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At least 1,027 records · Page 57Linked to original sources
USGS2006

Overview of the Opportunity Mars Exploration Rover Mission to Meridiani Planum: Eagle Crater to Purgatory Ripple

The Mars Exploration Rover Opportunity touched down at Meridiani Planum in January 2004 and since then has been conducting observations with the Athena science payload. The rover has traversed more than 5 km, carrying out the first outcrop‐scale investigation of sedimentary rocks on Mars. The rocks of Meridiani Planum are sandstones formed by eolian and aqueous reworking of sand grains that are composed of mixed fine‐grained siliciclastics and sulfates. The siliciclastic fraction was produced by chemical alteration of a precursor basalt. The sulfates are dominantly Mg‐sulfates and also include Ca‐sulfates and jarosite. The stratigraphic section observed to date is dominated by eolian bedforms, with subaqueous current ripples exposed near the top of the section. After deposition, interaction with groundwater produced a range of diagenetic features, notably the hematite‐rich concretions known as “blueberries.” The bedrock at Meridiani is highly friable and has undergone substantial erosion by wind‐transported basaltic sand. This sand, along with concretions and concretion fragments eroded from the rock, makes up a soil cover that thinly and discontinuously buries the bedrock. The soil surface exhibits both ancient and active wind ripples that record past and present wind directions. Loose rocks on the soil surface are rare and include both impact ejecta and meteorites. While Opportunity's results show that liquid water was once present at Meridiani Planum below and occasionally at the surface, the environmental conditions recorded were dominantly arid, acidic, and oxidizing and would have posed some significant challenges to the origin of life.

Journal of Geophysical Research E: Planets↗
USGS2006

Form drag in rivers due to small-scale natural topographic features: 1. Regular sequences

Small-scale topographic features are commonly found on the boundaries of natural rivers, streams, and floodplains. A simple method for determining the form drag on these features is presented, and the results of this model are compared to laboratory measurements. The roughness elements are modeled as Gaussian-shaped features defined in terms of three parameters: a protrusion height, H; a streamwise length scale, ??; and a spacing between crests, ??. This shape is shown to be a good approximation to a wide variety of natural topographic bank features. The form drag on an individual roughness element embedded in a series of identical elements is determined using the drag coefficient of the individual element and a reference velocity that includes the effects of roughness elements further upstream. In addition to calculating the drag on each element, the model determines the spatially averaged total stress, skin friction stress, and roughness height of the boundary. The effects of bank roughness on patterns of velocity and boundary shear stress are determined by combining the form drag model with a channel flow model. The combined model shows that drag on small-scale topographic features substantially alters the near-bank flow field. These methods can be used to improve predictions of flow resistance in rivers and to form the basis for fully predictive (no empirically adjusted parameters) channel flow models. They also provide a foundation for calculating the near-bank boundary shear stress fields necessary for determining rates of sediment transport and lateral erosion.

Journal of Geophysical Research F: Earth Surface↗
USGS2006

Form drag in rivers due to small-scale natural topographic features: 2. Irregular sequences

The size, shape, and spacing of small-scale topographic features found on the boundaries of natural streams, rivers, and floodplains can be quite variable. Consequently, a procedure for determining the form drag on irregular sequences of different-sized topographic features is essential for calculating near-boundary flows and sediment transport. A method for carrying out such calculations is developed in this paper. This method builds on the work of Kean and Smith (2006), which describes the flow field for the simpler case of a regular sequence of identical topographic features. Both approaches model topographic features as two-dimensional elements with Gaussian-shaped cross sections defined in terms of three parameters. Field measurements of bank topography are used to show that (1) the magnitude of these shape parameters can vary greatly between adjacent topographic features and (2) the variability of these shape parameters follows a lognormal distribution. Simulations using an irregular set of topographic roughness elements show that the drag on an individual element is primarily controlled by the size and shape of the feature immediately upstream and that the spatial average of the boundary shear stress over a large set of randomly ordered elements is relatively insensitive to the sequence of the elements. In addition, a method to transform the topography of irregular surfaces into an equivalently rough surface of regularly spaced, identical topographic elements also is given. The methods described in this paper can be used to improve predictions of flow resistance in rivers as well as quantify bank roughness.

