bluuebunny/crossref_metadata_2025 huggingface dataset:
On the thermal effects of three‐dimensional groundwater flow
Authors: Woodbury Allan D. , Smith Leslie
Published: April 2008

Numerical solutions of the coupled equations of fluid flow and heat transport are used to investigate how the near‐surface thermal regime is perturbed by groundwater flow in a basin with a three‐dimensional water table configuration. We consider specifically those conditions where the hydraulic gradient on the water table drives the flow system, thermally induced buoyancy forces modify but do not control the flow field. The hydrologic disturbance of the thermal field and the significance of a water table gradient transverse to the regional slope depend upon the interplay of the three‐dimensional water table configuration, the basin geometry including the depth to the basal impermeable boundary, the anisotropy, and the permeability of the subsurface formations. These factors act together to determine groundwater flow patterns, depths of circulation along individual flow lines, and areal distribution of groundwater recharge and discharge. The uniformity of surface heat flow values determined from a series of shallow boreholes in an advectively disturbed regime will depend on the location of the measurement sites relative to the hinge line separating areas of groundwater recharge and discharge and on the extent of the region centered about the hinge line where fluid inflow/outflow rates are insufficient to perturb the thermal field.
Porosity of sediments in accretionary prisms and some implications for dewatering processes
Authors: Bray C. J. , Karig D. E.
Published: April 2008

Rapid dewatering, largely through loss of porosity, is one of the most striking changes affecting deformed sediments in accretionary prisms. Analysis of porosity from sample and seismic data confirms that porosity in accretionary prisms is significantly less than that in basinal strata of similar lithology. More specifically, the porosity depth gradient increases arcward across the prism, until porosities less than 10% occur within a few kilometers of the surface. In deeper parts of the prism, dewatering and volume reduction occur not only during loss of porosity but also during dehydration and metamorphic reactions. Calculations involving volume change lead us to define a function, Γ, which reflects total volume reduction. The distribution of the relative intensity of dewatering within the prism can be calculated from the porosity contours and trajectories of sediment elements. An initial attempt indicates that in the toe region the rate of dewatering is accelerated at depth relative to the basinal section and may be greatest near the base of the prism. Toward the rear of the prism the maximum rate of dewatering occurs at progressively shallower depths. Several lines of evidence suggest that, in addition to gradual diffuse dewatering, localized zones of strong dewatering are associated with zones of intense shear. This model of shear dewatering, based on critical state soil mechanics, suggests that sediments adjacent to the basal decollement should be strongly dewatered.
First principles versus spherical ion models of the B1 and B2 phases of NaCl
Authors: Bukowinski M. S. T. , Aidun John
Published: April 2008

The electron density‐functional formalism is used to compute first‐principles equations of state and charge distributions for the B1 and B2 phases of NaCl. The B1 equation of state is in excellent agreement with available static, acoustic, and shock wave data. The theoretical B2 equation of state differs significantly from available diamond anvil data but agrees well with shock wave data. Analysis of the theoretical and experimental isotherms suggest that the former may be a little low in pressure, while the latter is almost certainly too stiff. The computed charge distribution and valence band energies indicate that Cl‐Cl interaction may dominate the energetics of the B2 phase and that, consequently, the Cl ions are significantly distorted. The theoretical zero pressure Cl radius is smaller than the classical crystallographical radius, is sensitive to coordination, and expands significantly across the B1–B2 transformation. In contrast, the Na ion is insensitive to coordination and seems to retain classical ionic properties (+1 charge, spherical charge distribution) up to the highest pressures investigated (∼70 GPa). These properties are largely responsible for the failure of Born‐Mayer‐type structure independent pair potentials to account accurately for the changes in elastic properties of NaCl across the B1–B2 transformation.
Near‐surface stress and displacement in a layered elastic crust
Authors: Leary Peter C.
Published: April 2008

In situ stress measurements on the San Andreas fault are modeled in terms of a solution to the equations of three‐dimensional elastic equilibrium in a layered medium. Crustal stresses are expressed in terms of surface deformations. In particular, vertical stress gradients are shown to be proportional to horizontal gradients of vertical surface deformation. In light of constraints derived by this relation it is proposed to reinterpret the observed near surface in situ vertical stress gradients on the San Andreas fault as partly a manifestation of a layered crust with vertically increasing rigidity. Extrapolation to depth of the revised stress gradients suggests that the San Andreas fault can be regarded as a low‐stress phenomenon compatible with estimates of shear stress obtained from heat flow data.
Airborne microwave measurements of the Southern Greenland Ice Sheet
Authors: Swift C. T. , Hayes P. S. , Herd J. S. , Jones W. L. , Delnore V. E.
Published: April 2008

Microwave remote sensing measurements were collected over Greenland with the NASA C‐130 aircraft used as a platform. The principal instruments were a C band radiometer and an X band scatterometer, which simultaneously collected both active and passive microwave remote sensing data. The results indicate that subsurface inhomogeneities control the scattering and emission process, including anisotropic effects. The results strongly suggest that microwave remote sensing techniques can provide a relative measure of the density and orientation of the volume scatters.
Comparison of SSC magnitudes at Magsat altitudes (325–550 km) and at ground locations
Authors: Kane R. P.
Published: April 2008

