bluuebunny/crossref_metadata_2025 huggingface dataset:
Neptune's magnetic field: Calculation of field geometric invariants derived from the I8E1 GSFC/BRI model
Authors: Acuña M. H. , Connerney J. E. P. , Ness N. F.
Published: February 2008

A simplified description of the Neptunian magnetic field derived from the Goddard Space Flight Center/Bartol Research Institute I8E1 model which includes internal terms up to and including the octupole (but no external terms) has been used to compute numerically L shell values along the Voyager 2 encounter trajectory and those associated with the N1 through N6 moons and N1R through N6R rings. The complex magnetic field geometry and the rapid rotation of the planet combine to produce an expectation of numerous times when absorption signatures should be observed as a result of the interaction between the trapped particle population and the Neptunian moons and rings. Thus a complete understanding of the observed charged particle fluxes and their variation along the Voyager 2 trajectory must take into account these geometrically anticipated interactions. Similar to Uranus, the large tilt between the dipole term and the rotation axis causes the moons and rings to sweep a very large range of L shells. Furthermore, their orbital motion introduces additional periodicities, causing the maxima and minima in L space to vary systematically with time. The unambiguous association of predicted and observed absorption signatures and times with a particular ring or moon is made very difficult by the superposition of many individual effects. The extremely rapid excursion in L space by Voyager 2 near closest approach, a region most significant for magnetic field models, apparently does not facilitate the unique associations previously used at other planets to provide an independent assessment of the global accuracy of the mathematical models of the field which are inherently nonunique.
Thermospheric tides simulated by the National Center for Atmospheric Research Thermosphere‐Ionosphere General Circulation Model at equinox
Authors: Fesen C. G. , Roble R. G. , Ridley E. C.
Published: February 2008

Simulations of the equinox thermospheric structure by the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) are presented. Calculations were made for both solar cycle minimum and solar cycle maximum under geomagnetically quiet conditions. Emphasis here is on the tidal structures at geographic longitude 70°W. Comparisons are made with observations and empirical model predictions of the thermospheric tides from the extended mass spectrometer and incoherent scatter radar model (MSIS) (Hedin, 1991) and the harmonic wind model (HWM) (Hedin et al., 1991). In general, the TIGCM tidal simulations are broadly consistent with available data. Agreement is best for the neutral temperatures and worst for the zonal winds. Investigation of the latitudinal structure of the tidal meridional velocities and temperatures near 300 km reveals important differences between the TIGCM and the empirical MSIS and HWM models. For example, the TIGCM predicts nearly a threefold increase in the diurnal temperature amplitude from solar cycle minimum to maximum with little latitudinal variation in the amplitudes from the equator to about 50°. The empirical MSIS model predicts a somewhat smaller variation of the the diurnal amplitudes over the solar cycle with the amplitudes decreasing with latitude. Resolution of the discrepancies requires acquisition of simultaneous observations from widely separated locations. Ideally, observations would be made in both the lower and upper thermosphere in order to help determine the contribution of the tides excited in the lower atmosphere to the thermospheric structure.
Pioneer Venus and IMP 8 observations of the latitude dependence of the solar wind
Authors: Gazis P. R.
Published: February 2008

Solar wind speeds and magnetic field data from the Pioneer Venus orbiter (0.7 AU) and IMP 8 spacecraft (1 AU) are compared to infer the latitudinal structure of the solar wind in the inner heliosphere between 1984 and 1987. It is found that solar wind streams and the interplanetary magnetic field (IMF) exhibited an unusual configuration during the last solar minimum (1986‐1987). This configuration only appeared during a limited period around the time of solar minimum, and was similar to the configuration that appeared during the previous solar minimum in 1975. Prior to solar minimum, the solar wind speed did not appear to vary with heliographic latitude, and the same high‐speed streams appeared at both spacecraft even when they were widely separated. The IMF had a two‐sector structure at all latitudes. During solar minimum, this situation changed, and high‐speed streams were excluded from low (<3°) latitudes, while the vicinity of the solar equator was characterized by slow solar wind and irregular variations in speed. The sector structure of the IMF also changed during solar minimum, and a single polarity predominated at latitudes greater than 4°. The onset of these changes appears to be related to a decrease in the inclination of the coronal neutral line; from approximately 40° at the end of 1984 to approximately 10° in the beginning of 1986. This is consistent with suggestions by previous observers that the coronal neutral line is associated with a sheet of low‐speed solar wind without high‐speed streams.
Micromechanics of rock friction 2: Quantitative modeling of initial friction with contact theory
Authors: Boitnott G. N. , Biegel R. L. , Scholz C. H. , Yoshioka N. , Wang W.
Published: February 2008

