bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal stress cracking in granite
Authors: Wang H. F. , Bonner B. P. , Carlson S. R. , Kowallis B. J. , Heard H. C.
Published: February 2008

We have developed a comprehensive experimental capability to study thermally induced microfractures in rock. We count acoustic emissions and measure compressional wave velocities while a sample is heated to a maximum temperature of 300°C under confining pressures up to 55 MPa. After a heating cycle we measure crack porosity and crack compressibility. We count the cracks under an SEM, sorting them by grain boundary and intragranular types and by mineral pair occurrence. Our observations reveal that (1) significant thermal cracking occurs in granites heated above 200° or 250°C under 28 and 55 MPa and above 100°C in granites heated under 7 MPa, (2) most newly created thermal cracks close at pressures below 40 MPa (1.5 km depth), (3) grain boundaries between quartz grains or between quartz and another mineral are preferentially cracked, (4) the threshold temperature for acoustic emissions is a positive, linear function of pressure, (5) total acoustic emission counts in Westerly granite heated to 300°C are slightly lower under 55 than 28 MPa confining pressure and are substantially lower under 28 than 7 MPa, (6) the compressional velocity decreases 22% as Westerly granite is heated to 300°C at 7 MPa, 15% at 28 MPa, and 10% at 55 MPa, and (7) greater hysteresis in compressional velocities occurs in samples heated at lower pressures. The acoustic emission counts in Westerly granite during heating show one or more peaks of activity as a function of temperature, with more peaks at lower pressures. We interpret this result to mean that additional crack populations are able to open at lower confining pressures. The curves of acoustic emission count rates can be fit by summing one to three Gaussian density functions.
A physical model for earthquakes: 3. Thermodynamical approach and its relation to nonclassical theories of nucleation
Authors: Rundle John B.
Published: February 2008

If a theory of earthquakes, sliding, and frictional slip is to have general validity, it should ideally arise out of fundamental thermodynamical considerations. We construct just such a theory using the principle that physical systems, including fault systems, tend toward a state of minimal Helmholtz free energy. If the system is not in a state of minimal free energy, thermodynamic forces arise which drive the system toward a minimum. Allowed minima in the free energy may be either global, representing static (absolute) equilibrium, or local, representing states of metastable equilibrium. Within the context of this theory, fault systems are understood to be in states of metastable equilibrium, kept there by action of the far‐field plate‐driving stresses, which continually keep the far‐field displacement ahead of the state of slip on the fault. Earthquakes therefore represent a sudden decay of the fault system from a state of metastable equilibrium to a state closer to static (absolute) equilibrium. The major difficulty is the formulation of an appropriate expression for the Helmholtz free energy of the system. The solution of this problem is facilitated by starting from classical nucleation theory, which was first formulated by Gibbs for general thermodynamical systems and later applied by Griffith for the specific case of the fracture of solids. Much more recently, significant advances have been made in understanding the physics of nucleation. In accordance with these ideas, in this paper the free energy is therefore written as a sum of two terms: one which describes the elastic potential energy contained by the solid (i.e., the internal energy) and one which describes the energy necessary to rupture and/or reconnect atomic and cohesive bonds (i.e., an entropy term). The former term describes thermodynamically reversible processes, and the latter more generally describes thermodynamically irreversible processes. In the Griffith theory of fracture the latter term is a surface energy term, since the creation of an element of surface requires the rupture of atomic or cohesive bonds. Together, these two terms most generally define a nonclassical free energy functional, which, when minimized, leads to two equations. The first equation is the classical equation obtained by Gibbs and Griffith, which relates the size of the slip region to the applied field. The second equation is a nonclassical Euler‐Lagrange equation, which can be solved to obtain the distribution of slip on the slip surface. In addition to developing these ideas, it is shown that the assumption of thermodynamic irreversibility, that is, nonnegative entropy production, leads to a specific requirement for the kinetic growth of slip and the growth of the slip region. A discussion is then given of how these ideas relate to the kind of hysteresis effects commonly observed in frictional experiments and of how these effects may be interpreted in terms of the physics of states of metastable equilibrium.
Interpretation of synthetic three‐dimensional magnetotelluric data
Authors: Robertson R. Clark
Published: February 2008

The magnetotelluric (MT) method is an electromagnetic sounding technique used in geophysical exploration that is based on the measurement of surface electric and magnetic fields. The presence of three‐dimensional conducting heterogeneities near the surface of the Earth obscures conducting regions buried deeper in the Earth and leads to interpretational errors. In this paper a procedure referred to as “wave number filtering” is developed that dramatically reduces the undesirable effects of three‐dimensional heterogeneities on the Zxy and Zxx components of the MT impedance tensor, where the x axis is arbitrarily determined by the line of the MT survey. The Zyx and Zyy components of the MT impedance tensor are improved to some extent by wave number filtering, but the amount of improvement depends on the location with respect to the anomalous region and does not, in general, result in a correct interpretation. The wave number filtering process is evaluated by comparing the filtered and the unfiltered apparent resistivities derived from the MT impedance tensor for a particular three‐dimensional model consisting of a heterogeneous region on the surface of an otherwise homogeneous Earth.
Dynamic extraction of sediment attenuation from subbottom acoustic data
Authors: Guigné Jacques Y. , Pace Nicholas G. , Chin Voon H.
Published: February 2008

