bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic characterization of a highly stressed rock mass using tomographic imaging and induced seismicity
Authors: Young R. P. , Maxwell S. C.
Published: February 2008

A seismic tomography study was carried out at Falconbridge's Lockerby Mine, Sudbury, Canada, in April 1989. The objective was to determine the integrity of a rock mass at 1 km depth and to investigate the correlation between induced seismicity and P wave velocity structure. The tomographic image showed two distinct and significant P wave velocity anomalies: a low‐velocity zone at the source region of a MN 2.3 mining‐induced tremor recorded in 1985 and a high‐velocity zone at the source region of a MN 2.6 mining‐induced tremor recorded in 1988. Analysis of mining‐induced microseismicity showed that the high‐velocity zone was the focus for seismic activity prior to and after the 1988 tremor, in contrast to the low‐velocity zone which remained aseismic for the same period. The seismic anomalies were diamond drilled, following the tomographic imaging, to verify the physical characteristics of the zones. The seismically active high‐velocity zone showed severe core disking, indicative of a highly stressed strong rock, while the low‐velocity zone was characterized by extensive fracturing and failed rock. The results showed that the 1985 tremor and rock burst sequence resulted in a major failure of the rock mass in the source region of the event, and stress was redistributed and concentrated in the source region of the 1988 tremor. The 1988 event and rock burst sequence did not result in failure of the source region, which therefore remains a seismically active high‐velocity zone with a potential for significant seismic energy release and further rock bursting. The study has highlighted a correlation between P wave velocity structure and induced seismicity which can be used to characterize highly stressed rock masses in underground mines.
Field and laboratory studies of metal uptake and release by hydrothermal precipitates
Authors: Metz Simone , Trefry John H.
Published: February 2008

Data for suspended particles from hydrothermal plumes on the Mid‐Atlantic Ridge (MAR) support uptake of dissolved V and As during formation of Fe oxides and release of Cu and Cd during alteration of sulfides. To complement these field observations, we carried out laboratory time series experiments to determine the stability of V, As, Cu, and Cd in precipitates from hydrothermal plumes on the MAR and Juan de Fuca Ridge. For V and As, the metal/Fe ratio in plume particles remained uniform after sampling. In contrast to V and As, particulate Cu and Cd values decreased relative to Fe over time in laboratory studies using both plume particles and mound‐derived sulfides. The combined field and laboratory results show that scavenging is an important removal mechanism for seawater V and As, whereas oxidative dissolution of hydrothermal precipitates in seawater adds Cu and Cd to the ocean.
Upwelling asthenosphere beneath western Arabia and its regional implications
Authors: Camp Victor E. , Roobol M. John
Published: February 2008

Two distinct phases of continental magmatism are evident in western Arabia. The first, from about 30 to 20 Ma, produced tholeiitic‐to‐transitional lavas emplaced along NW trends. The second, from about 12 Ma to Recent, produced transitional‐to‐strongly‐alkalic lavas emplaced along N‐S trends. The older phase is attributed to passive‐mantle upwelling during extension of the Red Sea Basin, whereas the younger phase is attributed to active‐mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision. The younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the West Arabian Swell after about 14 Ma. A variety of evidence suggests that the West Arabian Swell is thermally supported by hot, upwelling asthenosphere. In contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the Red Sea Basin, these processes were symmetric across a N‐S line marking the central axis of the West Arabian Swell. This axis coincides with two fundamental features: the Ha'il‐Rutbah Arch in the north, and the Makkah‐Madinah‐Nafud (MMN) volcanic line in the south. The symmetry of magmatism is demonstrated by petrochemical evidence that the MMN harrats were derived by greater degrees of partial melting, at shallower depths, than those harrats lying to the west and east of the MMN line. The potential temperature of the asthenosphere is estimated to be about 1436°C beneath the MMN line and about 1354°C beneath the flanking, more undersaturated harrats. The source of upwelling is either a mantle plume centrally located beneath the West Arabian Swell or an elongated and extended lobe of hot asthenosphere emanating from the Ethiopian mantle plume. Convective flow may have been channelled along a preexisting, regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the AfroArabian Dome. This crest of mantle upwelling underlies the MMN line in the north and the Danakil Depression and Ethiopian rift system in the south. It also passes through the Red Sea Basin at the midpoint of an unusual, doubly propagating rift system where axial seafloor spreading began 4–5 m.y. ago. The NW trend of the Red Sea Basin was well established by crustal attenuation during the older magmatic phase. The Pliocene invasion of this basin by a N‐S zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions.
Seismic constraints on the nature of lower crustal reflectors beneath the extending Southern Transition Zone of the Colorado Plateau, Arizona
Authors: Parsons Tom , Howie John M. , Thompson George A.
Published: February 2008

