bluuebunny/crossref_metadata_2025 huggingface dataset:
On the preferred source location for the convective amplification of ion cyclotron waves
Authors: Horne Richard B. , Thorne Richard M.
Published: February 2008

The propagation, growth and absorption of electromagnetic ion cyclotron waves in the Pc 1 frequency range is investigated using the HOTRAY ray tracing program for a realistic distribution of thermal plasma (H+, He+ and O+) that is assumed to be in diffusive equilibrium inside the plasmasphere and collisionless in the low‐density region outside the plasmapause. Free energy for L‐ mode wave growth is provided by a bi‐ Maxwellian distribution of energetic H+ and O+ with a temperature and density modelled on satellite observations. Solutions to the hot plasma dispersion relation show that inside the plasmasphere the spatial growth rates are small whereas they increase outside the plasmapause with increasing L shell. Ray tracing shows that inside the plasmasphere guided L‐ mode waves only grow during one crossing of the magnetic equator and only achieve small path‐integrated wave gain (≤ 2 e‐ foldings). At the plasmapause the density gradient enables guided mode waves to grow during several equatorial crossings and the net path‐integrated gain is much larger (≃ 8.7 e‐foldings). For the largest observed ring current densities of 4 × 106 m−3 at L = 4 the gain is above the critical level (10 e‐foldings) for amplification to observable levels. Just outside the plasmapause the waves only grow during the first equatorial crossing and the gain is smaller. In the absence of nonconvective instabilities the path‐ integrated amplification of the guided mode tends to increase with L shell and reaches the critical level for observable waves only in the outer magnetosphere (L ≥ 7). Unguided L‐ mode waves have very small wave gain. For L ≥ 7 the plasma beta becomes large (β⊥ > 1) and should lead to the onset of nonconvective instabilities. However, we suggest that inhomogeneities in the medium and quasi‐linear scattering will prevent absolute instabilities from occurring and that in reality the waves are propagating with very low group velocities. We suggest that the waves observed by Anderson et al. (1990, 1992a, b) beyond L = 7 near local noon are influenced by the enhanced wave gain due to these very low group velocity waves.
Effects of variable normal stress on rock friction: Observations and constitutive equations
Authors: Linker M. F. , Dieterich J. H.
Published: February 2008

We investigate the effects of variable normal stress on frictional resistance by performing quasi‐static sliding experiments with 5 × 5 cm blocks of Westerly granite in a double‐direct shear apparatus under servo‐control. The observed response to a change in normal stress mimics that which occurs in response to a change in slip velocity. In particular, a sudden change in normal stress results in a sudden change followed by a transient change in the resistance to sliding. We interpret these changes within the previously established constitutive framework in which frictional resistance is determined by the current slip speed V, the current normal stress, and the state of the sliding surface (Dieterich, 1979a, 1981; Ruina, 1980, 1983). Earlier work demonstrated that the state of the sliding surface depends on prior slip speed. Our observations indicate that the state of the sliding surface also depends on prior normal stress. In our model the functional dependence of state on normal stress is expressed in terms of the same state variable, θ, used previously to represent slip rate history effects. We assume that the steady state value of θ is independent of normal stress and that θSS = Dc/V, where Dc is a characteristic slip distance. We interpret the variable θ as a measure of effective contact time. At constant slip speed and from an initial steady state, a sudden change in normal stress results in a sudden change in θ followed by a gradual change in θ back toward the initial θSS, as sliding proceeds. The magnitude of the sudden change in θ is determined by a newly identified parameter that we call α. Earlier workers have established that stability is influenced by stiffness, dτSS/dV, Dc, and slip rate history (Rice and Ruina, 1983). We conclude that stability will also be influenced by normal stress history and by α.
Ultrasonic tomography and acoustic emission in hydraulically fractured Lac du Bonnet Grey granite
Authors: Falls S. D. , Young R. P. , Carlson S. R. , Chow T.
Published: February 2008

Ultrasonic tomography and acoustic emission (AE) data were obtained during laboratory hydraulic fracturing tests on two large, unconfined cylinders of Lac du Bonnet grey granite. The cylinders were internally pressurized over four to five cycles prior to final failure. Compressional velocities were measured before and after each pressurization cycle with an array of 16 evenly spaced transducers around the central, cross‐sectional plane of each sample. Sixteen channels of whole waveform AE data were recorded during most pressurization cycles and for a period of about 1 hour after final failure in one sample. Compressional velocities were found to be strongly anisotropic, with the in situ vertical direction being the most rapid direction in both samples. The velocity anisotropy is related to the rock's preexisting microcrack fabric. Owing to radial penetration of fluid into the rock, compressional velocities rose over the course of the experiment. A regression analysis showed that the velocity changes can be explained by variations in crack density, inferred from initial velocities, and radial distance from the borehole. Saturation levels consistent with the observed velocity changes were calculated on the basis of the O'Connell and Budiansky theory. Acoustic emissions reoccurred in a few distinct zones over several pressurization cycles. The AE locations allowed two distinct fracture planes to be sharply delineated in one sample. The fracture plane orientations were controlled by the preexisting microcrack fabric in both samples. AE occurred too rapidly during peak pressure failure to permit us to isolate distinct events. Source mechanism analysis of the AE which occurred prior to peak pressure failure in both samples, and during postfailure monitoring in one sample, showed a predominance of double‐couple sources. Compressive sources, thought to be related to crushing of asperities during crack closure, and tensile sources, related to mode I crack growth, were also recorded, as well as more complex sources that could not be modeled by the simple source types listed above.
Effects of crystalline anisotropy on fluid distribution in ultramafic partial melts
Authors: Waff Harve S. , Faul Ulrich H.
Published: February 2008

