bluuebunny/crossref_metadata_2025 huggingface dataset:
A thermodynamical study of the Martian permafrost
Authors: Coradini Marcello , Flamini Enrico
Published: February 2008

An outstanding result of the Viking mission has been the detection of water vapor in the martian environment. The only possible reservoir for the water, where no evidence of surface frost has been detected, is the soil. Comparative morphologic analyses of periglacial terrestrial and martian terrains suggested that permafrost can play an important role in forming many surficial structures as the Valles Marineris landslides or the collapse structures in the volcanic region of Tharsis. In this paper a study of the thermodynamics which regulate the processes of thawing and freezing of the layers of permafrost on Mars has been carried out. The equation of the heat propagation has been solved under martian conditions both in the case of periodic variations of the surface temperature and in the case of an anomalous heating of the ground. Seasonal temperature variations do not allow survival of CO2 or clathrate permafrost down to a depth of 15 m. We have demonstrated the existence of a secular ‘active layer’ of about 100‐m depth.
Viking gas exchange reaction: Simulation on UV‐irradiated manganese dioxide substrate
Authors: Blackburn Thomas R. , Holland Heinrich D. , Ceasar Gerald P.
Published: February 2008

Exchange of O2 for H2O, analogous to that recorded on Mars by the Viking GEX experiment has been observed on humidifying powdered β‐MnO2 (pyrolusite) which had been irradiated by UV in a humidified analog of the Martian atmosphere. Pyrolusite irradiated in a dry atmosphere did not release O2 on humidification. The XPS spectra of Mn and O of the reactive pyrolusite were shifted toward higher binding energies during UV irradiation. These shifts are consistent with the creation of a surface layer of a Mn(V) or Mn(VI) compound. The destruction of such a layer on humidification could account for the observed O2 release. Although manganese has not been identified in the Martian regolith, the upper limit of the Mn concentration is sufficiently high that O2 release from pyrolusite could have been responsible for the results of the Viking GEX experiment.
Phase changes and the frequency‐magnitude distribution in the upper plane of the deep seismic zone beneath Tohoku, Japan
Authors: Anderson Roger N. , Hasegawa Akira , Umino Norihito , Takagi Akio
Published: February 2008

The frequency‐magnitude distribution (b value) in the upper plane of the deep seismic zone beneath Tohoku, Japan, shows significant variation with depth which we attribute to dewatering of the oceanic crust during the subduction process. High b value from the surface to 40 km in the upper seismic plane and overlying mantle wedge is thought to be caused by the advection of free water from the oceanic crust. The fact that the b value drops to <0.7 in the upper seismic plane from 40‐ to 70‐km depth reflects the low pore pressures left in the subducting oceanic crust after the advection of this free water. From 70 to 90 km an increase in b to >1.0 is attributed to the passing of the hydrous oceanic crust through a dehydration front. The resulting increase in pore pressure lowers the effective stress and increases the b value in the upper plane. Beneath the volcanic line, b drops back to <0.7, implying that the newly released water advects immediately into the asthenosphere, lowering the mantle melting temperature and producing the calc‐alkaline volcanism on Tohoku. At depths greater than that beneath the volcanic line, the b value again rises, indicating a water source at 100–125 km which might be the further dehydration, of talc, chlorite, and amphibole in the oceanic crust.
Production profile of cosmogenic 53Mn in the Marjalahti pallasite
Authors: Bhattacharya S. K. , Bhandari N. , Perelygin V. P.
Published: February 2008

Cosmic ray induced 53Mn and tracks have been measured in the Marjalahti pallasite. The observed depth profile of 53Mn agrees with the production profile based on the calculations of Kohman and Bender (1967) within ±10%.
Remanent magnetization in the contact zone between Columbia Plateau flows and feeder dikes: Evidence for groundwater layer at time of intrusion
Authors: Buchan K. L. , Schwarz E. J. , Symons D. T. A. , Stupavsky M.
Published: February 2008

