bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionospheric effects of the solar eclipse of September 23, 1987, around the equatorial anomaly crest region
Authors: Cheng Kang , Huang Yinn‐Nien , Chen Sen‐Wen
Published: February 2008

The ionospheric responses to the solar eclipse of September 23, 1987, in the equatorial anomaly crest region have been investigated by using ionospheric vertical sounding, VLF propagation delay time, and differential Doppler shift data observed at Chungli, which is located near the northern equatorial anomaly crest region. It has been found that temporal variations of the F1 layer and D region are mainly controlled by local solar radiation. Quantitative analysis of the variations of the F1 layer critical frequency, ƒ0F1, shows that electrons are removed from the F1 layer through ionic recombination. However, the temporal variations of ƒ>0F2 and electron density above 200 km show that the variations of the F2 layer around the equatorial anomaly region are controlled not by local solar radiation but by solar radiation at the equator. The fountain effect plays an important role even during the solar eclipse. The VLF propagation time delay is controlled by the variations of average path obscuration. Atmospheric gravity waves produced by the moving bow wave front of the solar eclipse are found with a period around 17‐23 min and wavelength about 293 km.
Two‐day oscillation of the equatorial ionization anomaly
Authors: Chen Pei‐Ren
Published: February 2008

The day‐to‐day variability of the equatorial ionization anomaly (EIA), especially the 2‐day oscillation of the EIA, has recently been studied in China. We suggest that the main driving source of the 2‐day oscillation (or the day‐to‐day variation) of the EIA is a planetary wave with a 2‐day period (or the superposition of planetary waves with different periods). The planetary wave modulates the tidal wind, and this results in the 2‐day oscillation of the EIA via the dynamo effect and fountain effect. This suggestion, derived from the observational data, has been confirmed by numerical simulations.
Adiabatic cooling of solar wind electrons
Authors: Sandbæk Ørnulf , Leer Egil
Published: February 2008

In thermally driven winds emanating from regions in the solar corona with base electron densities of n0 ≥ 108 cm−3, a substantial fraction of the heat conductive flux from the base is transferred into flow energy by the pressure gradient force. The adiabatic cooling of the electrons causes the electron temperature profile to fall off more rapidly than in heat conduction dominated flows. Alfvén waves of solar origin, accelerating the basically thermally driven solar wind, lead to an increased mass flux and enhanced adiabatic cooling. The reduction in electron temperature may be significant also in the subsonic region of the flow and lead to a moderate increase of solar wind mass flux with increasing Alfvén wave amplitude. In the solar wind model presented here the Alfvén wave energy flux per unit mass is larger than that in models where the temperature in the subsonic flow is not reduced by the wave, and consequently the asymptotic flow speed is higher.
Polytropic indices in collisionless plasmas: Theory and measurements
Authors: Belmont Gerard , Mazelle Christian
Published: February 2008

All theories and models based upon fluid methods are dependent upon p pairs of parameters (p=1 for MHD) which are the polytropic indices γ⫽ and γ⊥ of each of the p fluid populations, and their results can sometimes be greatly affected by changes in these indices. For these fluid descriptions of collisionless plasmas, there was no general method up to now for determining the right values γ⫽ and γ⊥ to be injected, depending on the local physical situation. Are the different populations isothermal (γ⫽ = γ⊥ = 1), isobaric (γ⫽ = γ⊥ = 0), adiabatic (γ⫽ = γ⊥ = 5/3 if the variations are isotropic, but this hypothesis of isotropy is generally not verified in a magnetoplasma) or anything else? These choices may be made by going back to complete kinetic calculations in each specific situation, thereby losing the ease of the fluid methods. This paper shows that the use of polytropic laws is justified for plasma variations belonging to any linear low‐frequency wave, and it provides the correct values to be used for the polytropic indices in these cases, thanks to a general kinetic calculation. In these terms, the polytropic indices γ⫽ and γ⊥ must be considered as the interface between kinetic and fluid sides of the physics. The dependence of the indices upon the mode and upon the particle population considered is emphasized. An experimental case concerning a fast magnetosonic mode observed on board the Giotto spacecraft close to comet Halley is completely analyzed from this point of view. Electron measurements are used, simultaneously with magnetic field ones, and all the correlations between the four parameters p⫽, p⊥, n, and Bz are explained; for doing so, the theory is made more sophisticated to account for the fact that the coldest and densest part of the electron distribution function is not, in this case, accessible to the detector.
Observations of plasma dynamics in the coma of P/Halley by the Giotto Ion Mass Spectrometer
Authors: Goldstein B. E. , Goldstein R. , Neugebauer M. , Fuselier S. A. , Shelley E. G. , Balsiger H. , Kettmann G. , Ip W. ‐H. , Rosenbauer H. , Schwenn R.
Published: February 2008