Reliable and accurate measurements of acoustic attenuation enable correlations between the acoustic responses and properties of sediments to be attempted. This paper proposes a dynamic modeling technique which allows for a direct assessment of attenuation from changes in the signal spectral shape caused by absorption losses in the sediment. Using the frequencies corresponding to the signal spectral maximum and a point 3 dB below the maximum, attenuation data can be dynamically extracted. Laboratory experiments on three types of sand were conducted to validate the proposed method. Although the concept can be applied using any broadband sound source, a parametric array source was used in the study because of its broad bandwidth and definable wave shape. A comprehensive discussion on the different sets of data collected is given with an emphasis on the attenuation coefficient k and the exponent N of frequency. The results of the study demonstrate close agreement when compared with the accepted literature values.
The electrical resistivity of solid and liquid Fe at pressures up to 7 GPa
Authors: Secco R. A. , Schloessin H. H.
Published: February 2008

The electrical resistivity of pure solid and liquid Fe has been measured at pressures up to 7 GPa in a large volume cubic anvil press. In conjunction with the four‐wired method, a novel technique of potential lead attachment was employed to measure the resistivity of 0.020‐in‐diameter Fe wire samples. Both the temperature and pressure coefficients of resistivity have been determined for solid and liquid Fe. The temperature coefficients of resistivity (TCR) in the liquid state are of the order of 10−4 K−1 and show an abrupt increase at approximately 5.2 GPa corresponding to the δ‐γ‐liquid triple point. We have interpreted this discontinuity in the TCR to be a reflection of local atomic structure in the liquid state which is reminiscent of the parent solid structure. By analogy, this result can be applied to the geophysically more important γ‐ε‐liquid triple point. By examining the fundamental effects of pressure and temperature on the density of states function and their ranges we arrive at an estimate of 1.2–1.5 × 10−6 Ωm for the electrical resistivity of pure Fe at the pressures and temperatures expected in the Earth's core.
Stonewall Mountain Volcanic Center, southern Nevada: Stratigraphic, structural, and facies relations of outflow sheets, near‐vent tuffs, and intracaldera units
Authors: Weiss Steven I. , Noble Donald C.
Published: February 2008

Directly south and southeast of Stonewall Mountain, Nevada, a depression and north facing caldera scarp were formed during and(or) after eruption of the Spearhead Member of the late Miocene Stonewall Flat Tuff. Abundant large lithic and juvenile blocks are present in the Spearhead Member within 0.5 km of this topographic margin but absent elsewhere in the ash‐flow sheet, consistent with eruption from vents in the Stonewall Mountain area. Within about 100,000 years, comendite tuff of the overlying Civet Cat Canyon Member of the Stonewall Flat Tuff buried the depression and associated scarp. The Civet Cat Canyon Member is traceable continuously to the north from an outflow sheet capping northwestern Pahute Mesa, into near‐vent tuff on the southeastern flank of Stonewall Mountain. Proximal outflow‐sheet tuff locally exhibits strong rheomorphic disruption and is overlain without a cooling break by surge, flow, and fall deposits of trachytic composition. Much of Stonewall Mountain is composed of welded tuff and megabreccia interpreted as intracaldera tuff of the Civet Cat Canyon Member, strongly suggesting that the vent area of the member was largely within Stonewall Mountain. Welded tuff of trachytic composition comprises an important part of the intracaldera Civet Cat Canyon Member, which was intruded by dikes and plugs of trachyte and rhyolite. Juvenile inclusions of basalt dispersed in near‐vent facies trachyte tuff provide direct evidence for the high‐level involvement of basaltic magma in the evolution of the highly potassic Stonewall Mountain center. Complex discordant compaction foliations and the widespread presence of megabreccia within the intracaldera tuff suggest, following Foley (1978), cauldron subsidence by piecemeal collapse during eruption of the Civet Cat Canyon Member. The elevation of intracaldera tuff and intrusions in Stonewall Mountain above the surrounding ashflow sheet suggests a significant amount of magmatic uplift, perhaps involving the emplacement of plugs, dikes, and small stocks within the intracaldera tuff prism.
Travel time and amplitude analysis in seismic tomography
Authors: Bregman Nina D. , Chapman Christopher H. , Bailey Richard C.
Published: February 2008

