bluuebunny/crossref_metadata_2025 huggingface dataset:
A global geodetic reference frame from LAGEOS ranging (SL5.1AP)
Authors: Smith David E. , Christodoulidis Demos C. , Kolenkiewicz Ron , Dunn Peter J. , Klosko Steven M. , Torrence Mark H. , Fricke S. , Blackwell S.
Published: April 2008

A summary of the results obtained for a new comprehensive geodetic parameter solution from the analysis of LAGEOS laser ranging data for the period May 1976 to the end of 1982 is presented. Estimates of each component of the polar motion and earth rotation, the station coordinates, the value of the earth's gravitational constant GM, and the elements of the LAGEOS orbit comprise this SL5.1AP solution. The results differ from previously published values primarily through incorporation of more rigorous dynamic models for the ocean and solid earth tides. The precision of the geodetic parameters are on average 5‐marc sec polar motion, 0.2‐ms length of day, better than 5‐cm center‐of‐mass geodetic positioning, 3‐cm global baselines, and 2‐cm regional baselines. An assessment of the contribution of systematic errors in the interstation distance determination is presented.
GRIM gravity model improvement using LAGEOS (GRIM3‐L1)
Authors: Reigber C. , Balmino G. , Müller H. , Bosch W. , Moynot B.
Published: April 2008

Improved modeling of the earth's geopotential is one of the major tasks of satellite geodesy. The availability of a new set of precise LAGEOS satellite laser ranging (SLR) data as well as Seasat‐acquired observations enabled us to compute an improved solution in the GRIM model series. The new geopotential GRIM3‐L1 recombined the satellite perturbation normals of the former GRIM3 solution with new normal equation subsystems derived from an additional 16 months of LAGEOS SLR data, oceanic 1°×1° mean free air anomalies recovered from Seasat altimetry, an updated set of terrestrial 1°×1° mean free air anomalies, and several condition equations for zonal and resonant harmonics. The combined normal equations were solved for a complete set of spherical harmonics up to degree and order 36 and the coordinates of 109 tracking stations. Comparisons with altimetry‐derived quantities, terrestrial gravity, anomalies from satellite‐to‐satellite tracking, longitude accelerations of 24‐hour satellites, and satellite orbit fits show a substantial improvement over previous GRIM solutions and indicate that the LAGEOS data have contributed considerably to the reliable estimation of the long‐wavelength portion of the gravity field.
Earth orientation from lunar laser ranging and an error analysis of polar motion services
Authors: Dickey J. O. , Newhall X X , Williams J. G.
Published: April 2008

The rotational orientation of the earth has been determined from lunar laser ranging (LLR) measurements acquired at McDonald Observatory in Texas. The values and uncertainties of UT0 are calculated from 1971 through May 1982 in four forms: raw results on 799 separate days, Kalman‐filtered values, Gaussian‐smoothed values, and Fourier‐smoothed values. The effects of the different smoothings are assessed. LLR, being dominated by a single‐station operation, is sensitive to one projection of polar motion: the component along the McDonald meridian. Variation‐of‐latitude results are presented. For the polar motion component along the McDonald meridian, analyses of the LLR data assess the errors of polar motion sources: Bureau Internationale de l'Heure (BIH), Defense Mapping Agency Hydrographic/Topographic Center (DMAHTC Doppler), the International Polar Motion Service (IPMS), LAGEOS/Goddard Space Flight Center (GSFC), LAGEOS/University of Texas (Texas), and the Lerch analysis of artificial satellite data. The rms errors, of both the LLR data and the determination of polar motion by the various services, decrease in recent times. For data taken from August 1976 through May 1982, Doppler and BIH values are of comparable quality (estimated error: Doppler, 20±5 cm; BIH, 22±4 cm), with IPMS results being somewhat larger (38±5 cm). LAGEOS polar motion error drops in early 1979 in both analyses (GSFC, 14±7 cm; Texas, 18±6 cm, for data from February 1979 through May 1981). This drop parallels instrumental improvements and the increased number of stations in the laser system. The Doppler polar motion error (17±5 cm) is especially low for the period May 1977 through February 1981; the analysis of Lerch (February 1979 through May 1981) gives an error estimate of 14±7 cm. All analyses are consistent with 11 cm as an estimate for the combined LLR modeling, fitting, and instrumental error (noise) for August 1976 to May 1982.
Deformation induced by polar motion
Authors: Wahr John M.
Published: April 2008

Variations in the geocentric position of the earth's rotation axis (polar motion) cause deformation within the earth. We estimate the effects of this deformation on radial and horizontal positions of points on the earth's surface, on baseline lengths between those points, and on surface gravity. The effects of the oceans and of the earth's anelasticity on this deformation are found to be negligible. Peak‐to‐peak variations in radial motion are typically 10–20 mm over a year. Horizontal displacements are less than 7–8 mm. Variations in baseline length can be as large as 30–40 mm. Variations of up to 13 μGal in surface gravity are possible. These numbers are all small enough that they can probably not be used to learn about the earth. However, they are large enough to affect noticeably present high‐quality geodetic observations.
Relativistic effects on an Earth‐orbiting satellite in the Barycenter Coordinate System
Authors: Martin C. F. , Torrence M. H. , Misner C. W.
Published: April 2008