A comparison of the total ssc magnitudes at MAGSAT altitudes (325–550 km) and at ground observatories indicated equality, implying virtually no ionospheric contribution and instead, a predominantly magnetospheric origin, at dusk and dawn hours.
Scalar magnetic anomalies of Canada and northern United States derived from Magsat data
Authors: Arkani‐Hamed J. , Urquhart W. E. S. , Strangway D. W.
Published: April 2008

The total magnetic field of the earth over Canada and the northern United States is sampled along individual orbits (passes) of the magnetometer satellite (Magsat) on the basis of data extracted from the Magsat chronicle tapes. The main magnetic field at a given sample point is calculated using the main field model by Langel et al. (1982a) and then subtracted from the sampled value. The external magnetic field contribution is partially removed by a quadratic function fitted to each individual pass. The resulting magnetic residuals are resampled at 55 by 55 km grid points on an equal‐area projection and subsequently contoured. The dawn passes yield a magnetic map that is different than the dusk passes, implying that the remaining external magnetic field component has a dawn‐dusk asymmetry. The component is further suppressed by applying a band‐pass filter in the frequency domain. The magnetic anomaly map derived by averaging the dawn and the dusk band‐pass‐filtered maps shows the main features seen on the aeromagnetic maps of Canada and the United States. The map also shows that the Appalachian and the Cordilleran orogens and the Grenville‐Superior suture zone are magnetically low regions, whereas the craton basins such as the Michigan, the Thelon, and the NE British Columbia Basin are magnetically high areas.
Viscous remanent magnetization model for the Broken Ridge Satellite Magnetic Anomaly
Authors: Johnson B. David
Published: April 2008

A range of crustal models for the interpretation of the Broken Ridge satellite magnetic anomaly have been constructed from bathymetric data. The source magnetizations were assumed to be constant in direction throughout the source region and parallel to the present earth's field at the center of the source region. The model calculations were made at the observation points of a selected set of Magsat Investigator B passes, with a variable magnetic field direction calculated from the MGST (4/81) field model. The horizontal perimeter of all the models was defined by the departure of the Broken Ridge bathymetry from an average oceanic depth of 4.5 km. A topographic model, with an upper surface defined by the bathymetry of the ridge and a lower surface defined by the average oceanic depth, requires a magnetization of 42 A/m to account for the observed anomaly amplitude. An Airy‐type isostatic model, with an upper surface defined by the bathymetry of the ridge and a lower surface predicted from the bathymetry, requires a magnetization of 6 A/m to account for the observed anomaly amplitude. A midcrust model, with an upper surface at the average oceanic depth and a lower surface at the average oceanic Moho, requires a magnetization of 13 A/m to account for the observed anomaly amplitude. A comparison of the locations of the model and observed anomaly peaks shows that the Broken Ridge anomaly can be modeled assuming present field directions. There is not sufficient sensitivity in the data to distinguish between the models on the basis of anomaly location. The model magnetizations vary from 6 A/m, where the whole crust is uniformly magnetized, to 42 A/m, where the topographic expression of the ridge is the source of the anomaly. The strength of the required magnetization contrasts cannot be accounted for by induced magnetization if normal oceanic crustal materials are assumed to be the source for the observed anomaly. Measured and modeled remanent magnetizations of seafloor rocks are typically within the lower part of the range of required values, but the higher magnetizations are associated with very young rocks and dredge samples. It is concluded that the source magnetizations have been enhanced by the development of a viscous remanence effect since the last reversal of the earth's field.
Lateral variations of apparent magnetic susceptibility of lithosphere deduced from Magsat data
Authors: Arkani‐Hamed J. , Strangway D. W.
Published: April 2008

A method for determining the lateral variations of the apparent magnetic susceptibility of lithosphere from a global scalar magnetic anomaly map is presented in this paper. The method is then applied to a magnetic anomaly map of the earth deduced from Magsat data, and a global magnetic susceptibility anomaly map of the earth's lithosphere is derived. The main features of the susceptibility anomalies are as follows: (1) Oceanic regions younger than about 80 Ma, except the South Indian Ocean, have almost no significant susceptibility variations; the anomalies are seen mainly in the older regions. (2) The Circum‐Pacific subduction zones have positive susceptibility anomalies. These phenomena can arise from different thermal states beneath the regions. High temperatures beneath the younger oceanic regions cause demagnetization, while low temperatures beneath the subduction zones enhance the effective magnetization of the lithosphere. (3) Continents tend to have relatively stronger magnetic susceptibility anomalies than oceans, indicating that the oceanic lithosphere is magnetically more uniform than the continental lithosphere. (4) Small‐scale Archean nuclei of shields have positive susceptibility anomalies, while there is no obvious correlation between anomalies and major shields. (5) Most of the aulacogens associated with the rupture of the Atlantic Ocean have negative susceptibility anomalies. These phenomena can be explained by models of the magnetized major shields, which are locally demagnetized and fragmented into several small segments by the hot asthenospheric intrusions into the recent aulacogens and by the underlying hot spots. (6) Some of the sedimentary basins associated with the old rifts have positive susceptibility anomalies, while some of the mountain ranges and modern uplifts have negative anomalies.
Thermal response of a moving lithosphere over a mantle heat source
Authors: Nakiboglu S. Mete , Lambeck Kurt
Published: April 2008