A constitutive model is developed which predicts the mechanical properties of two rough surfaces in contact under shear load during the early stages of the development of frictional sliding. The model includes the development of slip at the contacts, a phenomenon which begins immediately upon shear loading. Upon initial application of the normal load, the model predicts that the joint consists of a finite number of contacts which are subject to a wide variety of local normal loads. As the shear load is increased, sliding of the contacts develops progressively, with the contacts under low local normal load sliding first. This gradual development of sliding is the cause of the experimentally observed nonlinear force‐displacement relation for deformation of the joint in shear. Two asperity scale strength laws are examined, one based on the adhesion theory of friction and the other based on observations of frictional strength for brittle elastic solids. The model is tested with experiments on lapped surfaces of Westerly Granite with a variety of surface roughnesses and under a range of normal loads from 10 to 35 MPa. Geometric parameters used in the model are constrained by direct measurement of surface profiles. For both strength laws, the model quantitatively predicts the shear compliance and development of slip for the first few microns of shear displacement, successfully describing the effect of surface roughness and normal load. This model helps explain the initial yield in the friction curve which corresponds to the gradual transition from the elastic deformation and partial slip of asperity contacts to a condition of fully sliding contacts. When a large population of contacts are fully sliding, the model under‐estimates the frictional strength, indicating that displacement strengthening mechanisms are important to the ultimate frictional strength of rocks.
Lithospheric structure of northern California from teleseismic images of the upper mantle
Authors: Benz H. M. , Zandt G. , Oppenheimer D. H.
Published: February 2008

Teleseismic P wave travel time residuals from 120 earthquakes recorded across the U.S. Geological Survey California seismic network were used to determine the lithosphere P wave velocity structure beneath northern California, a region characterized by complex interactions between the Pacific, North American, and Gorda plates. Lateral P wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals. Inversion results for the crust show strong correlations to volcanic features. The active volcanic fields, Shasta‐Medicine Lake, Lassen, and Clear Lake, are characterized by crustal low‐velocity anomalies that average approximately −6%, possibly identifying partially molten magma bodies. Cooled, solidified magma bodies beneath the extinct volcanic fields, Sonoma, southern Clear Lake, and Sutler Buttes, are denoted by relative velocity highs averaging +3%. The largest upper mantle velocity variations occur in the depth range 30–110 km, where velocities vary from −5.5% to +9.5%. These velocity variations reflect changes in the thickness and geometry of the Pacific, North American, and Gorda plates where they interact at the Mendocino Triple Junction. North of the Mendocino Triple Junction, the steep 70° east dipping portion of the Gorda plate is imaged as a +5% velocity high to depths near 270 km. A presumed segment of the Gorda plate, observed beneath the northern Great Valley and south of the inferred edge of the plate, is characterized by a +9% velocity high in the depth range 30–70 km. Beneath the northern Coast Ranges, shallow asthenosphere is imaged in the depth range 30–100 km as a pronounced southward tapering −4% low‐velocity zone, which we interpret as the slab window. Results from this study provide improved constraints on Gorda plate subduction, evolution of the San Andreas fault system, and development of the lithosphere beneath western North America.
Evaluation and prediction of shear wave velocities in calcareous marine sediment and rocks
Authors: Wilkens R. H. , Cheng C. H. , Meredith J. A.
Published: February 2008

Shear wave velocities from full waveform acoustic logs recorded in calcareous oozes and chalks were determined using the spectral ratio inversion method. Data were recorded during DSDP Leg 95 drilling off the east coast of the United States. DSDP Site 613 sampled a progression of carbonaceous‐siliceous oozes through partially lithified chalk. Discrete shear wave velocities within a 350 meter interval were calculated. The results of a method for predicting shear wave velocities using Wood's equation correlate well with inversion results at shear wave velocities greater than 800 m/sec. Predictions using Wood's equation provide a theoretical maximum value for shear wave velocity to compare with inversion results. Inverted data fall just below the predicted theoretical maximum value from Wood's equation and agree quite well with trends in Wood's results and other related logging measurements. At slower shear wave velocities the physical contrast between borehole fluid (drilling mud) and the formation (ooze) is not enough to excite normal borehole modes used in inversion processing, resulting in inversion velocities greater than Wood's maximum.
Horizontal lithosphere compression and subduction: Constraints provided by physical modeling
Authors: Shemenda Alexander I.
Published: February 2008

Physical modeling of the subduction is performed with a two‐layered mantle model (elastico‐plastic lithosphere and low‐viscosity asthenosphere) and is governed by the criteria of similarity. Compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average. Later, a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting. The plate then experiences a failure along the inclined zone, and subduction starts. The inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section. If there is an old inclined fault striking across the compression of an oceanic lithosphere, it is on that fault that a subduction zone is initiated. The inner trench slope has then a different structure and forms, due to normal faulting, in the frontal part of the overriding plate. The formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3. The subducting plate experiences an elastico‐plastic bending and (under specific conditions) thrusting along the zone, dipping from under the overriding plate oceanward and crossing the entire lithosphere. The best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit τs = 1.3 × 108 Pa, modulus of elasticity E about a few times 1011 Pa, and a thickness H = 60 km, are adopted for the real lithosphere.
Nucleation and growth of dip‐slip faults in a stable craton
Authors: Wallace Michelle H. , Kemeny John
Published: February 2008

