bluuebunny/crossref_metadata_2025 huggingface dataset:
Bistatic auroral radar system and three‐receiver‐ionospheric‐motions velocities: A comparison
Authors: MacDougall J. W. , Hall G. E. , Moorcroft D. R.
Published: February 2008

Observations of the NW‐SE component of F region convection obtained with a scintillation drift experiment have been compared with power and Doppler velocity measurements of auroral E region coherent backscatter at 50 MHz made with the Bistatic Auroral Radar System (BARS), which is able to observe only at large magnetic aspect angles. It was found that E region backscatter was observed only when the NW–SE component of the F region drift was in the SE direction. This and other observations are shown to be consistent with a recently proposed explanation for these large aspect angle VHF backscatter observations, based on refraction through auroral ionization structures in the E region. In most cases the vector velocity derived from BARS observations had a magnitude substantially below that inferred from the scintillation measurements. Observations during one period were noticeably different from the others, with unusually small Doppler velocities.
Spatial relationship between large aspect angle VHF radio aurora and 557.7‐nm emissions: Evidence for refraction
Authors: Hall G. , Moorcroft D. R. , Cogger L. L. , Andre D.
Published: February 2008

A recently proposed explanation for large aspect angle VHF backscatter observations, based on refraction through auroral ionization structures in the E region (Moorcroft, 1989), has been confirmed by an experimental comparison of observations of auroral E region coherent backscatter at 48.5 MHz (made with the Bistatic Auroral Radar System or BARS) with simultaneous 557.7‐nm emissions detected with a CCD‐based all‐sky imager. The apparently large aspect angle radio auroral echoes are found to occur predominantly on the side of the visual aurora away from the radar for both the premidnight and the postmidnight sectors. For the Red Lake BARS radar this means that the strongest echoes occur to the north of the visual form, in sharp contrast with previous comparisons, particularly for the premidnight sector, which found that radar echoes occur predominantly southward of the visual aurora. These results are consistent with the proposed explanation based on refraction in the auroral E region.
A mathematical model of the structure and evolution of small‐scale discrete auroral arcs
Authors: Seyler Charles E.
Published: February 2008

A three‐dimensional fluid model which includes the dispersive effect of electron inertia is used to study the nonlinear macroscopic plasma dynamics of small‐scale discrete auroral arcs within the auroral acceleration zone and ionosphere. The motion of the Alfven wave source relative to the magnetospheric and ionospheric plasma forms an oblique Alfven wave which is reflected from the topside ionosphere by the negative density gradient. The superposition of the incident and reflected wave can be described by a steady state analytic solution of the model equations with the appropriate boundary conditions. This two‐dimensional discrete auroral arc equilibrium provides a simple explanation of auroral acceleration associated with the parallel electric field. Three‐dimensional fully nonlinear numerical simulations indicate that the equilibrium arc configuration evolves three‐dimensionally through collisionless tearing and reconnection of the current layer. The instability evolves nonlinearly to produce complex transverse structure that may be the origin of the folds and curls observed to be associated with small‐scale discrete arcs. At late times the plasma becomes turbulent having transverse electric field power spectra that tend toward a universal k−5/3 spectral form.
Plasma density enhancement at the comet Halley diamagnetic cavity boundary
Authors: Keller C. N. , Cravens T. E.
Published: February 2008

A one‐dimensional single‐fluid MHD model was used by Cravens (1989a) to predict the existence of a narrow (50 km) layer of enhanced plasma density (factor of 3 over background) at the boundary of the comet Halley diamagnetic cavity. The existence of such a layer was confirmed by measurements made by the Giotto ion mass spectrometer (Goldstein et al., 1989), although with a relative density enhancement of more than a factor of 4. We solve the time‐dependent coupled continuity equations for several species including H3O+, H2O+, OH+, O+, NH4+, NH3+, CH4+, and CH3+. For several ion species we investigate factors affecting the magnitude of this enhancement and its relationship to the thickness of the transition layer. For example, ion species with short chemical lifetimes are shown to have smaller density enhancements than species with longer lifetimes. The cometocentric distance of the cavity boundary also strongly affects the magnitude of the density enhancement; the enhancement increases with increasing distance.
Multipoint observations of a small substorm
Authors: Lopez R. E. , Lühr H. , Anderson B. J. , Newell P. T. , McEntire R. W.
Published: February 2008