The mid‐ocean bathymetric swells observed around some volcanic island chains can be modeled using the thermal conduction equations of an oceanic lithosphere moving over a heat source located at the base of the plate. Some previous studies of this problem have suggested that the heat source must lie well within the lithosphere in order to explain the observed bathymetry and surface heat flow. These conclusions are based on an isothermal boundary condition at the base of the lithosphere, but this choice requires that the heat source is maintained above this boundary and can actually lead to a flux of heat back into the mantle, particularly in the vicinity of the heat source. If the lower boundary is insulated insofar as the anomalous temperature field is concerned, flux from the heat source back into the mantle is prevented, resulting in a more rapid uplift of, and in an increased heat flow across, the seafloor. Previous solutions have considered only the steady state solutions, whereas the present model also evaluates transient solutions. For rapidly moving oceanic plates the transient effects can be neglected provided that the heat source has been active for at least 107 years and that the direction of motion and the intensity of the heat source have remained constant in this interval. Solutions, with the heat source located near the base of the conductive layer, of the conduction equations for heat flow, bathymetry, and geoid heights in the spatial domain reproduce the principal observational characteristics of the Hawaiian swell at distances away from the immediate origin of the swell, and this part of the swell can be used to estimate the thermal conduction model parameters. The model does not wholly explain the observations near the origin and the mismatch may reflect a dynamic component in the support of the swell.
Quantitative electron probe microanalysis of oxygen in titanomagnetites with implications for oxidation processes
Authors: Furuta Toshio , Otsuki Masayuki , Akimoto Takatoshi
Published: April 2008

The quantitative electron probe microanalysis of oxygen and other elements in variably oxidized titanomagnetites has been used to measure the degree of oxidation in individual grains from margin to center. Low‐temperature oxidation processes of titanomagnetite have thus been demonstrated with natural samples. During oxidation, decreases in iron and increases in oxygen and in titanium have been observed. This finding is consistent with previous studies. The oxidation of titanomagnetite proceeds parallel to an equi‐Ti/O line rather than equi‐Ti/Fe line.
Analysis of regional slope characteristics on Venus and Earth
Authors: Sharpton Virgil L. , Head James W.
Published: April 2008

Regional slope values for earth and Venus are calculated for each 3° × 3° region of topography, and the global characteristics of these magnitudes are examined and compared. The effects on terrestrial regional slope due to ocean loading are treated by approximating the component of seafloor depth due to the weight of the overlying water column and removing this component from the ocean topography. Although regional slopes on Venus and earth span the same range, 0.0°–2.4° (±0.07°), the slope frequency distributions for the two planets differ significantly. The most abundant regional slope values for the terrestrial cases (earth and unloaded earth) are 0.0° and reflect the presence of vast plains associated with the continental interiors and ocean abyssal plains. In contrast, Venus exhibits a peak in slope frequency at approximately 0.09°, perhaps indicating less effective planation processes in the absence of a hydrosphere. Surfaces with regional slope values of 0.07°–0.24° are substantially more common on Venus than on earth. Slopes in this range are associated with discrete features in the lowland and upland rolling plains provinces of Venus and may be indicative of a geologic process that is not commonly observed on earth. Only 14±1% of all Venus surfaces have regional slopes above 0.24°, compared to 26±1% for earth. When mean regional slope is calculated as a function of elevation, a distinctive positive correlation is expressed on Venus. In contrast, the terrestrial cases exhibit a complex relationship marked by four modes related to the presence on earth of an active hydrosphere‐driven weathering regime, a distinctive crustal dichotomy, and plate tectonics. The distinctions in the regional slope characteristics between Venus and earth point to a simpler, less diversified geology on Venus.
Source parameters and faulting processes of the 1959 Hebgen Lake, Montana, earthquake sequence
Authors: Doser Diane I.
Published: April 2008