We determine the reflection polarity and exploit variations in P and S wave reflectivity and P wave amplitude versus offset (AVO) to constrain the origin of lower crustal reflectivity observed on new three‐component seismic data recorded across the structural transition of the Colorado Plateau. The near vertical incidence reflection data were collected by Stanford University in 1989 as part of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that traversed the Arizona Transition Zone of the Colorado Plateau. The results of independent waveform modeling methods are consistent with much of the lower crustal reflectivity resulting from thin, high‐impedance layers. The reflection polarity of the cleanest lower crustal events is positive, which implies that these reflections result from high‐velocity contrasts, and the waveform character indicates that the reflectors are probably layers less than or approximately equal to 200 m thick. The lower crustal events are generally less reflective to incident S waves than to P waves, which agrees with the predicted behavior of high‐velocity mafic layering. Analysis of the P wave AVO character of lower crustal reflections demonstrates that the events maintain a constant amplitude with offset, which is most consistent with a mafic‐layering model. One exception is a high‐amplitude (10 dB above background) event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets. The event has a pronounced S wave response, which along with its negative AVO trend is a possible indication of the presence of fluids in the lower crust. The Arizona Transition Zone is an active but weakly extended province, which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers. Instead, we favor horizontal basaltic intrusions as the primary origin of high‐impedance reflectors based on (1) The fact that most xenoliths in eruptive basalts of the Transition Zone are of mafic igneous composition, (2) indications that a pulse of magmatic activity crossed the Transition Zone in the late Tertiary period, and (3) the high regional heat flow observed in the Transition Zone. The apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion. We present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust.
Tomographic inversions for mantle P wave velocity structure based on the minimization of l2 and l1 norms of International Seismological Centre Travel Time Residuals
Authors: Pulliam R. Jay , Vasco D. W. , Johnson Lane R.
Published: February 2008

We use International Seismological Centre (ISC) P arrival data (1964–1987) and iterative algorithms which minimize the l1 and l2 residual norms to solve simultaneously for three‐dimensional P velocity variations in Earth's mantle, source mislocations, and station corrections. We find that the maximum velocity perturbations produced by the l1 minimization (approximately ±4% in our final model) are relatively insensitive to smoothing and damping constraints. Therefore, using an l1 norm criterion allows us to keep the bias introduced to the inversion to a minimum. Among the well‐resolved features contained in both the l1 and the l2 velocity models are a fast anomaly in the lower mantle beneath the Tonga‐New Hebrides subduction zone to a depth of 1670 km and another fast anomaly beneath the Japanese island arc and eastern Asia. Continuity between these anomalies and shallower fast anomalies is not clear. A fast anomaly extending from 670 km to 2070 km depth appears beneath eastern North America, the Caribbean, and north central South America. A broad, fast anomaly appears beneath eastern Asia just above the core‐mantle boundary as well as several slow anomalies under the Pacific basin of comparable size. Both models contain a circum‐Pacific ring of 2% lower velocities in the depth range 0–200 km, associated with back arc basins. High velocities (over 2%) associated with the continental shields tend to disappear below 400 km, though a significant region of high velocity remains beneath the Siberian platform in the 400 to 670 km depth interval.
Upper mantle structure beneath central Eurasia using a source array of nuclear explosions and waveforms at regional distances
Authors: Goldstein Peter , Walter William R. , Zandt George
Published: February 2008

We have observed very consistent broadband, regional, three‐component P waveforms from a set of 11 explosions that occurred at the former Soviet test site in Kazakhstan during 1988 and 1989 and have modeled most of the prominent features in these waveforms to determine upper mantle structure beneath central Eurasia. Using a subset of events as a source array, we have identified four consistent arrivals during the first 15 s of P waves recorded at the Soviet/Incorporated Research Institutions for Seismology (IRIS) stations Arti (ARU) (d ≈1500 km) and Garm (GAR) (d ≈1380 km). These arrivals show consistent variations in frequency content, relative timing, and amplitudes from event to event despite a range in source magnitude (mb) between 4.9 and 6.1. We have also identified consistent features in P waveforms of these events recorded at Obninsk (OBN) (d ≈2880 km). We argue that most of the prominent features in these waveforms can be explained by reflections at or refractions near discontinuities or large velocity gradients in the upper mantle. We show that a model with discontinuities of approximately 3.0% and 6.5% near 200 km and 400 km, respectively, produces a better fit to the broadband data at ARU and GAR than previous models for this region. This model also produces a good match to waveforms recorded at OBN and NORESS, however, this model is not unique. For example, we show that it is also possible to produce similar waveforms by replacing the 200 km discontinuity with a large, notch shaped, low velocity zone between approximately 100 and 140 km. Based on reflectivity synthetics, we suggest that additional data, especially from distances less than 1000 km, should indicate which of these models is more accurate. Although our models are not unique, our observations do constrain the following features. (1) There must be reflectors between about 140 and 200 km and near 400 km depth in the upper mantle to explain the large secondary arrivals at both ARU and GAR. (2) Based on the relative size of some of the secondary arrivals at ARU and GAR, the discontinuity near 400 km must be large. However, it cannot be much greater than about 6.5% because there is no evidence, in our data, of a reflection from 400 km at OBN. (3) The relatively small amplitude of the first arrivals at both ARU and GAR suggests that either the gradient in the uppermost mantle is very small or that a shallow low velocity zone is present. Comparison of our results with other upper mantle studies shows that the central Eurasian upper mantle is similar to the upper mantle of the central and eastern United States.
Seismic velocity structure of the subducting Pacific Plate in the Izu‐Bonin Region
Authors: Iidaka Takashi , Mizoue Megumi , Suyehiro Kiyoshi
Published: February 2008