In this paper we present multipoint observations of a small substorm which occurred just after 0110 UT on April 25, 1985. The observations were made by spacecraft (AMPTE CCE, AMPTE IRM, DMSP F6, and DMSP F7), ground auroral stations (EISCAT magnetometer cross, Syowa, Narssarssuaq, Great Whale River, and Fort Churchill), and mid‐latitude stations (Furstenfeldbruck, Toledo, and Argentine Island). These data provide us with a broad range of observations, including the latitudinal extent of the polar cap, visual identification of substorm aurorae and the magnetic perturbations produced directly beneath them, in situ magnetic field and energetic particle observations of the disruption of the cross‐tail current sheet, and observations concerning the spatial expansion of the current disruption region from two radially aligned spacecraft. The DMSP data indicate that the event took place during a period when the polar cap was relatively contracted, yet the disruption of the current sheet was observed by CCE at 8.56 RE. We have been able to infer a considerable amount of detail concerning the structure and westward expansion of the auroral features associated with the event, and we show that those auroral surges were located more than 10° equatorward of the boundary between open and closed field lines. Moreover, we present evidence that the current sheet disruption observed by CCE in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at Syowa. Furthermore, the observations strongly imply that disruption of the cross‐tail current began in the near‐Earth region and that it had a component of expansion which was radially antisunward.
Electrons in the ionopause boundary layer of Venus
Authors: Gan Lu , Cravens T. E. , Horanyi M.
Published: February 2008

Measurements made by the retarding potential analyzer on board the Pioneer Venus orbiter (PVO) have indicated the presence of both magnetosheath electrons and atmospheric photoelectrons in the plasma mantle region of Venus, which lies just outside the ionopause. A theoretical model of the spatial and energy distribution of electrons in the plasma mantle has been constructed using the two‐stream electron transport method and the time‐dependent electron energy equation. Magnetic field lines parallel to the ionopause are assumed. Transport and collisional processes, including electron scattering by neutrals and ions, are taken into consideration. The calculated electron spectra in the mantle show signatures of both solar wind electrons and photoelectrons. Cold ionospheric electrons are present only for altitudes below the ionopause. The calculated electron temperatures agree very well with the PVO data both in the mantle region and in the magnetized ionosphere.
Ionospheric electron content within the equatorial F2 layer anomaly belt
Authors: Rastogi R. G. , Klobuchar J. A.
Published: February 2008

This paper describes some new results on the day‐to‐day behavior of the location of the equatorial anomaly in the columnar electron content. The measurements were made from a unique network of stations covering dip latitudes from 0°N to 25°N during the period in 1975–1976 near sunspot minimum when the ATS 6 satellite was visible from India. The latitudinal distribution of the total electron content on a particular day was found to depend only on the strength of the electrojet current, as determined by the difference of the horizontal magnetic field at stations on the magnetic equator and another outside the equatorial electrojet, rather than on the horizontal magnetic field measured only at the equatorial station. During magnetically disturbed periods the presence or absence of the anomaly was also found to depend only upon the strength of the electrojet current. The varying electrojet current, that is, the equatorial electric field, is very effective in moving the ionization to the anomaly latitudes of 15°–20°, but the columnar electron content over the magnetic equator remains relatively constant.
A geophysical study of the Wharton Basin near the Investigator Fracture Zone
Authors: Larson Roger L. , Carpenter George B. , Diebold John B.
Published: February 2008

Most of the anomalously deep portion of the central Wharton Basin in the eastern Indian Ocean appears to be the result of a 55° change in spreading direction that occurred between magnetic anomalies 34 (80 m.y.) and M‐0 (108 m.y.). An Early Cretaceous transform fault known as the Wallaby Plateau Scarp or Wallaby Fracture Zone evolved in the Late Cretaceous into the Investigator Fracture Zone during this radical change in plate motion. What is now the southern end of the Investigator Fracture Zone formed as a substantially wider and deeper feature than most fracture zones. Seismic refraction results within the anomalously deep portion of the fracture zone show an essentially normal oceanic crustal section that is slightly thinned. The western boundary of the Investigator Fracture Zone can be easily traced south of (older than) anomaly 34 with magnetic, seismic reflection, and gravity data. This boundary is marked by a magnetic anomaly edge effect whose polarity depends on the presence of normally or reversely magnetized crust adjacent to the fracture zone. The polarity reversal of the boundary edge effect is evidence that the crust within the fracture zone has a greatly reduced remanent magnetization.
Fault creep and stress drops in saturated silt‐clay gouge
Authors: Bombolakis E. G. , Hepburn J. Christopher , Roy D. C.
Published: February 2008

An analysis of physicochemical processes in saturated silt‐clay gouge indicates that this type of fault zone material can account for the following phenomena: (1) the nonlinear mechanical behavior indicated by certain geophysical measurements along the San Andreas fault zone, (2) the low stress drops associated with earthquakes to several kilometers' depth, and (3) the recurrence of creep‐induced instabilities at shallow depths along fault zones. A rheological model is described for a gouge consisting of colloidal size clay platelets with adsorbed water, brittle silt size particles, and ‘free’ pore water. Recurrence of shallow earthquakes or accelerated creep is explained in the model by thixotropic hardening of the colloidal phase following shear deformation, i.e., by electrochemical reorientation of clay platelets from a dispersed structure to a face‐to‐edge type of structure during a quiescent period. The silt phase must support part of the effective mean stress for thixotropic hardening to occur at several kilometers' depth. The peak shear strength of the gouge in this case is expressed in functional form by Sp = f[κPe +
tan ψe;
tan ψs], where κ, Pe, and ψe are Hvorslev parameters;
is the effective stress in the colloidal phase, acting normal to the shear zone;
is the effective stress in the silt phase, acting normal to the shear zone; and ψs is the ‘friction angle’ of the silt phase. The peak shear strength is time dependent owing to viscous type contacts between adsorbed water layers surrounding the colloidal platelets. The time‐dependent nature of Sp may be responsible for certain nonlinear behavior noted in fault zones and for the small stress drops associated with earthquakes occurring at several kilometers' depth.
An easily implemented algorithm for the tropospheric range correction
Authors: Black Harold D.
Published: February 2008

