bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport of dust to high northern latitudes in a Martian polar warming
Authors: Barnes Jeffrey R.
Published: February 2008

The transport of dust to high northern latitudes in a Martian polar warming (and global dust storm) is investigated. General considerations of the dynamics of such an atmospheric warming indicate that transport of “tracers” such as dust to polar latitudes should take place, regardless of the specifie dynamical mechanism involved. Attention is focused upon a planetary wave induced polar warming, with a simplified transport model employed in conjunction with a beta‐plane dynamical model to simulate the transport of dust during a warming event. Numerical experiments are performed to examine the influence of different warming dynamics and to assess the sensitivity of the transport to the initial (that existing in the early stages of a global dust storm) dust distribution. Several simulations incorporating sedimentation are carried out, allowing examination of dust deposition at polar latitudes. The results of the numerical simulations indicate that substantial amounts of dust could be transported and deposited at high northern latitudes (extending to the pole) in a polar‐warming event. For a planetary wave warming the net transport is not very sensitive to the initial dust distribution (or to the details of the dynamics), as horizontal diffusive mixing plays a dominant role. Without such mixing (as might be the case if the dynamics does not involve planetary waves) the net transport is much more dependent on the initial state, with relatively elevated dust distributions favoring enhanced poleward transport. The potential for the transport of water is also examined, through several simulations in which condensation is neglected. These indicate that large amounts of water could be transported to the north polar region in a warming, given that substantial condensation and precipitation do not take place in subpolar regions. The warmth of the Martian atmosphere during global dust storms makes the latter plausible. The implications and significance of the transport of dust, as well as water, to high northern latitudes in global dust storm/polar warming events are discussed, in the context of the Mars climate system and the polar layered terrains.
On the origin of inclination shallowing in redeposited sediments
Authors: Levi Shaul , Banerjee Subir
Published: February 2008

We studied the detrital remanent magnetization of laboratory redeposited lake sediments, previously shown to be excellent recorders of the paleomagnetic field. The redeposited and original/natural sediments have indistinguishable stabilities to alternating fields (AF) demagnetization and remanence intensities, indicating that their remanences reside in the same magnetic particles. Thermomagnetic behavior, hysteresis parameters, and relatively high AF stabilities of the detrital and anhysteretic remanences are essentially identical throughout the sediment column, suggesting uniform distributions of submicron magnetite grains. The declination is accurately recorded, but inclination shallowing averaging 18° is observed for routinely hand‐stirred, rapidly redeposited sediments. However, sonically disaggregated sediments settle more slowly with anomalous shallowing averaging 7°, decreasing upward in the redeposited column. These results indicate that thorough disaggregation of the sediment reduces the magnitude of the inclination shallowing probably by more efficiently isolating the magnetic grains from the matrix, thus diminishing the dominance of the gravitational over geomagnetic aligning torques during deposition. The progressive decrease of the inclination shallowing upward in the sediment column might be due to a parallel upward increase in the fraction of isolated magnetic particles, which undergo Brownian motion and alignment by the geomagnetic field. The dominance of gravitational over geomagnetic aligning torques during deposition might be a cause for inclination shallowing in some coarser sediments. In contrast, inclination shallowing would be minimized for slow deposition of isolated submicron magnetite grains during natural sedimentation.
Compaction and inclination shallowing in deep‐sea sediments from the Pacific Ocean
Authors: Arason Pordur , Levi Shaul
Published: February 2008

Progressive downcore shallowing of the remanent inclination has been observed in a 120‐m section of marine sediments at Deep Sea Drilling Project site 578 in the northwest Pacific, representing the past 6.5 m.y. Near the top of the section the average inclination corresponds to the expected geocentric axial dipole value of 53° but shallows down section by about 6° to 8°. Northward translation of the Pacific plate accounts for only about a quarter of the inclination shallowing. Moreover, no inclination shallowing was observed at neighboring site 576, which has considerably lower sedimentation rates, and the last 5 m.y. are represented by a 26‐m sedimentary section. We correlate the inclination shallowing at site 578 with an average decrease in porosity of 3–4%. The porosities in the top 26 m at site 576 are slightly higher than at site 578 and show no definite trend downhole. We interpret these results to suggest that both the downhole inclination shallowing and decrease of porosity in site 578 are caused by sediment compaction. Compaction does not play a significant role for the section at site 576 due to its much shorter length.
Compaction‐induced inclination shallowing in synthetic and natural clay‐rich sediments
Authors: Deamer Gay A. , Kodama Kenneth P.
Published: February 2008