Although numerous studies have been made on the contrast between subducting slabs and surrounding mantle, little is known about the internal structure of the oceanic lithosphere after it descends into the mantle at subduction zones. The velocity structure within the slab can be expected to be heterogeneous, based on thermal and petrological considerations. In this study, observed travel time residual data from a spatially dense seismic network above deep earthquakes in the Izu‐Bonin region are compared with three‐dimensional ray tracing calculations. The data are inconsistent with a homogeneous slab model and consistent with a heterogeneous slab model with regional velocity variations. The residual data can be explained by a model that has a velocity gradient within the slab. In the subducting Pacific plate, the velocity near the center of the slab is faster than that near the upper boundary, and gradually decreases toward the bottom of the plate. A model with a velocity decrease of 3%, as predicted by a thermal profile, explains the observed data.
Simulations of the structure and thermodynamic properties of water at high pressures and temperatures
Authors: Brodholt John , Wood Bernard
Published: February 2008

Precise pressure‐volume‐temperature (PVT) data for water exist to pressures of only 8.9 kbar and temperatures of 900°C. Since aqueous fluids are important in many processes at much higher pressures and temperatures, there is a need to develop reliable extrapolations of the low pressures experimental data. In this paper we present the results of molecular dynamics simulations of water properties over a wide P‐T range. It has been shown that at a density of 1.0 g/cm3, the TIP4P intermolecular potential (Jorgensen et al., 1983) very accurately reproduces the available thermodynamic and structural properties of water at pressures of 1 bar to 10 kbar (Brodholt and Wood, 1990). This is extended here by testing TIP4P at temperatures to 2500 K and 350 kbar. The PVT predictions are compared to the experimental data of Bumham et al. (1969) at low pressures, and with the data of Kormer (1968) and Bridgman (1942) at higher pressures. We conclude that there is good reason to be confident of the predictive powers of the TIP4P intermolecular potential in the high‐pressure and high‐temperature regime. Heat capacities and structural properties have also been calculated and compared to available data. High‐pressure simulations have also been made with two other intermolecular potentials which accurately represent water properties at low pressures, SPC/E (Berendsen et al., 1987) and WK (Watanabe and Klein, 1989). SPC/E predicts PVT properties that are similar to those of TIP4P, while WK is accurate only at densities of about 1.0 g/cm3. At high pressures the spherical potential of Belonoshko and Saxena (1991) predicts volumes that are within 0.5 m of those predicted by TIP4P. A modified Redlich Kwong (MRK) type equation of state based on the molecular dynamics simulations is presented which can be used at pressures in excess of 10 kbar. This is used to predict the location of the brucite dehydration reaction which has recently been determined to 120 kbar (Johnson et al., 1991).
On the relation between transform fault resistance and plate motion
Authors: Stoddard Paul R.
Published: February 2008

A simple model for power dissipation due to friction along transform faults is used to examine the relations between relative velocity and power dissipation across transforms, transform lengths, and plate motions. Comparison is made between ridge transforms, i.e., those that are part of spreading systems, and major boundary transforms, such as the San Andreas fault. Power dissipation is calculated using velocity across the transform area above the 400°C (oceanic) or 350°C (continental) isotherm. Transform thermal structure is modeled by averaging the temperatures on either side of the transform. Whereas ridge transforms across which the relative plate velocity is slow can be of any length, fast relative velocities only appear across short transforms. When all transforms are used in this comparison, a maximum speed, as a function of transform length, is suggested. A minimum speed of ∼13.5 mm/yr across ridge transforms was found. In a case study of the Pacific plate, the transform lengths were found to have varied with time which has minimized the total power dissipation across the plate's transforms.
Effective stress for transport properties of inhomogeneous porous rock
Authors: Berryman James G.
Published: February 2008