Journal of Geophysical Research F: Earth Surface↗
USGS2007

Seismic detection and analysis of icequakes at Columbia Glacier, Alaska

Contributions to sea level rise from rapidly retreating marine-terminating glaciers are large and increasing. Strong increases in iceberg calving occur during retreat, which allows mass transfer to the ocean at a much higher rate than possible through surface melt alone. To study this process, we deployed an 11-sensor passive seismic network at Columbia Glacier, Alaska, during 2004–2005. We show that calving events generate narrow-band seismic signals, allowing frequency domain detections. Detection parameters were determined using direct observations of calving and validated using three statistical methods and hypocenter locations. The 1–3 Hz detections provide a good measure of the temporal distribution and size of calving events. Possible source mechanisms for the unique waveforms are discussed, and we analyze potential forcings for the observed seismicity.

Journal of Geophysical Research↗
USGS2007

Sorted bed forms as self-organized patterns: 2. complex forcing scenarios

We employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions. Results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated, a priori unpredictable, behavior. This happens because the system responds to a change in wave characteristics by attempting to self-organize into a patterned seabed of different geometry and spacing. The new wavelength might not have enough time to emerge before a new change in wave characteristics occurs, leading to less regular seabed configurations. The new seabed configuration is also highly dependent on the preexisting morphology, which further limits the possibility of predicting future behavior. For the same reasons, variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms. Spatial variability in grain size distribution and different types of net sediment aggradation/degradation can also result in the development of sorted bed forms characterized by a less regular shape. Numerical simulations qualitatively agree with observed geometry (spacing and height) of sorted bed forms. Also in agreement with observations is that at shallower depths, sorted bed forms are more likely to be affected by changes in the forcing conditions, which might also explain why, in shallow waters, sorted bed forms are described as ephemeral features. Finally, simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms. Instead, variations in sorted bed form characteristics may result from variations in local hydrodynamic and/or sedimentary conditions.

Journal of Geophysical Research F: Earth Surface↗
USGS2007

Nutrient (N, P) loads and yields at multiple scales and subbasin types in the Yukon River basin, Alaska

Loads and yields of dissolved and particulate nitrogen (N) and phosphorus (P) were measured and modeled at three locations on the Yukon River (YR) and on the Tanana and Porcupine Rivers in Alaska during 2001-2005. Total export of N and P upstream of Yukon Delta averaged 120 Gg N a-1 and 56 Gg P a-1, respectively, with 43.5% of total N (TN) as dissolved organic N, and 98% of total P (TP) as particulate phosphorus. Approximately half of the annual export of TN and TP occurred during spring. Hydrologic yields ofTN (5.6-13.3 mmol N m-2 a-1) and TP (0.8-9.0 mmol P m-2 a-1) were least in the Porcupine basin and greatest in the Tanana basin and were proportional to water yield. Comparison of current and historical dissolved organic matter (DOM) export from the basin indicates decreased DON export with respect to total water discharge during summer and autumn in recent decades. Any possible climate-related change in annual water discharge will result in proportional changes in N and P export.

Journal of Geophysical Research G: Biogeosciences↗
USGS2007

The role of historical fire disturbance in the carbon dynamics of the pan-boreal region: A process-based analysis