Experiments using single‐clay slurries consisting of acicular or equidimensional magnetite mixed with either kaolinite, illite, montmorillonite, or chlorite were conducted to determine the effect of compaction on inclination. A natural marine sediment and a “reconstructed” marine sediment, made up of the clays used in our single‐clay experiments, were also studied. A laboratory postdepositional remanence, imparted to the slurries by stirring, had its inclination shallowed by compaction for both the single‐clay samples, as seen previously (Anson and Kodama, 1987), and the natural and reconstructed marine sediments, suggesting that our single‐clay sediments are valid analogues of natural sediments. Magnetic intensity also decreased, by 30–50%, during compaction, the greatest decrease occurring for samples with the steepest initial inclination. Altering the pH of a slurry, so that the magnetite would have a negative surface charge and be repelled by clay particles, did not affect a slurry's inclination‐shallowing behavior, suggesting that electrostatic attraction between magnetite and clay is probably not a major contributor to inclination shallowing. However, most of the inclination shallowing observed in our experiments occurs at low pressures when void ratios are high. Comparison with a clay microfabric/compaction study (McConnachie, 1974) suggests that volume loss at these pressures is accommodated by reorientation of clay particles or clay‐particle aggregates (clay domains). Since pore spaces are large at these low pressures, inclination shallowing is probably caused by magnetite particles either being attached to clay particles, perhaps by van der Waals forces, or having been incorporated into clay domains. Alternating field demagnetization of compacted samples, which contain magnetite with a range in grain sizes, supports this model, indicating that the smallest magnetic grains are slightly more shallowed by compaction than larger grains. Our results suggest that a sediment's clay content may be an important contributor to inclination shallowing.
Slant stack velocity analysis of shot point 16 from the 1986 PASSCAL Ouachita Experiment
Authors: Lyslo J. A. , Nowack R. L.
Published: February 2008

A τ‐P slant stack algorithm is applied to shot point 16 of the 1986 PASSCAL Ouachita experiment to determine the optimal one‐dimensional velocity structure. Several enhancements to a standard slant stack algorithm are used to image a one‐dimensional velocity function from the shot point gathered data. Because only postcritical reflections and refractions are needed to image a velocity function, significant muting of precritical energy is performed, which reduces the effect of noise in calculating the stack. For data sets with long offsets, scaling the data by a factor equal to or greater than offset distance compensates for the effects of geometrical spreading and attenuation. This enhances imaging of the deeper regions of the velocity model and balances amplitudes across the shot point gather. Finally, ray parameter‐depth images can have difficulty imaging higher velocities, since for higher velocities, a small change in ray parameter results in a large increment in velocity. This problem is overcome by slant stacking in equal increments of velocity instead of ray parameter. The enhanced slant stack algorithm is applied to shot point 16 of the 1986 PASSCAL Ouachita data set, which sufficiently satisfies the assumption of lateral homogeneity based on previous studies of the region. Other velocity models are found to bie consistent with the envelope of velocity function imaged by the slant stack method. Maximum amplitudes on the τ‐P image are used as a constraint to delineate a more detailed velocity model. A seven‐layer velocity model is derived for shot point 16 which is found to be similar to those of others who have studied the area. The top three layers are sediments associated with the gulf coastal plain, followed by a thick layer derived from sedimentation associated with the closing of the proto‐Gulf of Mexico and the associated Ouachita orogeny. The determination of a more detailed velocity function from the envelope suggests somewhat higher velocities in the lower crustal layer than previous studies. However, given the one‐dimensional assumptions of the method, as well as the signal to noise ratio of the data, the velocity envelope is the most important constraint on the velocity models and tectonic interpretation provided by this analysis.
Tomographic imaging of local earthquake delay times for three‐dimensional velocity variation in western Washington
Authors: Lees Jonathan M. , Crosson Robert S.
Published: February 2008

Tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the Puget Sound region of Western Washington. The 37,500 square km region is represented by nearly cubic blocks of 5 km per side. P‐wave arrival time observations from 4,387 crustal earthquakes, with depths of 0 to 40 km, were used as sources producing 36,865 rays covering the target region. A conjugate gradient method (LSQR) is used to invert the large, sparse system of equations. To diminish the effects of noisy data, the Laplacian is constrained to be zero within horizontal layers, providing smoothing of the model. The resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure. Results of the inversion are correlated with some known geologic features and independent geophysical measurements. High P‐wave velocities along the eastern flank of the Olympic Peninsula are interpreted to reflect the subsurface extension of Crescent terrane. Low velocities beneath the Puget Sound further to the east are inferred to reflect thick sediment accumulations. The Crescent terrane appears to extend beneath Puget Sound, consistent with its interpretation as a major accretionary unit. In the southern Puget Sound basin, high velocity anomalies at depths of 10–20 km are interpreted as Crescent terrane and are correlated with a region of low seismicity. Near Mt. Rainier, high velocity anomalies may reflect buried plutons.
Geothermal anomalies in the Valles‐Penedes Graben Master Fault: Convection through the Horst as a possible mechanism
Authors: Fernàndez M. , Banda E.
Published: February 2008