The general relativistic equations of motion and range measurement relations for an earth‐orbiting satellite such as LAGEOS were investigated in the barycenter coordinate system. In comparison with the Newtonian equations, modifications must be made to the satellite acceleration, the light time measurement, station clock times, and station position coordinates. Modifications to the measurement computation are also necessary in order to continue to operate formally in an earth‐centered coordinate system. Limited comparisons of data reduction results using Newtonian equations with results using the Einsteinian formulation show that estimated baselines between LAGEOS tracking stations are affected at the several centimeter level. In three of four cases, rms of fit was slightly improved using the relativistic formulation.
The LAGEOS constraint on deep mantle viscosity: Results from a new normal mode method for the inversion of viscoelastic relaxation spectra
Authors: Peltier W. R.
Published: April 2008

The recently observed acceleration in the node of the orbit of the LAGEOS satellite has previously been shown to provide a good constraint on the viscosity of the earth's mantle beneath the 670‐km seismic discontinuity. This paper explores the extent to which the previously inferred lower mantle viscosity may be traded off against variations of other properties of the radial viscoelastic structure. We show explicitly that the nontidal acceleration of rotation inferred from the LAGEOS data, and predicted as a consequence of the earth's response to the last deglaciation event of the current ice age, is relatively insensitive both to variations of lithospheric thickness and to the presence or absence of plausible chemical heterogeneity across the 670‐km interface. It therefore provides a high‐quality constraint on the viscosity of the deep mantle. With the upper mantle viscosity fixed at the nominal value of 1021 Pa s, the bounds on lower mantle viscosity νLM are such that 2×1021 Pa s ≤ νLM ≤ ∼6×1021 Pa s even when allowance is made for the presence of a viscous boundary layer at the base of the upper mantle such as would exist if the mantle convective circulation were layered.
Iceland geochemical anomaly: Origin, volcanotectonics, chemical fractionation and isotope evolution of the crust
Authors: Oskarsson N. , Steinthorsson S. , Sigvaldason G. E.
Published: April 2008

Iceland is an oceanic plateau produced by channeled flow from the Iceland hotspot. The Icelandic rift system moves WNW relative to the hotspot. About magnetic anomaly 7 time the rift system overrode the hotspot from the east, and since that time the south segment of the Eastern Rift Zone of Iceland has kept position over the hotspot by the initiation of new spreading centers at its east margin. As a result, the volcanism across Mid‐Iceland reworks old crust of the hotspot trail. The Iceland geochemical anomaly is the result of magmatic processes within a chemically layered crust. That layering results from the kinematics of crustal accretion in the rift zones where material, hydrated in the zeolite facies, is brought down into the amphibolite facies where partial melts are formed and separated from the subsiding material. Assimilation of crust‐derived partial melts into ascending mantle‐derived olivine tholeiites is a feedback process resulting in the accumulation of large ion lithophile elements in the crust. That process is most extensive in silicic volcanic centers. The evolution of high 87Sr/86Sr ratios and low 143Nd/144Nd ratios in Iceland reflects long standing accumulation of Rb and light rare earth elements in the crust. The reworking of the hotspot trail, both along the southward propagation of the opening rift in south Iceland and along the eastward migration of the rift segment above the hotspot, involves regeneration of extinct volcanic centers. That process accounts for the observed increase in the 87Sr/86Sr ratio of intraplate centers as they are drifted away from their parental rift zones and for high ratios observed in silicic centers ahead of the propagating rift in south Iceland.
Scarp degraded by linear diffusion: Inverse solution for age
Authors: Andrews D. J. , Hanks T. C.
Published: April 2008

Under the assumption that landforms unaffected by drainage channels are degraded according to the linear diffusion equation, a procedure is developed to invert a scarp profile to find its “diffusion age.” Diffusion age, having dimension [length]2, is the product of diffusivity times chronological age. The second moment of scarp slope grows linearly with age. This fact, together with an assumption about initial scarp shape, allows the inverse determination of diffusion age. Age found assuming a vertical initial scarp is termed “apparent age”; any nonvertical initial scarp profile has a nonzero initial apparent age. True diffusion age differs from apparent age by a fraction of scarp offset squared. The inverse procedure applied to synthetic data yields the following rules of thumb. Evidence of initial scarp shape has been lost when apparent age reaches twice its initial value. If a scarp is formed by two events, the inversion gives their offset‐weighted‐mean age with an error that is a fraction of offset squared. A scarp that appears to have been formed by one event may have been formed by two with an interval between them as large as apparent age. After scarps of two fault traces have diffused to appear as one, the error in inferred age may be as large as half the apparent age. Variation of apparent age along strike would indicate multiple fault traces. The simplicity of scarp profile measurement and this inversion makes profile analysis attractive. If linearity of the flow law, time for a free face to be reduced to the angle of repose, and variation of diffusivity with climate and material could be established, profile analysis would become a reliable dating technique.
A spherical source model for low frequency volcanic earthquakes
Authors: Crosson R. S. , Bame D. A.
Published: April 2008