The textures of experimentally produced ultramafic partial melts show consistent and significant deviations from the morphology predicted by isotropic equilibrium theory. Flat crystalline interfaces are pervasive in these systems and they coexist with smoothly curved boundaries which are predicted by the isotropic theory. Long‐duration experimental runs on olivine‐basalt mixtures held at pressures between 1.0 and 2.0 GPa and temperatures from 1350°C to 1400°C were evaluated. Scanning electron microscope images of samples with 2.5 or more volume percent melt showed at least 20% of observable grain boundaries to be wetted by the melt. In addition, approximately 60% of the melt tubules occurring along triple junctions in this sample were found to have at least one flat interface, an effect which increases the ratio of permeability to porosity. Both the experimental evidence and theoretical considerations indicate that the flat faces are stable equilibrium or steady state features in these partial melts, and that they are crystallographically controlled. The crystal‐melt morphology is influenced by the crystalline equilibrium habit (obtained from minimization of surface energies of individual crystals) under the constraints of polycrystalline aggregates. A dependence of the style of melt distribution on melt fraction was observed in these runs. At very low melt fractions (less than 1 vol %) the texture is dominated by melt‐filled triple junctions and mostly dry grain boundaries, whereas at higher melt fractions (but below 5 vol %) more melt pockets and melt films along grain boundaries appear. An interpretation of the observed texture is made, applying established crystal growth and interface theories to steady state partially molten systems. The extensive occurrence of flat or faceted crystallographic faces in partial melts requires major changes in the modeling of their permeabilities, as well as bulk elastic, anelastic, and electrical properties, from existing models of melt distribution. In regions of the upper mantle where olivine lattice preferred orientation is expected (e.g., in the vicinity of mid‐ocean ridges) the presence of faceted faces and associated changes in melt distribution will produce anisotropic permeabilities and changes in seismic attenuation.
The rupture process of the 1989 offshore ito earthquakes preceding a submarine volcanic eruption
Authors: Takeo Minoru
Published: February 2008

An earthquake swarm at the eastern offshore region of Ito, the Izu peninsula, started on June 30, 1989, and became more active on and after July 4. The largest earthquake among the swarm (MJMA= 5.5) occurred at 1109 JST (0209 UT) on July 9, 1989, and a second significant event followed 45 s later. A remarkable volcanic tremor started on July 11, and a submarine eruption occurred just above the source region of the largest earthquake on July 13. In this paper we present a new inversion method to derive the source time function of rupture process. The source time function is divided into several sections following an automatic algorithm and is interpolated with a cubic spline function. The parameters, which are the values and first derivatives at knots of the spline function, are determined by minimizing a squared error between the observed and synthetic seismograms. To demonstrate the performance of this method, some numerical experiments (simulations) are carried out on near‐field seismic records. Applying this method to analysis of the two large earthquakes, the detailed rupture processes are obtained. A heterogeneous crustal structure near the volcanic area affects the rupture processes of these earthquakes. The large stress drop due to the largest earthquake just beneath the submarine eruption point would assist a magma ascending and may have triggered the eruption 4 days after these earthquakes.
Geology of a vigorous hydrothermal system on the Endeavour Segment, Juan de Fuca Ridge
Authors: Delaney John R. , Robigou Véronique , McDuff Russell E. , Tivey Margaret K.
Published: February 2008