The remanent magnetization of basaltic flows in the contact zones of two feeder dikes of the Columbia Plateau series was investigated in detail by af and thermal methods with the aim of estimating the depth of the sampling levels at the time of dike intrusion. In these cases these depths are known from geological and geochemical correlation of the dikes and certain flows stratigraphically higher in the series. In both cases the host rock direction differs strongly from the younger dike direction. In the Fields Spring contact zone, one sample in the profile across the contact displays a hybrid direction between that of the flow and that of the dike. Both remanence components show distinct blocking temperature spectra with an interface at 540°C. This implies that the maximum temperature to which this sample (taken at 60 cm from the contact with the 17.7‐m‐thick dike) was heated was 540°C. Applying heat conduction theory, the maximum temperature increase due to the cooling dike at 60 cm from the contact is calculated to be 614°C, which would imply a significant negative ambient temperature in the host rock prior to the intrusion. Although complicated by the absence of a hybrid sample in the profile across the Almota contact and the occurrence of spontaneous oxidation during heating of the samples, a comparable negative ambient temperature is derived. No evidence for multiple intrusions is observed, and the Almota dike is only 1.8 m thick. Furthermore, substantial heat leakage to the surface can be ruled out because of the relatively large depths of the sampling levels (250 m for Almota and 850 m for Fields Spring) with respect to the short distances to the contacts. For this reason a model is set up based on heat conduction theory allowing for 5% porosity in the flows. Assuming stagnant water in the flows, a reduction of about 9°C of the temperature increase is calculated. Consequently, if the discrepancy is to be explained by evaporation of water, high permeability would be required to allow perhaps 10 times the volume of the stagnant water to be evaporated. High permeability is indicated by the high‐density jointing in the lavas and the present dry condition of the surface. In this case the conclusion is that the groundwater layer extended from a minimum depth of 250 m (Almota sampling level) to at least 850 m (Fields Spring sampling level) at the time of dike intrusion about 15 m.y. ago. A semiquantitative model for convecting groundwater is also presented and, using published data for basaltic flows, is shown to represent a useful model to account for the derived temperatures in the hybrid zone.
129Xe on the outgassing of the atmosphere
Authors: Thomsen Leon
Published: February 2008

Because of the short half‐life of its parent (129I, 17 m.y.) and its own chemical inertness, 129Xe offers unique insight into the problem of the outgassing history of the earth's atmosphere. Because the current atmosphere is different in Xe isotopic patterns than is the interior, the present atmosphere must have been largely outgassed before these patterns finished changing, that is, very early in earth history. Furthermore, there can have been no significant delay in the onset of outgassing; that is, the earth must have been initially hot. Because ancient sedimentary rocks indicate that the atmospheric Xe patterns have not changed substantially for over 3 b.y., models presuming a separate, lower mantle reservoir do not alter these conclusions. Similar conclusions have been reached before but have always carried a heavier burden of debatable assumptions.
A note on the effect of viscosity on double‐diffusive processes
Authors: Kantha Lakshmi H.
Published: February 2008

Double‐diffusive processes, both of the ‘salt fingers’ and ‘double‐diffusive layers’ type have been investigated in the laboratory using glycerine‐water mixture as the fluid medium. The experiments indicate great disparities in the time scales and some in the length scales of resulting structures, when compared with those using water under identical salinity and sugar contrasts. The results are of interest in fields of science other than physical oceanography, such as geology, where double‐diffusive phenomena appear to have their counterparts and could be important in such problems as small‐scale mixing in hot viscous bodies of magma.
Statistics for the one‐norm misfit measure
Authors: Parker Robert L. , McNutt Marcia K.
Published: February 2008

Confidence tables and other parameters are provided for a misfit statistic based upon the sum of the absolute discrepancies between a model and observation. This one‐norm misfit is compatible with the application of linear programing in geophysical modeling, a technique of considerable value when inequality constraints are to be applied in the mathematical model.

Tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Ave., N.W., Washington, D.C. 20009. Document J80‐009; $1.00. Payment must accompany order.
Mapping seismic attenuation within geothermal systems using teleseisms with application to the Geysers‐Clear Lake Region
Authors: Ward Ronald W. , Young Chi‐Yuh
Published: February 2008

Though the exact relationship between temperature and seismic attenuation has not been defined, the thermal regime of geothermal systems exhibit high seismic attenuation. The attenuation of a seismic wave is proportional to Q−1 the reciprocal of the quality factor. The reduced spectral ratio technique has been used to infer Q structure. This technique is adapted to study a geothermal system. A generalized discrete linear inversion technique is applied to the observed differential attenuation data δt* of teleseismic P waves to obtain a discrete Q model. Errors have a large effect on the Q model in a local area. Five sources of errors are involved in the estimation of δt*. Four levels of random errors are added to synthetic data for eight different assumed models, using 12 events and 11 stations. These data are inverted to assess the effect of the observation errors on the inferred Q structures. For signal to error ratio (S/E) above 5 the models are reproducible; between 5 and 1 a dramatic degradation of fidelity occurs; and at 1 or below, numerous low Q cells are identified as high Q and vice versa. Acquisition of more numerous, better distributed data with lower error improves the quality of the inversion. A two‐dimensional Q model of the Geysers‐Clear Lake geothermal area is obtained using previously reported attenuation data of six teleseismic events with S/E of 4. A high/attenuation (low Q) zone is found located in the middle crust centered southeast of Mount Hannah, roughly corroborating the gravity and P wave travel time residual models.
A geomagnetic variation anomaly coincident with the Cascade Volcanic Belt
Authors: Law L. K. , Auld D. R. , Booker J. R.
Published: February 2008

A geomagnetic depth sounding profile in the western Cordillera has revealed concentrations of electric current related to the tectonics of the region. The east‐west profile at approximately 46°30′N extends from the coast to just east of the Cascade volcanic belt. In the western region of the profile the geomagnetic coast effect at a 50‐min period was similar to results in other tectonically active areas such as California and southeastern Australia. Stations near the Cascade volcanic zone had anomalous response functions; a clearcut reversal in both the in‐phase and quadrature components was observed in the direction of the induction arrows across the zone. Based on the similarity between the analyzed data curves and the response of a line current model, the anomaly can be approximated by a long, narrow conductor trending north‐south in direction. This conductive zone does not extend as far north as a profile previously measured in southern British Columbia.
Rate of Cenozoic explosive volcanism in the North Atlantic Ocean inferred from deep sea cores
Authors: Donn W. L. , Ninkovich D.
Published: February 2008

On the basis of all available piston and DSDP cores taken from the seafloor around Iceland an attempt is made to establish a history of major explosive North Atlantic Cenozoic volcanism from the distribution of volcanic ash layers. The earliest sediment reached is early Eocene. After interpolating for missing data and correcting for effects of prevailing winds and regional plate tectonics, the analysis provides an estimate of the rate of explosive Cenozoic volcanism. Two epochs appear outstanding in rates of volcanism; middle Eocene shows the highest rate, and Pliocene, next highest, has about half the Eocene rate. These are followed in decreasing order by Pleistocene, Miocene, and Oligocene. The analysis further suggests that the Cenozoic ash layers originated in subaerial volcanism related to the growth of Iceland.
A gravity traverse across northern Africa
Authors: Brown C. , Girdler R. W. , Renner R. G. B.
Published: February 2008

A military expedition across the Sahara Desert in 1975 provided an opportunity to obtain gravity data where there were previously few measurements. The expedition crossed northern Africa from the Atlantic Ocean to the Red Sea; 608 gravity measurements were made with a LaCoste Romberg geodetic gravimeter. Apart from filling a gap in world gravity data, the traverse revealed a large negative Bouguer anomaly associated with the Jebel Marra volcanic complex and provided a profile of the negative Bouguer anomaly as the Red Sea is approached. Comparison with the U. S. Defense Mapping Agency Aerospace Center (DMAAC) Bouguer gravity anomaly map of Africa lends confidence to the gravity prediction methods used by the DMAAC.
Lithospheric structure in the North Atlantic from observations of Love and Rayleigh waves
Authors: Evans J. R. , Sacks I. S.
Published: February 2008