Observations in the coma of P/Halley by the Giotto ion mass spectrometer (IMS) are reported. The high‐energy range spectrometer (HERS) of the IMS obtained measurements of protons and alpha particles from the far upstream region to the near ionopause region and of ions of mass 12–32 at distances of about 250,000 to 40,000 km from the nucleus. Plasma parameters from the high‐intensity spectrometer (HIS) of the IMS obtained between 150,000 and 5000 km from the nucleus are also discussed. The distribution functions of water group ions (“water group” will be used to refer to ions of 16–18 m/q, where m is in atomic mass units and q is in unit charges) are observed to be spherically symmetric in velocity space, indicating strong pitch angle scattering. The discontinuity known as the magnetic pileup boundary (MPB) is apparent only in proton, alpha particle, and magnetometer data, indicating that it is a tangential discontinuity of solar wind origin. HERS observations show no significant change in the properties of the heavy ions across the MPB. The issue of whether a cometopause was unambiguously observed at comet Halley is discussed; it is concluded that the observations do not convincingly support the idea of a boundary due to internal cometary processes (as opposed to convected solar wind discontinuities). A comparison of the observations to MHD models is made. The plasma flow directions at all distances greater than 30,000 km from the nucleus are in agreement with MHD calculations. However, despite the agreement in flow direction, within 200,000 km of the nucleus the magnitude of the velocity is lower than predicted by MHD models, and the density is much larger (a factor of 5 or more for distances 60,000–160,000 km from the comet). Within 30,000 km of the nucleus there are large theoretical differences between the MHD model flow calculations for the plane containing the magnetic field and for the plane perpendicular to the magnetic field. The observations agreed much better with the pattern calculated for the plane perpendicular to the magnetic field. The data obtained by the HERS that are published herein have also been provided to the International Halley Watch archive.
Variations of the polar cap potential measured during magnetospheric substorms
Authors: Weimer D. R. , Kan J. R. , Akasofu S. ‐I.
Published: February 2008

Measurements of the polar cap potential drop and size have been obtained during magnetospheric substorms. Using double‐probe electric field measurements on the DE 2 satellite, 148 measurements have been obtained at random times preceding, during, and after 64 substorms. The polar cap potentials are graphed as a function of the difference between the time of the polar cap measurement and the time of the expansion onset of the corresponding substorm. The ratios of the auroral electrojet (AE) indices and the potential are also determined. The results show that on the average the polar cap potential starts to increase at 1.5 hours before onset, reaches a level of 70 kV in the 30‐min period before onset, and starts to decline at 1.5 hours after onset. However, on a case‐by‐case basis there are substantial variations from the average, as polar cap potentials over 120 kV were measured as early as 1 hour before substorm onset and values as low as 40 kV were observed during the expansion phase. The size of the polar cap ranged from 23° to 38° invariant latitude at the time of onset, and had an average value of 31°. The AE/ΦPC ratio is nearly constant before and after substorms, but decreases slightly during the substorm growth phase and increases greatly during the expansion phase. This increase is most likely due to a higher conductivity and westward electric field within the electrojet during expansion, which causes AE to increase without a corresponding change in the polar cap potential.
Nucleation process of unstable rupture in the brittle regime: A theoretical approach based on experimentally inferred relations
Authors: Yamashita Teruo , Ohnaka Mitiyasu
Published: February 2008

Ohnaka and Kuwahara investigated the nucleation process of unstable rupture in their elaborate laboratory experiment with a very high resolution, using a rock sample with a simulated fault. They found, among other things, that the crack growth resistance such as the stress σp — σi (σp being the peak stress at the crack tip and σi being the shear stress at the propagating tip on the verge of slip) and the breakdown stress drop increase with crack growth in the nucleation zone. They also found that the critical slip displacement is inhomogeneously distributed on the fault in the nucleation zone. We investigate whether inhomogeneous distribution of the crack growth resistance can necessarily cause the stable and quasi‐static nucleation process in the brittle regime. The slip‐dependent breakdown zone model has been assumed in the present theoretical analysis of crack. It has theoretically been shown that an increase in the critical slip displacement with the distance is required for the occurrence of stable crack growth. Its increase rate must be larger than a certain threshold value. The transition to unstable rupture growth occurs at a location where the increase rate falls short of the threshold value. The inhomogeneous distribution of the critical slip displacement may physically be attributed to that of fault surface roughness, as shown in laboratory experiments. The stress distributions on the crack plane prescribe the rate of stable crack growth. The stable rupture growth process observed by Ohnaka and Kuwahara can be explained quantitatively assuming inhomogeneously distributed stresses and critical slip displacement, which have also been observed in their experiments. The present theoretical study shows that both the distribution of the crack growth resistance, particularly that of the critical slip displacement, and the size of preexisting initial crack play important roles in creating stable and quasi‐static nucleation in the brittle regime. We show that diverse rupture phenomena can be caused by the differences in the distribution of critical slip displacement and in the size of preexisting initial crack.
Qp beneath the Rio Grande and East African Rift Zones
Authors: Halderman Thomas P. , Davis Paul M.
Published: February 2008