A resonant magma chamber is modeled by a spherically symmetric whole space consisting of a small gas‐filled chamber surrounded by magma which is encased in country rock. Source boundary conditions are specified on the cavity boundary, simulating a possible sudden gas expansion in the magma. The simplicity of this model leads to an analytical closed form solution for the Fourier transform of radial displacement which predicts a peaked spectrum and an oscillating, decaying seismogram. For physically reasonable values of the rock and magma elastic properties and dimensions of the model, the dominant frequency of the signal is in the range of 1 to 5 Hz, near the values observed for low‐frequency volcanic earthquakes and volcanic tremor. Furthermore, the seismograms and spectra bear resemblance to those observed for low‐frequency earthquakes. The fundamental mode of oscillation is a pure radial “breathing” mode which has markedly different properties from single‐cavity solutions. The fundamental frequency is insensitive to the magma chamber boundary radius when both the inner and outer radii are greater than specified values. This may help to explain the apparent similarity of the dominant frequency of volcanic earthquakes over a wide range of physical situations. The presence of an inner effective “bubble” of a certain minimum size is necessary for the existence of the dominant low fundamental frequency spectral peak. Although highly idealized, we believe that this fundamental fluid‐solid cavity mode may provide an explanation for the observations of tremor and low‐frequency volcanic earthquakes.
Layered convection induced by phase transitions
Authors: Christensen Ulrich R. , Yuen David A.
Published: April 2008

We report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection. Two‐dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the “extended Boussinesq approximation,” i.e., considering the effects of adiabatic gradient, latent heat, and frictional heating in the energy equation. We find that the critical value of the negative Clapeyron slope, which must be surpassed in order to induce layered convection, decreases in magnitude with increasing Rayleigh number Ra in the range 104 ≤ Ra ≤ 2×106. Near the critical Clapeyron slope, vacillations between double‐ and single‐layer convection or strongly leaking double‐layer convection are possible. The breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences. The value of the critical Clapeyron slope also seems little affected by the width of the transition zone or by its depth. A possible superplastic rheology within the transition zone would tend to favor layered convection. Scaling the model results to the 670‐km discontinuity in the earth's mantle as a possible endothermic phase boundary, we estimate the critical Clapeyron slope to be in the range of −4 to −8 MPa/K (−40 to −80 bar/K). The possibility that the spinel → perovskite + periclase transition is within this range appears to be remote but certainly cannot be neglected.
Strain accumulation in the Rocky Mountain states
Authors: Savage J. C. , Lisowski M. , Prescott W. H.
Published: April 2008

Strain accumulation in trilateration networks in Montana, Utah, and New Mexico has been measured for over a decade. The network in New Mexico spans the Rio Grande rift near Socorro. The absence of any observed strain accumulation in that network places an upper limit (2 standard deviations) of 3 mm/a on east‐west spreading across the 100‐km‐wide rift. The network in Utah spans the Wasatch fault near Ogden. Although the 1972–1984 average rate of strain accumulation in that network is not significant at a 2‐standard‐deviation detection threshold of 0.03 μstrain/a, short‐term strain fluctuations of marginal significance have been detected in the network. The network in Montana spans the epicentral area of the 1959 Hebgen Lake earthquake (magnitude 7.1). The principal strain rates observed in that network for the 1973–1984 interval were
and
, extension reckoned positive. The principal extension is perpendicular to the 1959 rupture trace. The deformation of this network suggests rather rapid reloading of the 1959 rupture zone, but whether this is steady strain accumulation or still postseismic relaxation is not known. The strain rate does appear to be constant over the 1973–1984 interval.
Prediction of elastic moduli via crack density in pressurized and thermally stressed rock
Authors: Wang Herbert F. , Heard Hugh C.
Published: April 2008

The elastic moduli of crystalline rocks show marked reductions when heated to high temperatures (T), even when subjected to confining pressure (P). This degradation of moduli has been attributed to the creation of new microcracks or opening of preexisting microcracks from thermal stresses due to differential thermal expansion between mineral grains. A similar lowering of moduli is observed on depressurization of crystalline rock and is also thought to be due to microcrack opening. A spherical inclusion model has been used to predict whether a given change in T or P will introduce a tensile crack between a mineral constituent and the rock matrix. By using the principal thermal expansions and compressibilities of individual minerals, and simplifying assumptions, we are able to predict crack densities as a model rock is subjected to changes of P and T. Experimental crack densities for three granitic rocks, a gabbro, and two basalts are calculated from high temperature, high pressure Young's modulus data. The predicted and inferred crack densities are then compared for these six rocks. The results are dependent upon the P,T path to which the rock is subjected, and comparisons are for the case in which the rocks are heated only at maximum P, and then decompressed isothermally. Considering the simplicity of the model, we believe the comparisons are quite close for the three granitic rocks. The model underestimates the crack densities of the quartz‐free gabbro and basalts.
Geochemical and sedimentological control of the magnetic properties of hemipelagic sediments
Authors: Karlin Robert , Levi Shaul
Published: April 2008