Utilizing the Hopfield model for the altitude variation of the refractive index, we have found a convenient approach to the necessary integrals. The approach leads to simple analytic forms. The forms clearly illustrate that the dry term correction is directly proportional to surface pressure and quite insensitive to surface temperature: For elevation angles above 5° the range correction is proportional to P·(Tc + 269.04)/(Tc + 273.16), where Tc is the surface temperature in degrees Celsius and P is the surface pressure. The resulting equations and their derivations and limitations are described.
Surface waves and the structure of the Great Basin of Nevada and western Utah
Authors: Priestley Keith , Brune James
Published: February 2008

Fundamental mode Rayleigh wave (4‐ to 120‐s period) and Love wave (4‐ to 40‐s period) phase velocities have been measured for several paths wholly within the Great Basin of Nevada and western Utah. An average structural model for this area was derived from the dispersion data and existing refraction data. This model consists of a three‐layer, 35‐km‐thick crust and a 29‐km‐thick, higher‐velocity (Vs = 4.5 km/s) cap at the top of the mantle, below which lies a broad (180‐km‐thick) low‐velocity zone for shear waves (Vs = 4.05–4.12 km/s). Intermediate‐period (15‐ to 32‐s) Rayleigh wave phase velocities measured over shorter paths indicate the existence of significant local variations in the shallow (20‐ to 30‐km‐deep) structure which may result from varying crustal thickness or varying depths to a zone of partial melting. Comparison of our Great Basin mantle structure with the mantle structures of other areas (oceanic, shield, alpine, and rise) shows greatest similarity with the East African Rift and the East Pacific Rise. Like these areas the Great Basin may be a tensional region where deep mantle material has intruded near the surface and created a zone of partial melting.
Paleomagnetic results from the Lassiter Coast, Antarctica, and a test for oroclinal bending of the Antarctic Peninsula
Authors: Kellogg Karl S. , Reynolds Richard L.
Published: February 2008

Paleomagnetic results from 17 magnetically stable units of Upper Cretaceous (‘Andean’) plutons and dikes of the Lassiter Coast, on the southern Antarctic Peninsula, define a mean paleomagnetic pole at 87°S, 131°W (α95 = 11.5°). This indicates that little latitudinal movement of the southern Antarctic Peninsula has occurred during the past 100 m.y. All magnetically stable intrusives are normally polarized and are believed to have been emplaced during the Late Cretaceous epoch of predominantly normal polarity. There is no evidence of postemplacement remagnetization. The uncertainty in declination at the 95% confidence level is computed for both the Lassiter Coast data and those data available from other Andean sites in the Antarctic Peninsula. Within the limits of uncertainty, data from four localities north of 68°S support the contention of Dalziel et al. (1973) that there has not been any apparent post‐Late‐Cretaceous oroclinal bending in the northern half of the peninsula. For sites to the south in the Lassiter Coast, the uncertainty in declination, due to steep inclinations, is too large to support reliably or deny any large‐scale structural bending.
Gravity study of great impact
Authors: Sweeney J. F.
Published: February 2008

The residual Bouguer gravity field associated with the 65‐km‐diameter Manicouagan circular structure in eastern Quebec consists of a peripheral −4‐ to −10‐mGal ring that grades gently upward to a central high of 0 mGal. This pattern of gravity anomalies is consistent with the distribution of upper crustal density contrasts predicted by a hypervelocity meteorite impact origin for the structure. Excavation of a transient cavity produced by meteorite impact is limited to crustal depths between 2 and 8 km and is most likely between 3 and 5 km at Manicouagan on the basis of gravity model calculations of the vertical component of cavity infilling.
Asthenosphere readjustment and the earthquake cycle
Authors: Savage J. C. , Prescott W. H.
Published: February 2008

A simple two‐dimensional model of the earthquake cycle (preearthquake strain accumulation, coseismic strain release, and postseismic readjustment) has been constructed from the Nur‐Mavko solution for a screw dislocation in an elastic plate (lithosphere) overlying a viscoelastic substrate (asthenosphere). The deformation at the free surface is calculated for an earthquake cycle imposed by prescribed slip on a transform fault. This deformation is compared to that produced by a similar cycle in an elastic half space so that the effects of viscoelastic relaxation in the asthenosphere may be isolated. The following conclusions are drawn: (1) The surface deformation produced by viscoelastic relaxation in the asthenosphere can be duplicated identically by a reasonable distribution of slip at depth on a vertical fault in an elastic half space. Thus differentiation of two possible modes of postearthquake readjustment will be difficult. (2) The effect of asthenosphere relaxation is important only if the depth of the seismic zone is comparable to the thickness of the lithosphere. If the seismic zone is 15 km deep and the lithosphere is 75 km thick, as commonly estimated for the San Andreas fault zone, asthenosphere relaxation is not particularly significant in determining surface deformation. (3) In a periodic sequence of earthquakes the principal observable effects of viscoelasticity in the asthenosphere are to produce a rapid postearthquake deformation and to concentrate strain accumulation and relaxation even closer to the fault than in the elastic half‐space model.
Simultaneous inversion of surface wave phase velocity and attenuation: Love waves in western North America
Authors: Lee Wook Bae , Solomon Sean C.
Published: February 2008