We analyze the conditions for nucleation of a dip‐slip fault in intact crust and its subsequent growth using linear elastic fracture mechanics and the finite element method. We assume a fault can be modeled as a mode II shear crack in a layered elastic crust, and we investigate fault behavior under conditions of a variable dip and shear fracture energy. Two shear fracture energy gradients,Gc, were evaluated; both have surface values of 1 × 105J/m2and increase linearly to values of 1 × 106or 1 × 107J/m2at 15 km depth, the base of the seismogenic zone. We assume that the vertical and horizontal components of the gravitational stress are both equal to the lithostatic load, effectively removing them from the calculation of shear stress on the fault Strain required for failure varied with respect to shear fracture energy gradient and rupture direction but not with dip. Rupture upward from the base of the seismogenic zone required more than twice the initial strain to cause fault growth and would result in very large earthquakes. Rupture downward in models associated with the higherGcgradient resulted in stable growth of the fault; that is, additional strain was needed to allow for continuous fault growth from the surface to the base of the seismogenic zone. We suggest that fault nucleation and growth are a stable process which would preferentially occur by rupturing downward under stable conditions. Rerupturing of an established fault may initiate at depth or the surface depending on the degree of healing between rupture events and the tectonic stress available. A vast majority of shallow crustal earthquakes nucleate at the base of the seismogenic zone and rupture upwards. This suggests that most events are a reactivation of an old established fault and not growth of a new or developing fault.
Velocity structure near the inner core boundary from waveform modeling
Authors: Song Xiaodong , Helmberger Don V.
Published: February 2008

A velocity model of the inner core boundary (ICB) region is developed from broadband wave‐form modeling. The data consist of long period and short period record sections gathered from the U.S. arrays, WWSSN, and LRSM. Deep events beneath Indonesia were used; these sampled the ICB structure beneath the North Pacific. The Cagniard‐de Hoop technique was used to generate the synthetics, which allowed considerable flexibility in forward modeling these individual PKP phases. Our preferred model was developed from modifications of the Preliminary Reference Earth Model (PREM). It contains a relatively low velocity gradient above the ICB, a sharp jump at the ICB of about 0.78 km/s, and a relatively large gradient underneath. These features are constrained largely by the differential travel times and amplitude ratios of PKIKP and PKiKP of the short period section. Further modification of PREM in the fluid core was required to explain the long‐period observations associated with PKP waveforms. Our modeling results suggest a low‐velocity gradient at the bottom (roughly 400 km) of the fluid core. A low Q zone at the top of the inner core is required to fit the relative amplitudes of PKIKP phases similar to earlier studies. This model indicates possible inhomogeneity above the ICB, as suggested by other authors.
Crustal velocities from geodetic very long baseline interferometry
Authors: Fallon F. W. , Dillinger W. H.
Published: February 2008

We have used very long baseline interferometry observations from the International Radio Interferometric Surveying and Crustal Dynamics Project programs taken over a time span of 5–8 years (through August 1990) to derive relative velocities of 16 sites on the North American, Eurasian, Pacific, and African plates. The data reduction scheme simultaneously estimates Earth orientation parameters and nutation for each session, local atmosphere and clock correction terms, source positions, and initial site positions, as well as the site velocities. No a priori geophysical crustal model whatsoever is imposed to obtain the velocities. Instead we introduce a minimal set of geometric constraints to obtain the solution. Two alternative constraint formulations are considered; they are shown to be equivalent in that they yield equivalent velocity sets with allowance for translation and rotation. These are (1) setting the secular motion of the pole and mean length of day to fixed values and (2) fixing the net rotation of the sites. The resulting velocities have formal standard errors typically <0.2 cm/yr, and most velocities are significantly different from zero. They agree closely to within 0.5 cm/yr, with velocities predicted by the Minster‐Jordan and NUVEL‐1 plate motion models, after allowing for the arbitrary rotation and translation between the systems. The velocity of Kashima, Japan, is much more consistent with the North American plate than the Eurasian plate where current plate model boundaries place it. We suggest another possibility: Kashima does lie on the Eurasia plate, but its motion is so modified by interaction with the nearby Pacific plate as to result in the observed velocity. Sites near the active plate boundary in California also show motions intermediate between those of the adjoining plates.
A study of the contributions of various gravimetric data types on the estimation of gravity field parameters in the mountains
Authors: Tziavos I. N. , Sideris M. G. , Schwarz K. P.
Published: February 2008