The August 1959 (Ms = 7.5) Hebgen Lake, Montana, earthquake is the largest earthquake to have occurred in the intermountain region in historic time. Studies of waveforms at regional and teleseismic distances indicate that the main shock of the sequence was a double event consisting of a shock of mb = 6.3 (Mo = 3 × 1018 N m) followed 5 s later by one of m = 7.0 (Mo = 1 × 1020 N m). Comparisons between fault plane solutions from short‐period first motion data, seismic moment tensors determined from the inversion of long‐period body wave data, and observed surface faulting indicate that rupture occurred along one or more fault planes with strike orientations slightly discordant with the trace of surface faulting. A close association between the surface scarps and Laramide thrust faults also suggests that the events may represent normal faulting along reactivated older thrust faults. Focal mechanisms from short‐period first motion data for aftershocks with Ms > 5.5 in northwestern Yellow‐stone National Park showed strike‐slip, normal, and reverse mechanisms with a variety of nodal plane orientations that reflect the complex tectonics of the Yellowstone Plateau. Limited focal depth information suggests a decrease in the maximum focal depth of earthquakes from 15 km at Hebgen Lake to 6–10 km in northwestern Yellowstone National Park.
Flexure of the North American lithosphere above the subducted Mendocino Fracture Zone and the formation of east‐west faults in the transverse ranges
Authors: Glazner Allen F. , Schubert Gerald
Published: April 2008

During the middle and late Tertiary, several prominent topographic scarps, the Farallon plate equivalents of present‐day fracture zones in the Pacific plate, were subducted under the western margin of North America. If subduction occurred at a shallow angle so that the underthrust Farallon plate was in isostatic balance with the overlying North American plate, then differential uplift of the North American plate would have occurred. The Mendocino fracture zone was the tallest of the subducted scarps, and isostasy calculations indicate that the surface of the North American plate south of the trace of the subducted fracture zone should have been 500 to 1000 m higher than the surface north of the trace. Flexure calculations in which the North American plate is treated as a thin slab predict that this uplift should have been spread over a north‐south distance of 70 to 120 km, yielding an average slope of around 0.5°. A regional slope of this magnitude would have been great enough to account for observed late Oligocene and Miocene disruptions of drainage and sedimentation in southern California. Calculated stresses in the upper part of the flexed plate are 50 to 80 MPa. These stresses exceed the tensile failure strength of crustal rocks and suggest that the plate should have failed by tensile cracking. Tensile fracture would decrease the elastic thickness of the plate, enhance bending, and perhaps lead to failure by normal faulting. This mechanism may explain the formation of east‐west faults in southern California.
Gravity changes before and after the Tangshan earthquake of July 28, 1976, and possible interpretation
Authors: Wei Menghua , Zhao Wei , Li Ma
Published: April 2008

The data of 10 years from 1971 to 1981 were calculated and analyzed for this paper. Significant gravity changes were observed before the Tangshan earthquake of July 28, 1976 (M = 7.8), in the epicentral area along two lines that were independent of each other and intersected at Tangshan. The latest repeated surveys before the shock in the two lines were finished on July 6 and 25, 1976, only 22 and 3 days before the event, respectively. Maximum gravity changes at Tangshan of 142 μGal occurred on April 24, 1976, in the line from Beijing to Shanhaiguan and of 194 μGal occurred on July 6, 1976, in the line from Sanhe to Leting. These changes are much larger than 3 times the standard deviation. Because all possible causes of errors and disturbances were considered and corresponding methods were adopted to correct them in the analysis, it is impossible to attribute such large anomalies entirely to any kind of error. After the earthquake, gravity values at most stations decreased sharply, but the anomalies persisted at two stations in Tangshan. Remarkable difference between the gravity changes before and after the earthquake shows strongly that the anomalous gravity changes were associated with the earthquake. The relationship between the changes in gravity and in elevation and groundwater table was studied. The result indicates that the gravity changes were not caused by either the elevation change or the groundwater table.
In situ stress, natural fracture distribution, and borehole elongation in the Auburn Geothermal Well, Auburn, New York
Authors: Hickman Stephen H. , Healy John H. , Zoback Mark D.
Published: April 2008

Hydraulic fracturing stress measurements and a borehole televiewer survey were conducted in a 1.6‐km‐deep well at Auburn, New York. This well, which was drilled at the outer margin of the Appalachian Fold and Thrust Belt in the Appalachian Plateau, penetrates approximately 1540 m of lower Paleozoic sedimentary rocks and terminates 60 m into the Precambrian marble basement. Analysis of the hydraulic fracturing tests indicates that the minimum horizontal principal stress increases in a nearly linear fashion from 9.9±0.2 MPa at 593 m to 30.6±0.4 MPa at 1482 m. The magnitude of the maximum horizontal principal stress increases in a less regular fashion from 13.8±1.2 MPa to 49.0±2.0 MPa over the same depth range. The magnitudes of the horizontal principal stresses relative to the calculated overburden stress are somewhat lower than is the norm for this region and are indicative of a strike‐slip faulting regime that, at some depths, is transitional to normal faulting. As expected from the relative aseismicity of central New York State, however, analysis of the magnitudes of the horizontal principal stresses indicates, at least to a depth of 1.5 km, that frictional failure on favorably oriented preexisting fault planes is unlikely. Orientations of the hydraulic fractures at 593 and 919 m indicate that the azimuth of the maximum horizontal principal stress at Auburn is N83°E±15°, in agreement with other stress field indicators for this region. The borehole televiewer log revealed a considerable number of planar features in the Auburn well, the great majority of which are subhorizontal (dips < 5°) and are thought to be bedding plane washouts or drill bit scour marks. In addition, a smaller number of distinct natural fractures were observed on the borehole televiewer log. Of these, the distinct steeply dipping natural fractures in the lower half of the sedimentary section at Auburn tend to strike approximately east‐west, while those in the upper part of the well and in the Precambrian basement exhibit no strong preferred orientation. The origin of this east‐west striking fracture set is uncertain, as it is parallel both to the contemporary direction of maximum horizontal compression and to a late Paleozoic fracture set that has been mapped to the south of Auburn. In addition to these planar features the borehole televiewer log indicates paired dark bands on diametrically opposite sides of the borehole throughout the Auburn well. Processing of the borehole televiewer data in the time domain revealed these features to be irregular depressions in the borehole wall. As these depressions were consistently oriented in a direction at right angles to the direction of maximum horizontal compression, we interpret them to be the result of stress‐induced spalling of the borehole wall (breakouts).
Well bore breakouts and in situ stress
Authors: Zoback Mark D. , Moos Daniel , Mastin Larry , Anderson Roger N.
Published: April 2008