General effective‐stress rules are derived for various physical properties of inhomogeneous porous rocks. Some rigorous relations arising in the analysis show that the fluid (pore) pressure pf is least effective at counteracting the changes caused by confining pressure for the solid (grain) volume; pf is more effective for the total (solid plus pore) volume; pf is still more effective for the pore volume; and pf is most effective at maintaining the fluid content of the pores. Although these results are expected intuitively, this analysis provides the first rigorous demonstration. During analysis of coefficients, care is taken to distinguish between rigorous inequalities (following from thermodynamics) and empirical inequalities (commonly observed, but not required by thermodynamics). For microscopically homogeneous rocks (the Gassmann limit), it is shown that the confining pressure is always at least as effective as the fluid pressure at changing the fluid permeability; therefore it is impossible to use any “equivalent homogeneous rock” to explain experimental results of Zoback and Byerlee (1975) and others (wherein it has been shown experimentally that the permeability sometimes is more strongly influenced by fluid pressure than confining pressure). We show that the equivalent homogeneous rock paradigm may be successfully replaced by the “two‐constituent porous medium” paradigm. In principle, the new paradigm can explain the data, but new measurements of pore compressibilities are required before quantitative comparisons can be made.
Effect of thermal regime on growth increment and spacing of contraction joints in basaltic lava
Authors: Degraff James M. , Aydin Atilla
Published: February 2008

Columnar joints in volcanic rocks are thermal contraction cracks that bound elongate rock columns. The joints grow incrementally toward the interior of a cooling extrusion by successive formation of new segments on the joint edges. Joint spacing as measured by column diameter is often smaller in the upper portion of a lava flow and larger in the lower portion. We investigated incremental growth of columnar joints and factors controlling their spacing by measuring growth increments and spacings of joints in basaltic lava and by modeling the observations using fracture mechanics principles. The field data show that growth increment and spacing are linearly correlated and increase up to a limit with distance from the cooling surface on which the joints start. The models of incremental joint growth combine fracture mechanics with thermomechanical models of layers cooling by conduction or water‐steam convection in joints. A joint propagates when tensile stress concentrated at its tip builds up during cooling and exceeds the lava's resistance to fracture. The joint arrests when its tip reaches hot ductile lava with a very high resistance to fracture. Blunting of the arrested tip by ductile flow reduces joint tip stresses and thus stabilizes the joint until further cooling restores critical stress at the tip. The models predict that the joint growth increment will increase slightly from cycle to cycle up to a limit as observed in the field. Faster solidification rates with higher thermal gradients produce smaller growth increments because the level to which a joint propagates will have migrated relatively little when the arrested joint tip cools sufficiently to restart joint growth. An analysis accounting for the mechanical interaction of joints shows that a joint extended beyond its neighbors reduces the joint tip stress of its neighbors, thereby inhibiting their growth and determining a minimum joint spacing. The predicted minimum spacing increases with growth increment in agreement with field measurements. The data and model results therefore indicate that faster solidification promotes smaller joint growth increments that, in turn, produce smaller joint spacings.
Room temperature creep in saturated granite
Authors: Lockner David
Published: February 2008

Cylindrical samples of granite were deformed at 26ºC, constant confining pressure (60 MPa), and constant pore pressure (20 MPa). Axial and volumetric strain were determined from changes in the output of resistance foil strain gauges bonded to the rock surface. In addition, dc electrical resistivity was measured parallel to the sample axis. During each experiment (typically lasting from 1–2 weeks), the deviatoric stress σd applied to the sample was cycled between 70% and 90% of the short–term failure strength. The bulk of the experiments were conducted in the secondary or “steady state” creep regime. Inelastic volumetric strain rate was found to obey the law , where and (compressive stresses are negative). The C coefficient represents a strain‐hardening‐like term. The stress dependence is of the same form as the stress dependence measured for mode I crack growth in double cantilever beam experiments. The observed creep behavior is analyzed in terms of stress corrosion and crack growth models including a formulation based on energy release rate for characteristic microcracks.
The stretched exponential function as an alternative model for aftershock decay rate
Authors: Kisslinger Carl
Published: February 2008

The stretched exponential (Williams‐Watts) relaxation function is N*(t)=N*(0)exp[−(t/t0)q], 0<q≤1. As applied to aftershocks, N* (0) is the number of aftershocks that have not yet occurred at time t, starting with a finite number of potential events, N* (0). This function describes slow relaxation in a number of distinct physical systems. Whether it is the appropriate function for aftershocks, as an alternative to the modified Omori power law function, depends on the long‐time behavior of the mean interevent time. The behavior of this function has been tested on synthetic sequences in comparison with the Omori relation to determine the feasibility of determining which is the better model of aftershock rate decay. The differences are practically indistinguishable for very slowly decaying sequences but are measurable for normally and rapidly decaying sequences if excellent data are available. Maximum likelihood estimates of the parameters in the stretched exponential function were determined for 29 sequences and the fit of the model to the data was compared with the modified Omori fit by the Akaike Information Criterion. If the time of the rnainshock is taken as the start time for the sequence, the Omori fit is almost always better. If a later start time is used, 15 min to 2.5 hours for the cases tested, the stretched exponential fit is better in about half the cases. The selected start time is the time at which the rate of occurrence begins to decrease, after an initial apparent increase probably due to missed events during the very active interval. As expected, the relaxation time t0 is inversely related to the Omori p value and may prove to be more easily interpretable in terms of the physics of the aftershock‐generating process.
Analysis of scarp profiles: Evaluation of errors in morphologic dating
Authors: Avouac Jean‐Philippe
Published: February 2008