In this paper, new rigorous, simpler, and more efficient formulas are used to estimate the effect of the terrain on geoid undulations and deflections of the vertical. The new formulas are based on the successive application in the spectral domain of Molodensky's vertical derivative operator to powers of heights. A number of numerical tests are carried out in the area of British Columbia (BC) in western Canada. Geoid undulations and deflections of the vertical are computed by combining a geopotential model complete to degree and order 360, mean 5 arc min × 5 arc min free air gravity anomalies and height data on a 1 km × 1 km grid. In geoid undulation computations, the indirect effect of Helmert's second condensation reduction is taken into account. To assess the quality of the predicted deflections of the vertical and geoid undulations, the computed quantities are compared to a set of astronomic deflections of the vertical and geoid undulations derived from a combination of Global Positioning System with leveled orthometric heights. Test results present a precision of about ±2.0 arc sec for deflections, thus fulfilling the requirements for the reduction of geodetic measurements in any case. The achievable precision for geoid undulation differences, close to ±0.50 m, is also acceptable to determine an accurate relative geoid in the test area useful for various geodetic applications.
Global positioning system measurements of strain accumulation across the Imperial Valley, California: 1986–1989
Authors: Larsen Shawn , Reilinger Robert
Published: February 2008

Global Positioning System (GPS) data collected in southern California from 1986 to 1989 indicate considerable strain accumulation across the Imperial Valley. Displacements are computed at 29 stations in and near the valley from 1986 to 1988, and at 11 sites from 1988 to 1989. The earlier measurements indicate 5.9 ±1.0 cm/yr right‐lateral differential velocity across the valley, although the data are heavily influenced by the 1987 Superstition Hills earthquake sequence. The 1988–1989 GPS displacements are best modeled by 5.0±0.9 cm/yr of valley crossing deformation, but rates calculated from 15 years of terrestrial geodetic measurements (3.7 ± 0.1 cm/yr) fit the data nearly as weh1. The relatively high GPS rates are suspect because some measurements, especially the east‐trending displacements, have large errors. There is evidence from GPS and very long baseline interferometry observations that the present strain rate along the southern San Andreas fault is smaller than the long‐term geologic estimate, suggesting a lower earthquake potential than is currently assumed. Correspondingly, a higher earthquake potential is suggested for the San Jacinto fault.
The effects of fluid escape on accretionary wedges 2. Seepage force, slope failure, headless submarine canyons, and vents
Authors: Orange Daniel L. , Breen Nancy A.
Published: February 2008

The high pore pressure gradients inherent to accretionary complexes affect the force balance of the wedge via seepage force, which acts in the direction of flow and is proportional to the pressure (head) gradient. If sufficiently large, this seepage force can offset gravity and friction and lead to failure. At the toe of the wedge sediments are weak, slopes are over‐steepened by folding and faulting, and fluid pressure gradients can be high; these conditions are conducive to seepage‐induced failure. For the 14–16° slope at the toe of the southern Cascadia wedge, the pore pressure gradient necessary to initiate failure is λ=0.74–0.86. The gradient necessary to cause failure is sensitive to surface slope and sediment strength, but is insensitive to porosity. Reasonable estimates of sediment strength for most accretionary wedges require pore pressure gradients ranging from 10 to 60% of lithostatic to cause failure. These values are within the range of modeled and measured pore pressures in accretionary complexes, suggesting that seepage‐induced slope failure should be an expected feature in this environment. If these failure features are observed, then their presence can be used to constrain the pore pressure gradient within the wedge, independent of any assumptions regarding fluid discharge or permeability. If seepage failure repeats and is localized in the same region, then it can lead to channel, gully, and canyon formation. Two convergent margins, southern Cascadia and northern Hispaniola, show many regularly spaced headless canyons that cannot be attributed to downslope erosive flow. We suggest that these canyons are forming from internally driven seepage‐induced failure. Both the Oregon and Hispaniola accretionary wedges also contain evidence for non‐uniform fluid flow based on the observed and inferred presence of vents. Using Darcy's Law, the pore pressure constraint from the slope failure analysis and an estimate of the total fluid discharge, we examine the relationship between wedge and vent permeabilities, the areal extent of focused fluid venting, and the percent of the total fluid discharge that flows out of vents. Given reasonable estimates of the total fluid discharge out of the southern Cascadia wedge, we find that the wedge must be less permeable than 2 × 10−17m2in order for focused fluid venting to occur at all. If the permeability of the vents is much higher than the wedge permeability, then the vents will occur over a very small percentage of the wedge; these vents, however, could accommodate much of the fluid flowing out of the wedge. Using permeability measurements from samples collected at the toe of the Oregon margin [Horath, 1989], we estimate that vents at the toe of the southern Cascadia accretionary complex comprise less than 0.2% of the wedge area, but that these vents can accommodate up to 60% of the total fluid discharge.
Small‐scale heterogeneity and large‐scale velocity structure of the continental crust
Authors: Levander Alan R. , Holliger Klaus
Published: February 2008