In 1983 teleseismic short‐period P waves were recorded in the southwest United States along a linear array which stretched 1000 km across the Rio Grande rift. In 1985 a similar experiment was conducted across the East African rift, in which stations were installed along a 600 km array which ran east‐west across Kenya. We have developed a spectral comparison technique to calculate the attenuation at each station. For a given event we simultaneously solve for the effective source spectrum, which includes mantle path effects as well as instrument response, an amplitude factor, and an attenuation operator t* at each station. We use downward projection of P wave travel time residuals to define the lithosphere‐asthenosphere boundary, which we infer to upwarp beneath both rifts. The velocity structure so determined is then used to determine the Q of the asthenosphere relative to that of the lower lithosphere. When we assume a lower lithosphere Q of 100, we obtain asthenospheric Q values of 57 beneath the Rio Grande rift and 26 beneath the East African rift, with corresponding velocity contrasts of −8% and −12%, respectively. The low Q values as well as low velocity and density contrasts support the interpretation that the upwarped asthenosphere contains partial melt and that beneath the East African rift the concentration is higher than beneath the Rio Grande. Comparison of the results with experiments on peridotite (Sato et al., 1989a, b) suggests that the asthenosphere is at homologous temperatures of 1.01 and 1.03 with corresponding partial melt fractions of 1% and 3% beneath the Rio Grande and East African rifts, respectively.
Spatial variations in stress within the 1987 Whittier Narrows, California, aftershock sequence: New techniques and results
Authors: Michael Andrew J.
Published: February 2008

The ML = 5.9 Whittier Narrows, California, earthquake of October 1, 1987, triggered a complex aftershock sequence. The aftershocks had many different focal mechanisms including thrusting on E‐W striking, north dipping planes (similar to the main shock), right‐lateral motion on NW‐SE striking planes (similar to the ML = 5.3 aftershock on October 4, 1987), thrusting on N‐S striking planes, and left‐lateral motion on N‐S striking planes. I attempt to interpret these mechanisms in terms of a stress field that may have caused them. Previously, stress inversion methods based on focal mechanisms assumed that the stress field was spatially uniform. In order to test for the spatially varying effect of the main shock's dislocation on the aftershocks' focal mechanisms a new inversion technique was developed that solves for a uniform stress field that is superimposed on a given spatially varying stress field. A series of simulations were performed to test both the new method and an older method that uses only the uniform component of the stress field. These simulations show that both techniques work but will be limited by the amount of noise in the data and the accuracy to which the given spatially varying stress field resembles the actual spatially varying stresses. Most importantly, the simulations show that the older methods do work when the stress field has a uniform component that is at least as large as the spatially varying component. The simulations also show that when applying the older stress inversion techniques, one can determine if the results are valid, and the amount of spatially varying stresses present, by examining the misfit between the data and the model. Application of these techniques to the Whittier Narrows aftershocks suggests that the uniform component of the stress field, and the complex faulting, corresponds to the response of an elastic halfspace to a simple regional N‐S compression. The misfit between the data and the uniform model demonstrate the presence of spatially varying stresses at Whittier Narrows; however, their form, as shown by the simulations, can not be determined with this data set and these techniques.
Mechanics of extensional wedges
Authors: Xiao Hong‐Bin , Dahlen F. A. , Suppe John
Published: February 2008