Love and Rayleigh waves generated by earthquakes on and near the Gibbs Fracture Zone travel along paths nearly parallel to the Reykjanes Ridge. Observations of the dispersion of such wave trains are used to determine models of shear velocity against depth for crustal ages of 0, 20, and 40 m.y. We find that the lithosphere increases in thickness from about 35 km near the ridge axis to about 65 km at crustal age 40 m.y. The increase in thickness is accompanied by an increase in shear wave velocity in the lithospheric lid and probably also in the underlying low‐velocity channel.
Determining coordinates of the rotational pole using satellite data from four sites
Authors: Pisacane V. L. , Dillon S. C.
Published: February 2008

This study investigates how precisely the terrestrial coordinates of the rotational pole can be determined from satellite data from four sites in the United States. The five satellites used are those of the Navy Navigation Satellite System (Transit). Tracking data for these satellites are Doppler observations from sites in Maine, Minnesota, California, and Hawaii. The data used in the study are intermediate data products of the near‐real‐time orbit determination procedure, which is an integral part of the navigation system operations. Six 5‐day intervals were selected for study, one every other month during 1976. Coordinates were determined daily from five satellites. Five‐day averages were used as final estimates. Comparisons were made with extrapolated and final coordinates from the Bureau International de l'Heure (BIH) and final coordinates from the Doppler Polar Motion Service (DPMS). The final coordinates determined in the study agree as well with the final values of BIH as do the results of DPMS. The study implies that low‐cost and near‐real‐time estimates of the terrestrial coordinates of the pole can be made available daily as a by‐product of the routine support needed for the Navy Navigation Satellite System.
Magnetic field measurements in the aftershock region of the Coyote Lake earthquake
Authors: Davis Paul M. , Searls Craig A.
Published: February 2008

Within three days of the Coyote Lake earthquake, M = 5.7, of August 6, 1979, two recording magnetometers were installed on the Calaveras fault, 8 and 13 km south of the epicenter, in order to search for tectonomagnetic effects associated with aftershocks. These instruments supplemented the U.S. Geological Survey (USGS) permanent station and a temporary USGS station installed at about the same time. In addition, eight precise magnetic surveys were made during the month following installation at 30 sites in the region. After removal of ionospheric and magnetospheric disturbances from the records of the continuously recording magnetometers, no coseismic changes were evident greater than the noise level, which for hourly averages was less than 2.0 nanoteslas (nT) peak to peak at the time of the earthquakes. The majority of the differences in magnetic field between the survey sites and one of the continuously recording sites remained constant to ±2 nT. Differences at five sites which lie on the fault changed by slightly more than 4 nT. However, lack of coherence between the observed changes and the background seismicity casts doubt on interpreting them as tectonomagnetic effects.
Evidence for excess pore pressures in southwest Indian Ocean sediments
Authors: Abbott Dallas , Menke William , Hobart Michael , Anderson Roger
Published: February 2008

Brown clay cores from the Madagascar and Crozet basins show the following evidence of excess pore pressures: large amounts of flow‐in, increasing average sedimentation rate with age, and nonlinear temperature gradients. Additionally, many hilltops in these basins have no visible sediment cover. The bare hilltops may result from periodic slumping caused by excess pore pressures. Calculated excess pore pressures which equal or exceed the overburden pressure were inferred from water fluxes predicted by nonlinear temperature gradients and laboratory permeability measurements by using Darcy's law. Since pore pressures which exceed the overburden pressure are unreasonable, we attribute this discrepancy to laboratory measurements which underestimate the in situ permeability. The widespread presence of overpressured sediments in areas of irregular topography provides a process for resuspension of clay‐sized particles. This mechanism does not require high current velocities for the erosion of clay and therefore can be applied to many areas where no strong currents are evident. Carbonate‐rich sediments from the Madagascar Ridge, the Mozambique Ridge, and the Agulhas Plateau had almost no flow‐in and occurred in areas where all topography was thickly draped with sediment. Since the age and tectonic location of the ridges and plateaus preclude water circulation in the basement, we attribute these differences between the brown clay and the carbonate‐rich material to an absence of significant excess pore pressures in the plateau and ridge sediments.
Changes in P wave velocity during operation of a hot dry rock geothermal system
Authors: Fehler Michael
Published: February 2008