Since Conrad [1925] first identified a midcrustal refraction event in central Europe seismologists have developed layered models of the continental crust with velocity steps at various levels. Modern analysis of crustal refraction data, including automated inversion, relies on ray theoretical techniques or wave theory for plane layered media, which generally parameterize the Earth as a series of grossly horizontal layers. The bias toward layered models is in part based on physical and geologic intuition, but it is to a large measure a result of available interpretation methods, for example layer‐based reflectivity and ray tracing methods. In contrast, reflection seismology has produced a picture of a highly heterogeneous crust, with wavelength‐scale variations in velocity existing at different levels of the crust in different tectonic regimes. Finite difference modeling shows that zones of small amplitude random velocity fluctuations not only produce short discontinuous reflection segments as observed on deep seismic reflection data but also produce strong wide‐angle reflections. Some of the events identified as crustal wide‐angle reflections in field data may in fact be the result of scattering from wavelength‐scale heterogeneities and need not result from reflection from a first‐ or second‐ order velocity discontinuity. Significant changes in the large‐scale velocity structure associated with zones of random velocity fluctuations produce relatively small changes in the dynamic properties of the backscattered wave field. This observation is in disagreement with the common credo that the seismic refraction method is primarily sensitive to changes in the large‐scale velocity structure. As a consequence, the ray theoretical interpretation of such wide‐angle reflections as first‐order discontinuities leads to crustal velocity structures that are nonunique and may be erroneous in detail, while correctly predicting average crustal velocity and thickness. We are not suggesting that crustal velocity does not change with depth on the large‐scale; rather, we are proposing that crustal seismic events observed at intermediate and large offsets may be the result of scattering from wavelength‐scale velocity fabric rather than specular reflection or refraction from first‐order discontinuities.
Styles of Eruptive Activity on Intraplate Volcanoes in the Society and Austral Hot Spot Regions: Bathymetry, Petrology, and Submersible Observations
Authors: Binard N. , Hékinian R. , Cheminée J. L. , Stoffers P.
Published: February 2008

Submersible observations, carried out in the Society and Austral hot spot regions (South Pacific) during the Teahitia II cruise (1988–1989), enable us to propose a model relating The style of volcanism to the depth of eruption on large intraplate seamounts. (1) The bases of volcanic edifices are characterized by broad flat flows formed during a high rate of discharge of a very fluid magma on the sea floor. (2) The edifices' flanks deeper than 500 m are mainly made up of bulbous, tubular pillows and large drained lava tunnels occurring on slopes steeper than 60°–70°. These lava flows occur on lateral volcanic vents, about 200–300 m high, along linear rift zones which are formed on the flank of the major edifices. (3) Volcanic ejecta due to hydromagmatic eruptions occur at shallow depths (<500 m). Mixed explosive and quiet eruptions are marked by volcanic ejecta interbedded with pillows and intrusives (sills and dikes). (4) Volcanic shards, produced by the shattering of the vesiculated glassy margins of the flows, are observed on the edifices' slopes at all depths (from 3000 m depth to the sea surface), concentrated between pillows or covering large areas, and forming thin indurated layers of mixed shard, sediment, and hydrothermal deposits. On the basis of their morphological appearances, the volcanic edifices scattered on the hot spot‐affected seafloor are divided into two groups: (1) The small edifices (less than 500 m tall) consist of conical, linear, and truncated circular abyssal hills made up of old mid‐ocean ridge basalt (MORB) type basalts (depleted in K2O) inherited from volcanic activity near the East Pacific Rise and more recent conical vents made up of alkali‐enriched lavas extruded during “hot spot” magmatism. (2) The larger edifices (>500 m in height) are individual conical features built at a high rate of volcanic discharge localized along radial linear eruptive fissures called rift zones. The slopes located between the rift zones are covered by talus material are limited, and unimportant mass‐wasting (sector collapse) features were observed on the edifices. The conical‐shaped features are the primary morphology of the intraplate volcanoes formed during edifice growth which is closely linked to the hydrostatic pressure of the magmatic column. The mean values of the slopes (11°–17°) of intraplate seamounts are the same as those of subaerial intraplate shield volcanoes, when the seawater hydrostatic pressure is taken into account.
Climate change inferred from analysis of borehole temperatures: An example from western Utah
Authors: Chisholm Timothy J. , Chapman David S.
Published: February 2008