In cross‐hole tomography straight lines connecting the source and receiver are often used to approximate the true ray paths, thus linearizing the inverse problem. A recent paper by Bregman et al. (1989a) has shown that this approximation may be very poor in the regions of greatest interest where large velocity gradients cause significant ray curvature. The interpretation method presented by Bregman et al. (1989a) iteratively ray traces and updates the velocity model with model perturbations obtained by a damped least squares inversion of the travel time residuals. This paper presents a modification of the travel time tomography method so that it may be applied to the inversion of first motion amplitudes for inverse quality factor Q−1. After determining the velocity structure using travel time tomography the amplitudes are inverted using a similar scheme. No additional ray tracing is required for the amplitude inversions. The effect of the velocity structure previously determined through travel time tomography is taken into account so that amplitude variations due to the focusing and defocusing effects of the velocity structure are not interpreted as being due to attenuation structure. Both the travel time and amplitude tomography methods are applied here to field data from a cross‐hole experiment in crystalline rock (Wong et al., 1983). The frequency range of the seismograms is 1–6.6 kHz, allowing resolution of velocity structure on a scale of several meters. The resulting velocity image shows good agreement with other geological and geophysical data. Reversing the model by placing the model boreholes on opposite sides relative to the original reconstruction yields almost identical results, indicating that the interpolation scheme used in the forward modeling has not biased the final model. Synthetic Maslov seismograms calculated for the derived velocity model agree well with the waveform data, further confirming the validity of the model. In addition, the amplitude tomography results place absorptive regions in the same location as the most pronounced low‐velocity features which correlate with fractures in the boreholes. Highly transmissive regions also correlate well with high‐velocity regions.
Petrology and thermal evolution of the Tinaquillo peridotite (Venezuela)
Authors: Seyler Monique , Mattson Peter H.
Published: February 2008

The Tinaquillo peridotite complex is a layered and veined ultramafic‐mafic body cropping out in the Cordillera de la Costa in northern Venezuela. Whole rock compositions, textures, and mineral chemistry of the peridotite and associated mafic rocks suggest that the complex underwent two major events before being emplaced into the Cordillera de la Costa. The peridotite first rose adiabatically within the upper mantle, at about 1350°C and 15–21 kbar pressure. This rise caused a small to medium degree of partial melting, resulting in the formation of a stratified complex of clinopyroxene‐rich spinel lherzolites grading to harzburgites, interlayered with pyroxenite. The stratified complex then cooled and reequilibrated at about 800°–1000°C. A second upward movement of the peridotite brought it, still hot, from the upper mantle into lower crustal rocks (now called the Tinaco complex), at an estimated 7 kbar pressure, then cooling to 600°–700°C. The peridotite contains small amounts (2–5%) of homogeneously distributed pargasite. Locally, it is concentrated in amphibole veins grading into orthopyroxene‐ and spinel‐hornblendite layers. Amphibolization was associated with metasomatism, and this metasomatic enrichment seems greater in the most depleted peridotite. The Tinaquillo peridotite shares many characteristics with Alpine spinel lherzolites that are believed to be derived from the subcontinental upper mantle and are emplaced within a continental rift, or at the margins of an oceanic rift. These characteristics are low degree of depletion, highly aluminous spinels, abundance of metasomatic facies, and early high‐P equilibration followed by reequilibration with mafic‐silicic continental type rocks within the intermediate‐P granulite facies.
Lateral heterogeneity in the seismic structure of upper oceanic crust, western North Atlantic
Authors: Swift Stephen A. , Stephen Ralph A.
Published: February 2008

We carried out a borehole seismic experiment at Deep Sea Drilling Project hole 418A in order to study the vertical and lateral velocity variations in 100‐m.y.‐old oceanic crust. We clamped a seismometer successively at five depths between 41 m and 430 m within the extrusive basalt layer while air gun and explosive sources were fired on eight radial lines and four concentric circles to 8 km range. Our travel time data do not give consistent evidence of anisotropy. However, our experimental error may mask anisotropy of up to 0.2–0.3 km/s. Velocities from inflection point analysis, sonic log, and reflectivity synthetic seismograms support results of τ‐ζ inversion of radial line P arrival times. Velocity increases with an almost linear gradient of 1.3 s−1 from 4.6 km/s at the top of basement to 1.8 km depth. The τ‐ζ velocity profile at 418A does not differ significantly at the 95% confidence level from that at 417D located ∼7.5 km away. Seismic velocities at shorter‐length scales do vary laterally. Travel time anomalies indicate that seismic velocity in the upper 0.5 km increases laterally northwest to 3 km range. Other data indicate that this velocity anomaly in the extrusive layer may be due to alteration controlled by topography or to primary porosity variations.
The effective yield of a nuclear explosion in a small cavity in geologic material: Enhanced coupling revisited
Authors: King D. S. , Freeman B. E. , Eilers D. D. , Johnson J. D.
Published: February 2008