P wave arrival times have been picked on seismograms recorded during two dual well seismic experiments, separated in time by one year, that were carried out in the Los Alamos Scientific Laboratory Hot Dry Rock Geothermal Energy Reservoir. Average in situ P wave velocity is computed from arrival times measured along segments of the borehole. Velocity decreases as temperature drops owing to heat extraction from the reservoir. The velocity changes linearly with the change in temperature at a rate of 1.07 (km/s)/(100°C). The linear decrease in velocity leads to the important conclusion that P wave velocity can be used as an effective measure of change in in situ temperature during operation of the geothermal system. The change in velocity is attributed to an increase in the microcrack porosity which is a result of the heat extraction from the rock.
The thermal emission spectrum of Io and a determination of the heat flux from its hot spots
Authors: Sinton William M.
Published: February 2008

Observations of thermal emission from Io in the near infrared made during an eclipse were combined with unpublished 8‐ to 13‐μm intermediate band photometry and a 16‐ to 22‐μm spectrum to specify Io's emission spectrum from 2.2 to 22 μm. Models were calculated having ‘hot spots’ at several different temperatures superposed on a surface, the major part of which is assumed to be at the solar equilibrium temperature. It was possible to fit the entire composite spectrum with this model. It is argued that the total emission from the hot spots can be equated to the nonsolar energy input into Io. The disk‐averaged heat radiated by the hot spots is found to be 180 ± 60 μW cm−2 = 43 ± 14 μcal cm−2 s−l. A possible bimodal temperature distribution of the hot spots is discussed.
Seismicity surveys with ocean bottom seismographs off western Canada
Authors: Hyndman R. D. , Rogers G. C.
Published: February 2008

Three arrays of ocean bottom seismographs have been deployed to study the seismicity at the northern end of the Juan de Fuca ridge system off western Canada. Nearly 100 events were located with estimated accuracies generally better than ±10 km, all lying on or near the en echalon ridge‐transform fault plate boundaries as defined in this area by the magnetic anomalies, the seafloor morphology and by other geophysical data. The depths of 12 events were determined to lie between 2 and 6 km below the top of the crust. The seismograms exhibit clear P and S wave arrivals along with phases that involve P to S and sometimes S to P conversion probably at the base of the sediments beneath the instruments. The event magnitudes have been estimated from signal duration using four calibration events that were well recorded by a land station. The magnitude estimates permit the determination of rough magnitude‐frequency of occurrence relations over the magnitude range of 1 to 3 that are in surprisingly good agreement with the recurrence relations for the area at larger magnitudes from 75 years of land station data. The mean P wave velocity in the uppermost mantle from the earthquake data recorded by the sea floor arrays is 7.6 km s−1 and the mean Vp/Vs ratio is 1.71 or a Poisson's ratio of 0.24.
Isostasy, magma chambers, and plate driving forces on the East Pacific Rise
Authors: Lewis Brian T. R.
Published: February 2008

The density distribution within a cooling plate is calculated, which incorporates temperature and pressure effects. From this density distribution the pressure field within the plate and the gravity field at sea level are computed for various degrees of isostatic compensation. In this model the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a horizontal gradient toward zero age at greater depths caused by the loading of the ocean. Isostatic equilibrium is approached if one allows the loading due to the water to depress the seafloor and at the same time allows mass conservation by flow at depth toward zero age. A viscosity model based on a Newtonian rheology which included temperature and pressure effects has a high gradient close to the plane separating positive and negative pressure gradients which would facilitate the return flow and decouple the lithosphere from the asthenosphere. Addition of a crust to the homogeneous model does not substantially change these conclusions. Comparison of this model with examples of East Pacific Rise data suggests that some areas may not be in complete isostatic equilibrium, implying the existence of horizontal pressure gradients toward zero age in the asthenosphere. This model can be made to fit the general features of the East Pacific Rise but not the detailed gravity and topography near zero age. If one allows convective cooling of the crust by water, partial melting of the upper mantle, and intrusion of this partial melt into the crust, the water depths increase more rapidly near zero age, and an increased positive gravity anomaly is produced over the rise axis, both of which produce a better fit to East Pacific Rise data at 12°N. This study suggests that crustal magma chambers on the East Pacific Rise may be associated with anomalous positive gravity anomalies caused by a positive density contrast between magma and fractured porous crustal rocks.
Sedimentary and crustal velocities in the Norwegian‐Greenland Sea
Authors: Myhre Annik M. , Eldholm Olav
Published: February 2008