A high‐precision, high‐resolution geologic map explicitly documents relationships between tectonic features and large steep‐sided, sulfide‐sulfate‐silica deposits in the vigorously venting Endeavour hydrothermal field near the northern end of the Juan de Fuca Ridge. Water depth in the vent field varies from 2220 to 2200 m. Location of the most massive sulfide structures appears to be controlled by intersections of ridge‐parallel normal faults and other fracture‐fissure sets that trend oblique to, and perpendicular to the overall structural fabric of the axial valley. The fractured basaltic substrate is primarily composed of well‐weathered pillow and lobate flows. As presently mapped, the field is about 200 by 400 m on a side and contains at least 15 large (> 1000 m3) sulfide edifices and many tens of smaller, commonly inactive, sulfide structures. The larger sulfide structures are also the most vigorously venting features in the field; they are commonly more than 30 m in diameter and up to 20 m in height. Actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller. Maximum venting temperatures of 375°C are associated with the smaller structures in the southeastern portion of the field; highest‐temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20°–30°C lower. Hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture‐controlled black smoker activity near the top of the structures. A different type of high temperature venting takes place from the upper sides of the structures in the form of “overflow” from fully exposed, quiescent pools of buoyant 350°C vent water trapped beneath overhanging sulfide‐sulfate‐silica ledges, or flanges. These flanges are attached to the upper, outer walls of the large sulfide edifices. Two types of diffuse venting in the Endeavour field include a lower temperature 8°–15°C output through colonies of large tubeworms and 25°–50°C vent fluid that seems to percolate through the tops of overhanging flanges. The large size and steep‐walled nature of the these structures evidently results from sustained venting in a “mature” hydrothermal system, coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development.
Shallow water sediment properties derived from high‐frequency shear and interface waves
Authors: Ewing John , Carter Jerry A. , Sutton George H. , Barstow Noel
Published: February 2008

Low‐frequency sound propagation in shallow water environments is not restricted to the water column but also involves the subbottom. Thus, as well as being important for geophysical description of the seabed, subbottom velocity/attenuation structure is essential input for predictive propagation models. To estimate this structure, bottom‐mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf, usually where boreholes and high‐resolution reflection profiles give substantial supporting geologic information about the subsurface. This colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the “ground truth” properties. Measurements were made in 1986 with source/detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30–50 m of the seabed (and P wave profiles to lesser depths). Measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at 1‐m intervals to improve spatial sampling and resolution of shallow structure. These investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the shallow seabed are common and are principally created by erosional and depositional processes associated with glacial cycles and sea level oscillations during the Quaternary. When the seabed structure is relatively uniform over the length of the profiles, the shear wave fields are well ordered, and the matching of the data with full waveform synthetics has been successful, producing velocity/attenuation models consistent with the subsurface lithology indicated by coring results. Both body waves and interface waves have been modeled for velocity/attenuation as a function of depth with the aid of synthetic seismograms and other analytical techniques. Some results give strong evidence of anisotropy and lateral heterogeneity in shear velocity of the upper 5–10 m of sediments and of extremely high velocity gradients in the topmost 1–2 m, possibly exceeding 30 s−1.
The Upper Kaena sedimentary geomagnetic reversal record from southern Sicily
Authors: Van Hoof A. A. M. , Langereis C. G.
Published: February 2008

A detailed paleomagnetic and rock magnetic record of the upper Kaena polarity reversal was determined from late Pliocene marine marls in southern Sicily. Two separate characteristic magnetization components were distinguished: a low‐temperature (LT) component between 200° and 50°C and a high‐temperature (HT) component between 500° and 580°–610°C. The two components record the reversal in a slightly different way, but both were acquired in an early stage following deposition, most probably by authigenic (biogenic) formation of magnetic minerals. The actual (R‐N) reversal, via E and horizontal inclinations, seems to take 3.3 kyr and is followed by a stable normal interval (3.3 kyr) and subsequently by a significant excursion (50°–60°) in declination (3.6 kyr). Relative intensities were lower during a time‐span of 5.6 kyr and show a minimum of 10% of the nontransitional values, but during the excursion, no intensity changes are apparent. The VGPs of the HT component are confined to a longitude between India and Australia; those of the LT component oscillate between this same longitude and one passing over North and South America. We conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions. This has not only resulted in considerable smoothing but also in delayed NRM acquisition in specific sedimentary intervals. We believe that the observed “excursion” is a sedimentary artifact and not related to geomagnetic field behavior during transitions. We emphasize that many reported “geomagnetic” features may in fact be due to the sedimentary remanence acquisition process.
Rotational buoyancy tectonics and models of simple half graben formation
Authors: Reiter Marshall , Barroll Margaret W. , Cather Steven M.
Published: February 2008

A buoyancy model for half graben development, based on the stability of floating bodies, assumes parallel, high‐angle normal faults which penetrate the brittle crust and define half graben parallelograms. Buoyancy forces balance the weight of the half grabens, and torques generated by buoyancy forces are balanced by opposing torques arising from crustal stresses. Model results indicate that widths of developing half grabens depend on the state of stress in the brittle crust, thickness of the brittle crust, and spacing of boundary faults. For a given thickness of elastic crust in a constant state of stress, narrower half grabens will experience greater rotation. Increasing deviatoric tension and/or thickening of the brittle crust will enable rotation of wider half grabens. The effective coefficient of friction across the fault must be ≤ 0.1–0. 2 for half graben rotation to occur; and probably ≤ 0. 1 for wider half grabens to form or any half grabens to rotate through large angles. These conclusions contribute to an explanation of half graben formation in the Socorro, New Mexico, area, where decreasing heat flows and associated thickening of the brittle crust allowed for development of wider half grabens as rifting progressed.
Water throughflow and the physical effects of deformation on sedimentary glacier beds
Authors: Murray Tavi , Dowdeswell Julian A.
Published: February 2008