[1] Wildfire is a common occurrence in ecosystems of northern high latitudes, and changes in the fire regime of this region have consequences for carbon feedbacks to the climate system. To improve our understanding of how wildfire influences carbon dynamics of this region, we used the process-based Terrestrial Ecosystem Model to simulate fire emissions and changes in carbon storage north of 45°N from the start of spatially explicit historically recorded fire records in the twentieth century through 2002, and evaluated the role of fire in the carbon dynamics of the region within the context of ecosystem responses to changes in atmospheric CO 2 concentration and climate. Our analysis indicates that fire plays an important role in interannual and decadal scale variation of source/sink relationships of northern terrestrial ecosystems and also suggests that atmospheric CO 2 may be important to consider in addition to changes in climate and fire disturbance. There are substantial uncertainties in the effects of fire on carbon storage in our simulations. These uncertainties are associated with sparse fire data for northern Eurasia, uncertainty in estimating carbon consumption, and difficulty in verifying assumptions about the representation of fires that occurred prior to the start of the historical fire record. To improve the ability to better predict how fire will influence carbon storage of this region in the future, new analyses of the retrospective role of fire in the carbon dynamics of northern high latitudes should address these uncertainties.

Journal of Geophysical Research G: Biogeosciences↗
USGS2008

Monte Carlo method for determining earthquake recurrence parameters from short paleoseismic catalogs: Example calculations for California

Paleoearthquake observations often lack enough events at a given site to directly define a probability density function (PDF) for earthquake recurrence. Sites with fewer than 10-15 intervals do not provide enough information to reliably determine the shape of the PDF using standard maximum-likelihood techniques (e.g., Ellsworth et al., 1999). In this paper I present a method that attempts to fit wide ranges of distribution parameters to short paleoseismic series. From repeated Monte Carlo draws, it becomes possible to quantitatively estimate most likely recurrence PDF parameters, and a ranked distribution of parameters is returned that can be used to assess uncertainties in hazard calculations. In tests on short synthetic earthquake series, the method gives results that cluster around the mean of the input distribution, whereas maximum likelihood methods return the sample means (e.g., NIST/SEMATECH, 2006). For short series (fewer than 10 intervals), sample means tend to reflect the median of an asymmetric recurrence distribution, possibly leading to an overestimate of the hazard should they be used in probability calculations. Therefore a Monte Carlo approach may be useful for assessing recurrence from limited paleoearthquake records. Further, the degree of functional dependence among parameters like mean recurrence interval and coefficient of variation can be established. The method is described for use with time-independent and time-dependent PDFs, and results from 19 paleoseismic sequences on strike-slip faults throughout the state of California are given.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Finding concealed active faults: Extending the southern Whidbey Island fault across the Puget Lowland, Washington

The southern Whidbey Island fault zone (SWIF), as previously mapped using borehole data, potential field anomalies, and marine seismic reflection surveys, consists of three subparallel, northwest trending strands extending ∼100 km from near Vancouver Island to the northern Puget Lowland. East of Puget Sound, the SWIF makes landfall between the cities of Seattle and Everett but is concealed beneath a thick mantle of young glacial deposits and vegetation. A ∼20‐km‐wide, northwest trending swath of subparallel, low‐amplitude aeromagnetic anomalies crosses this region of the Puget Lowland and is on strike with the SWIF. The most prominent aeromagnetic anomaly, the Cottage Lake lineament, extends at least 18 km and lies approximately on strike with the SWIF on Whidbey Island. Subtle scarps and topographic lineaments on Pleistocene surfaces, visible on high‐resolution lidar topography at a number of locations along the SWIF, lie on or near these magnetic anomalies. In the field, scarps exhibit northeast‐side‐up and vertical relief of 1 to 5 m. Excavations across several lidar scarps lying on or near magnetic anomalies show evidence for multiple folding and faulting events since deglaciation, most likely above buried reverse/oblique faults. Excavations in areas away from magnetic anomalies do not show evidence of tectonic deformation. In total, paleoseismological evidence suggests that the SWIF produced at least four earthquakes since deglaciation about 16,400 years ago, the most recent less than 2700 years ago.