A formalism is developed for simultaneous inversion of surface wave phase velocity and attenuation to determine shear wave velocity and Q−1 in the earth. A simultaneous inversion takes full account of the dependence of surface wave velocity and attenuation on both the elastic and the dissipative part of an earth structure and permits inclusion of the physical relationship between anelasticity and intrinsic dispersion that arises from linearity. The procedure is mathematically more complete than the approach of correcting phase velocity data for the intrinsic dispersion due to anelasticity and inverting the corrected velocity data alone, and it gives different results. The proposed formalism, including resolution analysis, weighted least squares inversion, and extremal inversion, is applied to Love waves in western North America. Various intrinsic dispersion‐attenuation relations are tested, including Q independent of frequency, Q varying as a power of frequency, and Q specified by a sum of relaxation mechanisms. The results of the inversions confirm the coincidence of the low‐velocity and low Q zones beneath western North America for frequencies in the surface wave band. Compared with previous inversion of Q−1 data alone, the simultaneous inversion results in improved depth resolution of Qs−1 and the elimination of an apparent incompatibility of low‐attenuation data at 20‐ to 25‐s periods. The Love wave data do not discriminate among the various dispersion‐attenuation relations, though a constant Q leads to the removal of the requirement for a low‐velocity zone at frequencies above 1 Hz. The predicted intrinsic dispersion within the low‐velocity zone varies from 1% to 10% between 0.01 and 1 Hz for the various models; broadband measurements of body wave dispersion offer the greatest promise for choosing among the models.
Gravity anomalies, regional elevation, and the deep structure of the North Atlantic
Authors: Cochran James R. , Talwani Manik
Published: February 2008

The pattern of depth and gravity anomalies in the North Atlantic Ocean was examined by using 1° × 1° and 5° × 5° averages. The gravity field is dominated by two features: a broad high in the northern and central portion of the ocean and a large area of very negative anomalies in the western basin. The negative anomaly in the western North Atlantic has no expression in residual depth anomalies and does not appear to be related to surface features. The North Atlantic Gravity High is bounded on the south by an important and distinct boundary near 30°N which is present in both depth and gravity anomalies. North of this latitude, residual gravity anomalies (corrected for the ‘ridge anomaly’) take the form of a very broad high of about 20 mGal with the values along isochrons nearly constant from 30°N to at least as far north as 75°N. The depths within this region are consistently shallow with 5° × 5° average residual depth anomalies which vary greatly from a few hundred meters near the Charlie Gibbs fracture zone to a few thousand meters near Iceland and the Azores. We used three‐dimensional spherical earth models to investigate simple compensated mass distributions which could explain the observed depth and gravity anomalies. The acceptable family of models has the common characteristics that within the anomalous region north of the 30°N boundary a portion of the compensation must be distributed to depths of several hundred kilometers and that this deep mass deficiency must be nearly uniform throughout the area. However, the compensation for more local features within this region must be shallow (within the lithosphere). Thus the topographic highs surrounding Iceland and the Azores are compensated within the lithosphere, but the overall elevation is maintained at some greater depth by a mass deficiency in the asthenosphere. This mass deficiency can be explained by an increase in temperature of about 75°C. Thus the gravity data enable us to establish the presence of a very large upper mantle hot spot that might be associated with the broad overall compensation of the entire North Atlantic north of 30°N. Whether this upper mantle hot spot is associated with a deep mantle plume is uncertain, since the gravity effect of a plume is undetectable. However, its presence suggests that the immediate source of the unusual amount of material erupted at Iceland and the Azores is within the upper mantle.
A quantitative model of dilatancy in dry rock and its application to westerly granite
Authors: Holcomb David J.
Published: February 2008

A general model of dilatancy was developed based on the behavior of individual microcracks. Macroscopic effects were described by combining the effects of individual cracks using a statistical approach. The model was developed using distribution functions for crack size, crack strength, and local stress. Hysteresis in volumetric strain and stress‐strain data for partial unloading and reloading and general loading paths can be described. The model is best suited to describing rock which has been subjected to a number of loading cycles sufficient to remove the residual volumetric strain that is present in the initial cycles. This makes the rock more representative of the real earth and removes transient effects that obscure the dilatant effect. Volumetric strain data from triaxial experiments on Westerly granite were well described by the model for confining pressures from 0 to 4.11 kbar. Only one parameter was needed to describe the effect of changing the confining pressure, and this can be interpreted as an elastic effect on the crack volume. Some aspects of the crack behavior that were necessary to explain the data are incompatible with the usual sliding crack model of dilatancy. Combined with the lack of observable shear cracks, this casts serious doubts on the validity of the sliding crack model.
Initiation of intracontinental subduction in the Himalaya
Authors: Bird Peter
Published: February 2008