The sediment comprising a deformable bed beneath an ice mass is typically modeled as a homogeneous fluid without internal structure, although clearly, deformation must result from the spatial rearrangement of the particles that make up the sediment. Laboratory deformation experiments were used to simulate the effects of active deformation on subglacial sediments. These experiments indicate that deformation results in the rearrangement of particles and changes in sediment structure. Scanning electron and optical microscopy of postdeformation sediments revealed both (1) particle alignment parallel or subparallel to the direction of principal strain and (2) dilation where active deformation had occurred. Both of these effects will alter the permeability and hence hydraulic conductivity of the sediment body, increasing the rate of water flow through the bed parallel to deformation. Microscopic examination of these sediments allows estimation of postdeformation permeabilities. These permeabilities may be anisotropic. Furthermore, water throughflow by Darcian processes, enhanced by the effects of deformation, may increase the proportion of basal water that can be evacuated by throughflow. Water will tend to be retained in the high‐permeability actively deforming layer and may destabilize this layer.
Temporal variability of radon concentration in the interstitial gas of soils in Pennsylvania
Authors: Washington John W. , Rose Arthur W.
Published: February 2008

In order to determine the extent and causes of radon variability in soil gas, repeated measurements of 222Rn, 220Rn, and soil properties including moisture, temperature, permeability, and diffusivity have been made at five sites in central Pennsylvania. Concentrations of 222Rn and 220Rn to a depth of 2 m varied temporally in an annual, approximately sinusoidal pattern having an amplitude of twofold to tenfold at all five sites. The existence of the annual pattern is independent of monitoring method, soil drainage, or bedrock parent material. Radon variability is caused by changes in soil moisture content and distribution; the action of soil water on Rn is classified into two distinct regimes: (1) At low and moderate moisture contents, diffusion occurs dominantly in air‐filled pores, and Rn is distributed between air and water approximately at equilibrium. Under these conditions Rn in soil air at depth increases with increasing moisture. (2) At high moisture contents diffusion occurs largely in water‐filled pores, and air/water equilibrium exists only near interfaces. Under these conditions, Rn in soil air can be low. Since Rn partitioning between gas and water is temperature sensitive and because soil moisture and temperature change in annual cycles, much of the variability in 222Rn concentration occurs in annual cycles. As a result, knowledge of regional and temporal soil moisture and temperature patterns allows estimates of 222Rn concentration in soil gas. In areas where the soil substrate has a significantly lower Ra concentration, emanation coefficient, or dry bulk density than the soil, the substrate can act as a Rn sink with respect to the soil. Under extreme conditions the [Rn] profile in the soil may actually display a concentration gradient toward the rock. The process of horizon formation during soil genesis significantly affects the vertical distribution of Ra, emanation coefficient and porosity, and therefore Rn production in soil.
On the relation between seismic anisotropy and finite strain
Authors: Ribe Neil M.
Published: February 2008

Because seismic anisotropy in the upper mantle is due primarily to lattice preferred orientation (LPO) of olivine crystals induced by deformation, observations of anisotropy can provide constraints on mantle deformation and flow. In this paper, we use a previously published theory for deformation‐induced LPO (Ribe and Yu, 1991) to quantify the relation between seismic anisotropy and deformation for two model compositions: a pure olivine (dunite) aggregate, and a harzburgite comprising 70% olivine and 30% enstatite. Analytical and numerical solutions of the governing equations show that the predicted LPO (and hence also the seismic anisotropy) depends only on the finite strain, and not on the deformation path by which that strain is produced. The magnitude of the anisotropy depends only on the ratios c1/c2 and c2 /c3, where c1 > c2 > c3 are the lengths of the principle axes of the finite strain ellipsoid, and the directions of maximum and minimum compressional wave velocity Vp coincide with the axes c1. and c3, respectively. We calculate the percent anisotropies for compresional and shear waves propagating along the three principle axes of the finite strain ellipsoid, as functions of the axial ratios c1/c2 and c2 /c3, for our two model compositions. However, inferences of the magnitude of finite strain obtained using these results are likely to be minimum estimates because our theory neglects the effects of dynamic recrystallization. Finally, we present, as a byproduct of this study a new direct method for calculating the finite strain produced by an arbitiary deformation history.
Subsidence history, crustal structure, and thermal evolution of the Valencia Trough: A young extensional basin in the western Mediterranean
Authors: Watts A. B. , Torné M.
Published: February 2008