The detailed cross‐sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data. We show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations. Both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression. This work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ.
Orbital dynamics of a viscoelastic body
Authors: VanArsdale William E.
Published: April 2008

The equations of motion for a system of two deformable bodies are analyzed with a view toward describing orbital evolution. These bodies are modeled as incompressible, viscoelastic solids. The mean tidal moment exerted by each body in a slightly eccentric orbit is obtained directly from the deformation induced by gravitational attraction. The corresponding orbital evolution is described analytically by determining a slowly varying solution to the equations of motion. Results for the earth‐moon system indicate that the mean orbital angular velocity is decreasing more rapidly at present than in the distant past. This change in the angular velocity rate suggests a possible resolution of the time‐scale problem for lunar orbital evolution.
Toroidal Love numbers and transverse stress at the Earth's surface
Authors: Merriam J. B.
Published: April 2008

Stress fields at the earth's surface and the consequent strain fields require a harmonic description which contains both spheroidal and toroidal components. The elastic response of the earth to spheroidal stresses is completely described by well‐known Love numbers, and it is shown here that the Love number concept lends itself to the description of the forced toroidal response as well. The toroidal Love number of degree 1 is shown to bear on problems which involve a change in the rotation of the earth just as the spheroidal Love numbers of degree 1 are related to translations of the earth. Thus the effects of mantle elasticity on wind‐induced changes in the length of day are embodied in the degree 1 toroidal Love number. These effects are found to be below the level of detectability for all but the largest amplitude fluctuations of short period.
Theoretical study of solid NaF and NaCl at high pressures and temperatures
Authors: Hemley R. J. , Gordon R. G.
Published: April 2008

The static and dynamical properties of NaF and NaCl at high pressures in both the B1 (NaCl) and B2 (CsCl) phases are studied theoretically with an improved electron gas model. The model incorporates the effect of crystal stabilization of the ionic charge densities by way of a Watson sphere‐type potential. A series of such calculations as a function of sphere radius is performed to obtain the wave functions of the ions as a function of lattice constant and to obtain the volume dependence of the ion self energy. The latter is shown to make an important contribution to the equation of state. Elastic constants and phonons are calculated in the rigid ion quasi‐harmonic approximation. From the effects of compression on the elastic constants and lattice dynamics, mechanical instabilities in both the B1 and B2 phases are identified. The quasi‐harmonic frequencies are used to calculate the vibrational free energy and pressure contribution to the equation of state. The calculated (parameter‐free) equations of state are found to be in good agreement with the available experimental data for both NaF and NaCl, including the pressure dependence of the thermal expansivity of the B1 phases and recent measurements of the room‐temperature compression curves of the B2 phases. The B1–B2 phase transition is examined in detail, and comparisons with experimental and previous theoretical studies are discussed.
Frictional sliding of dolomite: A variation in constitutive behavior
Authors: Weeks John D. , Tullis Terry E.
Published: April 2008

Fault slip occurs via earthquakes or stable creep depending on the manner in which frictional resistance varies with sliding velocity and displacement. If the form of this dependence is a function of rock type, observed spatial variations in slip behavior along faults could be explained by lithologic variation. We have used a rotary shear apparatus to investigate the frictional response of dolomite marble to changes in sliding velocity. We have modeled the results using a state variable constitutive law for rock friction and a numerical method to solve for the interaction between the friction law and the compliance of our machine. Dolomite marble displays steady state velocity strengthening, as contrasted with granite and quartzite, which show velocity weakening at the same conditions. To model the dolomite results, it is necessary to use two state variables, and surprisingly, the parameters specifying the decay magnitudes associated with these state variables differ in sign from each other. The magnitude of the parameter associated with the longer decay is negative, leading to a downward decay of frictional strength associated with a decrease in velocity. If the shorter decay is of sufficient magnitude, as we observe at low velocity, it is possible for the machine‐rock system to exhibit instability and oscillatory behavior even though the steady state response is velocity strengthening. Another way in which dolomite contrasts with granite and quartzite is that the friction parameters vary strongly with velocity. A transition from unstable to stable sliding with increasing velocity has been observed previously, and our observed variation of the constitutive parameters with velocity can explain this behavior. There is some indication that the evolution of the state variables for dolomite occurs as a function of time rather than displacement, in contrast with granite and quartzite. The frictional behavior of dolomite shows that the constitutive laws that have been developed based on results from granite and quartzite are inadequate to describe all rock types and that more general laws are required.
Assessment of global and regional tectonic models for faulting in the ancient terrains of Mars
Authors: Schultz Richard A.
Published: April 2008