The extensional wedges that overlie low‐angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold‐and‐thrust belt, but with an opposite sense of shear on the basal fault. Such wedges are common in zones of crustal extension, such as the Basin and Range province of the North American Cordillera, the North Sea, and the U.S. Gulf Coast. This paper examines the mechanics of these upper crustal extensional wedges, using a modification of the compressional Coulomb critical taper model developed for fold‐and‐thrust belts and accretionary wedges. If a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer, then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline. A critically tapered extensional wedge is one that is on the verge of Coulomb failure everywhere. A wedge whose taper is narrower may be slid stably down the same incline without internal deformation; a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical. After testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges, we apply the theory to a pair of active extensional wedges in the Brazos area, offshore Texas. Both wedges are underlain by well‐imaged basal detachment faults dipping at 17° down to depths of 10 km; one wedge exhibits relatively little internal deformation, whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults. Direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical. Regional values for the coefficients of basal and internal friction in excess of 0.3 together with vertical cohesion gradients in excess of 1 kPa/m are consistent with the observed pore pressures and the geometry of the wedges, as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults. Such values are comparable to values inferred from mechanical and thermal models of compressional wedges, and they are consistent with laboratory rock mechanics data.
Analysis of geoid height versus topography for oceanic plateaus and swells using nonbiased linear regression
Authors: Marks Karen M. , Sandwell David T.
Published: February 2008

We have investigated the relationship between geoid height and topography for 53 oceanic plateaus and swells to determine the mode of compensation. The ratio of geoid height to topography was obtained from the slope of a best line fit by functional analysis (i.e. nonbiased linear regression), a method that minimizes both geoid height and topography residuals. This method is more appropriate than traditional least squares analysis that minimizes only geoid height residuals, because uncertainties are present in both data types. We find that approximately half of the oceanic and continental plateaus analyzed have low ratios that are consistent with Airy‐compensated crustal thickening. The remaining plateaus, however, have higher geoid/topography ratios than predicted by the simple Airy model, and the seismically determined Moho depths beneath some of these features are too shallow for crustal thickening alone. A two‐layer Airy compensation model, composed of thickened crust underlain by an anomalously low density “mantle root,” is used to explain these observations. The Walvis Ridge, and the Agulhas, Crozet, and north Kerguelen plateaus have geoid/topography ratios and Moho depths that are consistent with the two‐layer Airy model. The proximity of the Agulhas Plateau to a RRR triple junction during its early development, and the excessive volcanism at active spreading ridges that created the Crozet and north Kerguelen plateaus and the Walvis Ridge, may have produced regions of enhanced depletion and hence the low‐density mantle anomalies. If this explanation is correct, then the low‐density mantle anomaly persists over time and remains embedded in the lithosphere beneath the oceanic feature.
On the mechanics of earthquake afterslip
Authors: Marone Chris J. , Scholtz C. H. , Bilham Roger
Published: February 2008

We propose a model for earthquake afterslip based on rate and state variable friction laws. In the model, afterslip is attributed to the interaction of a velocity‐weakening region at depth (within which earthquakes nucleate) with an upper region of velocity‐strengthening frictional behavior. The existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments. In our model, afterslip is the result of relaxation of a stress perturbation within the velocity‐strengthening region, which arises when an earthquake propagates into that region from below. We derive the stress perturbation and its decay from the friction constitutive law using a simple, 1 degree‐of‐freedom approximation for the elastic interaction between the fault and its surroundings. This approximation is based on thickness‐averaged displacements and slip velocities within the velocity‐strengthening region, which is assumed to slip as a rigid block. Coseismic and postseismic slip are coupled through the thickness‐averaged stiffness k of the velocity‐strengthening region. We assume k to be inversely proportional to the thickness of this region, which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip. We model the afterslip‐time histories of the 1966 Parkfield and 1987 Superstition Hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults. In accord with field observations, our model predicts (1) that afterslip on some faults scales with the thickness of the (unconsolidated) sedimentary cover and (2) that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth. Velocity‐strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones (about >500 m) within their shallow regions (<3–5 km). Combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip.
Electromagnetic imaging and simulated annealing
Authors: Gibert Dominique , Virieux Jean
Published: February 2008

In contrast with acoustical imaging methods, for which the wave field is dominated by propagation effects, electromagnetic imaging of conductive media suffers from the diffusive behavior of the electromagnetic field. An important question to address when working toward the achievement of electromagnetic imaging concerns the possibility of resolving the diffusion damping. Exact inversion will be looking at the solvability of the integral equation relating a diffusive field to its dual wavefield. This equation is ill posed because its Laplace‐like kernel makes the inverse problem of finding the dual wave field a notoriously difficult (both numerically and mathematically) one. Stochastic inversion is another alternative based on least squares fitting. In this inverse problem approach, extracting the wave field is still a relatively instable process, although the L2 misfit function for data without noise presents a global minimum. The simulated annealing overcomes this instability for parameterization of this problem designed as follows. The unknown wave field is expected to be a sequence of impulsive functions. The number of impulsive functions can be determined by using a statistical criterion, called AIC, which comes from the Prony technique. The simulated annealing is applied to the positions of the reflections, while the amplitudes, which are not taken as parameters, are obtained by linear fitting. The simulated annealing method proves to be efficient even in the presence of noise. Furthermore, this nonlinear numerical inversion furnishes statistical quantities which allows an estimation of the resolution. Simple synthetic examples illustrate the performance of the inversion, while a synthetic finite element example shows the final pseudo‐seismic section to be processed by standard seismic migration techniques.
Brittle frictional mountain building 3. Low‐grade metamorphism
Authors: Barr Terence D. , Dahlen F. A. , McPhail D. C.
Published: February 2008