Owing to their high sedimentation rates and wide areal extent, hemipelagic sediments along continental borderlands are potentially important high‐resolution recorders of geomagnetic secular variation. To assess this possibility, we studied suboxic hemipelagic muds from the Oregon continental slope and anoxic laminated diatomaceous oozes from the Gulf of California. These sediments were rapidly deposited, with average sedimentation rates of 121 and 135 cm/kyr, respectively. Bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes. The remanence in both environments resides in fine‐grained magnetite particles, although minor amounts of hematite were observed in the topmost Gulf of California sediments. Despite first‐order differences, the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur (mainly as pyrite), dramatic decreases in natural (NRM), anhysteretic, and isothermal remanent magnetization, intensities, and accompanying shifts in the magnetic stabilities. These changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter. The magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed, then slowly grows finer as the remaining particles dissolve. No evidence was observed for authigenic formation of magnetite. Provided sufficient concentration and stability of the detrital magnetic particles, paleomagnetic directions can survive this dissolution diagenesis. However, relative paleointensity determinations based on normalization of NRM by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments.
Aftershock distribution related to the controversy regarding mechanisms of the May 1980, Mammoth Lakes, California, earthquakes
Authors: Lide Chester S. , Ryall Alan S.
Published: April 2008

Precise master event locations of 344 aftershocks of the May 1980 Mammoth Lakes earthquakes indicate a complex pattern of faulting within the hypocentral region. On the east three of the ML 6 + shocks lie along a N‐S line but are associated with en echelon, vertical clusters of aftershocks striking N15°E, in good agreement with swarms of fractures in the epicentral area and in general agreement with short‐ and long‐period fault plane solutions for the Mammoth Lakes sequence. To the northwest, one of the large events was located at the end of a WNW striking vertical zone of aftershocks along the southern boundary of Long Valley caldera. On the bottom, the hypocentral zone appears to be bounded by a plane striking NNW and dipping east, parallel to the Sierran frontal fault system. The results of this study do not support the idea that the large shocks were associated with the rapid formation of NNW striking vertical dikes.
Earthquake processes in the Long Valley Caldera Area, California
Authors: Julian Bruce R. , Sipkin Stuart A.
Published: April 2008

At least three large earthquakes in 1978 and 1980 near Long Valley caldera have unusual non‐double‐couple mechanisms, unlike those appropriate for shear faulting. This conclusion is supported by short‐period P first motions, long‐period P and SH first motions, surface wave spectral amplitudes and initial phases, and long‐period P and SH waveforms. Two explanations for these anomalous mechanisms have been proposed: (1) simultaneous strike slip and normal motion on separate faults and (2) tensile failure under high fluid pressure. To evaluate these conflicting hypotheses, we have inverted P and SH waveforms recorded by the Global Digital Seismograph Network (GDSN) to obtain moment tensors for the earthquakes and have also generated broadband seismograms by deconvolving short‐ and long‐period GDSN data. The largest event (1633:44 UTC, May 25, 1980) has a duration of about 20 s and can be resolved into three events, separated by intervals of about 7 and 10 s, each having a non‐double‐couple mechanism. This contrasts with previously reported inversion results for other complex earthquakes, all of which have double‐couple subevents. Furthermore, the earthquake of 1450:56 UTC May 27, 1980, appears to be simple and not composed of smaller events. Thus the non‐double‐couple mechanisms are probably intrinsic to the source process and not artifacts caused by misinterpretation of multiple ruptures as single events. Rapid opening of cracks under high fluid pressure is a likely possibility for this process, although its dynamics are complex and not yet fully understood.
Dynamics of an expanding fluid‐filled crack
Authors: Chouet Bernard , Julian Bruce R.
Published: April 2008