Structural landforms within parts of the heavily cratered terrain of Mars were studied in order to test various models of global fracturing. Landforms analyzed were scarps, grabens, simple ridges, and channel wall scarps. Many of these landforms predate graben formation in Tharsis. Very few structural landforms were found to be either radial to Tharsis or normal to principal stress trajectories calculated for Tharsis isostasy or flexural loading. These results suggest either that the structures are locally derived or, if they were produced by Tharsis activity, that the state of stress in Tharsis was different from that which produced the classic graben systems. Association of narrow valley networks with subdued grabens in one portion of Margaritifer Sinus suggests that one episode of narrow valley formation coincided with flexural loading in Tharsis. Crustal fracturing in Oxia Palus that predates outflow channel formation does not correlate with Tharsis tectonism and may reflect fracturing by the proposed Chryse multiring basin to the north. In contrast to previous studies, no evidence was found for either an orthogonal Martian grid or conjugate sets of shear fractures. It is concluded that Mars probably did not experience significant tidal despinning. Calculations of thermal stress within a cooling early lithosphere performed for varying lithospheric thicknesses predict large‐scale tensile near‐surface failure. Many of the large ridges in Memnonia that predate classical Tharsis‐centered tectonism may reflect normal faulting generated in part by thermal stress.
The 1968 Tokachi‐Oki and the 1969 Kurile Islands earthquakes: Variability in the rupture process
Authors: Schwartz Susan Y. , Ruff Larry J.
Published: April 2008

The May 16, 1968, Tokachi‐Oki (Mw = 8.2) and the August 11, 1969, Kurile Islands (Mw = 8.2) shallow thrust earthquakes occurred along the Japan‐Kurile arc about 500 km apart and possess similar static fault parameters. Their dynamic source processes were investigated using the observable directivity in the source time functions deconvolved from long‐period P waves. The rupture processes of both earthquakes can be understood in terms of a combination of the end‐member asperity and barrier models of earthquake rupture. The 1969 Kurile Islands earthquake ruptured in the sense of the asperity model; one region of the fault plane radiated most of the seismic moment (i.e., the asperity) and is surrounded by the rupture zones of large precursor events. The rupture process of the 1968 Tokachi‐Oki earthquake contains elements of both end‐member rupture models. A dominant asperity is bounded to the south by a region of the fault plane ruptured by a large precursor event and is bounded to the north by a geometric barrier where the plate margin changes direction. This geometric feature is responsible for the abrupt termination of the rupture front. It appears that the tectonic setting plays a vital role in determining the rupture mode. Earthquakes that occur along linear trenches, where no geometric or topographic complications exist, seem to rupture in the sense of the asperity model, whereas earthquakes that occur in the vicinity of such complications are more likely to possess rupture processes intermediate between the two end‐member models.
Regional change of coda Q in the oceanic lithosphere
Authors: Jin Anshu , Cao Tianqing , Aki Keiiti
Published: April 2008

Coda Q of 22 local earthquakes recorded on islands located in young and old oceanic lithosphere areas has been measured. We found that the Q values at periods of from 1 to ∼2 seconds increase from young to old ocean lithosphere in the same way as for the continent, where the intensity of current tectonic activity shows a strong negative correlation with the coda Q. The Q values increase with frequency proportional to fn, where n was found to be 0.46–0.61, with the higher values for younger oceanic lithosphere, which is again similar to the continents where the active regions have higher n values.
Hydrothermal models for the generation of massive sulfide ore deposits
Authors: Lowell Robert P. , Rona Peter A.
Published: April 2008

The discovery of massive sulfide ore deposits at certain sites at active seafloor spreading centers provides a basis for considering the energetics of processes that have concentrated similar large ore deposits that have been preserved in volcanic rocks on land. In this paper we construct transport models based on hydrothermal convection systems in the oceanic crust. We investigate models in which the circulation is driven by heat extracted from the permeable crustal rocks themselves as well as by heat conducted through the roof of a replenished, vigorously convecting crustal magma chamber. If the hydrothermal fluids carry 100 ppm dissolved iron as found in black smokers on the East Pacific Rise and exit at 350°C or greater, calculations show that the heat content of the oceanic crust is insufficient to account for ore deposits of 3 Mt or greater. Heat extraction through the roof of a magma chamber, in conjunction with the formation of a thin (∼1 km) layer of plated gabbro, may account for ore bodies of the order of 3 Mt, provided the permeability of the oceanic crust in the magma‐hydrothermal fluid contact zone is 10−15 m2 or less. If the conducting boundary layer were only 1 m thick, locally, for 103years, the permeability could be as high as 10−13 m2 The permeability in the discharge zone could, conceivably, be two orders of magnitude greater than the permeability at depth. More work is needed on the details of the effects of magma chamber replenishment and off‐axis dike injection and on the role of magmatic fluids. Very large ore deposits (i.e., ∼100 Mt) are probably formed in multiple hydrothermal cycles.
LAGEOS Scientific Results: Introduction
Authors: Cohen Steven C. , Smith David E.
Published: April 2008