Refraction and wide‐angle reflection velocities from the oceanic crust and the overlying sediments in the Norwegian‐Greenland Sea have been analyzed. There is a relatively rapid increase in velocity with depth in the uppermost sediments, whereas a low constant gradient is representative of the main sediment sequence. Two regions, the northeastern Greenland Sea and the oldest parts of the basins, have particularly high velocity gradients, 0.71 and 0.85 s−1 respectively. The high gradient in the northeastern Greenland Sea is associated with the shear‐rifted margin off Svalbard. Relatively high sea floor sediment sound velocities are typical of the Norwegian‐Greenland Sea. The crustal velocities fit the Houtz‐Ewing model which divides layer 2 into layers 2A, 2B and 2C. Average velocities in the Norwegian‐Greenland Sea are: 3.88(2A), 5.18(2B), 6.05(2C), 6.66(3), and 7.80(4) km/s. Layer 2A is age dependent and has not been observed on crust older than 36 m.y. The model is not representative of the uppermost crust near the active spreading axis where one has to introduce additional layers or assume continuous velocity changes with depth.
A comparison of the upper mantle structure beneath North America and Europe
Authors: Burdick L. J.
Published: February 2008

The techniques of modeling upper mantle structure by matching long‐period waveforms with synthetic seismograms have been applied to observations from the tectonically stable part of North America and from Europe. The consistent differences, which can be resolved by the long‐period data between Europe and North America, can be interpreted in terms of variations in the crust, lid, and low‐velocity zone. At epicentral ranges less than 15° the effects of shallow lateral variations are strong and body wave propagation is regionally dependent. Between ranges of 15° and 20°, regional effects are still observed, but they can be explained in terms of variations above 250 km. Beyond 20°, wave propagation appears to be stable and independent of region. Most of the observed long‐period P waves from 20° to 30° are consistent with a single model. This indicates that the relative depths and sizes of the major discontinuities do not vary substantially. A comparison of the upper mantle models of this study with those of other studies indicates that the shape of the P velocity profile is fairly uniform through the transition region (300–700km) though there may be differences in the absolute depth to the discontinuities. These differences could, however, be the result of systematic errors in travel time data.
The determination of displacement fields from geodetic data along a strike slip fault
Authors: Prescott William H.
Published: February 2008

Observations of angles or distances between stations of a geodetic network are commonly used to infer information about the movement of the surface of the earth. The absence of any observations external to the network leads to an ambiguous displacement field. Existing techniques for eliminating this ambiguity are all unsatisfactory in some respect. The best technique, an ‘inner coordinate’ solution, is not appropriate for networks located in a strike slip fault environment. The inner coordinate solution zeros the rotation of all stations about their center of mass. Along a strike slip fault like the San Andreas, however, motion normal to the fault is less likely than motion parallel to the fault. The solution presented here, an ‘outer coordinate’ solution, finds the rotation of the network that minimizes the components of displacement normal to the fault. Since motion along a strike slip fault is generally expected to be parallel to the fault, the displacements obtained with the outer coordinate solution are more reasonable than those obtained with other techniques. Examination of a trilateration network near San Francisco Bay demonstrates the large effect that the choice of adjustment technique can have on the inferred relative motion of the two sides of the fault. The inner coordinate solution gave a rate of about 1 mm/yr, whereas the preferred outer coordinate solution rate was 36 mm/yr.
Repeated precise gravity measurements on Vancouver Island, British Columbia
Authors: Dragert H. , Lambert A. , Liard J.
Published: February 2008