The hydrodynamic yield of an underground nuclear explosion in a small cavity has been recalculated using computer modeling. We have considered explosions in spherical cavities having scaled radii, R/W1/3, less than 3.5 m/kt1/3 (R is the cavity radius in meters and W is the explosion energy in kilotons) in quartz (SiO2) using an available equation of state (Ree, 1976). These calculations show, at a scaled radius of ≈1.5 m/kt1/3, a maximum hydrodynamic yield substantially greater than that of an explosion in a tamped cavity (enhanced coupling) as measured by the shock time of arrival in the near field. These results are in qualitative agreement with those of earlier investigators who utilized the reduced displacement potential to evaluate coupling. We found, however, that the coupling coefficient (ratio of yield for cavity model to yield for tamped model) is significantly reduced when an improved quartz equation of state is employed. We also calculated enhanced coupling in two additional geologic materials, salt and tuff. All of the calculations mentioned above (including our initial ones) were simplified through the omission of significant thermal radiation contributions to energy and pressure in the cavity and the diffusion by photon transport of energy from the explosion cavity into the adjacent rock. When we take these processes into account, we find that for a cavity radius of approximately 1.5 m/kt1/3, near the peak of enhancement for our models with no radiation, there is instead a decoupling of about 15% in SiO2, 20% in tuff, and about 15% in salt. At the largest scaled cavity radius considered, 3.42 m/kt1/3, there is a decoupling of 40% in SiO2, 35% in tuff, and 35% in salt. For all three media the coupling coefficients decrease monotonically as the cavity radius increases. This result is different from previous ones in which the radiation terms were neglected. The radiation processes not only provide another method for vaporization of the rock but also change the effective gamma of the cavity gas as a result of the reduced cavity temperature and the addition of radiation pressure and energy terms. We have also found that the major effect on coupling is due to the contribution of radiation to the equation of state. Computer calculations which omitted radiation diffusion showed that the major change in effective coupling is due to the radiation energy and pressure modifications of the equation of state and to the corresponding changes in the effective gamma of the cavity gas. Thus, while the radiation diffusion is responsible for minor changes in the coupling curves, the main trend of these curves does not depend on diffusion.
Possible use of pattern recognition for the analysis of Mars Rover X ray fluorescence spectra
Authors: Yin Lo I , Trombka Jacob I. , Seltzer Stephen M. , Johnson Robert G. , Philpotts John A.
Published: February 2008

On the Mars rover sample return mission the rover vehicle will collect and select samples from different locations on the Martian surface to be brought back to Earth for laboratory studies. It is anticipated that an in situ energy‐dispersive X ray fluorescence (XRF) spectrometer will be on board the rover. On such a mission, sample selection is of higher priority than in situ quantitative chemical analysis. With this in mind we propose pattern recognition as a simple, direct, and speedy alternative to detailed chemical analysis of the XRF spectra. The validity and efficacy of the pattern recognition technique are demonstrated by the analyses of laboratory XRF spectra obtained from a series of geological samples, in the form both of standardized pressed pellets and as unprepared rocks. It is found that pattern recognition techniques applied to the raw XRF spectra can provide for the same discrimination among samples as knowledge of their actual chemical composition.
The route to chaos in thermal convection at infinite Prandtl number: 1. Some trajectories and bifurcations
Authors: Stewart Cheryl A. , Turcotte Donald L.
Published: February 2008

An important issue is whether thermal convection in the Earth's mantle is chaotic and the mechanisms by which this chaos may be generated. In this paper, we conclude that the high Prandtl number/high Raleigh number thermal convection associated with mantle convection is chaotic, and we document the route to chaos. Deterministic chaos was first exhibited by the Lorenz equations, a three‐mode spatial Fourier expansion of the nonlinear thermal convection equations. This transition to chaos is characterized by the two nontrivial fixed points undergoing a reverse Hopf bifurcation into two unstable limit cycles. However, when the limit of infinite Prandtl number is taken for these equations, the fixed points are stable for all Rayleigh numbers. Is the stability of these fixed points a characteristic of infinite Prandtl number convection or is this an artifact of the truncation? To answer this question, we extend the expansion to 12 modes {n = 0, 1, 2; m = 1, 2, 3, 4} where n is the horizontal wave number and m is the vertical wave number to generate a set of 12 ordinary differential equations. We calculate the loci of roots of this system as a function of Rayleigh number, and we calculate several trajectories. At Rayleigh numbers between Ra = 0 and Ra = 4.140×104 we find pitchfork bifurcations to 14 separate nontrivial branches of solution, and we determine their stability by calculating the eigenvalues of the Jacobian matrix along each branch; we thus find 40 Hopf bifurcations. These fixed points and their bifurcations give excellent quantitative agreement with the trajectories, converging to stable fixed points at and below Ra = 4×104, and oscillating aperiodically at and above Ra = 4.5×104, where many of the Hopf bifurcations occur. Our conclusion is that thermal convection at infinite Prandtl number becomes chaotic by means of symmetry‐breaking pitchfork bifurcations and the appearance of the Hopf bifurcations which produce unstable periodic orbits.
A magnetotelluric transect in the Oregon Coast Range
Authors: Young Charles T. , Kitchen Mark R.
Published: February 2008

As part of EMSLAB, broadband magnetotelluric data have been collected and analyzed at 11 sites along an east‐west transect in the Oregon Coast Range. Using one‐ and two‐dimensional modelling, the Coast Range was found to consist of marine sediments about 4 km deep near the coast and volcanic rocks about 8 km deep farther inland. Willamette Valley consists of surficial conductive sediments to a depth of 1 km, which overlie an eastern extension of the rocks that compose the Coast Range. An east dipping conductor, at a depth of approximately 20 km, is associated with the upper surface of the subducting Juan de Fuca plate. Possible mechanisms for the high conductance are water‐saturated fractured basalts forming the surface of the plate, subducted sediments, and/or the dehydration of the subducting slab trapped by metamorphic layering within the lower crust. This conductor correlates with an east dipping reflector, 35–40 km below the eastern flank of the Coast Range, defined by COCORP profiling and thought to represent the surface of the subducting Juan de Fuca plate.
Active faulting and deformation of the Coalinga Anticline as interpreted from three‐dimensional velocity structure and seismicity
Authors: Eberhart‐Phillips Donna
Published: February 2008