Temperature‐depth profiles measured in a suite of boreholes in western Utah are used to infer climate change in the region over the past century and to document how effectively the solid earth records secular changes in surface air temperature. The data for this analysis consist of (1) high‐resolution temperature logs from six sites where terrain, hydrologic, and cultural disturbances to the temperature field are minimal, (2) surface air temperature records for the period 1891–1990 from seven meteorological stations geographically interspersed with the borehole sites, and (3) ground and air temperature records available from four weather stations. Deviations from linear temperature‐depth profiles in the boreholes, interpreted in terms of a linear change of surface temperature with time, suggest changes of −0.8°C to +0.6°C (average +0.3°C) in surface temperature for western Utah over the last several decades. These changes are consistent in trend but smaller in amplitude than the 100‐year linear trends in the surface air temperature data (average +0.8°C) for the same region. If the last 100 years of surface temperature change is assumed to be given by a nearby meteorological station record, then borehole temperatures yield additional information about the mean temperature prior to 1891. For three western Utah borehole sites the pre observational means are within ±0.3°C of the mean air temperatures for this century, indicating that up to 50% of the temperature increase seen in the 100 year record constitutes recovery from a cold period toward the end of the last century. The veracity with which our observed borehole temperature profiles match synthetic temperature‐depth profiles computed from air temperature records leaves little doubt that the solid earth is a valuable recorder of climatic change.
Structure of axisymmetric mantle plumes
Authors: Olson Peter , Schubert Gerald , Anderson Charles
Published: February 2008

Results of high‐resolution numerical calculations of axisymmetric thermal plumes in an infinite Prandtl number fluid with thermally activated viscosity are used to infer the structure of axisymmetric subsolidus thermal plumes in the Earth's lower mantle. We calculate the velocity and temperature distribution for axisymmetric convection above a heated disk in an incompressible fluid cylinder, 2400 km in height and 1200 km in diameter, initially at a uniform temperature T∞. The lower boundary, representing the plume source region near the core‐mantle boundary, is stress‐free and maintained at temperature Tc> T∞. The upper boundary is permeable, allowing the plume to escape the cylinder. We determine the sensitivity of plume structure to viscosity variations η∞/ηc ranging from 102 to more than 104 and temperature differences ΔT = Tc ‐ T∞. in the range 400–850 K. Starting plumes consist of a large leading diapir and a trailing conduit connected to the thermal boundary layer above the heated surface. The leading diapir expands during ascent, to about 400 km diameter at 2200 km height. In steady state, the plume consists of a narrow, high‐velocity conduit imbedded within a broader thermal halo. The width of both these structures is proportional to height above the heated boundary and inversely proportional to ΔT and the viscosity ratio. Heat transport in the plume is nearly independent of viscosity variations. A simplified boundary layer model yields a heat transfer law, Nu ≈ Ra⅓ where Nu is the plume Nusselt number and Ra is the Rayleigh number based on, ΔT, η∞ and the radius of the heated circular region feeding the plume. Calculated steady state plume heat transports fit this law to within a few percent. We present several calculations of plumes with heat transport in the range 100–400 GW, similar to the advective heat transport by the Hawaiian hotspot. At the top of the lower mantle, plumes with large viscosity variations have high‐velocity conduits 50–60 km in diameter and 125‐km‐diameter thermal halos, a centerline temperature anomaly of about 500 K and a centerline velocity of about 75 cm yr−1. Plumes with weak viscosity variations are approximately twice as broad and have centerline temperature anomalies less than 160 K at the same height and much smaller velocity. Large viscosity variations and a minimum heat transport are both
On the strength of oceanic fracture zones and their influence on the intraplate stress field
Authors: Bergman Eric A. , Solomon Sean C.
Published: February 2008

We use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere. If fracture zones are weaker than the surrounding lithosphere, they should be sites of concentrated intraplate seismicity, and the lithospheric stress field should be perturbed in their vicinity. The 77 earthquakes selected for the study have well‐determined focal mechanisms and epicenters in regions where fracture zones are well mapped. We have searched for dependence of faulting style, fault orientation, or principal stress direction on the distance from the nearest fracture zone. If fracture zones were generally weaker than the surrounding lithosphere, one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone; we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner. There is a slight tendency for earthquakes to occur near fracture zones, and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress. These patterns are not demonstrable at high significance, however. If oceanic transform fault zones are weak relative to the surrounding lithosphere, these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere.
Cross correlation of the variations of the geomagnetic dipole moment and the fluctuations of Earth's rotation
Authors: Jin Rong‐sheng
Published: February 2008

The origin of the main geomagnetic field is related to the current in the fluid core of Earth. A close relationship between the fluctuations of the rotation of Earth and the secular changes of the geomagnetic field is suspected. Two‐channel maximum entropy spectral analyses are applied to the dipole moments of Earth from 1901 to 1980 and the rotational rate fluctuations from 1890 to 1987 in this study. The common spectral peaks are found with periods of 51, 34, 22, 13, and 9.5 years. The values of the coherence are above 0.99, 0.98, 0.95, 0.95, and 0.77 at these peaks. The 51‐ and 34‐year spectral peaks are the dominant ones. The time lags of the magnetic field are 3.1 ± 0.6 years and 22.4 ± 4.5 years, respectively. Two simulated time series are constructed with the obtained time lags. They exhibit features similar to the actual time series of the dipole moment variations and Earth's rotational fluctuations.
Rupture processes of the 1987–1988 Gulf of Alaska Earthquake Sequence
Authors: Hwang Lorraine J. , Kanamori Hiroo
Published: February 2008