Independent arguments based on topographic stress and crustal strength give upper limits of 200 bars and 300 bars, respectively, for the average shear stress on the intracontinental thrust fault that formed the Himalaya. According to either a one‐dimensional or a two‐dimensional fault model, such stresses could not have produced the Himalayan granites by friction, unless overthrusting velocity exceeded 30 cm/yr. More probably, Himalayan metamorphism was caused by exposure of continental crust to hot asthenosphere prior to the formation of the intracontinental thrust. Crust was exposed by peeling away of Indian subcrustal lithosphere in response to the force and moment exerted by the Tethyan slab. This detachment of buoyant crust from dense lithosphere better explains the metamorphic pattern and also explains why the distributed crustal shortening at the beginning of the collision orogeny was replaced by localized thrusting or intracontinental subduction.
On isostatic geoid anomalies
Authors: Haxby W. F. , Turcotte D. L.
Published: February 2008

In regions of slowly varying lateral density changes, the gravity and geoid anomalies may be expressed as power series expansions in topography. To a good approximation, geoid anomalies in isostatically compensated regions can be directly related to the local dipole moment of the density‐depth distribution. This relationship is used to obtain theoretical geoid anomalies for different models of isostatic compensation. The classical Pratt and Airy models give geoid height‐elevation relationships which differ in their functional form but which predict geoid anomalies of comparable magnitude. The thermal cooling model which explains ocean floor subsidence away from mid‐ocean ridges predicts a linear age‐goid height relationship of 0.16 m/m.y. Geos 3 altimetry profiles were examined to test these theoretical relationships. A profile over the mid‐Atlantic ridge is closely matched by the geoid curve derived from the thermal cooling model. The observed geoid anomaly over the Atlantic margin of North America can be explained by Airy compensation. The relation between geoid anomaly and bathymetry across the Bermuda Swell is consistent with Pratt compensation with a 100‐km depth of compensation.
The calculation of perturbation and induction arrows for a three‐dimensional Conductivity model and various two‐dimensional source fields
Authors: Hibbs R. D. , Jones F. W.
Published: February 2008

Nonuniform sources affect the calculation of transfer functions in the electromagnetic induction problem. Modeling the source field in terms of a number of two‐dimensional sheet currents with different current intensity distributions can determine the degree of this effect. The results show little source effect for either the Schmucker (1970) or Cochrane and Hyndman (1970) formulation of the transfer function calculations. However, for distant sources with small angular spread there is some skewing of the induction arrows for both formulations. The perturbation arrows, when vectorially combined, outline the spatial extent of the conductivity anomaly, especially the quadrature‐phase arrow.
Propagation of failure surfaces in natural slopes
Authors: Chowdhury Robin N.
Published: February 2008

The mechanism of slope failures has been the subject of extensive discussion and study in recent years. The role of progressive failure is now considered to be an important one in many situations. Failures which appear to occur quite suddenly are, in fact, preceded by the gradual formation of slip surfaces or shear bands. Deformation is often concentrated along these surfaces, and as a consequence of the strain‐softening behavior of slope materials, the strength falls from peak to lower values. The minimum or limiting shear strength is the residual strength, and this may be operative over part or the whole of the shear band at any given stage of its development. Concepts from crack mechanics based on energy considerations have been used to study the conditions for the propagation of shear bands. Planar shear bands entirely at the residual strength parallel to the surface of a slope with a cut have been considered. The energy for driving the shear band is provided by the release of stresses due to the cut as well as by the difference in shear stress and shear strength over the length of the shear band. In this paper, two extensions to this work are considered. The first concerns the variation of strength from peak to the residual along the length of the shear band. This variation may be quite arbitrary. It is shown that the proportion of the fall from the peak strength has a considerable influence on the criterion for failure propagation. Consequently, the manner in which strength actually varies along shear bands needs to be studied. The second extension concerns shear bands of arbitrary inclination within natural slopes. The energy approach is used again but with one important difference. There is no cut in the slope and thus no release of stress. The energy for driving the shear band is provided entirely by the difference between the shear stress and residual shear strength along the shear band. This extension is a necessary step toward consideration of slip surfaces of arbitrary shape within natural or altered slopes.
Local changes in gravity resulting from deformation
Authors: Walsh J. B. , Rice J. R.
Published: February 2008

The horizontal and vertical components of gravity change when tectonic stresses deform the earth because mass is redistributed relative to the gravity meter. We analyze the change in gravity resulting from deformation in a homogeneous elastic half‐space. We derive expressions in closed form which give the change in horizontal and vertical components of gravity measured at the surface for any specified distribution of dislocations at depth. For example, the change in the vertical component of gravity observed by a gravity meter fixed in space above an infinitely long thrust fault is found to be proportional to the local change in height, whereas the change due to a spherically symmetric source of dilatation is zero. Analysis of the change in the horizontal component shows that error in measurements of uplift resulting from changes in level is negligible for these sources.
Revised geomagnetic polarity time scale for the interval 0–5 m.y. B.P.
Authors: Mankinen Edward A. , Dalrymple G. Brent
Published: February 2008