This work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake. Interpretation of hypocenters and fault plane solutions, from the 1983 Coalinga, Coast Range California, earthquake sequence, in combination with the three‐dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher‐velocity material. Above the main listric blind thrust there is a conjugate fault, steeply northeast dipping, that provides the western limit of the aftershocks within the Coalinga Anticline and that corresponds in location and spatial extent with the adjacent Pleasant Valley syncline. The character of the seismicity varies with the degree of previous deformation on each section of the anticline. Where the previous uplift was largest, the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure. Diffuse seismicity characterizes the area with the least previous deformation. The mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features. The upward extent of the mainshock rupture ended at the approximate boundary between Franciscan and Great Valley Sequence rocks. Above that depth the main thrust appears to splay into a steeper segment and a near‐horizontal segment. Thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material. With the three‐dimensional velocity model each individual hypocenter moved slightly (0–2 km) in accord with the details of the surrounding velocity structure, so that secondary features in the seismicity pattern are more detailed than with a local one‐dimensional model and station corrections. The overall character of the fault plane solutions was not altered by the three‐dimensional model, but the more accurate ray paths did result in distinct changes. In particular, the mainshock has a fault plane dipping 30° southwest instead of the 23° obtained with the one‐dimensional model.
Full waveform inversion of P waves for Vs and Qp
Authors: Cheng C. H.
Published: February 2008

We present an indirect method of determining shear wave velocities from full waveform acoustic logs based on the inversion of the spectral ratio of the P wave trains at two source‐receiver separations. This method simultaneously inverts for the formation shear wave velocity and compressional wave attenuation. The P wave response is calculated by means of branch‐cut integration. This method is useful in “soft” formations where the shear wave velocity is lower than the acoustic velocity of the borehole fluid and thus there are no refracted shear wave or pseudo‐Rayleigh wave arrivals. The method is shown to give good estimates of formation shear wave velocity in both synthetic and field data. The inversion algorithm is sensitive to local minima; care must be taken to avoid them.
Crustal paleorheology of the southeastern Canadian Cordillera and its influence on the kinematics of Jurassic convergence
Authors: Murphy Donald C.
Published: February 2008

The kinematic evolution of the southeastern Canadian Cordillera (52°N–54°N) during the Jurassic is characterized by an eastward migration of the active boundary of convergence between North America and allochthonous terranes and by two reversals in regional structural vergence. To investigate possible mechanical controls on the kinematics of convergence, the thermal and geological structure of the Jurassic continental margin at three stages in its history of shortening were palinspastically reconstructed and analyzed with the use of strength versus depth profiles. The resulting models reproduce to a first order the observed distribution of brittle and ductile deformation and provide a mechanical basis for the localization and orientation of displacement zones during shortening of the continental margin. The initial eastward migration of the active boundary of convergence between North America and allochthonous terranes from outboard of the continental margin sediment prism into the thick central part of the continental margin prism is attributed to thermal weakening of the continental margin lithosphere during obduction of the oceanic terrane. The subsequent eastward migration of the convergence boundary into the inner part of the continental margin lithosphere is attributed to thermal weakening of the lithosphere during prograde regional metamorphism. Reversals in regional structural vergence accompanying each migration apparently occur because the push necessary to continue displacement on one displacement trajectory exceeds the failure strength of another displacement trajectory that dips in the opposite direction.
Models of the core‐mantle boundary and the travel times of internally reflected core phases
Authors: Doornbos D. J. , Hilton T.
Published: February 2008

Topographic maps of the core‐mantle boundary (CMB), obtained by inverting different subsets of International Seismological Centre arrival time data involving either waves refracted through the boundary (PKP) or waves reflected from the topside (PcP), do not agree. We report on the implications for models of CMB structure when data from bottomside reflections (notably PKKP) are included in the inversion. Travel time residuals with respect to a reference Earth model (PREM) of all core phases are positively biased, but the mean residual is different for different phases and must be accounted for prior to inversion. The new data illustrate that models of CMB topography can explain only a small fraction of the data variance. The new CMB model involves both topography and a laterally varying boundary layer. The CMB itself is smooth compared to previous results.
Relationship between hot springs and geothermal fields in Japan
Authors: Acharya Hemendra R.
Published: February 2008