An active fold‐and‐thrust belt in unchanging tectonic and climatic conditions exhibits a dynamic steady state, with the flux of rocks accreted at the toe balanced by the flux of rocks eroded off the top. Rocks entering the toe are buried and heated before they are uplifted and eroded; this results in a characteristic map pattern of low‐grade metamorphism on the surface. Metamorphic isograds are generally parallel to the regional strike of a fold‐and‐thrust belt, with the grade increasing progressively from unmetamorphosed and zeolite facies near the deformation front up to greenschist facies in the highest mountains; such a pattern is observed in the active fold‐and‐thrust belt of Taiwan. This paper examines the origin of this low‐grade syntectonic metamorphism, using a previously developed mechanical and thermal model of a steady state fold‐and‐thrust belt as a basis. To model the metamorphism, we develop a petrogenetic grid for rocks of basaltic composition, considering phases within the chemical system CaO‐MgO‐Al2O3‐SiO2‐H2O. Equilibria diagnostic of stable low‐grade mineral assemblages are mapped onto a cross section of the Taiwan fold‐and‐thrust belt, using the calculated fluid pressure and temperature distributions within the wedge. The variation of metamorphic grade along the surface is predicted by assuming that there is no retrograde metamorphism. The most important mechanical factor controlling the degree of metamorphism in an active fold‐and‐thrust belt is the amount of frictional heating, both on the basal decollement fault and within the deforming brittle wedge. Frictional heating raises temperatures in the deepest portions of the Taiwan fold‐and‐thrust belt by 200° C to 250° C; the resulting high temperatures, in excess of 400° C, are responsible for the extensive greenschist facies metamorphism. The amount of heating is well constrained by the observed heat flow anomaly, geochronological data, and the critical wedge taper; the best‐fitting coefficient of basal friction in Taiwan is μb = 0.5 ± 0.2. Underplating of footwall rocks by the assimilation of duplexes along the decollement fault is not considered to be a significant process in Taiwan, because of the observed balance between the accretionary influx at the toe and the erosive efflux off the top. In other fold‐and‐thrust belts, however, underplating can significantly increase the outcrop width of higher metamorphic grades on the surface by increasing the flux of rocks through the higher fluid pressure and temperature regions within the wedge.
The Rayleigh‐Taylor instability of an embedded layer of low‐viscosity fluid
Authors: Wilcock William S. D. , Whitehead J. A.
Published: February 2008

The properties of gravitational instabilities formed within two‐layer fluid systems are well known and have been applied to a variety of geophysical problems. We present theoretical and experimental results for the gravitational instability developed by a three‐layer system, comprising a thin low‐viscosity low‐density fluid layer sandwiched between two thick layers of equal properties. Linearized equations can be used to solve for the initial growth rates as a function of perturbation wavelength. As is the case for two‐layer systems, the results yield a fastest growing wavelength, termed the characteristic wavelength, whose value is much greater than the thickness of the low‐viscosity layer. The experimental results confirm the ability of the linearized equations to predict the dominant wavelength of the instability. However, for very thin layers or smaller viscosity ratios a second instability is also observed at a scale much greater than the characteristic wavelength. Numerical solutions show that the wavelength of this instability matches that of a fast growing but short‐lived mode arising from perturbations which predominantly involve thickening rather than translation of the buoyant layer. The analytical solution also shows that at the characteristic wavelength, the displacement of the lower interface will be initially a factor 2 − √3 = 0.268 that of the upper interface. As the instability develops the characteristic diapir structures, the experiments show that the relative magnitude of these displacements increase, with underlying fluid being drawn up into the head of the diapir.
Magmatic system geometry at Mount St. Helens modeled from the stress field associated with posteruptive earthquakes
Authors: Barker Sally E. , Malone Stephen D.
Published: February 2008