The dike intrusion mechanism proposed recently by B. R. Julian and his colleagues for several large earthquakes at Long Valley caldera has stimulated new interest in the mechanics of fluid injection. This study is an attempt to resolve some of the questions raised by this interpretation by numerically simulating the dynamics of a propagating fluid‐filled crack. The computations are based on the two‐dimensional finite difference method applied by Aki and coworkers to a similar problem of a fluid‐driven crack. We extend this earlier study by improving the boundary conditions applied in the plane of the crack and by analyzing a crack containing a viscous fluid that supports acoustic wave propagation. The problem has two time scales: the duration of the rupture, which is proportional to the distance the crack propagates, and the period of the acoustic resonance of the fluid, which is a function of the length of the crack and the acoustic velocity in the fluid. For a small extension of a long crack containing a fluid of low bulk modulus, these time scales are markedly different. The initial motion of the walls near the propagating crack tip is directed outward, so the radiated first motion is compressive everywhere. The increase of the crack tip volume, however, induces a pressure drop in the fluid which propagates over the length of the crack with the velocity of the acoustic wave, causing a partial collapse of the wall radiated as a long‐period dilatation. The dilatation following the short compressional first arrival is well marked in the vicinity of the crack plane, and for a buried vertical crack it is a conspicuous feature of the near‐field vertical ground motion close to the crack trace. These properties of the signal suggest that first motion studies of this frequency‐dependent source may be delicate without broadband instruments. Inhomogeneous waves propagating at the liquid‐solid interface produce high‐frequency vibrations which are observed only in the immediate vicinity of the crack. The source duration depends strongly on the fluid viscosity and associated viscous damping at the crack wall; damping of the motion by the radiation of elastic waves is a comparatively small effect.
Deformation of Long Valley Caldera between August 1982 and August 1983
Authors: Denlinger Roger P. , Riley Francis S. , Boling James K. , Carpenter Michael C.
Published: April 2008

Geodetic and seismic data collected between July 1982 and August 1983 document an important event in the recent evolution of Long Valley caldera. In January 1983 a substantial change in the pattern of deformation was associated with the most intense seismic swarm since May 1980. The earthquake hypocenters of the January 1983 swarm formed a vertical, roughly tabular zone beneath the south moat of the caldera. Analysis of extensive intracaldera horizontal control data indicates that the shear strain distributed throughout this zone could not all be attributed to slip along any single fault plane. The surface deformation of the resurgent dome and caldera floor is temporally associated with the swarm and was caused by an expansion of the upper levels of the residual magma chamber; analysis of the 1983 data indicates a pressure distribution 2–5 km shallower than could be fit by analysis of the 1979–1982 caldera deformation. The volume of this shallow expansion is 10–15% of the whole chamber expansion that occurred between 1979 and 1982. Most of the 1983 deformation occurred between December 14, 1982, and January 20, 1983, and thus was associated with the January 6–7, 1983, swarm, though right‐lateral shear strain continued to accumulate in the south moat through 1983. We conclude that a shallow pressure increase probably occurred within the upper levels of the residual magma chamber of Long Valley caldera during the 5‐week period following December 14, 1982. It is not clear, however, whether this pressure increase preceded or accompanied the January 1983 earthquake swarm.
Implications of silicic vent patterns for the presence of large crustal magma chambers
Authors: Bacon Charles R.
Published: April 2008

On the basis of the distribution of silicic vents, many volcanic fields can be grouped with (1) igneous systems that may be small and whose vent locations are controlled by regional tectonics, (2) those that include sizable crustal magma bodies which erupt at sites determined by their anomalous local stress fields, or (3) relatively small volume systems that are transitional between categories 1 and 2. Linear vent patterns that are aligned normal to the regional least principal stress (σ3) commonly are associated with absence of evidence for large, shallow magma bodies. The Coso volcanic field and the Inyo‐Mono domes in California and probably the South Sister area in Oregon are examples of such systems. The 1960 dacitic fissure eruption at Cordón Caulle in southern Chile evidently is linked to tectonic stress relaxation associated with the great earthquake that occurred 48 hours before the eruption began. Large, shallow magma chambers are thought to perturb the local stress field so that areal patterns of silicic vents are diffuse, radial, or arcuate. Such systems may erupt great volumes of pyroclastic material catastrophically and produce large calderas. Well‐preserved examples of late precaldera leaks of silicic magma occur at Long Valley, California, and Mount Mazama (Crater Lake), Oregon. Other possible examples are noted. Some of these, those which formed calderas smaller than Crater Lake, apparently were preceded by silicic eruptions from aligned vents. Bearing in mind that there exists a transitional group between tectonically controlled small systems and very large magma chambers, vent distributions can be useful in evaluating potential volcanic hazards for silicic volcanic fields.
Geophysical expression of the batholith beneath Questa Caldera, New Mexico
Authors: Cordell Lindrith , Long Carl L. , Jones David W.
Published: April 2008

Gravity gradients delineate uncharacteristically straight, north‐south trending graben faults in the Rio Grande rift west of Questa caldera, and gravity and audiomagnetotelluric (AMT) data show north‐south trending low‐density and high‐resistivity zones from the caldera southward. A gravity inversion technique was used to isolate the gravity anomaly of the caldera and related intrusive rocks from the complicated Bouguer gravity field. The residual gravity anomaly together with AMT data reported in a companion paper by C. L. Long seem to delineate the subcaldera batholith. Geophysical models indicate this to be a north‐south trending dikelike or beamlike body about 25 km long, 5 km wide, and >4 km thick, thickness estimate being limited by the penetration depth of the AMT soundings. The north‐south trend is not consistent with the regional pattern of early Miocene southwest directed extension and instead may reflect passive control by preexisting north‐south strike‐slip faults.
Regional audiomagnetotelluric study of the Questa Caldera, New Mexico
Authors: Long Carl L.
Published: April 2008