Hot springs usually are situated in the volcanic belt of a convergent zone and are utilized as indicators of geothermal potential in an area. Characteristics of hot springs in Japan have been examined in order to understand the relationship between hot springs and geothermal fields. The characteristics examined include locations of hot springs, their temperature, density of distribution, flow rate, heat flow in the vicinity, and rate of volcanic activity in the area during last 200 years. Geochemical thermometers have been used to estimate subsurface temperatures in the area. The object of the study was to determine subsurface conditions in the vicinity of existing geothermal fields from these characteristics of hot springs. This examination shows that geothermal fields are situated near clusters of three or more hot springs with temperatures in the subsurface ≥150°C in the vicinity of these clusters, and many hot springs have a higher than average flow rate. The study also shows that higher subsurface temperatures and heat flow in the vicinity of the clusters of hot springs are not the result of significant volcanic activity in the area. The examination also shows that clusters of hot springs in Japan are located along the extension of zones which mark discontinuities in plate underthrusting. Measured heat flow in many of these zones is higher than average for volcanic areas. Such zones, with higher than average crustal temperatures and permeability, may be favorable for the development of productive geothermal fields if local geological and hydrological conditions permit.
Limits on the Age of the Deccan Traps of India from paleomagnetic and plate reconstruction data and their uncertainties
Authors: Acton Gary D. , Gordon Richard G.
Published: February 2008

Revised limits on the age of the Deccan Traps of India are provided by recent improvements to reconstructions of India relative to surrounding plates since Late Cretaceous time, realistic estimates of the uncertainties in the plate reconstructions, and recent paleomagnetic data from the Deccans Traps and from North America. We estimate the age of the Deccan Traps by rotating its paleomagnetic pole into the North American reference frame. Different assumed ages for the Deccan Traps result in very different positions for its paleomagnetic pole relative to North America. Comparing these positions with the North American apparent polar wander path and accounting for uncertainties in both the paleomagnetic poles and in the reconstructions, we place limits on the age of the Deccan Traps of 62–72 Ma, significantly younger than found in several prior studies. The 62–72 Ma limits are consistent with ages obtained independently from radiometric dating and paleontology and are consistent with the hypothesis that the Deccan Traps were extruded rapidly during reversed polarity chron 29r, which contains the Cretaceous‐Tertiary boundary.
Discrepancies in the Earth‐atmosphere angular momentum budget
Authors: Rosen Richard D. , Salstein David A. , Wood Tamara M.
Published: February 2008

Modern geodetic and atmospheric data sets have demonstrated that changes in the solid earth's rotation rate are closely coupled to changes in the atmosphere's angular momentum over a range of subseasonal through interannual time scales. Nevertheless, limits to the correspondence between changes in the momenta of the two bodies exist at both ends of the spectrum resolvable by the data. At high frequencies we show that changes in the length of day as short as a fortnight can now be attributed almost entirely to atmospheric forcing; more rapid fluctuations in the length of day, however, cannot be shown to coincide with atmospheric behavior, a result we blame mostly on errors remaining in both the geodetic and atmospheric series rather than on the neglect of some other component of the momentum budget. On the decadal time scale, nonatmospheric processes, believed primarily to involve coremantle coupling, dominate the global momentum budget. Our analysis of the low‐frequency component of the difference between the momenta of the solid earth and atmosphere during 1976–1988 shows it to consist of a general trend interrupted by three major changes in slope. Whether this result indicates that coremantle coupling is a nonsteady phenomenon or that unmodeled oceanic processes are responsible for the intermittency of the difference series is presently uncertain.
Introduction to Special Section on the California‐Arizona Crustal Transect: Part 1
Authors: Howard K. A. , Simpson R. W. , Haxel G. B.
Published: February 2008

Recent research along the east half of a crustal geotraverse across the Cordillera from the Southern California Borderland to the Colorado Plateaus in Arizona was the subject of a workshop held in Flagstaff, Arizona, in May 1988. The diversity of earth science disciplines brought together by this stimulating CACTIS (California‐Arizona Crustal Transect—Interim Synthesis) workshop offered new insights toward an understanding of the crustal structure and evolution of this region. It was decided at the workshop that a set of papers should be assembled to present these new insights. This special section, part I, presents the first group of papers on the California‐Arizona crustal transect; part II will be published in early 1990, and part III is projected for late 1990.
Large‐magnitude miocene extension in the central Mojave Desert: Implications for Paleozoic to Tertiary paleogeography and tectonics
Authors: Walker J. Douglas , Bartley John M. , Glazner Allen F.
Published: February 2008