The Valencia Trough is an extensional basin that began rifting during the late Oligoceneearly Miocene, yet it is located within a region of compression between the European and African plates. Seismic reflection, seismic refraction, well, gravity, and geoid data have been used to determine the relative importance of extensional and compressional processes in controlling the structure and evolution of the trough. Although the trough appears symmetric on bathymetric maps, seismic and gravity and geoid data indicate a fundamental asymmetry in its deep crustal and upper mantle structure. Two dimensional flexural backstripping techniques have been used to estimate the total amount of crustal stretching along a “transect” of the trough between the Catalan Coastal Ranges and Mallorca. There is good agreement between the predicted depth to the Mono based on a stretching model and the seismically constrained depth beneath the Catalan margin, but poor agreement beneath Mallorca. The seismic Moho beneath Mallorca can be explained by a model in which the crust was modified by the flexural effects of thrust/fold loading. Geoid data suggest that the differences in the crustal structure between the two margins extend into the upper mantle. The Catalan margin is characterized by a thermal and mechanical boundary, which marks the transition between cold, unstretched lithosphere and hot, thin lithosphere, while Mallorca is located above a broad region of lithospheric thinning. The subsidence history and amount of crustal thinning in the Catalan margin resemble that of typical Atlantic‐type nonvolcanic margin. However, a narrow transition zone (<60 km), the lack of any evidence for a “conjugate” margin, and the absence of oceanic crust suggest fundamental differences in the way that extensional processes may occur in convergent plate settings.
Hydromechanical modeling of tectonically driven groundwater flow with application to the Arkoma Foreland Basin
Authors: Ge Shemin , Garven Grant
Published: February 2008

Deep groundwater flow can be driven by several mechanisms in sedimentary basins. In the case of evolving foreland basins, large‐scale compression and thrusting could develop abnormally high pressures in the foreland sag that would initiate transient fluid flow. The so‐called tectonic “squeegee” effect is thought to have caused basin‐wide migration of ore‐forming brines and hydrocarbons (Oliver, 1986). Two‐dimensional numerical models are developed here to quantify the role of compressional tectonics in driving regional fluid flow in the later stages of thrusting in a foreland basin. Poroelasticity theory coupled with regional groundwater flow form the basic elements of the mathematical model. We use the mathematical model to predict deformation and pressure dissipation in the unfaulted and nonfolded part of a foreland basin in front of a thrust belt as it is subjected to an instantaneous loading event. Sets of numerical experiments show that overpressure zones develop along the leading edge of the thrust belt near the loading front. Stress‐induced flow rates of the order of centimeters to meters per year are possible soon after compression of the foreland, and transient flow fields dissipate in about 103 and 104 years. Longer transients can exist in very low permeability strata. Large overpressures may be unable to buildup under conditions of gradual thrusting, as fluid pressures may dissipate too quickly. The general features of tectonically driven flow are also explored through a sensitivity study to consider effects of permeability, fault and stratigraphic heterogeneity, loading magnitude, and variations in rock compressibility. The sensitivity study is based mostly on numerical experiments. As these solutions suffered from stability problems in cases where bulk rock compressibility exceeded 10−9 Pa−1, some simple scaling arguments are used to extend the numerical results for squeezing of soft shale. One basin‐specific application to the Ouachita orogen suggests that tectonic squeezing could have caused transient flow systems with relatively large flow velocities in basal Cambro‐Ordovician aquifers. The volume of fluid expelled, however, is probably only a small fraction of the total brine volume needed to have formed the huge Mississippi Valley‐type lead‐zinc ore deposits fringing the northern margin of the Arkoma Basin on the Ozark Uplift.
A comparison of phase screen and finite difference calculations for elastic waves in random media
Authors: Fisk M. D. , Charrette E. E. , McCartor G. D.
Published: February 2008

Phase screen and second‐order finite difference calculations of elastic wave propagation in two‐dimensional (2‐D) random media are compared to assess the accuracy and efficiency of the former method. The phase screen method is a forward propagation algorithm which depends only on the local S and P wave velocities. It differs from similar methods for scalar waves by treating P/S conversion. In its current formulation, the phase screen method does not treat modal propagation. Synthetic seismograms are generated by both methods at 640 evenly spaced receiver positions for identical realizations of 512 × 2750 velocity grid points. Comparisons are made for 2‐D random media characterized by exponential and zeroth‐order von Karman (self‐similar) autocorrelation functions of varying strength and correlation length. Constant and varying Poisson ratios are considered. Early arrivals compare more favorably since the phase screen method does not treat backscatter. The waveforms are compared by computing relative differences and cross correlations as a function of time offset. Temporal energy centroids of bandpass filtered synthetic seismograms are also computed and compared. Execution times are compared on a SUN 4/330, an ELXSI 6400, a Cray‐2, and an nCUBE parallel computer. The phase screen algorithm is roughly 2 orders of magnitude faster than the second‐order finite difference algorithm for 2‐D grids. An estimate, based on the number of operations in each algorithm, suggests that the phase screen method may be 3 orders of magnitude faster for comparable three‐dimensional problems.
Tsunami earthquakes: Slow thrust‐faulting events in the accretionary wedge
Authors: Pelayo Aristeo M. , Wiens Douglas A.
Published: February 2008