Morphologic analysis of scarp degradation can be used quantitatively to determine relative ages of different scarps formed in cohesionless materials, under the same climatic conditions. Scarps of tectonic origin as well as wavecut or rivercut terraces can be treated as topographic impulses that are attenuated by surface erosional processes. This morphological evolution can be modelled as the convolution of the initial shape with erosion (or degradation) function whose width increases with time. Such modeling applies well to scarps less than 10m high, formed in unconsolidated fanglomerates. To a good approximation, the degradation function is Gaussian with a variance measuring the degree of rounding of the initial shape. This geometric parameter can be called the degradation coefficient. A synthetic experiment shows that the degradation coefficient can be obtained by least squares fitting of profiles levelled perpendicular to the scarp. Gravitational collapse of the free face is accounted for by assuming initial scarp slopes at the angle of repose of the cohesionless materials (30°–35°). Uncertainties in the measured profiles result in an uncertainty in degradation coefficient that can be evaluated graphically. Because the degradation coefficient is sensitive to the regional slope and to three‐dimensional processes (gullying, loess accumulation, stream incision, etc.), a reliable and accurate determination of degradation coefficient requires several long profiles across the same scarp. The linear diffusion model of scarp degradation is a Gaussian model in which the degradation coefficient is proportional to numerical age. In that case, absolute dating requires only determination of the propotionality constant, called the mass diffusivity constant. For Holocene scarps a few meters high, in loose alluvium under arid climatic conditions, mass diffusivity constants generally range between 1 and 6 m2/kyr. Morphologic analysis is a reliable method to compare ages of different scarps in a given area, and it can provide approximate absolute ages of Holocene scarplike landforms.
Depth of seismic coupling along subduction zones
Authors: Tichelaar Bart W. , Ruff Larry J.
Published: February 2008

Underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding along the plate interface. We have determined the depth of this stability transition for the circurn‐Pacific subduction zones of: Honshu, Kuriles, Kamchatka, Aleutians, Alaska, Mexico, and Chile. These subduction zones have experienced great interplate earthquakes and the aftershock regions are well‐located. Depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling. For an average P wave velocity of 6.7 km s−1 above the plate interface, we find that for most subduction zones the stability transition occurs at 40 ± 5 km depth. There are, however, several exceptions. At the Hokkaido trench junction, where the Japan trench and the Kurile trench intersect, seismic coupling is deep and extends down to 52–55 km. Deep coupling was also found in the Coquimbo region in central Chile. The Mexico subduction zone has shallow coupling: the transition occurs at 20–30 km depth. Previous studies of micro‐earthquakes in Honshu, Hokkaido, the Aleutians, and Alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find. Given our measurements of seismic coupling depth, we then explore the mechanism that may determine coupling depth. The concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments, thus we first test whether a critical temperature can explain our results. Temperatures at the plate interface are dependent on many variables; but two that are poorly determined are shear stress and radiogenic heat generation. Shear stress has been constrained by inversion of heat flow data. Assuming a crustal radiogenic heat production rate of 3.1 exp−z/8.5 μWm−3 and a constant coefficient of friction, we find two critical temperatures of about 400 ° C and 550 ° C. The lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust. On the other hand, one single critical temperature of about 250 ° C can explain the coupling depths if shear stresses are constant with depth.
Effects of curvature on two‐dimensional models of mantle convection: Cylindrical polar coordinates
Authors: Jarvis Gary T.
Published: February 2008

A two‐dimensional time‐dependent numerical model of convection in a cylindrical annulus is described. Model results are presented and compared with similar models in planar geometry. In curvilinear geometry the upper (outer) boundary has a larger surface area than does the lower (inner) boundary. This results in an asymmetry between the upper and lower thermal boundary layers which is not found in planar geometry. A boundary layer argument is developed for curvilinear geometry which accounts for differences in model predictions between the curvilinear and planar models. It is found that in cylindrical geometry the effects of curvature on Nusselt number, mean temperature, relative thickness of the inner and outer thermal boundary layers, and temperature drops across the thermal boundary layers can all be parameterized in terms of the fraction ƒ of the inner to outer radii of the cylindrical bounding surfaces.
Nondestructive measurements of fracture aperture in crystalline rock cores using X ray computed tomography
Authors: Johns Robert A. , Steude John S. , Castanier Louis M. , Roberts Paul V.
Published: February 2008