The main Cenozoic extensional structure in the central Mojave Desert is the Waterman Hills detachment fault, which places brittlely deformed synorogenic Miocene rocks on ductilely and cataclastically deformed footwall rocks. New data are presented regarding the timing, distribution, magnitude, and significance of early Miocene extension in the area. The mylonitic fabric in the lower plate was formed at 23 Ma, based on a zircon U/Pb age from a synmylonitic intrusion. Upper plate strata consist of rhyolite flows overlain by sedimentary rocks that were apparently deposited during extensional faulting. These strata were tilted, folded, and intruded by synkinematic rhyolite plugs that are cut off at the detachment fault. Potassium metasomatism of the rhyolitic rocks is pervasive. Upper plate detrital sediment was derived from the rhyolitic rocks and from metamorphic and plutonic basement rocks not present in the area. The probable source of the exotic basement clasts is the Alvord Mountain area, presently located 35 km east‐northeast of the Waterman Hills area. This source was probably much nearer to the Waterman Hills during deposition of the synorogenic deposits and has been subsequently shifted by extensional deformation. Distinctive Mesozoic plutonic rocks provide a possible tie between upper and lower plate rocks. Similar poikilitic gabbro bodies in the Goldstone area and the Iron Mountains suggest slip on the Waterman Hills detachment fault to be about 40–50 km. This is also consistent with other offset markers, such as the western edge of a Mesozoic dike swarm. When 15–20 km(?) of Tertiary extension is restored, Paleozoic eugeoclinal rocks are placed structurally above their miogeoclinal counterparts. Combined with the distribution of Triassic and Jurassic rocks, this implies post‐Early Triassic and pre‐Late Jurassic stacking of these lithologies.
Models of inclination shallowing during sediment compaction
Authors: Arason Pordur , Levi Shaul
Published: February 2008

We construct microscopic models of compacting sediment which lead to inclination shallowing of the magnetic remanence. The models can be classified as (1) rotation of elongated magnetic grains to more horizontal orientations; (2) rotation toward the horizontal of flat nonmagnetic fabric grains to which smaller magnetic grains are attached; (3) randomization of the sediment by grain rotations which lead to decreased intensity of magnetization and possibly also to inclination shallowing; and (4) finally, we show that the initial within‐sample dispersion of the magnetic moments dampens the amount of inclination shallowing of all the models and transforms any form of microscopic mechanism to an equation of a standardized form. The physically realistic models give rise to different magnitudes of inclination shallowing, which to the first order obey an equation of the form tan (I ‐ ΔI) = (1 ‐ a ΔV) tanI, where I is the inclination of the ambient field, ΔI is the inclination shallowing and ΔV the compaction. For these models we also calculate the effect of compaction on the intensity of magnetization, and the results show that considerable randomization is needed to offset the increased intensity due to higher concentrations of magnetic particles caused by compaction. If random rotations of the grains are biased toward rolling about horizontal axes and the randomization is sufficient to cancel the effect of greater concentrations, then the random grain rolling due to the compaction would give rise to considerable inclination shallowing.
Bathozonal tilt corrections to paleomagnetic data from Mid‐Cretaceous plutonic rocks: Examples from the Omineca Belt, British Columbia
Authors: Irving E. , Archibald D. A.
Published: February 2008

The Skelly Creek batholith (49.4°N, 116.6°W, 94–99Ma) and Summit stock (49.1°N, 117.0°W, 102 Ma) are intruded into the southern Omineca Belt of southeastern British Columbia. Bathozonal mineral assemblages from contact aureoles of these plutons and others of the same age indicate that post emplacement tilting of the Purcell anticlinorium has probably not exceeded about 5°, whereas the Summit Stock has been tilted 24° down to the west. The Skelly Creek batholith has a mean direction of magnetization (D,I) with respect to present horizontal 349°, 74° (13 sites, k = 53, α95 = 6°). The mean direction of magnetization of the Summit stock is 48°, 72° (nine sites, k = 290, α95 = 3°), and after correction for a westerly dip of 24°, the mean direction becomes 318°, 74°. Directions of the Skelly Creek batholith and tilt‐corrected direction from the Summit stock are in agreement with the reference field for cratonic North America. Evidently, this part of the eastern Omineca Belt was close to the margin of ancestral North America in mid‐Cretaceous time, essentially where it is today.
S wave coda Q from 3 to 20 Hz and P wave Q for foreshocks and aftershocks of the Petatlan Earthquake
Authors: Novelo‐Casanova David A. , Bero Eduard , Helsley Charles E.
Published: February 2008

In order to understand better the variations of mechanical properties close to the source and the failure process in the upper portion of the crust in a geologically very young subduction zone, the seismic S wave coda Qc from 3 to 20 Hz and P wave half pulse width Qp from 6 to 12 Hz were determined for selected foreshocks and aftershocks of the 1979 Petatlan, Mexico, Ms = 7.6 earthquake. To limit the region being sampled, the quality factor of S coda, Qc, was obtained from the first 10 s following the S onset. Qc exhibited a strong spatial, temporal, and frequency dependence, and in the small foreshock portion of the aftershock area, Qc and (to a lesser extent) Qp were significantly lower before the mainshock than afterward. The frequency dependence n, in Qc (ƒ) = Qo ƒn, was practically constant for aftershocks in four different regions (n = 1.32±0.03) and for foreshocks (n = 1.43±0.07). Only in one peripheral region‐station pair did we find a significantly different n (n = 0.97±0.09). Our observations indicate that Qc, and perhaps the frequency dependence n, are sensitive to the physical property changes in the source region and might be useful as predictors of large earthquakes in a shallow subduction zone environment.
Permeabilities, fluid pressures, and flow rates in the Barbados Ridge Complex
Authors: Screaton Elizabeth J. , Wuthrich Dennis R. , Dreiss Shirley J.
Published: February 2008