The November 20, 1960, Peru, October 20, 1963, Kurile and June 10, 1975, Kurile earthquakes are classified as tsunami earthquakes based on anomalously large tsunami excitation relative to earthquake magnitude. Long‐period surface wave analysis indicates double‐couple (faulting) mechanisms for all three events rather than single‐force mechanisms indicative of submarine landslides. The earthquakes have shallow depths (< 15 km) and are located near the trench axis and seaward of most other thrust zone events beneath the accretionary prism. Body waveform inversion indicates very shallowly dipping thrust faulting mechanisms for the three events, with dip angles of 6°–8°. Surface wave spectral amplitudes and deconvolution of SH waveforms suggests anomalously long source durations and large seismic moments relative to MS. Specifically, the 1963 Kurile event (MS 7.2) shows a duration of 85 s and a moment of 6.0 × 1027 dyn cm (MW 7.8), the 1975 Kurile event (MS 7.0) shows a duration of 60 s and a moment of 2.0 × 1027 dyn cm (MW 7.5), and the 1960 Peru event (MS 6.75) shows a time function consisting of four subevents with a total duration of 110–130 s and a seismic moment of 3.4 × 1027 dyn cm (MW 7.6). Estimated rupture velocities are about 1 km/s or less, but there is no evidence of unusually low stress drops. The August 1, 1968, Philippines event, previously classified as a tsunami earthquake, shows none of the anomalous source properties, and teleseismic tsunami height measurements are sparse; we do not consider this event a tsunami earthquake. Most of the “anomalous” tsunami excitation results from underestimation of earthquake size by MS due to the long source duration; the tsunami heights are not significantly anomalous relative to seismic moment. The slow nature of these events may result from rupture through the sedimentary rock along the basal decollement of the accretionary prism. Standard scaling laws when adjusted for the slow seismic velocity in the source region show an MW ‐ MS relationship similar to that observed for the tsunami earthquakes and predict MS saturation at about 7.3 rather than 8.0 for typical events.
Paleomagnetism of the Late Triassic Hound Island Volcanics: Revisited
Authors: Haeussler Peter , Coe Robert S. , Onstott T. C.
Published: February 2008

The collision and accretion of the Alexander terrane profoundly influenced the geologic history of Alaska and western Canada; however, the terrane's displacement history is only poorly constrained by sparse paleomagnetic studies. We studied the paleomagnetism of the Hound Island Volcanics in order to evaluate the location of the Alexander terrane in Late Triassic time. We collected 618 samples at 102 sites in and near the Keku Strait, Alaska, from the Late Triassic Hound Island Volcanics, the Permian Pybus Formation, and 23‐Ma gabbroic intrusions. We found three components of magnetization in the Hound Island Volcanics. The high‐temperature component (component A) resides in hematite and magnetite and was found only in highly oxidized lava flows in a geographically restricted area. We think it is primary, or acquired soon after eruption of the lavas, principally because the directions pass a fold test. The paleolatitude indicated by this component (19.2° ± 10.3°) is similar to those determined for various portions of Wrangellia, consistent with the geologic interpretation that the Alexander terrane was with the Wrangellia terrane in Late Triassic time. We found two overprint directions in the Hound Island Volcanics. Component B was acquired 23 m.y. ago due to intrusion of gabbroic dikes and sills. This interpretation is indicated by the similarity of upper‐hemisphere directions in the Hound Island Volcanics to those in the gabbro. Component C, found in both the Hound Island Volcanics and the Permian Pybus Formation, is oriented northeast and down, fails a regional fold test, and was acquired after regional deformation around 90 to 100 Ma. This overprint direction yields a paleolatitude similar to, but slightly higher than, slightly older rocks from the Coast Plutonic Complex, suggesting that the Alexander terrane was displaced 17° in early Late Cretaceous time. The occurrence of these two separate overprinting events provides a satisfying explanation of the earlier puzzling results from the Hound Island Volcanics (Hillhouse and Grommé, 1980). Finally, great‐circle analysis of the paleomagnetic data from the Pybus Formation suggests the Alexander terrane may have been in the northern hemisphere in Permian time.
Lithospheric necking and regional isostasy at extensional basins 1. Subsidence and gravity modeling with an application to the Gulf of Lions Margin (SE France)
Authors: Kooi Henk , Cloetingh Sierd , Burrus Jean
Published: February 2008