A change in the constants used in K‐Ar dating and a significant increase in new data have made a recompilation and recomputation of data used to define the Late Cenozoic K‐Ar polarity time scale highly desirable at this time. All available data in the range 0–5 m.y. have been recalculated using the refined constants, with 354 data points in this time interval now meeting the minimum criteria for acceptability. Recalculation of the major polarity epoch boundaries has yielded ages of 0.73 m.y. for the Brunhes‐Matuyama, 2.48 m.y. for the Matuyama‐Gauss, and 3.40 m.y. for the Gauss‐Gilbert boundaries. A revised polarity time scale has been constructed based on available K‐Ar data and information obtained from marine magnetic anomalies and deep‐sea sedimentary cores.
The Curie Point and the α–γ transition of iron to 53 Kbar — A reexamination
Authors: Mirwald Peter W. , Kennedy George C.
Published: February 2008

We have completed a study of the ferromagnetic‐paramagnetic phase transition (Curie transition) and the α‐γ phase transition in iron by differential thermal analysis. The investigation was carried out in a piston‐cylinder apparatus to 53 kbar. A low‐friction cell was employed that allowed the phase transitions to be detected by monitoring the change in compressibility. The ferromagnetic‐paramagnetic phase boundary was found to have a small negative slope and to intersect the α‐γ phase boundary in a triple point at 20±1.2 kbar. The precision of the measurements is discussed.
Modeling of rock friction: 1. Experimental results and constitutive equations
Authors: Dieterich James H.
Published: February 2008

Direct shear experiments on ground surfaces of a granodiorite from Raymond, California, at normal stresses of ∼6 MPa demonstrate that competing time, displacement, and velocity effects control rock friction. It is proposed that the strength of the population of points of contacts between sliding surfaces determines frictional strength and that the population of contacts changes continuously with displacements. Previous experiments demonstrate that the strength of the contacts increases with the age of the contacts. The present experiments establish that a characteristic displacement, proportional to surface roughness, is required to change the population of contacts. Hence during slip the average age of the points of contact and therefore frictional strength decrease as slip velocity increases. Displacement weakening and consequently the potential for unstable slip occur whenever displacement reduces the average age of the contacts. In addition to this velocity dependency, which arises from displacement dependency and time dependency, the experiments also show a competing but transient increase in friction whenever slip velocity increases. Creep of the sliding surface at stresses below that for steady state slip is also observed. Constitutive relationships are developed that permit quantitative simulation of the friction versus displacement data as a function of surface roughness and for different time and velocity histories. Unstable slip in experiments is controlled by these constitutive effects and by the stiffness of the experimental system. It is argued that analogous properties control earthquake instability.
Implications of earthquake triggering and rupture propagation for earthquake prediction based on premonitory phenomena
Authors: Brune James N.
Published: February 2008

A simple model of earthquake triggering and rupture propagation, based on concepts of earthquake mechanism commonly accepted, suggests that earthquake prediction (especially prediction of magnitude) might be very difficult, depending on the values of certain stress parameters. The concepts in the model suggest lines of research which may help to judge how successful the earthquake prediction effort might eventually be.
On the relation between seismic moment and stress drop in the presence of stress and strength heterogeneity
Authors: Madariaga Raul
Published: February 2008

The seismic moment is related by definition to the average slip on the fault plane of an earthquake. Here we derive an exact expression for the seismic moment in terms of a general heterogeneous stress drop distribution and the geometry of the fault of a complex event. We find that the seismic moment is proportional to a weighted integral of the stress drop on the fault. The weight in this linear relationship is the slip for a hypothetical event with the same source geometry but uniform stress drop. This relationship between seismic moment and stress drop depends on geometry. In particular, for multiple sources the weight is reduced by factors of the order of ρ/R, where ρ is the radius of a typical subfault and R is the radius of the total source area. As a consequence of these results we find that for a given stress drop, a simple fault generates a larger seismic moment than a multiple fault of the same total surface. Conversely, for a given moment and source area, a complex event would need higher stress drops on the subfaults than a simple smooth fault. We test these results with three rectangular models of faulting. The first is a simple, smooth fault with uniform stress drop. The second model is a simple fault with zero stress drop in the central section of the fault. The last model is a complex event where the central section of the fault remains unbroken. We show that the last two models are difficult to distinguish from their far‐field radiation.
Far‐field S‐wave spectra, corner frequencies, and pulse shapes
Authors: Brune James N. , Archuleta Ralph J. , Hartzell Stephen
Published: February 2008