Four different mechanisms (frictional heating, magmatic intrusion, free convection, and forced convection) have been tested as possible causes to explain the localized and relatively high geothermal anomalies found along master faults of graben structures. Forced convection is the only mechanism found to be generally able to generate the anomalies. The Vallès‐Penedès basin (NE Spain) is used to illustrate the results of the different mechanisms. In general, it is concluded that forced convection will generate geothermal anomalies when several conditions are met: water circulation has to be deep and slow in its descent; deep reaching faults must exist to allow a rapid ascension of water; and some sort of sealing along the ascending conduit has to prevent mixing with cold water. The hydraulic model of the Vallès‐Penedès graben rules out a possible circulation through the sediments filling the basin, and therefore a circulation model through the adjacent horst is proposed. This model is supported by isotopic data. Such a model is not restricted to horst and graben structures but can be used to explain anomalies found in compressive geological settings.
Effects of regolith/megaregolith insulation on the cooling histories of differentiated asteroids
Authors: Haack Henning , Rasmussen Kaare L. , Warren Paul H.
Published: February 2008

The cooling histories, and in particular the core cooling rates, of differentiated asteroids are calculated, employing a variety of thermal conductivity structures in order to simulate the potential insulating effects of regolith and megaregolith (brecciated impact rubble) layers. The regolith is assumed to have thermal conductivity 0.005×the unbrecciated mantle conductivity, and its thickness, in our most typical model, is assumed to be 0.003×the parent body radius. The megaregolith, if present, is assumed to have thermal conductivity 0.1×that of unbrecciated mantle. We find that a combination of relatively thick megaregolith and regolith can potentially reduce the core cooling rate by more than a factor of 10 below the rate predicted by models with conventional thermal conductivity structure. For any given measured cooling rate, the radius implied for the parent asteroid can potentially be reduced by a factor of 5 from the radius predicted by models with conventional thermal conductivity structure. Realistically, the regolith and megaregolith thicknesses required to engender these extreme effects were probably seldom generated, except perhaps within the largest asteroids (where, even if generated, they would quickly be reduced by compaction and sintering). Nonetheless, our results for models with plausible regolith and megaregolith thicknesses indicate that most of the iron meteorites and the pallasites probably formed in relatively small (R less than 50 km) parent bodies. We have parameterized our results so that effects of intermediate configurations can readily be calculated. The parameterization also facilitates evaluation of additional variables, e.g., the initial temperature, the concentrations of long‐lived radioactive heat sources, etc., albeit these variables are far less crucial than the thermal conductivity structure. Our results indicate that the insulation of even a thin regolith causes metallalographic cooling rates to be relatively uniform, and practically independent of depth, within a given asteroid. Stochastic differences in cratering (regolith production) histories may have engendered radically different cooling rates from asteroids of similar radius, or similar cooling rates from asteroids of different radius.
Age estimates and uplift rates for Late Pleistocene marine terraces: Southern Oregon portion of the Cascadia Forearc
Authors: Muhs Daniel R. , Kelsey Harvey M. , Miller Gifford H. , Kennedy George L. , Whelan Joseph F. , McInelly Galan W.
Published: February 2008

Marine terraces are prominent landforms along the southern Oregon coast, which forms part of the forearc region of the Cascadia subduction zone. Interest in the Cascadia subduction zone has increased because recent investigations have suggested that slip along plates at certain types of convergent margins is characteristically accompanied by large earthquakes. In addition, other investigations have suggested that convergent margins can be broadly classified by the magnitude of their uplift rates. With these hypotheses in mind, we generated new uranium series, amino acid, and stable isotope data for southern Oregon marine terrace fossils. These data, along with terrace elevations and two alternative estimates of sea level at the time of terrace formation, allow us to determine terrace ages and uplift rates. Uranium series analysis of fossil coral yields an age of 83±5 ka for the Whisky Run terrace at Coquille Point in Bandon, Oregon. A combination of amino acid and oxygen isotope data suggest ages of about 80 and 105 ka for the lowest two terraces at Cape Blanco. These ages indicate uplift rates of 0.45–1.05 and 0.81–1.49 m/kyr for Coquille Point and Cape Blanco, respectively. Late Quaternary uplift rates of marine terraces yield information about deformation in the overriding plate, but it is unclear if such data vary systematically with convergent margin type. In order to assess the utility of the southern Oregon uplift rates for predicting the behavior of the Cascadia subduction zone, we compared late Quaternary uplift rates derived from terrace data from subduction zones around the world. On the basis of this comparison the southern Oregon rates of vertical deformation are not unusually high or low. Furthermore, late Quaternary uplift rates show little relationship to the type of convergent margin. These observations suggest that local structures may play a large role in uplift rate variability. In addition, while the type of convergent margin may place an upper limit on possible uplift rate, greater upper limits serve to increase the range of possible uplift rates. In the case of the southern Oregon coast, variability in uplift rate probably reflects local structures in the overriding plate, and the rate of uplift cannot be used as a simple index of the potential for great earthquakes along the southern Cascadia subduction zone.
Earthquakes as self‐organized critical phenomena
Authors: Ito Keisuke , Matsuzaki Mitsuhiro
Published: February 2008