Three large earthquakes occurred in the Gulf of Alaska during 1987–1988: one on November 17, 1987 (Mw = 7.2), the second one on November 30, 1987 (Mw = 7.8), and the third one on March 6, 1988 (Mw = 7.7). These major intraplate, strike‐slip oceanic events occurred along conjugate trends in a region of no previous seismicity. Body wave modeling places their centroid depths in the upper mantle. The depth of the first two events is approximately 20 km and of the third is 15 km. As compared to other large events, the body wave models for the two largest earthquakes have short rupture lengths, 110 km and 40 km, respectively, and short duration times, 36 and 20 s, respectively, for their magnitude. The latter of the two events has a rupture length much shorter than that inferred from aftershock seismicity. The short rupture lengths and source durations may reflect differences between the strength of oceanic and continental lithosphere and suggest that events in oceanic lithosphere have a higher moment release per unit area than similar continental events. Most of the moment release occurred near the epicenter and/or regions of apparent structural complexities where seismicity trends intersect.
Frequency dependence of the groundwater discharge at an artesian well as recognized from tidal fluctuation records
Authors: Koizumi Naoji
Published: February 2008

Groundwater discharge, temperature, and electrical conductivity of an artesian well were measured at Nishiawakura, Okayama Prefecture, southwest, Japan. The discharge ranged from 2×10−4 to 5×10−4 m3/s and showed tidal fluctuations of about 5 ×10−5 m3/s induced by strain. Although the groundwater temperature reflected the same tidal fluctuations as the discharge, the electrical conductivity did not manifest tidal fluctuations. The discharge slightly leads the theoretical tidal strain, and its response to the strain of the diurnal constituents is different from that of the semidiurnal constituents. The discharge fluctuations induced by atmospheric loading also have some frequency dependence. These can be explained by the vertical or horizontal groundwater flow(leakage) from the aquifer which has been used in the explanation of the well water level fluctuations induced by tidal strain and atmospheric loading.
Signals and noise in magnetic observatory annual means: Mantle conductivity and jerks
Authors: McLeod Malcolm G.
Published: February 2008

Geomagnetic temporal variations can yield valuable information on the electrical conductivity of Earth's mantle and on motion of core fluid. The external‐source signal in first differences of magnetic observatory annual means is primarily due to a degree‐one spherical harmonic closely aligned with Earth's magnetic dipole axis. The transfer function between the electromagnetically induced degree‐one internal Gauss coefficient (Schmidt seminormalized) and the inducing degree‐one external Gauss coefficient is 0.089 ± 0.020 with a phase shift of 0° ± 45° for a 2‐year period. This transfer function is consistent with a nearly insulating mantle and a highly conducting core for which the theoretical transfer function is 0.082 with no phase shift. The temporal power spectrum for noise in first differences of magnetic observatory annual means is approximately white. Third differences of annual means are primarily noise and degree‐one external‐source signal; thus, when the degree‐one external‐source signal is removed from annual means third differences, the root‐mean‐square residuals for a given field component and time interval at a given observatory are a good indicator of noise for the relevant component, observatory, and time interval. These rms residuals were used as weights for construction of spherical harmonic models of geomagnetic secular variation. European secular variation graphs for the 1962–1983 time interval exhibit prominent changes of slope (geomagnetic jerks) in the geomagnetic east component at approximately 1970 and 1978. The jerk of 1970 (but not 1978) is evident on the geomagnetic north and vertical components. The vertical component exhibits additional slope changes at approximately 1966 and 1975.
Bering Sea Earthquake of February 21, 1991: Active faulting along the Bering Shelf Edge
Authors: Abers Geoffrey A. , Ekström Göran , Marlow Michael S. , Geist Eric L.
Published: February 2008