This study used X ray computed tomography (CT) to investigate the variability in the aperture field of natural fractures in two granitic cores; one core, 57 mm in diameter and 146 mm in length, was studied with a medical CT scanner, whereas the other core, 87 mm in diameter and 245 mm long, was characterized by means of an industrial CT scanner. A quantitative methodology for measuring the fracture‐aperture thickness from CT images was developed and applied to images of cores to reconstruct the distribution of fracture aperture in the cores, with a spatial resolution of 1.4 mm by 1.4 mm by 5 mm. The CT images of the natural fractures revealed that the aperture thickness (1) is not constant within the fracture plane, (2) varies over several orders of magnitude, and (3) approximately followed a lognormal distribution for one of the cores. The CT method, when applied to monitor the movement of contrast agents injected into the cores, demonstrated fingering of the flow. Higher contrast agent concentrations and earlier contrast agent arrivals correlated with larger aperture regions of the core. The results of this study demonstrate that CT can be used to characterize fractures nondestructively and to detect the movement of contrast agents in granitic cores.
Denudation of metamorphic core complexes and the reconstruction of the transition zone, West central Arizona: Constraints from apatite fission track thermochronology
Authors: Foster David A. , Gleadow Andrew J. W. , Reynolds Stephen J. , Fitzgerald Paul G.
Published: February 2008

Apatite fission track thermochronologic results from transects across the Basin and Range and Transition Zone provinces in west central Arizona provide constraints on the denudational history and structural framework of the region. Apatite fission track ages decrease from ∼21 to 14 Ma in the Harcuvar Mountains and from ∼16 to 13 Ma in the Buckskin‐Rawhide Mountains in the slip direction (SW ‐ NE) of detachment faults in the lower plates of the metamorphic core complexes. Mean lengths of confined fission tracks from the core complexes are all >14 μm, indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone (<110°C) to near‐surface conditions. These data give the time at which progressively deeper parts of the lower plates were drawn up through the apatite annealing zone and suggest that the slip rate on detachment faults in the Whipple tilt domain was ∼7–8 mm/yr. The apatite fission track results also indicate that detachment faulting ended at ∼13–14 Ma in this area and, when they are combined with other thermochronologic data, yield cooling rates of >40°C/m.y. for lower plate rocks. Apparent apatite ages in the Transition Zone province generally increase from ∼25 Ma to ∼100 Ma away from the Basin and Range province. This trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the Weaver Mountains and the Colorado Plateau. Gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the Phillips‐Kirkland drill hole reveal parts of denuded Mesozoic and Cenozoic apatite partial annealing zones. These paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks. The reconstructions indicate that the major faults in the Transition Zone province have relative displacements of >1 km and that offset on them occurred mostly after 25 Ma. These data also indicate that most of the rocks now exposed in the Transition Zone of west central Arizona were not exposed until Miocene time.
Inhibition and geomagnetic field reversals
Authors: McFadden P. L. , Merrill R. T.
Published: February 2008

Analysis of the geomagnetic reversal chronology indicates a positive correlation between reversal rate and reversal inhibition. Such a correlation is to be expected between reversal rate and any inhibition resulting from the fact that a reversal transition requires a finite time. The very nature of the way that reversal chronologies are constructed dictates the existence of an interval of about 5000 years, following the initiation of a successful reversal, during which the probability of observing a further reversal is zero. Modeling indicates that the observed estimates of inhibition are consistent with an interval of depressed probability much longer than this. Our preferred model is that reversals are triggered by internal instabilities of the fluid motion of the core and that, after the (approximately) 5000‐year interval required for successful reversal of the direction of the magnetic field, the probability for further reversals gradually recovers to its initial value over an interval of about 45,000 years. We suggest that this recovery period is related to subsequent diffusion of the magnetic field in the core. A corollary of this interpretation is that it is possible to make a rough estimate of the electrical conductivity of the core fluid from paleomagnetic data. The estimated value is 6 × 105 S m−1.
Three‐dimensional mantle flow beneath mid‐ocean ridges
Authors: Rabinowicz M. , Rouzo S. , Sempere J.‐C. , Rosemberg C.
Published: February 2008