A cellular automaton model with threshold elements, previously proposed by Bak et al.[1988], which generates 1/f noise and self‐organizes into a critical state, is studied as a model for earthquake occurrence. The model explains some of fractal features of earthquakes such as the power law distribution of the size of the earthquake. The model is modified so that every earthquake may trigger aftershocks. The modified trigger model explains other fractal features of earthquakes, such as the power law decay of aftershock frequency and the power law correlation of the hypocenter distribution.
Deconvolution of mineral absorption bands: An improved approach
Authors: Sunshine Jessica M. , Pieters Carle M. , Pratt Stephen F.
Published: February 2008

Although visible and near‐infrared reflectance spectra contain absorption bands that are characteristic of the composition and structure of the absorbing species, deconvolving a complex spectrum is nontrivial. An improved approach to spectral deconvolution is presented here that accurately represents absorption bands as discrete mathematical distributions and resolves composite absorption features into individual absorptions bands. The frequently used Gaussian model of absorption bands is first evaluated and shown to be inappropriate for the Fe2+ electronic transition absorptions in pyroxene spectra. Subsequently, a modified Gaussian model is derived using a power law relationship of energy to average bond length. This modified Gaussian model successfully depicts the characteristic 0.9‐um absorption feature in orthopyroxene spectra using a single distribution. The modified Gaussian model is also shown to provide an objective and consistent tool for deconvolving individual absorption bands in the more complex orthopyroxene, clinopyroxene, pyroxene mixtures, and olivine spectra. The ability of this new modified Gaussian model to describe the Fe2+ electronic transition absorption bands in both pyroxene and olivine spectra strongly suggests that it be the method of choice for analyzing all electronic transition bands.
Effective medium theory and network theory applied to the transport properties of rock
Authors: David Christian , Gueguen Yves , Pampoukis Georges
Published: February 2008

The effective medium theory (EMT) proposed by Kirkpatrick (1973) is known to be a useful tool to describe the conducting properties of heterogeneous media. We check the validity of the theory on several probability distribution functions. The accuracy of the theory is inferred by comparing the results given by the EMT to those obtained when using an exact method based on the resolution of two‐dimensional conducting networks. It turns out that the EMT approximation is quite good for quasi‐uniform distributions of conducting elements, whereas large discrepancies appear for highly contrasted distributions such as decreasing exponential functions. The reason is that ensemble averaging and spatial averaging are not equivalent for broad, nonuniform distributions. Therefore the effective medium theory may not be appropriate for the calculation of the transport properties of rocks which exhibit this type of distribution. The nature of the distribution varies, however, depending on the rock type and the length scale of interest. Critical path analysis can be a complementary tool to deal with highly heterogeneous conducting media. The accuracy of such a technique appears to be better than that of the EMT approximation. But it depends also on the representativity of the input data. Finally, an example of data relative to Fontainebleau Sandstone is developed in this paper. It is concluded that for this rock, network theory is the most accurate tool for calculating transport properties from statistically averaged observations on two‐dimensional sections.
Spectral properties of land surfaces in the thermal infrared: 2. Field method for spectrally averaged emissivity measurements
Authors: Nerry F. , Labed J. , Stoll M. P.
Published: February 2008

A large‐dimension experimental setup, relying on the principle of the box method, for measurements of infrared emissivities of ground surfaces is described and discussed. Emissivity is obtained over an area of 1 m2 and is spectrally integrated according to the radiometer bandwidth. Analysis of the different radiance contributions to the signal and a careful calibration procedure led to correction factors which have been experimentally determined. The absolute accuracy in the [10.5–12.5] μm band is 0.5%. Tests with sample material (SiO2, SiC) have shown excellent agreement with the corresponding quantities, calculated from the absolute emissivity signature measured in the laboratory. A similar observation was achieved for bare, flat, and dry surfaces studied in natural conditions as well. However, results involving agricultural soils have given rise to roughness and moisture considerations. They have also led to the enhancement of the complexity of field measurement interpretation and the need to consider heterogeneity aspects.
Low‐temperature and high‐temperature hysteresis of small multidomain magnetites (215–540 nm)
Authors: Argyle Kenneth S. , Dunlop David J.
Published: February 2008