Audiomagnetotelluric (AMT) soundings at Questa caldera were made in the course of a mineral resource study of the Columbine‐Hondo and Latir Peak Wilderness areas, New Mexico, in the fall of 1980 and 1981. This regional electrical survey covered an area of approximately 518 km2 and consisted of 92 AMT stations at an average spacing of approximately 3 km. An AMT apparent resistivity map and one‐dimensional models delineate several major lithologic and structural features. A large north‐south resistivity high is interpreted to outline a largely concealed batholith 20 km long, 2–6 km wide, and >4 km thick. Two high‐resistivity east‐west trends coincide with the mapped margins of the Questa caldera, indicating the electrically contrasting rocks at its boundary. Apparent resistivity lows coincide with areas of hydrothermal alteration known to contain stockwork molybdenum deposits.
Links between magma‐tectonic rate balances, plutonism, and volcanism
Authors: Shaw Herbert R.
Published: April 2008

With Smith's (1979) model of ash flow magmatism as the point of departure, quantitative regimes of crustal magmatic evolution are outlined. The average extrusive regime of ash flow magmatism operates at a rate of about 0.001 km3/yr; progressively larger episodes of increasing repose times characterize the evolution of caldera‐forming eruptions. The intrusive regime operates at an inferred average rate of about 0.01 km3/yr expressed in terms of the volumetric influx of mafic magma from the mantle, based on the record of silicic rates of extrusion and a 10:1 intrusive to extrusive ratio derived from evidence of zoned silicic systems. This rate is almost identical with the mean cumulative rate for the Hawaiian‐Emperor system of basaltic volcanism, which is determined to be 0.015 km3/yr. Fluctuations about a mean rate of 10−2 km3/yr lead to a threefold classification of magmatic regimes in terms of source power, depending on whether they are persistently greater than (HHER regime), about equal to (MHER regime), or lower than (LHER regime) the mean Hawaiian‐Emperor rate. Ash flow magmatism correlates with the MHER regime, averaged over a sufficiently long time. Thus the conditions of Hawaiian‐Emperor magmatism are identified with a characteristic power of magma transfer rates that optimize mechanisms for creation of high‐level chambers of derivative and fractionated silicic magma (terminology of Smith (1979)) within sufficiently localized regions of high residence times. These conditions represent a balance point, and a bifurcation, in dynamic paths of magmatic evolution leading to widely divergent types of plutonic‐volcanic associations. Categorical examples of the principal types of igneous path are discussed relative to continental basalt, silicic ash flow, batholithic granite, and intrusive porphyry systems. The Henry Mountains, Utah, are thought to represent an example of an intrusive porphyry generated in the LHER regime of a mafic source system lacking sufficient power for volcanic expression or significant mineralization. Long Valley, California (and possibly other systems along the eastern front of the Sierra Nevada), is an example operating at the mean rate with transient excursions to higher and lower rates. In regions of rapid extension and/or volcanic propagation (whether linear or areal) these regimes give rise to basaltic eruptions ranging from isolated cinder cones to flood basalts.
Can cometary bombardment disrupt synchronous rotation of planetary satellites?
Authors: Lissauer Jack J.
Published: April 2008

The impact crater distribution observed on the moons of Jupiter and Saturn is more symmetric than that expected from cometary bombardment, which should preferentially scar the leading hemisphere of a synchronously rotating moon. The possibility that cometary impacts may have “spun up” these satellites, resulting in an interchange of leading and trailing hemispheres, is investigated. It is found that such a “spinup” is unlikely for the Galilean satellites of Jupiter and for Saturn's inner midsized moons.
Applications and limitations of finite element modeling to glaciers: A case study
Authors: Nixon W. A. , Dowdeswell J. A. , Cooper A. P. R. , Drewry D. J. , Watts L. G. , Liestøl O. , Smith R. A.
Published: April 2008

A finite element model is used to calculate temperature and velocity along a two‐dimensional glacier flow line. The model incorporates automatic element mesh generation, interpolation from point input data to all boundary nodes, comprehensive output graphics, linkage between velocity and temperature under one control program to allow iteration between the two, and time stepping in discrete units. The model results are tested against analytical solutions (for ice temperatures and velocities in simple situations) and field data (ice thickness, velocity, temperature, and mass balance). Sensitivity analyses were conducted with respect to model resolution, changed time step interval, geothermal temperature gradient, and bedrock topography. Limitations and strengths of applying finite element techniques to the modeling of glaciers are discussed. Some problems are encountered with the technique and methods of computation (e.g., limitation of mesh size, discretizing of a continuous body, equation approximation). Others arise from an inability to handle complex ice conditions (e.g., basal sliding and non sliding/sliding transition, cavitation, basal water). The inadequacy of glaciological data sets to provide sufficiently accurate and numerous boundary conditions and input values is a further major problem. Nevertheless, finite element methods hold considerable promise for modeling studies of cold, creep‐dominated ice masses where there are marked gradients in temperature and velocity. These conditions are least well handled by conventional numerical techniques.
Mantle phase transitions and postglacial rebound
Authors: Christensen Ulrich R.
Published: April 2008