The LAGEOS spacecraft is unique in that both its design and orbital characteristics were specifically chosen for high‐resolution geodynamic studies using satellite laser ranging techniques. Since its launch in 1976, the precision of single shot ranging observations made on the satellite has improved from several decimeters to a few centimeters. With this level of precision it is possible to use LAGEOS data for studies of contemporary plate movement, polar motion, gravity, earth rotation, and earth and ocean tides. Inferences about mantle structure and dynamics, angular momentum coupling between the earth and its atmosphere, and perturbative effects acting on earth satellites are being extracted from the data. This special issue of the Journal of Geophysical Research presents the current research results derived from LAGEOS data.
A gravity model for crustal dynamics (GEM‐L2)
Authors: Lerch F. J. , Klosko S. M. , Patel G. B. , Wagner C. A.
Published: April 2008

Goddard earth model, GEM‐L2, has been derived using predominantly the precise laser ranging taken on many satellites, including LAGEOS, in support of NASA's crustal dynamics activities. Only conventional satellite tracking (ground to satellite: GEM 9's data plus LAGEOS ranging) has been employed. The solution consists of 566 gravitational harmonic coefficients (complete through degree 20) and the earth‐fixed coordinates of 173 tracking stations including about 20 used exclusively for LAGEOS. In addition, 5‐day averages of earth orientations (pole position and length of day) have been derived simultaneously from the LAGEOS data for 1979–1981. These latter results have been independently confirmed by very long baseline interferometric observations. The model improves the utility of the LAGEOS data for detecting global movements of the crust and earth orientation while also enhancing our understanding of the long‐wavelength gravity field. In addition to the LAGEOS data, the solution combined laser ranging from nine satellites and optical and electronic measurements on some 20 spacecraft for a total of 1,257,000 observations. GEM‐L2 has improved the modeling of the longest‐wavelength portion of the field by more than a factor of 2 over GEM's developed in the late 1970's. A full discussion of GEM‐L2's accuracy is found elsewhere in this issue. Use of GEM‐L2 yields smaller LAGEOS positioning errors from about 1 m with GEM 9 (1979) to a current value of about 25 cm.
Precision geodesy and geodynamics using Starlette laser ranging
Authors: Marsh James G. , Lerch Francis J. , Williamson Ronald G.
Published: April 2008

The Starlette satellite, launched by the French Centre National d'Etudes Spatiales in 1975, has provided an important means for deriving geodetic and geodynamic parameters from near‐earth satellite laser ranging data. We report here on the development of a gravity field specialized for Starlette, designated PGS‐1331, using the data from 1975 through 1978. We further report on the detailed analysis of the 1980 Starlette laser ranging data for the estimation of tracking station coordinates, polar motion, gravitational constant (GM), and other parameters. These analyses illustrate the improvement in our geodetic and geodynamic modeling provided by PGS‐1331. Data fits for 5‐day orbital arcs have been improved from the level of several meters to a few decimeters. Comparisons of the station coordinate values derived from Starlette with those from independent LAGEOS solutions indicate an rms agreement of about 25 cm. These analyses have provided an independent check of the LAGEOS‐derived center of mass tracking station coordinates.
The orbit of LAGEOS and solar eclipses
Authors: Rubincam David Parry , Weiss Nelson R.
Published: April 2008

An eclipse of the sun by the moon as seen by the LAGEOS satellite can affect the orbital semimajor axis at the centimeter level. The weakened radiation pressure acting on LAGEOS perturbs the orbit differently from that due to full sunlight. This difference amounted to less than 2 mm in the semimajor axis for 23 of the 30 eclipses LAGEOS experienced between launch in 1976 and the end of 1983. However, it was 17.6 mm for the eclipse on March 28, 1979, and 11.2 mm for the one on December 15, 1982. Differences such as these generate large enough along‐track errors to make it worthwhile to include eclipses in complex orbit determination programs such as GEODYN which integrate the orbit. Eclipses cannot explain the presently unmodeled variations in along‐track acceleration which have a magnitude of about 3×10−12 m s−2.
Magnetic and thermal effects of dike intrusions in Iceland
Authors: Kristjansson L.
Published: April 2008