Approximately equidimensional magnetite crystals, with mean sizes of 215, 390, and 540 nm, respectively, have been produced by reducing hematite crystals. Isothermal magnetic hysteresis properties show a clear progression toward multidomain‐like behavior as the mean grain size increases. Saturation remanences Mrs are only 5–10% of saturation magnetization Ms, coercive forces HC are low (5.5–8 mT), and both Mrs and HC have grain‐size dependences compatible with those previously established for smaller and larger hydrothermally produced magnetites. Coercivities during remanence acquisition are greater than those measured during demagnetization. The difference between acquisition and destructive fields increases in the larger grains as a result of the increasing importance of the internal demagnetizing field. The low‐temperature transition is well expressed in the Mrs and HC data of the 540‐nm sample but is more subdued for smaller grains. Magnetostrictive, magnetocrystalline, and magnetostatic mechanisms in turn govern coercivity as the temperature rises. Remanence and coercivity ratios, Mrs/Ms and HR/HC, are almost temperature independent up to 500°C, indicating that domain wall configurations resulting from saturating fields are about the same at any temperature. A thermal fluctuation analysis of high‐temperature coercive force data suggests that regions 200–250 nm in size are thermally activated as a unit in grains of all sizes; these are likely domain walls. Apparent demagnetizing factors calculated from both low‐ and high‐temperature data are consistent with a mixture of two‐domain (2D) and three‐domain (3D) grains in all samples. However, theories of remanence in conventional 2D and 3D grains or in mixtures of 2D, 3D, and metastable single‐domain grains do not explain the data in a satisfying way.
Venus Trough‐and‐Ridge Tessera: Analog to Earth oceanic crust formed at spreading centers?
Authors: Head James W.
Published: February 2008

One type of tessera terrain on Venus, the trough‐and‐ridge tessera, exhibits a distinctive morphology composed of throughgoing generally parallel linear valleys and shorter orthogonal valleys and ridges. The similarity of this pattern to oceanic crustal topography and morphology is examined. Oceanic crust on Earth is characterized by a distinctive orthogonal pattern, with transforms and fracture zones in one direction and linear abyssal hills developed parallel to the rise crest in the other. Similarities between the components of the tessera terrain and terrestrial seafloor fabric and structure are found in terms of linearity, parallelism, length, width, spacing, orthogonality, and general morphology. A difference is that trough and ridge tessera tend to occur as plateau‐like regions at intermediate elevations (average ∼2 km) above the surrounding plains, while terrestrial seafloor ranges in elevation from that observed at rise crests and oceanic plateaus such as Iceland, down to depths of several kilometers on the abyssal floors, where the orthogonal pattern is often masked by sedimentation. On the basis of the similarities it is proposed that trough‐and‐ridge tessera may have originated through processes analogous to those responsible for the ocean floor fabric on Earth, forming at a rise crest similar to divergent boundaries on the Earth's seafloor, and evolving to its present morphology and configuration through processes of crustal spreading. The plateau‐like nature of the trough‐and‐ridge tessera distribution could be the result of localized crustal thickening in the spreading process, producing elevated Iceland‐like plateaus whose texture is preserved from subsequent lowland volcanic flooding. Some regions of more complex tessera patterns may result from deformation of the basic trough‐and‐ridge pattern of tessera. This hypothesis can be tested with global high‐resolution imaging and altimetry data and gravity data from the Magellan mission.
Silicate versus trace mineral susceptibility in metamorphic rocks
Authors: Borradaile Graham , Mackenzie Allan , Jensen Eleanor
Published: February 2008

Rates of change of magnetic susceptibility during leaching can characterize the presence of certain common types of magnetic minerals in metamorphic rocks. In this study the results of leaching are confirmed by mineral separation, optical microscopy, scanning electron microscopy (SEM) and microchemical analysis under SEM. Leaching curves provide a simple, rapid way of determining the relative roles of oxides, sulfides, and silicates in carrying the susceptibility in metamorphic rocks.
Rayleigh convection, mass transport, and change in porosity in layers of sandstone
Authors: Palm E.
Published: February 2008

The critical Rayleigh number for the onset of thermal convection in a quartz‐rich sandstone layer is discussed for different thermal boundary conditions. The mass transfer of silica due to Rayleigh convection is computed, assuming that the solution is saturated with respect to quartz and that the silica concentration is a function of temperature only. Since quartz solubility increases with temperature, silica is dissolved from the sandstones in the lower part of the layer where the fluid is being heated and precipitated in the upper part where it is cooled. The change in porosity is found as function of the vertical coordinate, valid for small values of time. It is shown that for small supercritical Rayleigh numbers the time for the porosity to change up to 10% of its initial value is of the order 10 m.y. The spatial variation in porosity should be a characteristic signature of Rayleigh convection and should be easy to recognize if convection has taken place for significant geological time.
Effect of grain size on brittle and semibrittle strength: Implications for micromechanical modelling of failure in compression
Authors: Fredrich Joanne T. , Evans Brian , Wong Teng‐Fong
Published: February 2008