Earthquakes following the May 18, 1980, and June 12, 1980, explosive eruptions of Mount St. Helens were concentrated at depths below 6 km. The hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater. The timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume. The withdrawal of magma caused a pressure decrease within the reservoir, and the earthquakes occurred as a brittle response to the resulting stress change. Focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data. Results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km. This anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime. In the best fitting model the magma body is centered at 46.204°N, 122.187°W, 600 m to the north of the present dome, and is under a regional shear stress of 20 bars. The radius of the chamber is calculated to be between 0.65 and 0.75 km when using pressure drops within the cylinder of 220 and 150 bars respectively. The magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 Posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime. This fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust.
Crustal structure at the Blake Spur Fracture Zone from expanding spread profiles
Authors: Minshull T. A. , White R. S. , Mutter J. C. , Buhl P. , Detrick R. S. , Williams C. A. , Morris E.
Published: February 2008

We present results from WKBJ and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles (ESPs), along flow lines parallel to the Blake Spur fracture zone across 140 Ma Atlantic crust. These profiles provide detailed constraints on variations in crustal structure at the fracture zone. Seven profiles on either side of the fracture zone show a normal structure for old oceanic crust. A 2–3 km thick upper layer with a steep velocity gradient is underlain by 4–5 km of crust with velocities of 6.5–7.2 km/s and low gradient and a sharp transition to upper mantle velocities of 8 km/s. Several velocity discontinuities were detected within the upper 2–3 km, but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles. This structure shows little regional variation toward the fracture zone. Despite the small magnetic anomaly offset (∼12 km) and the indistinct topographic signature of the fracture zone, three ESPs within a 10–20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure, relative to normal oceanic crust. A 2–4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7.2–7.6 km/s and high Poisson's ratio (probably at least 0.29), which is consistent with 15–30% serpentinization of upper mantle peridotites. This interpretation requires the action of off‐axis hydrothermal circulation, penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km. The original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent “normal” crust, which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments.
Thermal alteration of the magnetic mineralogy in ferruginous rocks
Authors: Hirt A. M. , Gehring A. U.
Published: February 2008

The Northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite. The magnetic fabric of this sedimentary rock, as defined by the anisotropy of magnetic susceptibility, is controlled by bedding compaction but is inverse (i.e., the maximum susceptibility axes are normal to the bedding plane). This inverse fabric is attributed to the dominating presence of siderite. The rocks were incrementally heated to 600°C, and the magnetic fabric was measured at room temperature and at near liquid nitrogen temperature after each heating step; cooling the samples in liquid nitrogen enhances the paramagnetic contribution to the magnetic fabric. After heating the samples above 250°C, the room temperature magnetic fabric became normal with the minimum susceptibility axes perpendicular to the bedding plane. The low‐temperature magnetic fabric remains inverse until the samples are heated to about 500°C. After heating to higher temperatures a normal fabric was observed at both room temperature and low temperature. Rock magnetic evidence shows that magnetite is created during the heating. The start of mineralogical changes in the rock was detected by electron spin resonance spectroscopy, powder X ray diffraction, and acquisition of isothermal remanence. Berthierine starts to break down at approximately 250°C and is totally oxidized by 500°C. The creation of magnetite in the heating experiments corresponds with this mineralogical change. The change in the room temperature magnetic fabric is associated with the decomposition of berthierine, part of which alters to magnetite. The new magnetite phase has a normal magnetic fabric which overprints the inverse magnetic fabric of siderite.
Heck and Heckle Seamounts, northeast Pacific Ocean: High extrusion rates of primitive and highly depleted mid‐ocean ridge basalt on off‐ridge seamounts
Authors: Leybourne M. I. , Van Wagoner N. A.
Published: February 2008