Washington↗
USGS2008

Effects of supershear rupture speed on the high-frequency content of S waves investigated using spontaneous dynamic rupture models and isochrone theory

In this paper we achieve three goals: (1) We demonstrate that crack tips governed by friction laws, including slip weakening, rate- and state-dependent laws, and thermal pressurization of pore fluids, propagating at supershear speed have slip velocity functions with reduced high-frequency content compared to crack tips traveling at subshear speeds. This is demonstrated using a fully dynamic, spontaneous, three-dimensional earthquake model, in which we calculate fault slip velocity at nine points (locations) distributed along a quarter circle on the fault where the rupture is traveling at supershear speed in the in-plane direction and subshear speed in the antiplane direction. This holds for a fault governed by the linear slip-weakening constitutive equation, by slip weakening with thermal pressurization of pore fluid, and by rate- and state-dependent laws with thermal pressurization. The same is also true even assuming a highly heterogeneous initial shear stress field on the fault. (2) Using isochrone theory, we derive a general expression for the spectral characteristics and geometric spreading of two pulses arising from supershear rupture, the well-known Mach wave, and a second lesser known pulse caused by rupture acceleration. (3) We demonstrate that the Mach cone amplification of high frequencies overwhelms the de-amplification of high-frequency content in the slip velocity functions in supershear ruptures. Consequently, when earthquake ruptures travel at supershear speed, a net enhancement of high-frequency radiation is expected, and the alleged “low” peak accelerations observed for the 2002 Denali and other large earthquakes are probably not caused by diminished high-frequency content in the slip velocity function, as has been speculated.

Journal of Geophysical Research Solid Earth↗
USGS2008

Dislocation models of interseismic deformation in the western United States

The GPS-derived crustal velocity field of the western United States is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation. The interseismic velocity field is constrained by 1052 GPS velocity vectors spanning the ???2500-km-long plate boundary zone adjacent to the San Andreas fault and Cascadia subduction zone and extending ???1000 km into the plate interior. The GPS data set is compiled from U.S. Geological Survey campaign data, Plate Boundary Observatory data, and the Western U.S. Cordillera velocity field of Bennett et al. (1999). In the context of viscoelastic cycle models of postearthquake deformation, the interseismic velocity field is modeled with a combination of earthquake sources on ???100 known faults plus broadly distributed sources. Models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins, viscoelastic relaxation from distributed faulting in the plate interior, as well as lateral variations in depth-averaged rigidity in the elastic lithosphere. Resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the San Andreas fault (SAF) system. Primary deformation characteristics are captured along the entire SAF system, Eastern California Shear Zone, Walker Lane, the Mendocino triple junction, the Cascadia margin, and the plate interior up to ???1000 km from the major plate boundaries.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

A model of earthquake triggering probabilities and application to dynamic deformations constrained by ground motion observations

We have used observations from Felzer and Brodsky (2006) of the variation of linear aftershock densities (i.e., aftershocks per unit length) with the magnitude of and distance from the main shock fault to derive constraints on how the probability of a main shock triggering a single aftershock at a point, P(r, D), varies as a function of distance, r, and main shock rupture dimension, D. We find that P(r, D) becomes independent of D as the triggering fault is approached. When r ??? D P(r, D) scales as Dm where m-2 and decays with distance approximately as r-n with n = 2, with a possible change to r-(n-1) at r > h, where h is the closest distance between the fault and the boundaries of the seismogenic zone. These constraints may be used to test hypotheses about the types of deformations and mechanisms that trigger aftershocks. We illustrate this using dynamic deformations (i.e., radiated seismic waves) and a posited proportionality with P(r, D). Deformation characteristics examined include peak displacements, peak accelerations and velocities (proportional to strain rates and strains, respectively), and two measures that account for cumulative deformations. Our model indicates that either peak strains alone or strain rates averaged over the duration of rupture may be responsible for aftershock triggering.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Probabilistic seismic hazard in the San Francisco Bay area based on a simplified viscoelastic cycle model of fault interactions