The short‐term response of a univariant or divariant isochemical phase boundary to a pressure change is studied. The pressure change can be caused by vertical displacement of material in response to surface loading. The aim is to determine if the density discontinuity is nonadiabatic, i.e., will affect fluid flow, or adiabatic. For the basic state both a quasi‐static mantle with a smooth thermal structure in the transition region and the case of a steady convection current through the boundary are considered. In the latter case the peculiar temperature distribution near the boundary affects significantly the response to additional loading. A univariant transition behaves fully nonadiabatic at low convective velocities below the “Verhoogen limit” but increasingly adiabatic above. A divariant transition behaves partly adiabatic already in a static mantle, and the adiabatic character is strongly enhanced with increasing velocity of convection. With realistic combinations of the relevant parameters for a divariant mantle phase change the response will be hardly more than 10% nonadiabatic, and probably less than 1%. This result affects especially the interpretation of the gravity anomaly above formerly glaciated areas in terms of mantle viscosity and layering of mantle convection.
A Mid‐Ocean Ridge Thermal Model: Constraints on the volume of axial hydrothermal heat flux
Authors: Morton Janet L. , Sleep Norman H.
Published: April 2008

The depth to the axial magma chamber at three oceanic spreading centers, as indicated by seismic reflection surveys, is greater than that predicted by conductive cooling thermal models of ridge crests. The additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust. A theoretical model for the temperature distribution at spreading centers, which includes distributed heat sources and sinks, is presented. By representing the hydrothermal heat loss as a series of heat sinks, the depths to the axial magma chamber at the East Pacific Rise at latitude 9°N., the southern Juan de Fuca Ridge, and the Lau Basin spreading center are modeled. Heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys. The thermal modeling indicates that the axial hydrothermal heat flux is between 7.1×106 and 13.8×106 MJ/m2, or 10 to 20% of the total missing heat at the ridge axis, requiring extensive low‐temperature circulation off axis. High‐temperature vents that are spaced 1 km apart and are active between 4 and 10% of the time would account for the axial hydrothermal heat loss.
Inversion of intermediate‐period Rayleigh waves for source characteristics of the 1968 Tokachi‐Oki earthquake
Authors: Mori Jim , Shimazaki Kunihiko
Published: April 2008

A least squares inversion technique was applied to Rayleigh waves with periods of 10–25 s to determine the distribution of moment release within the source region of the 1968 Tokachi‐Oki earthquake. The inversion procedure attempts to produce synthetic seismograms which match the data by solving for the amount of moment generated from each point of a grid spaced at 40 km. The data were recorded on low‐gain mechanical seismometers that were located 70–200 km from the source region. Assuming simple unilateral and bilateral rupture models, the inversion was run for different values of constant rupture velocity, resulting in a best fit to the data at 3.0–3.1 km/s for both models. The calculated surface wave synthetics depended strongly on the crustal velocity model. By using a velocity structure obtained from a refraction survey, the waveforms of the synthetics became significantly different, depending on whether we used discontinuities or linear gradients to connect the depth points for which the survey gives a refractor P velocity. Neither a structure of only discontinuities nor a structure of only gradients was found to produce satisfactory synthetics. Therefore an intermediate structure with a linear gradient near the surface and discontinuities below was used. The results of the inversion show that the data can be modeled with a source consisting of two strong subevents which dominate the radiation of these surface waves. The relative location of the two subevents is consistent with the locations of the two subevents of high stress drop that were seen on strong motion records.
An ocean bottom seismometer study of shallow seismicity near the Mid‐America Trench offshore Guatemala
Authors: Ambos E. L. , Hussong D. M. , Holman C. E.
Published: April 2008