Recent measurements from Ocean Drilling Program leg 110 and Deep Sea Drilling Project leg 78a indicate that pore pressures near the toe of the Barbados accretionary prism may be close to lithostatic and that the décollement is a zone with relatively high rates of fluid flow and methane transport. We used a numerical model of fluid flow to estimate intrinsic permeabilities, pore pressures, and flow velocities that are consistent with these observations. Model results suggest that the permeability of the décollement may be 3–5 orders of magnitude greater than that of adjacent prism sediments. If permeabilities in the prism vary with depth in a manner similar to those in sedimentary basins, the average intrinsic permeability of the décollement, kd, must be about 10−14 m2. When kd is 10−13 m2, high pore pressures do not develop near the deformation front in the model. If kd is 10−15 m2, simulated pressures are unrealistically high in both the prism and underthrust sediments arcward of the deformation front. Water originating from compaction in the décollement and underthrust sediments flows laterally seaward, while water expelled from prism sediments flows upward to the ocean floor. However, flow velocities are small, and the net motion of pore water in prism and underthrust sediments is arcward relative to the deformation front because of tectonic transport. Pore water migrates seaward in spite of tectonic transport only in discrete zones with higher permeability, in this case the décollement.
Ray series modeling of seismic wave travel times and amplitudes in three‐dimensional heterogeneous anisotropic crystalline rock: Borehole vertical seismic profiling seismograms from the Mojave Desert, California
Authors: Li Y.‐G. , Leary P. C. , Aki K.
Published: February 2008

A 600‐m borehole vertical seismic profiling (VSP) survey conducted in crystalline rock in the Mojave Block stress province, southern California, yielded 100 oriented three‐component seismograms from a fan‐shooting geometry with radius of 100 m using both vertical and horizontal vibrators and a vertical impact source. The seismograms show up to 12 ms of shear wave splitting and 5% lateral velocity heterogeneity. First‐ and second‐order ray tracing was used to model the travel times and amplitudes of P‐, SV‐ and SH‐ waves in a three dimensional mildly heterogeneous medium with elastic anisotropy induced by aligned fractures. A least squares inverse procedure was used to improve model parameters determined by forward modeling; the final root mean square travel time residual was ±2 ms. For purposes of amplitude computation we assume that the wavelets observed in a VSP section derive from a smooth intermediate interface (base of weathering layer or sedimentary/basement interface) that is illuminated by a narrow range of ray angles originating at a point source on the surface. Source‐derived amplitude complications are thus minimized. Our results strongly indicate a population of vertical cracks with an average crack density of 0.035 oriented in the N31.5°W direction. Such a fracture population is consistent with regional principal strain and stress determinations. Ray series geometric amplitudes were consistent with the observation that wave motion parallel to the aligned fractures decayed more slowly than wave motion transverse to the aligned fractures. We estimate that Q ≥ 75–100; we were unable to measure a polarization‐dependent Q.
Nondestructive, three‐dimensional density measurements of ice core samples by X ray computed tomography
Authors: Kawamura Toshiyuki
Published: February 2008

Conventional methods for measuring ice bulk density do not allow one to detect three‐dimensional changes in density in a nondestructive manner. An X ray computed tomography (CT scanner) was used to determine ice core density. This scanner has led to remarkable achievements in the field of medical science to obtain a nondestructive cross‐sectional representation of the human body. In this paper the relation between density and the CT value ordinarily obtained by the CT scanner is discussed for the ice/air mixtures similar to ice core samples. The influence of impurities on the density determination is also considered. Images, taken at intervals of 2 mm, detected the internal air bubble distribution of an ice core. Three‐dimensional density distribution was also evaluated on the volume elements with dimensions of 1.1 by 1.1 by 2 mm. Results of CT scanner measurements were compared with those of the conventional hydrostatic method. Comparisons made on compressed ice samples with various densities showed close agreement. The results show that the CT scanner can be used to nondestructively determine the three‐dimensional distribution of ice core density and that the CT scanner is an extremely valuable tool for making detailed measurements of spatial variation in density of ice cores as well as for examining the internal structure.
Source complexity of the 1987 Whittier Narrows, California, earthquake from the inversion of strong motion records
Authors: Hartzell Stephen , Iida Masahiro
Published: February 2008

Strong motion records for the Whittier Narrows earthquake are inverted to obtain the history of slip. Both constant rupture velocity models and variable rupture velocity models are considered. The results show a complex rupture process within a relatively small source volume, with at least four separate concentrations of slip. Two sources are associated with the hypocenter, the larger having a slip of 55–90 cm, depending on the rupture model. These sources have a radius of approximately 2–3 km and are ringed by a region of reduced slip. The aftershocks fall within this low slip annulus. Other sources with slips from 40 to 70 cm each ring the central source region and the aftershock pattern. All the sources are predominantly thrust, although some minor right‐lateral strike‐slip motion is seen. The overall dimensions of the Whittier earthquake from the strong motion inversions is 10 km long (along the strike) and 6 km wide (down the dip). The preferred dip is 30° and the preferred average rupture velocity is 2.5 km/s. Moment estimates range from 7.4 to 10.0×1024 dyn cm, depending on the rupture model.