Oriented core samples were collected from 18 lava‐dike contacts in Iceland, in locations of low to moderate (<100°C) regional hydrothermal alteration. Alternating field demagnetization of these samples shows that they generally carry a strong and stable remanence. Magnetic directions in the lavas follow a simple pattern, demonstrating that the remanence of the contact zone has only been reset within very short distances from the contact surface. On average, this distance may be only one tenth of the thickness of a typical dike (4 m). Other magnetic properties (remanence intensity, susceptibility, Curie point) of the contact zone are affected to a lesser or a less regular degree than the remanence direction. It is concluded that for the crustal depth of this study, thermal conduction models do not apply to temperature anomalies set up by dike intrusions or fissure eruptions in Iceland, presumably because of rapid advection of groundwater into the active fissures. These results also indicate that the magnetic anomalies observed over Iceland are only to a small extent caused by dikes.
Dynamics of crustal rifting in NE Iceland
Authors: Björnsson Axel
Published: April 2008

Magnetotelluric measurements have revealed a crustal thickness of 8–10 km in the axial rift zone of NE Iceland and above the proposed hot spot in central east and north Iceland. The crust thickens with age and is 20–30 km thick in the older Tertiary areas to the east and west of the axial rift zone. It also thickens toward north with increasing distance from the hot spot. The crust is underlain by a diapiric updoming asthenosphere over large parts of the country. At the crust asthenosphere interface there is a partially molten basaltic layer a few kilometers thick. This layer partially decouples the crust from the anomalous mantle. Its low viscosity may explain the spatial and temporal instability of the rift axis and the high mobility of the crust during technically active periods. A major volcano‐tectonic rifting episode started in the Krafla area in 1975 involving an 80‐km‐long segment of the plate boundary. Several meters of opening accompanied by subsidence was observed in the center of the axial fissure swarm and contraction and uplift occurred on the sides. The rifting occurred in several short events. Magma flowed horizontally from an upper crustal magma chamber and mixed with magma ascending directly from the semifluid layer at the base of the crust. Crustal accretion was mainly engineered by dike intrusions. Only a minor part of the magma transported reached the surface. During technically quiet epochs, plate‐driving forces, caused by the upwelling asthenosphere, result in long‐term stretching and accumulation of tensile stress in the thinnest part of the crust. The vertical and horizontal displacements during rifting episodes can be explained by elastic rebound, where fissure opening accompanied by magma flow causes the release of tension.
Regional and local vertical crustal movements in northern Iceland, 1965–1980
Authors: Torge Wolfgang , Kanngieser Erich
Published: April 2008

Since 1965, repeated gravity and height measurements have been carried out in northern Iceland across the exposed part of the constructive plate boundary in order to detect vertical crustal movements. Regional movements have been observed across the neovolcanic zone between the western and eastern Tertiary basalt zones. The center of activity is situated below the Krafla caldera, and since 1975 a rift process is affecting the 100‐km‐long and 5‐km‐wide fissure zone associated with this central volcano. Local vertical movements have been investigated by reobserving transverse profiles approximately 10 km south and 15 km north of Krafla. The recent rifting process is characterized by regional uplift and gravity decrease on the flanks and local subsidence and gravity increase in the central fissure zone. The results of repeated levelings and gravity measurements are used to produce common and continuous models of vertical crustal movements.
The relocation of microearthquakes in the northern Mississippi Embayment
Authors: Andrews Mary C. , Mooney Walter D. , Meyer Robert P.
Published: April 2008

Three‐component seismograms, recorded by a small array of digital instruments in the northern Mississippi embayment, consistently show a high‐amplitude phase on the vertical component that arrives approximately 0.8 s before the shear wave. On the basis of its timing and apparent velocity, this phase is identified as an S‐P conversion from the boundary between the unconsolidated Cenozoic sediments and the underlying Paleozoic rocks. Synthetic models of ground motion in the Mississippi embayment indicate that vertical displacement amplitudes are higher for S‐P conversions than for transmitted shear waves at all angles of incidence. The models and their agreement with observations of the three‐component seismograms suggest that true shear waves cannot be reliably identified from vertical component seismograms recorded in this area. The travel times of converted phases are used, together with crustal velocity models derived from a recent seismic refraction survey, to relocate approximately 500 microearthquakes recorded by the Central Mississippi Valley regional seismic network. Since the network data are recorded by vertical component seismographs, we assume that the S phases, observed at the stations sited on unconsolidated sediments, are S‐P conversions. This assumption significantly reduces the average rms residual and provides well‐constrained hypocentral locations for 350 earthquakes. The relocated microearthquakes cluster tightly along previously identified epicentral trends. Focal depths range from 0.5 to 22 km, but events deeper than 14 km are rare, and events shallower than 3 km are confined, almost exclusively, to the area between Ridgely, Tennessee, and New Madrid, Missouri. Focal mechanisms are generally consistent with the results of previous studies. The axial seismicity trend, which extends from Caruthersville, Missouri, to Marked Tree, Arkansas, is characterized by right‐lateral strike‐slip motion on a northeast trending, nearly vertical fault plane. The events between Ridgely, Tennessee, and New Madrid, Missouri, commonly show strike‐slip focal mechanisms, with minor components of both normal and reverse dip slip.