We analyzed, petrographically and chemically, basalt from eight dredge hauls from the Heck and Heckle seamounts, northeast Pacific Ocean. Major elements were determined for mineral, glass, and whole rock samples, and trace and rare earth elements were determined for glass and whole rock samples. The dredge hauls included hyaloclastites and fragments from sheet flows and pillows. The clinkery fragments are interpreted to be deformed sheet flow tops, characteristic of high effusion rates. The hyaloclastites recovered are reworked deposits, as indicated by the wide compositional range of the glass shards, abundance of clay and calcite matrix, and bedding. Most rocks are aphyric, but the analyzed plagioclase and olivine phenocrysts and microcrysts are equilibrium compositions and show minor compositional zonation (up to 7.5% An, <1 %Fo). Calculated quenching temperatures range from 1228° to 1216°C, decrease with decreasing Mg #, and are consistent with phase equilibria. The seamount lavas are primitive (Mg # 68.7–59.5) and restricted in compositional range ([La/Ce]n = 0.65–0.77). The basalts are equivalent to highly depleted mid‐ocean ridge basalt ([La/Ce]n = 0.29–0.41) and are at the depleted end of the range in composition of basalts from East Pacific seamounts. They are more depleted than basalts from the adjacent West Valley spreading axis. The lavas can be produced by partial melting of a depleted source. Mineralogical and geochemical evidence is consistent with rapid ascent of the Heck and Heckle lavas through oceanic crust with little residence time in magma chambers. Lavas of the Heck and Heckle Seamount chains have a limited range of incompatible element ratios, whereas the adjacent West Valley Segment of the Juan de Fuca Ridge is highly heterogeneous. In contrast, lavas from the East Pacific near‐ridge seamounts exhibit a wider range of incompatible element ratios than do the adjacent East Pacific Rise basalts. On the West Valley Segment, magma supply is less robust associated with lower spreading rates compared to the East Pacific Rise at 10°N. In contrast, at fast spreading centers robust melting produces a mixed mantle signature in axial lavas, while suppressed melting at the seamounts reveals the heterogeneities. We suggest that at some spreading ridges, more fertile portions of the mantle are preferentially melted such that the outwelled portions of the mantle tapped by the seamounts are more depleted.
Mechanisms of dike propagation in layered rocks and in massive, porous sedimentary rocks
Authors: Baer Gidon
Published: February 2008

The mechanisms of dike propagation are analyzed through detailed examination of small structures associated with the dike intrusion. Exposures in the Ramon area, Israel, offer an opportunity to evaluate theoretical models of tensile fracture propagation and to better understand the factors governing the dike geometry. Dike‐related structures are studied in two Jurassic units: the Ardon Formation, a well‐stratified sequence of shales and dolomites, and the Inmar Formation, which is composed of porous, massive sandstones with a few shale lenses. It is shown that for the different host rocks the propagation mechanisms and structures differ significantly. Dikes in the Ardon Formation are dominated by 1–20 m wide segments, confined to distinct layers. The mechanisms of segment containment are determined by calculating the stress intensity factor for an extension crack approaching a mechanical interface. It is apparent that dike segmentation and containment is controlled mainly by in situ stress and shear moduli differences between adjacent layers and partly by bedding plane slippage. Dikes in the Inmar Formation are dominated by smaller segments and by 1–10 cm wide and 10–100 cm long fingers with intermittent smooth patches and slickensides. Opposite dike walls display exact mirror images of these features. Dikes propagate in alternating stages of viscous flow and brittle deformation. The viscous stages include fluidization of the sandstone by the dike‐related fluids and increase of the pore pressure in the rock. A viscous instability develops between the highly viscous fluidized sandstone and the less viscous dike‐related fluids, forming a zone of viscous fingering in front of the dike. Stages of brittle fracturing occur when the pore pressure is released and the flux of fluids is insufficient to fluidize the rock.
Lg and Rg waves on the California Regional Networks from the December 23, 1985 Nahanni Earthquake
Authors: Wald Lisa A. , Heaton Thomas H.
Published: February 2008

We investigate Lg and Rg propagation in California using the central and southern California regional networks. Approximately 550 stations constitute these two short‐period networks providing a dense coverage of almost the entire state. The waveforms recorded from the December 23, 1985, Nahanni, Canada, earthquake are used to construct three profiles along the propagation path (almost N‐S) and three perpendicular to the propagation path (almost E‐W) to look at the nature of propagation of these two types of surface waves. Groups of records from stations in various geological and tectonic provinces in California are also examined in order to establish regional characteristics of the surface waves. We find that the propagation characteristics of Lg differ from those of Rg across California; Lg waves are apparently more sensitive to crustal heterogeneities. The most striking observations are the similarity of coda for both the Lg and the Rg waves within geologic provinces and the marked difference in coda between regions. These differences are seen in the amplitudes, coda duration, shape of the energy envelope, frequency content, and sharpness of the phase initiation. In general, a decrease in the Moho depth near the Pacific Coast is correlated with a decrease in the surface wave amplitude, especially at higher frequencies (0.15–0.2 Hz). Most interesting is the association of the San Andreas fault with abrupt changes in the wave train amplitudes. The surface waves are amplified in the vicinity of the fault zone and then decrease in amplitude after the zone is crossed. In the Coast Ranges, amplitudes are low and waveform coherence is poor. The Rg phase dominates the record in the Sierra Nevada, and both surface waves are amplified by the thick sedimentary sequence of the Great Valley.
Polarities of P and S waves, and shear wave splitting observed from the Bucaramanga Nest, Colombia
Authors: Shih Xiao R. , Schneider John F. , Meyer Robert P.
Published: February 2008