We have studied the development of multiscalar mantle flow beneath mid‐ocean ridges using a three‐dimensional numerical technique. Modeling includes various modes of heating and the shear flow induced by plate motion away from a spreading center with and without transform offsets. The computational box consists of two layers differing in their viscosities. It is shown than when flow is generated by heat sources located for the most part in the lower layer, convective cells have horizontal dimensions proportional to the entire thickness of the box. It is only when flow is driven by buoyancy forces in the top layer, or by transient cooling along the top interface, that transverse rolls are found to be confined within the lop low‐viscosity layer. These rolls remain generally located far from the ridge plane, except in the case of a viscosity contrast of 3 orders of magnitude between the bottom and top layers. We conclude that segmentation beneath mid‐ocean ridges requires both strong viscosity contrasts and the lack of deep convective flow. Marine geophysical observations suggest that mantle upwelling beneath mid‐ocean ridges is two‐dimensional (sheetlike) beneath intermediate, fast and hotspot‐influenced spreading centers and three‐dimensional (plumelike) beneath slow‐spreading centers and ridges overlying cold regions of the mantle, such as the Australian‐Antarctic Discordance. We propose that these observations can be explained by variations in the viscosity structure (and temperature) of the mantle along the mid‐ocean ridge system. Models including a transform offset show that the active portion of the transform is a region of mantle downwelling. The toroidal flow induced by the offset shifts the plumes away from the ridge crests. The effects of the toroidal field are strongly magnified when the return flow cannot penetrate in the lower layer due to a large viscosity contrast. Stresses generated by the flow partly explains the development of inside corner highs and the evolution of transform faults.
Validation and assimilation of Seasat altimeter wave heights using the WAM wave model
Authors: Bauer Eva , Hasselmann Susanne , Hasselmann Klaus , Graber Hans C.
Published: February 2008

The mutual consistency of the Seasat global data sets of scatterometer winds and altimeter wave heights is investigated for the complete Seasat period using the third‐generation wave model WAM. The wave model was driven by surface (1000 hPa) wind and surface stress fields constructed by the Goddard Laboratory for Atmospheres (GLA) by assimilation of the scatterometer winds in an atmospheric model. For the 10‐day period September 7–17 the intercomparison was extended to two further scatterometer wind fields: a 1000‐hPa assimilated wind field from the European Centre for Medium‐Range Weather Forecasts and a subjectively analyzed 19.5‐m‐height surface wind field from the Jet Propulsion Laboratory. On the global average, the modeled and observed wave heights agree reasonably well. Regional differences, however, can be large and sometimes exceed 40%. The errors are attributed mainly to deficiencies in the forcing wind fields. Low wind speeds are found to be overestimated and high wind speeds underestimated by the Seasat scatterometer algorithm. The friction velocities of the GLA model are found to be significantly underestimated in the high‐wind belt of the southern hemisphere. The analysis demonstrates the diagnostic advantages of applying a wave model for the quality assessment of satellite wind and wave data. A preliminary wave data assimilation scheme is presented in which the wave field is updated without changing the forcing wind field. A considerable improvement of the computed wave field is achieved, particularly in regions in which the wave energy is dominated by swell. However, a more general assimilation scheme including modifications of the wind field is needed to upgrade wind sea forecasts.
Upper ocean response to Hurricane Gilbert
Authors: Shay Lynn K. , Black Peter G. , Mariano Arthur J. , Hawkins Jeffery D. , Elsberry Russell L.
Published: February 2008

The evolving upper ocean response excited by the passage of hurricane Gilbert (September 14–19, 1988) was investigated using current and temperature observations acquired from the deployment of 79 airborne expendable current profilers (AXCPs) and 51 airborne expendable bathythermographs from the National Oceanic and Atmospheric Administration WP‐3D aircraft in the western Gulf of Mexico. The sea surface temperatures (SSTs), mixed layer depths, and bulk Richardson numbers were objectively analyzed to examine the spatial variability of the upper ocean response to Gilbert. Net decreases of the SSTs of 3°–4°C were observed by the profilers as well as by the airborne infrared thermometer (AIRT) along the flight tracks and advanced very high resolution radiometer (AVHRR) imagery. The AXCPs indicated a marked cooling from 29°C to about 25.5°C on September 17, 1988, which was about 1.2 inertial periods (IP) following storm passage. This pool of cooler water (3.5°) was located further downstream in the hurricane wake by September 19 (2.7 IP following the storm) as a result of the near‐inertial currents in the mixed layer. While there was a bias of about 0.6°C and 1.7°C between the in situ and AVHRR‐derived SSTs, respectively, both the AVHRR images and the objectively analyzed fields indicated a rightward bias in the upper ocean cooling that extended from the storm track to about 4Rmax (where Rmax, the radius of maximum winds, is equal to 50 km). The larger SST offset of 1.7°C was due to the difference between the time of the AVHRR image and the time of the aircraft experiment on September 19. The SSTs derived from the AVHRR images and the AIRT also indicated large gradients between the cold wake and the warm eddy in the central Gulf of Mexico. The mixed layer deepened by about 30–35 m on the right side of the track during the storm and 1.2 IP later, with little evidence of continued deepening afterward. The mixed layer current vectors demonstrate that a strong, near‐inertially rotating current was excited by the passage of Gilbert, with maxima of about 1–1.4 m s−1. The currents, observed during and subsequent to (1.2 IP) the storm, diverged from the storm track, whereas the mixed layer current vectors 2.7 IP after storm passage converged toward the track, with relative maxima of 0.8–1 m s−1. This alternating pattern of convergence and divergence of the mixed layer current was associated with the upwelling and downwelling cycles of the baroclinic response. Considerable current shear existed between the mixed layer and the thermocline currents in the cool wake between the storm track and the 4Rmax. Estimates of the bulk Richardson numbers ranged between 0.2 and 1.0 during Gilbert and at 1.2 IP, which suggests that enhanced current shears were responsible for some of the mixed layer deepening.
Ocean interactions with the base of Amery Ice Shelf, Antarctica
Authors: Hellmer Hartmut H. , Jacobs Stanley S.
Published: February 2008