Triaxial experiments were performed at room temperature and confining pressures up to 450 MPa on four pure, dense calcite rocks whose average grain sizes range over four orders of magnitude. Volumetric strain was measured during some of the experiments and microstructural studies were conducted to identify the active deformation mechanisms. The brittle fracture strength and macroscopic initial “plastic” yield stress in the semibrittle field follow empirical Hall‐Petch relations. The confining pressure at the brittle‐ductile transition depends inversely on grain size, but the stress ratio σ3/σ1 at the transition is nearly the same for the different rocks. We assume that the initial flaw size scales with grain size and compare our experimental data to the fracture mechanics models of Ashby and Hallam (1986) for brittle fracture and Horii and Nemat‐Nasser (1986) for the brittle‐plastic transition in compression. The first model predicts that small confining pressures are sufficient to inhibit work softening behavior; however, our data indicate that localization occurs for significantly higher values of confining pressure than predicted. Furthermore, we find that localization is inhibited with increased confining pressure because of the increased activity of plastic flow mechanisms, rather than because of the increased difficulty of crack propagation alone. With certain assumptions, the model predicts the experimentally determined slope of the Hall‐Petch relation in the brittle field, although it underestimates the compressive strength of the rocks. The second model predicts that the stress ratio σ3/σ1 at the brittle‐plastic transition scales with the, square root of the grain size; however, the experimental data do not corroborate the model unless the square of the ratio of the mode I fracture toughness to the plastic yield stress in shear scales with the grain size. The stress ratio at the brittle‐ductile transition is apparently a constant for many different rock types; we suggest that the physical basis for this relationship is that the ductility of most mineral aggregates falls within a small range.
Sediment volume in the north polar sand seas of Mars
Authors: Lancaster N. , Greeley R.
Published: February 2008

Data from studies of the cross‐sectional area of terrestrial transverse dunes have been combined with maps of dune morphometry derived from Viking orbiter images to generate new estimates of sediment thickness and dune sediment volume in the north polar sand seas of Mars. A relationship between dune spacing and equivalent sediment thickness (EST) was developed from field data on Namibian and North American dunes and was applied to data on dune spacing and dune cover measured on Viking orbiter images to generate maps of dune sediment thickness for Martian north polar sand seas. There are four major sand seas in the north polar region of Mars, covering an area of 6.8×105 km2. Equivalent sediment thickness ranges between 0.5 and 6.1 m with a mean of 1.8 m. The sand seas contain a total of 1158 km3 of dune sediment, which may have been derived by erosion of polar layered deposits and concentrated in its present location by winds that change direction seasonally.
Origin of crustal anisotropy: Shear Wave splitting studies in Japan
Authors: Kaneshima Satoshi
Published: February 2008

Shear wave splitting manifested as leading shear wave polarization, that is, parallel alignment of leading shear wave particle motions from a variety of sources, has been observed at a number of seismograph stations in Japan. Detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source‐receiver azimuths, the shear wave splitting can be attributed to crustal anisotropy. This paper discusses the relation between leading shear wave polarization directions and tectonic features of Japan. To explain the observed shear wave splitting, we propose that at least three phenomena should be taken into account: stress‐induced microcracks primarily aligned in vertical or subvertical planes; cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes; and intrinsic rock anisotropy resulting from preferred orientation of minerals. Travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15–25 km. This implies that the density of nonhorizontally aligned cracks or fractures below 15–25 km and into the upper mantle is much smaller than that in the crust above 15–25 km.
Shear wave anisotropy and stress direction in and near Long Valley Caldera, California, 1979–1988
Authors: Savage M. K. , Peppin W. A. , Vetter U. R.
Published: February 2008

We observed shear wave splitting (birefringence) for two temporary surface deployments of three‐component, digital seismographs, that were in place before and after M = 6+ earthquakes near the Long Valley caldera, California. In one of these deployments, the data sample precedes the large events of the May 1980 Mammoth Lakes earthquake sequence by 6 months and cover the two most active months of the May 1980 aftershock sequence; one of the stations (WIT) from this deployment was reoccupied with identical instrumentation in 1988. Another deployment preceded the 1986 Chalfant Valley mainshock by 2 days and recorded events for 6 days. The polarization of the faster shear wave changes from N30°W at Mammoth to due north at Chalfant Valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of P axes determined from focal mechanism groupings. Observations from nearby stations yield fast directions nearly at 23° from each other, and time separations of fast and slow shear waves show considerable station‐to‐station variation, showing no correlation with earthquake‐station distance or earthquake depth. These observations suggest that the observed anisotropy results primarily from near‐station (presumably shallow) effects rather than from widespread aligned microcracks. The records at Long Valley station WIT show a slight variation of average fast shear wave polarization from 1979 to 1980, but with the limited amount of data available, this difference is not statistically significant. Time separation measurements of up to 0.48 s at station WIT yield estimates of 15–30% velocity anisotropy, among the highest reported for local earthquakes. Secular variation in the time separation of fast and slow shear waves between events recorded at station WIT in 1979 and 1980 is swamped by the scatter in the data, if it is present. The five events recorded in 1988 showed similar S wave polarizations and time separations to those in 1980, but there are not enough events for a statistical comparison. Unlocated events at Long Valley station MRS show a slight increase in time separation after the May 1980 M = 6.0+ earthquakes, but the increase is not statistically significant.
Analytic waveforms for a line source in a transversely isotropic medium
Authors: Sakai Yukifumi , Kawasaki Ichiro
Published: February 2008