We present a numerical pure‐shear stretching model to study the effect of the “depth of lithospheric necking” on the state of flexure at extensional basins. This model avoids the need for low flexural rigidities during synrift basin subsidence. The model also accounts for long‐standing rift flank uplifts in the absence of significant thermal anomalies or underplating beneath rift shoulders, and it predicts low β factors beneath basin centers. Numerical modeling of synthetic continental rift zones and rifted continental margins together with their gravity characteristics demonstrates that isostatic residual anomalies are sensitive indicators of the state of flexure at extensional basins, particularly at basin margins. Isostatic residual anomalies are particularly useful to study the state of lithospheric flexure because the results are not biased by assumptions on the loads that have to be made in a forward modeling approach. Previous studies of lithospheric flexure and gravity modeling might have underestimated flexural rigidities at extensional basins. Comparison of predicted gravity anomaly signatures with gravity data suggests that at narrow rift basins and young rifted continental margins, upward flexure occurs more commonly than does downward flexure. This can be explained by intermediate or deep levels of necking (>15 km). For rifted continental margins, the apparent transition in the state of flexure during basin evolution from upward to downward flexure points to intermediate levels of necking (15–20 km). The occurrence of intermediate to deep levels of necking at narrow rift basins and intermediate levels of necking at rifted margins is in accordance with brittle‐ductile rheological models of the lithosphere. An intermediate depth range for the level of lithospheric necking at rifted continental margins is consistent with a scenario in which rifting preferentially occurs in an area where the lithosphère is weakened by a thickened crust Rifting in areas of stronger lithosphere, as for example cratons, is probably more often associated with a deep level of necking, explaining the widespread occurrence of rift shoulders at failed rift basins. We have tested several depths of necking and a model of local isostasy for the Gulf of Lions margin in the northwestern Mediterranean. At positions of deep synrift grabens, models with depths of necking of 25–35 km predict crustal thicknesses that are more in accordance with observations than are models with a shallower level of necking. It appears that local isostasy cannot account for the present‐day basin configuration and at the same time reproduce the observed strongly laterally varying bathymetry at the end of rifting, supporting a significant postrift flexural rigidity.
Fine structure of methane hydrate‐bearing sediments on the Blake Outer Ridge as determined from deep‐tow multichannel seismic data
Authors: Rowe M. M. , Gettrust J. F.
Published: February 2008

High‐resolution, deep‐tow multichannel seismic data are used to investigate the detailed structure of sediments containing methane hydrate. These data support thick, laterally extensive layers of methane hydrate‐bearing sediment underlain by a bottom simulating reflector (BSR) and spatially discontinuous zones of hydrate within the sediments above the BSR depth where no BSR is present. These data resolve normal faults which extend from the surface through the BSR with apparent offsets of up to 20 m. A phase inversion identified at the top of the BSR shows that the material immediately beneath the BSR has anomalously low velocity, consistent with a layer of sediment containing free methane gas. The fault offsets along the BSR are consistent with a pressure change of ∼200 kPa (∼2 bars) across the fault zone.
Attenuation of high‐frequency seismic waves beneath the central Andean Plateau
Authors: Whitman Dean , Isacks Bryan L. , Chatelain Jean‐Luc , Chiu Jer‐Ming , Perez Alejandro
Published: February 2008

Observed patterns of high‐frequency seismic wave attenuation suggest that near 22°S, the upper mantle structure beneath the central Andean plateau changes along strike to the south. Contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of P and S waves propagating beneath the plateau to a portable seismic network deployed in Jujuy Province, Argentina (24°S, 65°W). Ray paths from intermediate depth earthquakes located north and northwest of the network transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network. The estimated apparent Q values fall into two categories: Qp >500 and Qs >350 for the high‐Q paths, and Qp <350 and Qs < 200 for the low‐Q paths. In addition, Sn phases from regional crustal earthquakes in the Subandean foreland fold‐thrust belt to the north propagate efficiently to the Jujuy network, while Sn is not observed from foreland earthquakes located at similar distances to the south of the network. These observations combined with data from previously reported wave propagation studies suggest that south of about 22° S, the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north. Forward modeling of the Q measurements made at Jujuy indicates that the observations can be explained either by a variable thickness high‐Q upper plate beneath the plateau, or by a thin, variable width, very low‐Q zone in the asthenospheric wedge above the subducted slab. This lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central Andean plateau and its adjacent foreland thrust belt to the east: the Bolivian Altiplano and Subandean ranges in the north and the Argentine Puna and Santa Barbara system in the south. We interpret the north‐south change in upper mantle attenuation and the corresponding changes in physiography, topography, and tectonic style at the surface to be due to a mantle lid that is thicker beneath the Altiplano and the Subandean belt than beneath the Puna and the Santa Barbara ranges.
Seismic and electrical properties of sandstones at low saturations
Authors: Knight Rosemary , Dvorkin Jack
Published: February 2008