Recent results concerning the relationship of S‐wave far‐field corner frequencies to stress drop are discussed. It is suggested that the corner frequency as picked by Madariaga [1977] from his theoretical spectra is not consistent with the way in which corner frequencies were picked by Tucker and Brune [1973, 1977]. When the difference is taken into account, the stress drops inferred from the Madariaga relationship and his method of picking corner frequencies are roughly the same as inferred using the Brune [1970, 1971] relationship and the Tucker and Brune method of picking corner frequencies (for those relatively few experimental spectra which are similar in shape to the Madariaga theoretical spectra). Far‐field spectra for a number of new finite element models of fault ruptures in a half‐space are presented, and for these data, new values for the relationship between source dimension and corner frequency are obtained. Fault models used include semicircular faults with rupture initiation at the surface and at depth and rectangular faults with unilateral and bilateral rupture propagation. The results indicate that there is a considerable variation in corner frequency with respect to type of rupture and position around the rupture. Because of the variation it is not possible to conclude, without more calculations, what the best average relationship between corner frequency and source dimension is; however, a value of K about 1/3 is reasonable where K = fcr/β (fc = corner frequency, r = radius, and β = shear wave velocity). Dahlen [1974] speculated that the corner frequencies picked experimentally could be significantly altered by scattering. For the San Fernando aftershocks, it is possible to make a case that this is not so. Many of the San Fernando aftershocks show very simple pulse shapes, with a pulse duration consistent with the spectral corner frequency and little later arriving energy ‐ a direct indication that scattering is not radically affecting the results.
Seismic velocity structure below the island of Hawaii from local earthquake data
Authors: Crosson Robert S. , Koyanagi Robert Y.
Published: February 2008

P wave arrival time data from 45 earthquakes and three explosions recorded on the 37‐station Hawaii network were inverted simultaneously for hypocenter parameters, compressional wave velocity as a function of depth, and station corrections. Hypocenter depths range from near surface to approximately 35 km, with most events near Kilauea volcano. The structure model obtained has a distinct and well‐constrained low‐velocity zone at the base of the crust. This feature is most readily explained as a serpentinized transition layer, although it is difficult to completely rule out a thermal origin. Upper mantle velocity at the M discontinuity is about 8.3km/s with a gradient estimated to be about a 0.01‐km/s/km increase in velocity with depth. Evidence is found for a crust which is depressed beneath the island, presumably in response to the volcanic load. Station corrections exhibit an inverse correlation with regional Bouguer gravity variations. The summit of Mauna Loa volcano in particular has arrival times anomalously early by as much as 0.5 s compared to flank stations, indicating that a high‐velocity core in the volcano extends nearly to the base of the volcanic pile.
A moment magnitude scale
Authors: Hanks Thomas C. , Kanamori Hiroo
Published: February 2008

The nearly coincident forms of the relations between seismic moment M0 and the magnitudes ML, MS, and Mw imply a moment magnitude scale M = ⅔ log M0 ‐ 10.7 which is uniformly valid for 3 ≲ ML ≲ 7, 5 ≲ Ms ≲ 7½, and Mw ≳ 7½.
Horizontal crustal deformation from historic geodetic measurements in southern California
Authors: Thatcher Wayne
Published: February 2008

Data from horizontal control surveys are used to determine the regional deformation pattern across the southern San Andreas fault system and the history of movements since ∼1930. The interseismic straining is spread over a wide zone, with no sharp concentration across major faults and with significant deformation extending out to distances on the order of 100 km from the San Andreas fault. The maximum right‐lateral shear straining parallels the trends of the northwest striking faults of the region and tensor rates of from 0.1 to 0.2×10−6/yr are observed. A slight decrease in strain rate away from the San Andreas is indicated by the data, but shallow seismic slip on this fault alone is incapable of releasing all accumulated strains. The substantial residual deformation must then be accommodated by other means, and the range of possibilities includes (1) permanent inelastic deformation over a broad region, (2) massive pre‐ and/or post‐seismic slippage on the San Andreas beneath the brittle seismic zone, and (3) earthquake faulting on several subparallel active strands of the San Andreas system. Although available data are insufficient to exclude any of these possibilities, the diffuse seismicity and numerous active faults of the region suggest the third mechanism is the predominant one. Aseismic fluctuations in the rate of deformation have been inferred during the 1959–1974 southern California uplift, and long‐lived, wideranging postseismic effects have been observed following several major earthquakes and are particularly well‐documented for the 1940 El Centro shock (M = 7.1). Such effects are large and unexpected, complicate determination of the pattern of secular deformation, and are difficult to separate from suspected earthquake precursors. Determination of the rate of relative plate motion across southern California is made difficult by both the great breadth of the zone of secular straining and the observed irregularities in deformation rate.
Distribution of heat flow and radioactive heat generation in northern Mexico
Authors: Smith Douglas L. , Nuckels C. Edward , Jones Ronald L. , Cook Gregory A.
Published: February 2008