On February 21, 1991, an MS = 6.8 shallow earthquake occurred in the eastern Bering Sea in an area which previously has shown no significant seismic activity. The earthquake was located on the Bering Shelf close to the shelf edge, near one of the elongate basins (Zhemchug Basin) that follow the outer shelf. To better understand the causes of this unusual event, we relocated the earthquake and its aftershocks, determined its source mechanism and depth, and examined multichannel seismic observations of structures near the epicenter. The event was relocated from regional and teleseismic P and S arrival times using a nonlinear inverse technique, as were the 19 previous well‐recorded Bering Sea earthquakes in global catalogs (1964–1987). The 1991 mainshock epicenter was located on the western flank of Zhemchug Basin. The only four previous events with small (<1°) location errors relocated at the outer shelf near the Pribilof Islands. The focal mechanism for the 1991 event, determined by inversion of teleseismic, broadband body waves and centroid‐moment tensor data, shows oblique normal faulting with the T‐axis oriented north‐south. One nodal plane dips steeply to the northeast and strikes parallel to the basin axis, subparallel to faults that bound Zhemchug Basin. This earthquake may be due to slip on one of these normal faults, several of which are seen to offset young strata on a multichannel seismic line 30 km south of the earthquake. It has previously been suggested that the large canyons that cut the Bering Shelf and the adjacent outer‐shelf basins are fault controlled, but this event and its aftershocks provide the first strong evidence to support the hypothesis that these structures are currently active and produce earthquakes. Seismic activity along the Bering Shelf edge may provide a good explanation for felt reports of several large events in the 1800s that have so far been difficult to associate with interplate activity along the Aleutian trench or with any other seismically active structure.
Modeling the closure of volcanic conduits with an application to Mount Vesuvius
Authors: Quareni Francesca , Mulargia Francesco
Published: February 2008

The eruptive activity of a volcano is controlled by the opening and closure of conduits through which magma ascends to the surface. Assuming that the conduit is filled with magma only immediately before and during an eruption, and disregarding any cooling of magma in this state, we develop a model to study the deformation of a cylindrical conduit surrounded by a viscoelastic cylindrical region in an infinite, elastic, homogeneous space. The viscoelastic behavior of the zone around the conduit is due to heat conduction from the hot magma, which raises the temperature beyond the brittle‐ductile transition point. The effect of a tectonic regional stress which favors (compressive) or acts against (tensile) conduit closure is taken into account. The sensitivity of the model is checked with respect to the governing parameters, namely size of the viscoelastic region, ratio between its rigidity and that of the elastic medium, Poisson's ratio, and tectonic stress. Conduit closure is found to be ruled essentially by the extension of the viscoelastic region and by the ratio between its rigidity and the rigidity of the surrounding elastic medium, while tectonic stress is much less important. The model is applied to the last eruptive cycle of Mount Vesuvius. Using realistic values for the elastic and viscoelastic moduli, we find that an open conduit condition has been possible from 1631 to 1944, while the quiescence from 1944 on implies a closed conduit state.
Strain accumulation in the Santa Barbara Channel: 1970–1988
Authors: Larsen Shawn C. , Agnew Duncan Carr , Hager Bradford H.
Published: February 2008

Geodetic observations between 1970 and 1988 indicate appreciable strain accumulation in the Santa Barbara Channel, California. Eleven line‐length changes from a six‐station geodetic network spanning the eastern two‐thirds of the channel were determined from electronic distance measurements in 1970/1971 and Global Positioning System observations in 1987/1988. Within this network the strains observed are spatially nonuniform. In the easternmost channel the strain is nearly uniaxial, with convergence of 6.4 ±0.9 mm/yr oriented N25°E ±5 °; this direction is consistent with the seismicity, which is dominated by thrust mechanisms with P axes directed to the northeast. In the central channel the strain is less well determined, but appears to include a significant component of shear that is left‐lateral when resolved on an east‐west plane.
Slip systems in calcite single crystals deformed at 300–800°C
Authors: de Bresser J. H. P. , Spiers C. J.
Published: February 2008

In an attempt to resolve questions recently raised regarding the principal high‐temperature slip systems in calcite, optical quality single crystals have been uniaxially compressed at 300–800°C and at a constant strain rate of ∼ 3×10−5 s−1. In addition, a single strain rate cycling test was performed at 650°C. The tests were all carried out with the compression direction parallel to
, which lies at ∼30° to the c axis and makes angles of 52° and 23° with the poles to two rhombohedral (r) planes. Axial strains of 5–16% were achieved in the tests. The stress‐strain curves obtained showed three‐stage hardening behavior, while the strain rate cycling data showed flow stresses to be rather insensitive to strain rate with empirical power law fits yielding a conventional stress exponent of 22–30 at strains ≥ 5%. The active glide systems were identified by slip line analysis. Slip on r
in the so‐called negative sense was found to be an important deformation mechanism across the entire range of temperatures investigated, while slip on a single f system in the negative sense became important at 650°C and above. The active f slip direction was of
type, confirming the existence of a new set of f slip systems, namely, f
, as opposed to the generally accepted f
systems. In addition, clear evidence was found for significant slip on the basal (c) plane at temperatures above 600°C, probably in the 〈a〉 direction. By comparison with earlier data on crystals from the same batch, no evidence was found for significant strength asymmetry on the r
and f
slip systems in the positive and negative senses. Notably, the presently observed f and c sup systems have not been taken into account in previous modelling studies of plasticity and texture (i.e., crystallographic preferred orientation) development in calcite rocks, and r slip is usually taken as stronger in the positive than negative sense. Our results therefore imply a need for renewed modelling work on calcite, particularly regarding texture development at relatively high temperatures.