Laboratory measurements of elastic wave velocities, attenuation, and electrical resistivity of sandstones all show distinct changes in the measured properties at very low levels of water saturation. Velocity decreases, attenuation reaches a maximum, and electrical resistivity decreases over a limited range in saturation. We assume that all the water at low saturations is present in the rock as a layer of uniform thickness covering the entire internal surface area of the sample. With this model, we calculate the thickness of surface water that corresponds to the distinct low saturation region. In all cases, for both the seismic and the electrical data, the thickness is equal to one to four monolayers of water. This is interpreted as marking the transition in the behavior of water in a sandstone from that of a surface adsorbed phase to a bulk water phase. The changes in seismic properties can be modeled by considering the effect of adding surface and bulk water to grain contacts. In a similar treatment, the role of contacts is shown to be a dominant factor in the electrical resistivity of sandstones at low saturations, such that the addition of water to the contact area causes a dramatic decrease in the electrical resistivity.
Plio—Pleistocene time evolution of the 100‐kyr cycle in marine paleoclimate records
Authors: Park Jeffrey , Maasch Kirk A.
Published: February 2008

To constrain theories for the dynamic evolution of global ice mass through the late Neogene, it is important to determine whether major changes in the record were gradual or rapid. Of particular interest is the evolution of the near 100‐kyr ice age cycle in the middle Pleistocene. We describe and apply a new technique based on multiple taper spectral analysis which allows us to model the time evolution of quasi‐periodic signals. This technique uses both phase and amplitude information and enables us to address the question of abrupt versus gradual onset of the 100‐kyr periodicity in the middle Pleistocene. We analyzed three long (> 2.6 m.y.) time series from Deep Sea Drilling Project (DSDP) site 607 (midlatitude Atlantic) and Ocean Drilling Program (ODP) site 677 (equatorial Pacific). The published time scales for these sites differ significantly, influencing the interpretation of the mid‐Pleistocene climate transition. When the data series are expressed on the same time scale, there is evidence for a coherent δ18O signal at both sites in the eccentricity and obliquity frequency bands, consistent with variations in global ice volume as the causative factor. If the Shaddeton et al. (1990) time scale for ODP 677 is accepted, the amplitude match between the δ18O obliquity cycle and the 65°N insolation derived from the recent astronomical solution of Laskar (1988, 1990) is excellent for times t≲2.3 Ma. If the Ruddiman/Raymo time scale for DSDP 607 is accepted, the 41‐kyr δ18O cycle has enhanced amplitude between 1.0 and 1.5 Ma, relative to the late Pleistocene (t < 1.0 Ma), implying a nonlinear Earth system response to obliquity insolation cycles. We do not find compelling evidence for an abrupt change in the 100‐kyr δ18O signal, although we cannot rule it out at this time. If the Shackleton et al. (1990) time scale for ODP 677 is accepted, our three δ18O records are consistent with a low‐amplitude 100‐kyr cycle between 1.2 and 2.6 Ma, whose local period of oscillation alternates between the 95‐ and 124‐kyr eccentricity periods, and a gradual increase in the 100‐kyr δ18O signal between 0.5 amd 1.0 Ma. The DSDP 607 time scale is more favorable to an abrupt jump in amplitude for the 95‐kyr δ18O envelope, but not in the 124‐kyr envelope. Rather, long‐period δ18O fluctuations appear phase‐locked with the 124‐kyr eccentricity cycle some 300–400 kyr prior to its growth in amplitude and phase‐lock with the 95‐kyr eccentricity cycle in the late Pleistocene.
Modeling radon transport in dry, cracked soil
Authors: Holford Diana J. , Schery Stephen D. , Wilson John L. , Phillips Fred M.
Published: February 2008

A two‐dimensional finite element code was used to investigate the effect of changes in surface air pressure on radon flux from soil with parallel, partially penetrating cracks. A sensitivity analysis investigates the effects of various crack dimensions, soil characteristics, and surface air pressure on radon flux from the soil surface to the atmosphere. Simulation results indicate that radon flux is most sensitive to soil properties; the diffusion coefficient is most important, followed by permeability and porosity. Radon flux is also sensitive to changes in barometric pressure, which cause variations in radon flux above and below the average diffusive flux. Sinusoidal variations in barometric pressure cause a net increase in the average radon flux from the soil, because increases in flux during periods of decreasing pressure are greater than the decreases in flux during periods of decreasing pressure of equal magnitude. Cracks were found to significantly increase radon flux from soils of low permeability.
Three‐axis static alternating field demagnetization of rocks and the identification of natural remanent magnetization, gyroremanent magnetization, and anisotropy
Authors: Stephenson A.
Published: February 2008

A theoretical basis is given for two empirically derived methods of three‐axis alternating field (AF) demagnetization of static samples used initially by Zijderveld and Dunlop to remove “spurious” components of magnetization produced by the AF. It is shown theoretically that both methods of treating the experimental results to reveal the natural remanent magnetization (NRM) vector are precisely those which algebraically eliminate gyroremanent magnetization (GRM). By implication, the “spurious” remanences found by these and other workers are almost certainly of gyromagnetic origin and not remanences produced as a result of shortcomings in instrumentation. It is further demonstrated that by suitably treating the data obtained by either three‐axis method, GRM can be identified and used to provide information about the anisotropy of the rock.