We construct a viscoelastic cycle model of plate boundary deformation that includes the effect of time-dependent interseismic strain accumulation, coseismic strain release, and viscoelastic relaxation of the substrate beneath the seismogenic crust. For a given fault system, time-averaged stress changes at any point (not on a fault) are constrained to zero; that is, kinematic consistency is enforced for the fault system. The dates of last rupture, mean recurrence times, and the slip distributions of the (assumed) repeating ruptures are key inputs into the viscoelastic cycle model. This simple formulation allows construction of stress evolution at all points in the plate boundary zone for purposes of probabilistic seismic hazard analysis (PSHA). Stress evolution is combined with a Coulomb failure stress threshold at representative points on the fault segments to estimate the times of their respective future ruptures. In our PSHA we consider uncertainties in a four-dimensional parameter space: the rupture peridocities, slip distributions, time of last earthquake (for prehistoric ruptures) and Coulomb failure stress thresholds. We apply this methodology to the San Francisco Bay region using a recently determined fault chronology of area faults. Assuming single-segment rupture scenarios, we find that fature rupture probabilities of area faults in the coming decades are the highest for the southern Hayward, Rodgers Creek, and northern Calaveras faults. This conclusion is qualitatively similar to that of Working Group on California Earthquake Probabilities, but the probabilities derived here are significantly higher. Given that fault rupture probabilities are highly model-dependent, no single model should be used to assess to time-dependent rupture probabilities. We suggest that several models, including the present one, be used in a comprehensive PSHA methodology, as was done by Working Group on California Earthquake Probabilities.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Noise in GPS displacement measurements from southern California and southern Nevada

Time series of position changes estimated from data from 236 continuously recording GPS receivers operating in Southern California and Southern Nevada are evaluated for noise models that characterize their temporal correlations. The lengths of the time series range between 3.5 and 10 years. After adjusting these data for postseismic deformation, offsets, and annual periodicities, I find that about one-half of the time series have temporal correlations that are categorized as either flicker or random-walk noise. The remaining time series can be best categorized as either a combination of flicker and random-walk; power law noise; first-order Gauss-Markov plus random-walk noise; or power law plus broadband, seasonal noise. A variety of geodetic monuments are used in Southern California and Nevada, including deeply braced designs, cement piers, pins drilled in outcrop, and buildings. When I evaluate the noise for each time series in terms of an estimate of the standard error in velocity, I find that the sites with the smallest errors are those located in Nevada using deeply braced monuments. Sites that are installed within regions of active pumping, both for groundwater and oil, had the largest standard errors in velocity. Comparison of monument stability, as measured by standard error in rate, with average, annual rainfall nearby indicates a marginally significant correlation. In addition, even though regional filtering removed much of the common-mode signals in these time series, there still remains a common-mode seasonal signal which can and should be removed.

California, Nevada↗
USGS2008

Eruption dynamics at Mount St. Helens imaged from broadband seismic waveforms: Interaction of the shallow magmatic and hydrothermal systems

The current eruption at Mount St. Helens is characterized by dome building and shallow, repetitive, long-period (LP) earthquakes. Waveform cross-correlation reveals remarkable similarity for a majority of the earthquakes over periods of several weeks. Stacked spectra of these events display multiple peaks between 0.5 and 2 Hz that are common to most stations. Lower-amplitude very-long-period (VLP) events commonly accompany the LP events. We model the source mechanisms of LP and VLP events in the 0.5-4 s and 8-40 s bands, respectively, using data recorded in July 2005 with a 19-station temporary broadband network. The source mechanism of the LP events includes: 1) a volumetric component modeled as resonance of a gently NNW-dipping, steam-filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2) a vertical single force attributed to movement of the overlying dome. The VLP source, which also includes volumetric and single-force components, is 250 m deeper and NNW of the LP source, at the SW edge of the 1980s lava dome. The volumetric component points to the compression and expansion of a shallow, magma-filled sill, which is subparallel to the hydrothermal crack imaged at the LP source, coupled with a smaller component of expansion and compression of a dike. The single-force components are due to mass advection in the magma conduit. The location, geometry and timing of the sources suggest the VLP and LP events are caused by perturbations of a common crack system.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