Using a two‐dimensional ocean thermohaline circulation model, we varied the cavity shape beneath Amery Ice Shelf in an attempt to reproduce the 150‐m‐thick marine ice layer observed at the “G1” ice core site. Most simulations caused melting rates which decrease the ice thickness by as much as 400 m between grounding line and G1, but produce only minor accumulation at the ice core site and closer to the ice front. Changes in the seafloor and ice topographies revealed a high sensitivity of the basal mass balance to water column thickness near the grounding line, to submarine sills, and to discontinuities in ice thickness. Model results showed temperature/salinity gradients similar to observations from beneath other ice shelves where ice is melting into seawater. Modeled outflow characteristics at the ice front are in general agreement with oceanographic data from Prydz Bay. A freshwater flux across the grounding line, derived from melting beneath the grounded ice sheet, would have to be anomalously large to produce the basal marine ice layer and account for the Ice Shelf Water outflow. We concur with Morgan's inference that the G1 core may have been taken in a basal crevasse filled with marine ice. This ice is formed from water cooled by ocean/ice shelf interactions along the interior ice shelf base.
Modeling mixed‐layer photochemistry of H2O2: Optical and chemical modeling of production
Authors: Sikorski Richard J. , Zika R. G.
Published: February 2008

We have developed a vertical‐flux model of undersea irradiance (300–500 nm) using measured absorption, scattering, midday spectral irradiance, and midday vertical attenuation (Kd) for stations in the eastern Caribbean in 1988. The model varies the pathlength and geometry of undersea light with solar zenith angle according to scattering in the water column, and according to the ratio of direct sunlight to diffuse skylight. Modeled UV and visible Kd have diel variations up to 67%, which are greatest above 400 nm. Using an experimentally determined photoproduction quantum yield (normalized to total dissolved organic matter absorption) which decreases exponentially as wavelength increases, modeled H2O2 photoproduction is primarily due to 320–360 nm irradiance. A significant fraction of total production occurs above 400 nm, especially at depth and at smaller zenith angles. The diel distribution of H2O2 photoproduction is notably deeper at midday than would be predicted by UV excitation in a simple nonscattering model.
Autoregressive modeling for ocean wave — radar modulation transfer function
Authors: Haimov Samuel , Hesany Vahid , Moore Richard K.
Published: February 2008

The standard technique for computing ocean wave — radar modulation transfer functions (MTFs) depends on nonparametric fast Fourier transform (FFT) methods. In this paper we suggest a new approach applying an autoregressive (AR) parametric spectral estimator. The analysis shows that the two‐channel autoregressive model produces smooth estimates of the MTF and permits better frequency resolution than FFT techniques. The feed‐across effect inherent in the methods developed to evaluate the multichannel AR coefficients causes a negligible effect on MTF. The AR‐based MTF depends weakly on the record length; for steady meteorological conditions the AR model of the MTF for a 2‐min data run is comparable with ones calculated using 10 min or more of data. In the same situation the FFT technique is unreliable for records of less than 10 min. The appropriate order of the multichannel AR model can vary without causing significant change in the MTF. In most cases any order between 8 and 16, depending on radar system parameters and meteorological conditions, will give acceptable results.
Measurements of atmospheric dimethylsulfide, hydrogen sulfide,and carbon disulfide during GTE/CITE 3
Authors: Cooper David J. , Saltzman Eric S.
Published: February 2008

Measurements of atmospheric dimethylsulfide (DMS), hydrogen sulfide (H2S), and carbon disulfide (CS2) were made over the North and South Atlantic Ocean as part of the Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE 3) project. DMS and CS2samples were collected and analyzed using an automated gas chromatography/flame photometric detection system with a sampling frequency of 10 min. H2S samples were collected using silver nitrate impregnated filters and analyzed by fluorescence quenching. The DMS data from both hemispheres have a bimodal distribution. Over the North Atlantic this reflects the difference between marine and continental air masses. Over the South Atlantic it may reflect differences in the sea surface source of DMS, corresponding to different air mass source regions. The median boundary layer H2S and CS2levels were significantly higher in the northern hemisphere than the southern hemisphere, reflecting the higher frequency of samples influenced by pollutant and/or coastal emissions. Composite vertical profiles of DMS and H2S are similar to each other, and are consistent with a sea surface source. Vertical profiles of CS2have maxima in the free troposphere, implicating a continental source. The low levels of H2S and CS2found in the southern hemisphere constrain the role of these compounds in global budgets to significantly less than previously estimated.