo states as thin mantles of dust are deposited and later stripped off. Hence the face of Mars remains recognizable after a century of telescopic observations, in spite of the enormous extent of alteration that has taken place during the era of spacecraft exploration.

Journal of Geophysical Research E: Planets↗
USGS2005

Control of impact crater fracture systems on subsurface hydrology, ground subsidence, and collapse, Mars

Noachian layered materials are pervasively exposed throughout the highlands of Mars. The layered deposits, in places many kilometers thick, exhibit impact craters of diverse morphologic characteristics, ranging from highly degraded to pristine, most of which formed during the period of heavy bombardment. In addition, exhumed impact craters, ancient channels, and fluvial and alluvial fans are visible in the layered deposits through MOC imagery. These features are more abundant in Noachian terrains, which indicates relatively high erosion rates during ancient Mars that competed with heavy meteoritic bombardment. The Noachian layered materials are thus expected to contain numerous buried impact craters in various states of preservation. Here, we propose that impact craters (buried and exposed) and associated fracture systems dominate the basement structural fabric of the ancient highlands and that they have significantly influenced the hydrogeology. Diversity in the occurrence of high and low densities of impact craters and associated fracture systems controls the magnitude of the local effects of magmatic-driven hydrothermal activity. In and surrounding the Tharsis region, for example, the formation of chaotic terrains (the source regions of the circum-Chryse outflow channel system) and a large diversity of collapse structures, including impact crater moats and pit chains, appear to be the result of enhanced hydrothermal activity. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗
USGS2005

Nitrogen and carbon flow from rock to water: Regulation through soil biogeochemical processes, Mokelumne River watershed, California, and Grand Valley, Colorado

Soil denitrification is an ecologically important nitrogen removal mechanism that releases to the atmosphere the greenhouse gas N2O, an intermediate product from the reduction of NO3- to N 2. In this study we evaluate the relationship between soil carbon and denitrification potential in watersheds with bedrock acting as a nonpoint source of nitrogen, testing the hypothesis that nitrate leaching to stream water is in part regulated by denitrification. Two sites, one in a Mediterranean climate and the other in an arid climate, were investigated to understand the interplay between carbon and denitrification potential. Both sites included carbonaceous bedrock with relatively high nitrogen concentrations (> 1,000 mg N kg-1) and had low background nitrogen concentrations in surface and groundwater. There was a net accumulation of carbon and nitrogen in soil relative to the corresponding bedrock, with the exception of carbonaceous shale from the arid site. There the concentration of carbon in the soil (15,620 mg C kg-1) was less than the shale parent (22,460 mg C kg-1), consistent with the bedrock being a source of soil carbon. Rates of denitrification potential (0.5-83 ??g N kg-1 hr-1) derived from laboratory incubations appeared to be related to the ratio of dissolved organic carbon and nitrate extracted from soils. These data indicate that microbial processes such as denitrification can help maintain background nitrogen concentrations to tens of ??M N in relatively undisturbed ecosystems when nitrogen inputs from weathering bedrock are accompanied by sufficient organic carbon concentrations to promote microbial nitrogen transformations.

Journal of Geophysical Research F: Earth Surface↗
USGS2005

Critical shear stress for erosion of cohesive soils subjected to temperatures typical of wildfires

[1] Increased erosion is a well-known response after wildfire. To predict and to model erosion on a landscape scale requires knowledge of the critical shear stress for the initiation of motion of soil particles. As this soil property is temperature-dependent, a quantitative relation between critical shear stress and the temperatures to which the soils have been subjected during a wildfire is required. In this study the critical shear stress was measured in a recirculating flume using samples of forest soil exposed to different temperatures (40??-550??C) for 1 hour. Results were obtained for four replicates of soils derived from three different types of parent material (granitic bedrock, sandstone, and volcanic tuffs). In general, the relation between critical shear stress and temperature can be separated into three different temperature ranges (<175??C; 175??C-275??C; >275??C), which are similar to those for water repellency and temperature. The critical shear stress was most variable (1.0-2.0 N m-2) for temperatures <175??C, was a maximum (>2.0 N m-2) between 175?? and 275??C, and was essentially constant (0.5-0.8 N m-2) for temperatures >275??C. The changes in critical shear stress with temperature were found to be essentially independent of soil type and suggest that erosion processes in burned watersheds can be modeled more simply than erosion processes in unburned watersheds. Wildfire reduces the spatial variability of soil erodibility associated with unburned watersheds by eliminating the complex effects of vegetation in protecting soils and by reducing the range of cohesion associated with different types of unburned soils. Our results indicate that modeling the erosional response after a wildfire depends primarily on determining the spatial distribution of the maximum soil temperatures that were reached during the wildfire. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗
USGS2006

Influence of a dam on fine-sediment storage in a canyon river

Glen Canyon Dam has caused a fundamental change in the distribution of fine sediment storage in the 99-km reach of the Colorado River in Marble Canyon, Grand Canyon National Park, Arizona. The two major storage sites for fine sediment (i.e., sand and finer material) in this canyon river are lateral recirculation eddies and the main-channel bed. We use a combination of methods, including direct measurement of sediment storage change, measurements of sediment flux, and comparison of the grain size of sediment found in different storage sites relative to the supply and that in transport, in order to evaluate the change in both the volume and location of sediment storage. The analysis shows that the bed of the main channel was an important storage environment for fine sediment in the predam era. In years of large seasonal accumulation, approximately 50% of the fine sediment supplied to the reach from upstream sources was stored on the main-channel bed. In contrast, sediment budgets constructed for two short-duration, high experimental releases from Glen Canyon Dam indicate that approximately 90% of the sediment discharge from the reach during each release was derived from eddy storage, rather than from sandy deposits on the main-channel bed. These results indicate that the majority of the fine sediment in Marble Canyon is now stored in eddies, even though they occupy a small percentage (???17%) of the total river area. Because of a 95% reduction in the supply of fine sediment to Marble Canyon, future high releases without significant input of tributary sediment will potentially erode sediment from long-term eddy storage, resulting in continued degradation in Marble Canyon. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗
USGS2005

Nearshore wave-induced cyclical flexing of sea cliffs

[1] Evolution of a tectonically active coast is driven by geomorphically destructive energy supplied by ocean waves. Wave energy is episodic and concentrated; sea cliffs are battered by the geomorphic wrecking ball every 4-25 s. We measure the response of sea cliffs to wave assault by sensing the ground motion using near-coastal seismometers. Sea cliffs respond to waves in two distinct styles. High-frequency motion (20 Hz) reflects the natural frequency of the sea cliff as it rings in response to direct wave impact. Low-frequency motion in the 0.1-0.05 Hz (10-20 s) band consistently agrees with the dominant nearshore wave period. Integrating microseismic velocities suggests 50 ??m and 10 ??m displacements in horizontal and vertical directions, respectively. Displacement ellipsoids exhibit simultaneous downward and seaward sea cliff motion with each wave. Video footage corroborates the downward sea cliff flex in response to the imposed water load on the wave cut platform. Gradients in displacement amplitudes documented using multiple seismometers suggest longitudinal and shear strain of the flexing sea cliff on the order of 0.5-4 ?? strains during each wave loading cycle. As this sea cliff flexure occurs approximately 3 million times annually, it has the potential to fatigue the rock through cyclical loading. Local sea cliff retreat rates of 10 cm/yr imply that a given parcel of rock is flexed through roughly 109 cycles of increasing amplitude before exposure to direct wave attack at the cliff face. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