In order to clarify anisotropy effects on body wave propagation, exact transient solutions radiated from horizontal line sources of stress discontinuities in a transversely isotropic medium with a vertical axis of symmetry are formulated by a modified Cagniard method which gives the inverse Laplace transform in an ingenious manner. The advantage of the exact transient solutions is the clear onset of elastic displacements at the time of body wave arrivals that are defined by corresponding group velocities. The exact transient solutions display three principal effects of anisotropy: (1) noncircular propagation of wave fronts; (2) deviation of polarizations of particle motions of qP, qSV, and qSH waves from those in an isotropic medium; and (3) separation of group velocity from phase velocity, depending on the relative orientation of anisotropy and wave propagation directions.
Magnetic properties and opaque mineralogy of rocks from selected seafloor hydrothermal sites at oceanic ridges
Authors: Wooldridge Anita L. , Haggerty Stephen E. , Rona Peter A. , Harrison Christopher G. A.
Published: February 2008

Magnetic properties (natural remanent magnetization (NRM), susceptibility (χ), Curie point temperature (Tc), saturation isothermal remanent magnetization (IRMs), and Köenigsberger ratio (Q)) and opaque mineralogy were determined for basalts, diabases, gabbros, peridotites, and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites (15°N, 17°N, 23°N (the Snake Pit hydrothermal field), 26°N (the TAG hydrothermal field) on the Mid‐Atlantic Ridge, and 42°N (the Sea Cliff hydrothermal field) on the northern Gorda Ridge). The magnetomineralogy is interpreted in terms of deuteric, hydrothermal, metasomatic, and ambient seawater alteration. Evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases, gabbros, and ultramafic rocks. This lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low‐temperature alteration in young surface basalts to more pervasive high–temperature alteration in diabases, gabbros, and peridotites. The studies reveal distinct correlations between magnetization intensity, thermomagnetic behavior, and magnetic mineralogy, grain size, style, and intensity of alteration and rock type. Basalts have the highest NRM intensities and the lowest Curie points (105°–294°C); basalts with NRM/IRMs ratios less than 10−2 may have been remagnetized and have NRM intensities and Q values much lower (one–third and one–half, respectively) than basalts with NRM/IRMs ratios of 10−2 or greater. In the oceanic lithospheric suite examined, remanent (Q > 1) magnetization is present in basalts and induced magnetization (Q < 1) occurs in metabasalts and the other rock types. Layer 2A basalts are the source of median valley magnetic anomalies. The magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2B dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites. This shift is influenced by fluctuations in the Curie isotherm related to the duration of active magma chambers and convective hydrothermal cells. The magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere, magmatic cooling history, and hydrothermal circulation.
Times of increased probability of strong earthquakes (M ≥ 7.5) diagnosed by Algorithm M8 in Japan and adjacent territories
Authors: Keilis‐Borok V. I. , Kossobokov V. G.
Published: February 2008

This paper continues to explore the possibility of identifying the times of increased probability (TIPs) of occurrence of a strong earthquake by premonitory intermediate‐term seismic activation in lower‐magnitude range, as defined in the algorithm M8. A TIP refers to a 5‐year period and an area whose linear size is several times larger than that of an incipient earthquake. In the course of the global test of the algorithm we applied it retrospectively to the data from the National Earthquake Information Center worldwide earthquake catalog (since the regional Japan Meteorological Agency catalog was unavailable to authors), related to the segment of circum‐Pacific belt between latitudes 20°N and 48°N, from the south of Kuril arc to the Taiwan and Izu‐Bonin arc. The worldwide catalog allows us to diagnose TIPs for earthquakes above magnitude 7.5 from 1975 on. Five out of six earthquakes with M ≥ 7.5 occurred within the TIPs diagnosed by the algorithm M8 with a priori fixed parameters. The missed earthquake occurred at the final stage of formation of the corresponding TIP. Altogether, the TIPs occupy less than 20%, and times of expectation occupy about 8%, of the total space‐time domain considered. There are also current TIPs covering about half of the territory. Application of the additional algorithm of localization, named “Mendocino Scenario,” indicates that the southern part of the Japan trench is an area deserving particular attention. The results obtained here by no means constitute the definitive earthquake prediction but merely outline the territories for more detailed and comprehensive analysis. If confirmed by regional data, the results may be used for implementation of some safety measures and evaluation of other precursors, including short‐term ones.
Thermodynamically consistent decompression: Implications for lower mantle composition
Authors: Bukowinski Mark S. T. , Wolf George H.
Published: February 2008