Five ocean bottom seismometers recorded seismicity near the Mid‐America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench, in the area investigated during Deep Sea Drilling Project legs 67 and 84. Approximately 170 events were recorded by three or more seismometers, and almost half were located with statistical hypocentral errors of less than 10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone northwest of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate‐plate boundary. Their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake‐generating stresses. The data permit construction of a magnitude‐duration scale, calibrated with mb magnitudes for events located by the World‐Wide Standard Seismograph Network (WWSSN) and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 Petatlan earthquake. Most magnitudes ranged between 3.0 and 4.0 mb, and the threshold magnitude of locatable events was about 2.8 mb. Two distinct composite focal mechanisms were determined. One appears to indicate high‐angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone northwest of the array, seems to show normal faulting along possible fault planes oriented quasi‐perpendicular to the trench axis. The normal faulting is consistent with the segmentation of the Cocos plate that has been proposed from land evidence. Such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike. Alternatively, the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the San Jose Canyon, a prominent submarine canyon located farther upslope. Finally, projection of our seismicity sample and of well‐located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati‐Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling, but we suggest that it requires further investigation.
A preliminary harmonic spline model from Magsat data
Authors: Shure L. , Parker R. L. , Langel R. A.
Published: April 2008

We present a preliminary main field model for 1980 derived from a carefully selected subset of Magsat vector measurements using the method of harmonic splines. This model (PHS (80) for preliminary harmonic splines) is the smoothest model (in the sense that the rms radial field at the core surface is minimum) consistent with the measurements (with an rms misfit of 10 nT to account for crustal and external fields as well as noise in the measurement procedure). Therefore PHS (80) is more suitable for studies of the core than models derived with the traditional least squares approach (e.g., GSFC (9/80)). We compare characteristics of the harmonic spline spectrum, topology of the core field and especially the null‐flux curves (loci where the radial field is zero) and the flux through patches bounded by such curves. PHS (80) is less complex than GSFC (9/80) and is therefore more representative of that part of the core field that the data constrain.
An experimental study of chemical remanent magnetizations of synthetic monodomain titanomaghemites with initial thermoremanent magnetizations
Authors: Özdemir Özden , Dunlop David J.
Published: April 2008

We have carried out a laboratory simulation of the chemical remanent magnetization (CRM) of titanomaghemites produced by seafloor oxidation of titanomagnetites carrying a primary thermoremanent magnetization (TRM). The starting material was well‐characterized synthetic monodomain titanomagnetite of composition Fe2.4Ti0.4Al0.2O4, dispersed in CaF2 and sealed in evacuated quartz capsules. First a TRM, JTRM, was induced in a field of either 50 or 100 μT Then the samples were removed from their capsules and oxidized by long heatings in air between 100 and 230°C, well below the Curie temperature of 270°C. Titanomaghemites with oxidation parameters of 0.32, 0.46, 0.62, 0.72, 0.91 and 0.97 resulted. For 0.15 ≤ z ≤, 0.72, single‐phase spinels were produced. Faint hematite X‐ray lines were observed only for the most oxidized samples. During oxidation, a field HCRM of either 50 or 100 μT was applied perpendicular to JTRM The resulting CRM, HCRM was parallel to JTRM and uninfluenced by HCRM throughout most of the single‐phase oxidation range (0.15 ≤ z ≤ 0.62), and remained directionally stable during alternating field (AF) demagnetization to 100 mT. For z ≥ 0.72, JCRM was increasingly deflected toward HCRM For z = 0.97, the deflection amounted to 23.5°, but AF cleaning revealed that JCRM consists of a harder component (coercivities > 35 mT) with deflection 14.5° and a softer component with deflection 38°. We conclude that low and medium degrees of oxidation (z ≲ 0.7) of single‐domain titanomagnetites should not change the original NRM directions of submarine basalts nor the pattern of magnetic stripes over the oceans.
Hydraulic and acoustic properties as a function of porosity in Fontainebleau Sandstone
Authors: Bourbie Thierry , Zinszner Bernard
Published: April 2008

Laboratory measurements have been made of the permeability (k), free porosity (ϕL), compressional velocities (VPorVE), and compressional attenuations (QPorQE) in Fontainebleau sandstone over a continuous range of porosities ϕ from 3 to 28%. This large variation was achieved without any composition change: Fontainebleau sandstone is made of fine quartz grains with regular grain size (≈250 μm). Permeability was measured with a falling head permeameter. Velocities and attenuations were obtained either through an ultrasonic experiment for frequencies around 500 kHz or through a resonant bar technique experiment for frequencies around 5 kHz and in both cases with varying water saturation. The results show an excellent correlation between permeability k and total porosity ϕ for all our samples. For low porosities (ϕ = 3% to 9%), permeability (in millidarcies) is 2.75×10−5(ϕ)7.33, while for high porosities (ϕ = 9% to 28%), permeabilityk(in millidarcies) is given by 0.303(ϕ)3.05. The correlation is also excellent between free porosity and total porosity. On the other hand the correlation between acoustic properties and total porosity is not as clear as for hydraulic properties whatever the frequency (500 kHz or 5 kHz) or the water saturation. On the average, velocity decreases, and attenuation roughly increases with increasing total porosity. Velocity and attenuation values are related to the variation of grain contact structure, and two samples with the same porosity and permeability may exhibit different velocities and attenuations. The clear correlation between hydraulic properties and porosity is related to constant grain size, while the lack of correlation for acoustic properties emphasizes the importance of the microstructure.