A global search for stress shadows

[1] Debate continues regarding the relative proportion of earthquakes triggered by passing seismic waves versus static stress changes from a main shock. Static stress changes are expected to have long‐term effects on earthquake probabilities, whereas dynamic stress changes due to the passing of seismic waves should not. Both mechanisms are expected to raise seismicity rates in some areas, but only static stress change calculations predict rate decrease shadows. Thus, identification of post‐main‐shock earthquake suppression is diagnostic of a static stress change process. We note that in principle, static stress change theory predicts suppression of particular earthquake mechanisms in a shadow zone rather than an overall rate reduction. A stress shadow can therefore be characterized by a change in the average earthquake focal mechanism before and after a main shock that results from suppression of a given mechanism type. We examined average mechanisms from ±2° radii and 5‐year periods before and after 119 M s ≥ 7 main shock earthquakes drawn from the Harvard Centroid Moment Tensor (CMT) catalog. Significant average mechanism changes caused by earthquake suppression were found in only two cases. However, by stacking the data, we were able to resolve statistically significant suppression of particular post‐main‐shock focal mechanisms. This indicates that, while static stress shadows are subtle, they are indeed present in the global catalog.

Journal of Geophysical Research Solid Earth↗
USGS2008

Physical property changes in hydrate-bearing sediment due to depressurization and subsequent repressurization

Physical property measurements of sediment cores containing natural gas hydrate are typically performed on material exposed, at least briefly, to non-in situ conditions during recovery. To examine the effects of a brief excursion from the gas-hydrate stability field, as can occur when pressure cores are transferred to pressurized storage vessels, we measured physical properties on laboratory-formed sand packs containing methane hydrate and methane pore gas. After depressurizing samples to atmospheric pressure, we repressurized them into the methane-hydrate stability field and remeasured their physical properties. Thermal conductivity, shear strength, acoustic compressional and shear wave amplitudes, and speeds of the original and depressurized/repressurized samples are compared. X– ray computed tomography images track how the gas-hydrate distribution changes in the hydrate-cemented sands owing to the depressurizaton/repressurization process. Because depressurization-induced property changes can be substantial and are not easily predicted, particularly in water-saturated, hydrate-bearing sediment, maintaining pressure and temperature conditions throughout the core recovery and measurement process is critical for using laboratory measurements to estimate in situ properties.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Exponential decline of aftershocks of the M7.9 1868 great Kau earthquake, Hawaii, through the 20th century

The remarkable catalog of Hawaiian earthquakes going back to the 1820s is based on missionary diaries, newspaper accounts, and instrumental records and spans the great M 7.9 Kau earthquake of April 1868 and its aftershock sequence. The earthquake record since 1868 defines a smooth curve complete to M 5.2 of the declining rate into the 21st century, after five short volcanic swarms are removed. A single aftershock curve fits the earthquake record, even with numerous M 6 and 7 main shocks and eruptions. The timing of some moderate earthquakes may be controlled by magmatic stresses, but their overall long‐term rate reflects one of aftershocks of the Kau earthquake. The 1868 earthquake is, therefore, the largest and most controlling stress event in the 19th and 20th centuries. We fit both the modified Omori (power law) and stretched exponential (SE) functions to the earthquakes. We found that the modified Omori law is a good fit to the M ≥ 5.2 earthquake rate for the first 10 years or so and the more rapidly declining SE function fits better thereafter, as supported by three statistical tests. The switch to exponential decay suggests that a possible change in aftershock physics may occur from rate and state fault friction, with no change in the stress rate, to viscoelastic stress relaxation. The 61‐year exponential decay constant is at the upper end of the range of geodetic relaxation times seen after other global earthquakes. Modeling deformation in Hawaii is beyond the scope of this paper, but a simple interpretation of the decay suggests an effective viscosity of 10 19 to 10 20 Pa s pertains in the volcanic spreading of Hawaii's flanks. The rapid decline in earthquake rate poses questions for seismic hazard estimates in an area that is cited as one of the most hazardous in the United States.

Hawaii↗