Estimates of the composition of the lower mantle are commonly obtained by a process of adiabatic decompression, in which zero‐pressure mantle parameters are determined with a least squares fit of an equation of state to mantle densities. We show that this procedure may lead to biased estimates of the zero‐pressure density and compressibility of the mantle. The bias can be eliminated or minimized by subjecting the equation of state fit to a thermodynamic constraint at zero pressure. Such inversions provide equations of state that are compatible with the lower mantle as well as the thermal properties of selected mineral models. We used this method to evaluate assemblages containing (Mg, Fe)SiO3 perovskite and (Mg, Fe)O, with Fe/(Fe+Mg) ratios as high as 0.2. Thermodynamic systematics were used to place bounds on the Anderson‐Gruneisen parameter δs for perovskite. We find a fairly large range of acceptable models for the composition of the lower mantle. They range from uppermantle‐like compositions to assemblages that are enriched in both Si and Fe relative to the upper mantle. However, a pure perovskite lower mantle, whether enriched in Fe or not, seems improbable. Small Fe enrichments of the order of 1%, given density contrasts across the 670‐km discontinuity that are probably insufficient to prevent convective mixing of the upper and lower mantles. We therefore suggest that unless the mantle is homogeneous in its major oxide components, the lower mantle is likely to be enriched in Fe by at least several percent.
Axial seamount: An active ridge axis volcano on the Central Juan De Fuca Ridge
Authors: Johnson H. Paul , Embley Robert W.
Published: February 2008

Axial Seamount (some of the manuscripts in this special section refer to the edifice with the more precise name of “Axial Volcano”), a large ridge axis volcano, is located on the central segment of the Juan de Fuca Ridge approximately 250 nautical miles west of the Washington/Oregon/British Columbia coast. Currently both volcanically and hydrothermally active, Axial lies directly at the intersection of the Cobb‐Eickelberg Seamount Chain and the Juan de Fuca Ridge (Figure 1). The volcanic activity associated with the seamount formation strongly interacts with, and is affected by, the normal seafloor spreading processes at the intersection. Because of this unique geologic setting, its proximity to west coast ports and oceanographie institutions, and its shallow depth, Axial has become the focus of a large number of scientific investigations over the past decade.
Consequences of phase separation on the distribution of hydrothermal fluids at ASHES Vent Field, Axial Volcano, Juan de Fuca Ridge
Authors: Fox Christopher G.
Published: February 2008

Recent studies of hydrothermal fluid chemistry at the Axial Seamount Hydrothermal Emissions Study (ASHES) vent field of Axial Volcano indicate separation of liquid and vapor phases during the migration of fluids to the seafloor. The brine phase liquid emanates from discrete sulfide chimneys, while fluid which has been through vapor phase separation flows diffusely from fissures in the surrounding basalt. One possible mechanism to explain this spatial segregation of the fluid phases is based on the relative permeability terms in the equations of two‐phase flow. A qualitative model for the distribution of fluids observed at ASHES is postulated in which brine phase fluids are confined within flow conduits by a surrounding relative permeability barrier, while vapor phase fluids flow diffusely into the surrounding host rock.
Discontinuities in the shallow Martian crust at Lunae, Syria, and Sinai Plana
Authors: Davis Philip A. , Golombek Matthew P.
Published: February 2008

Detailed photoclinometric profiles across 125 erosional features and 141 grabens in the western equatorial region of Mars indicate the presence of three discontinuities within the shallow crust. Pits, troughs, and wall valleys (tributary canyons) within Noctis Labyrinthus and Valles Marineris and escarpments within the fretted terrain of Sacra Fossae and Kasei Valles show distinct erosional base levels at depths of 0.3–0.6 km, 1 km, and 2–3 km. The shallowest discontinuity corresponds to thickness estimates for the ridged plains unit in this region, and thus the discontinuity probably is the contact between a sequence of layered rock making up this unit and the underlying megaregolith. The 1‐km discontinuity is reflected in the base levels of erosion of all the features studied, and it may correspond to the base of the proposed layer of ground ice. Model calculations using photoclinometric profiles of simple grabens (corrected for the effects of mass wasting) and the 60° dip of bounding faults (measured from the exposed traces of faults on trough walls) show that graben‐bounding faults consistently intersect at the mechanical discontinuity at about 1 km depth. This discontinuity may represent an interface between ice‐laden and dry regolith, ice‐laden and water‐laden regolith, or pristine and cemented regolith. A correlation between wall valley head depth and local thickness of the faulted layer suggests that the 1‐km discontinuity also controlled the depth of the heads of sapping canyons. There is no apparent relation between the pit and trough depths and the local thickness of the faulted layer, which can be explained if pits and troughs developed by subsidence into underlying tension cracks that disrupted the 1‐km discontinuity. The data do suggest, however, that erosion of shallow pits and troughs was influenced by the 1‐km discontinuity. The third discontinuity, at a depth of 2–3 km, corresponds to the proposed base of the Martian megaregolith and is probably the interface between overlying, ejected breccia and in situ, fractured basement rocks.