Twenty‐five new heat flow measurements from northern Mexico range from 0.6 HFU (1 HFU = 1 μcal/cm2s = 41.8 mW/m2) at Los Plomosas, Chihuahua, to 4.2 HFU about 30 km east of Mazatlan, Sinaloa. The new values, in conjunction with previous data, confirm the Baja peninsula as an area of low to normal heat flow and demonstrate an irregular decrease of heat flow eastward from the Gulf of California across the Sierra Madre Occidental and a separate pattern of decreasing heat flow eastward from the Central Plateau across the Sierra Madre Oriental. An area of high heat flow immediately east of the Gulf of Calfornia is identified and is tentatively related to the spreading ridges in the gulf. Abundances of the radioactive‐heat‐generating elements uranium, thorium, and potassium increase from Baja California to the eastern border of the Sierra Madre Occidental, abruptly decrease within the Central Plateau, and then increase again eastward through the Sierra Madre Oriental. Although a general positive correlation between heat flow and radioactive heat generation is observed, adherance to the expected linear patterns cannot be demonstrated. This lack of linearity prohibits the definition of separate thermal provinces in northern Mexico and suggests the assignment of the Sierra Madre Occidental as a southerly extension of the Basin and Range thermal province that is modified by sea floor spreading in the Gulf of California. Heat flow measurements in the states of Chihuahua, Durango, and Zacatecas are similar to those associated with the Rio Grande Rift thermal anomaly in New Mexico, but an identification of a continuous extension of the rift thermal conditions into northern Mexico cannot be made.
Paleomagnetism of the Morro Rock‐Islay Hill Complex as evidence for crustal block Rotations in central coastal California
Authors: Greenhaus Michael R. , Cox Allan
Published: February 2008

Paleomagnetic directions obtained from seven hypabyssal intrusive bodies of the late Oligocene Morro Rock‐Islay Hill igneous complex of central coastal California display clockwise deflections from the expected mid‐Tertiary direction. Both alternating field and thermal demagnetization experiments show these magnetization directions to be stable and essentially free from secondary overprinting. The mean paleomagnetic direction for all sites is rotated clockwise 49°. Moreover, different bodies show different amounts of clockwise rotation, with declinations ranging from 27° to 76°. These results can best be explained by the clockwise rotation of several local crustal blocks. It is suggested that these rotations are the result of right lateral shearing acting on extensionally loosened crustal fragments within a pull‐apart basin. This combines two previously suggested ideas, the ‘ball bearing’ rotation hypothesis of Beck (1976) and the pull‐apart basin process discussed by Crowell (1974), both of which appear to be important tectonic processes in the extensively sheared continental margin in California.
Frontal systems during passage of the Martian north polar hood over the Viking Lander 2 site prior to the first 1977 dust storm
Authors: Tillman James E. , Henry Robert M. , Hess Seymour L.
Published: February 2008

Analysis of a 12‐sol period of wind speed, wind direction, temperature, pressure, and optical depth at the Viking lander 2 site presents the first in situ evidence of high‐ and low‐pressure systems, complete with fronts, on the surface of Mars. The discovery of these systems in the landed data occurred while analyzing a period during which the north polar hood was advected over the site at midday, dramatically decreasing the surface illumination and surface‐to‐atmosphere heat flux. This obscuration immediately preceded a global dust storm in the southern hemisphere and low latitudes of the northern hemisphere. The direct effects of the dust storm, other than pressure variations, reached 48°N, the lander 2 latitude, later and more gradually than they reached the 22°N latitude of lander 1. The system responsible for the polar hood passage is a major disturbance, and it appears that radiational damping is inadequate to stop strong frontal formation. The front analyzed, although it was the strongest observed in this sol, is characteristic of a repetitive series of systems that pass roughly every 3.3 sols. These systems are similar to those predicted by theoretical analyses and by general circulation modeling of the Martian atmosphere and those observed in laboratory experiments.
Thermal tides and Martian dust storms: Direct evidence for coupling
Authors: Leovy Conway B. , Zurek Richard W.
Published: February 2008

Observations of surface pressure oscillations at the Viking 1 and Viking 2 lander sites on Mars indicate that the thermally driven global atmospheric tides were closely coupled to the dust content of the Martian atmosphere, especially during northern fall and winter, when two successive global dust storms occurred. The onset of each of these global storms was marked by substantial, nearly simultaneous increases in the dust opacity and in the range of the daily surface pressure variation observed at both lander sites. Although both the diurnal and semidiurnal tidal surface pressure components were amplified at Lander 1 during the onset of a global dust storm, the semidiurnal component was greatly enhanced in relation to the diurnal tide. Semidiurnal wind components were prominent at both lander sites during the height of the global dust storm. We have attempted to interpret these observations using simplified dynamical models. In particular, the semidiurnal wind component can be successfully related to the observed surface pressure variation using a simplified model of a semidiurnally forced Ekman boundary layer. On the other hand, a classical atmospheric tidal model shows that the preferential enhancement of the semidiurnal surface pressure oscillation at Lander 1 can be produced by a tidal heating distribution which places most of the heating (per unit mass) above 10‐km altitude. Furthermore, when a dust storm expands to global scale, it does so rather quickly, and the total atmospheric heating at the peak of the dust storm can represent more than 50% of the available insolation. The Viking observations suggest that a number of mechanisms are important for the generation and decay of these episodic Martian global dust storms.