The Bucaramanga nest, Colombia, is an intense seismic source centered at 6°48′N, 73°10′W and 161‐km depth beneath the eastern Andes. During a 3‐week period, 161 microearthquakes (mb ≤ 4.3) were recorded by a 260‐by‐140‐km array of 12 three‐component University of Wisconsin (UW) seismographs centered above the nest. Eighty‐eight percent of the events recorded were from the nest, providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths. Shear wave splitting analysis shows that the average polarization direction of the first‐arriving shear waves from the nest events at nine of the seismographs is N12°E±18°. After corrections based on the amount of discrepancy between the observed average P wave azimuth and the expected event‐to‐station azimuth, average S wave polarization is N6°E ±15°. Variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system. Shear wave splitting observed from 17 events along the Wadati‐Benioff zone outside the nest shows that the time delays increase with increasing depth, and defines a wedge‐shaped anisotropic volume above the subducting plate. The anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip. The anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a localized mantle flow in the lithosphere above the Wadati‐Benioff zone, a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab.
The Hechiceros Caldera: A recently identified Mid‐Tertiary caldera in eastern Chihuahua, Mexico
Authors: Ritter Steven P. , Cepeda Joseph C.
Published: February 2008

Geologic mapping of Tertiary volcanic rocks in the Sierra Hechiceros, 50 to 80 km south of Big Bend National Park has delineated a 20 by 25 km Mid‐Tertiary caldera, herein named the Hechiceros Caldera. This caldera was formed by the eruption of the Palomas tuff which has a minimum volume of approximately 170 km3, and is distributed radially around the caldera. The caldera was filled with more than 500 meters of postcaldera rhyolitic lavas which overflowed the caldera margin and caused additional subsidence. The rocks of the Sierra Hechiceros are subalkalic and include rhyolites and dacites. The chemistry of these rocks is transitional between the calc‐alkalic province of the Sierra Madre Occidental and the alkalic rocks of the eastern Trans‐Pecos magmatic province. The potassium argon ages both the Palomas tuff and the postcaldera El Jobero rhyolite are about 29±0.6 Ma.
Frequency dependence of the Q factor in random media
Authors: Jannaud L. R. , Adler P. M. , Jacquin C. G.
Published: February 2008

An investigation of the frequency evolution of the quality factor in Gaussian media is conducted. By means of a source with a restricted spectrum, a method is developed to measure accurate values of Q−1. When the fluctuations of the velocity are less than 4%, it is shown that the results are in good agreement with the formula of Wu. However, it is noticed that for such media the attenuation is too weak to be measured in a real seismic survey. It is shown that codas and the attenuation of the first arrival are closely related. The evolution of the function Q−1 (ƒ) is studied for large fluctuations in the nonlinear range.
Investigation of deep slab structure using long‐period S waves
Authors: Gaherty James B. , Lay Thorne , Vidale John E.
Published: February 2008

Travel times and amplitudes of long‐period SH, ScSH, sSH, and sScSH phases from several deep focus earthquakes in the northwest Pacific are analyzed for evidence of lithospheric slab penetration into the lower mantle. Inclusion of amplitude observations in the analysis provides constraints on lateral velocity gradients present in the deep slabs which are not resolvable using travel times alone. Travel time and amplitude residual spheres are presented for two deep focus events with good azimuthal coverage, but while interesting patterns are present, quantitative analysis is precluded by the lack of accurate methods for calculating synthetic long‐period seismograms for three‐dimensional slab models. Therefore, we focus our analysis on a two‐dimensional approximation to the downdip geometry of the Kurile and Japan slabs, which allows comparison of a much larger data set with accurate two‐dimensional synthetics, as well as the use of sS and sScS travel time and amplitude patterns as empirical corrections for deep mantle structure. While the limited azimuth range required for this approximation reduces the diagnostic capability of the data, it also increases our confidence that the corrected data are most sensitive to the near‐source region. The sS and sScS observations indicate that the downdip shear wave travel time pattern previously attributed to a deep slab extension is primarily caused by broad‐scale lower mantle heterogeneity. Once corrected for this structure, the S wave observations do not support a simple, undeformed slab steepening in dip and penetrating deep into the lower mantle beneath the Kurile and Japan Islands, as proposed by Jordan (1977) and Creager and Jordan (1984, 1986). Rather, the observations support shorter and/or broader slab models man those previously hypothesized: models which can probably be reconciled with P wave data. Further analysis of the complete three‐dimensional patterns will be required for more precise resolution of the penetration depths of these slabs, if, indeed, any deep slab heterogeneity is actually seismically detectable.