Over the past 3 years, repeated precise gravity measurements have been made on Vancouver Island, British Columbia, to test a practical field technique for precise gravimetric surveys and to monitor possible gravity changes associated with contemporary crustal deformation. In order to obtain a precision for measured gravity differences of a few microgals (1 μGal = 10 nm/s2), rigorous field survey procedures were followed and the operating characteristics and calibration nonlinearities of the LaCoste and Romberg model D gravimeters were taken into account. Instrument evaluations indicate that although the standard error of the mean of several gravity ties is 1.5–2.0 μGal for an individual instrument, the present achievable accuracy of the model D gravimeter for the measurement of temporal gravity changes is 3–4 μGal, provided the dial settings of previous surveys are recovered or both long and short wavelength nonlinearities in calibration can be specified. Semiannual surveys of the central Vancouver Island region indicate significant temporal variations over the past 3 years. Gravity changes as large as 60 μGal at sites adjacent to lake shorelines are caused by the mass effect of changing water levels and can be accounted for by simple model calculations. A gravity effect of seasonal changes in groundwater levels is not identifiable in the data, although easternmost network sites are underlain by porous sediments subject to seasonal water table fluctuations of 1 or 2 m which are expected to cause gravity variations of the order of 10μGal. A spatially coherent pattern of longer‐term gravity trends is indicated in the northern network after corrections have been made for known mass changes. These trends can be interpreted in terms of relative elevation changes of the order of 2 cm/a between Buttle Lake and the area to the northwest. Alternatively, a change in density caused by secular changes of water content in fracture zones or by compression through an accumulation of stress could also cause the observed gravity trends.
Shock‐induced microfractures in six terrestrial igneous rocks characterized with differential strain analysis
Authors: Siegfried Robert W. , McQueen Robert G. , Simmons Gene
Published: February 2008

Samples from a suite of 10 low porosity igneous rocks were shock loaded to low pressures (5 to 10 kbar). The specimens were recovered and analyzed with differential strain analysis (DSA) in order to characterize the microcracks and thus determine the influence of initial sample properties and shock pulse characteristics on shock‐induced microcracks. The DSA results reveal the following: The duration of a shock pulse is an important parameter; those samples that had been subjected to a shock of longer duration had a higher postshock crack porosity. If a sample initially contains a high concentration of cracks closing at a particular pressure, then such a concentration is also present in the postshock sample in addition to a concentration of cracks closing between 100 and 300 bars which is generally produced by the shock process. The shock‐induced cracks in samples with low initial crack porosity close over a wider range of pressures than those in samples with higher preshock crack porosity. Neither the grain size nor the high pressure (2 kbar) compressibility of the samples exerted any influence on the postshock crack distributions. Thus, the distribution of crack porosity with closure pressure of shock‐induced cracks reflects only the shock history and the initial crack distribution of a rock.
The P wave velocity of the uppermost mantle of the Rio Grande Rift region of north central New Mexico
Authors: Murdock James N. , Jaksha Lawrence H.
Published: February 2008

A network of seismograph stations has operated in north‐central New Mexico since 1975. The network is approximately 200 by 300 km in size and encompasses the Rio Grande rift there. Several seismic refraction experiments have been reported in the literature for the region of the network and adjacent areas. Because all of the seismic refraction lines are unreversed, Pn velocities reported were mainly of the inverse travel time slope for the direction of the corresponding line. The values of the inverse slope for those studies range from 7.6 to 8.2 km/s. The purpose of our study is to estimate the P wave velocity of the uppermost mantle by using the time term method. First, we timed the Pn waves of strong signals from five explosions and eight shallow earthquakes recorded by the network. The main data set, which contains 87 time‐distance pairs, was processed by using the time term method. The Pn velocity estimated by this method is 8.0 ± 0.1 km/s. To corroborate this estimate, we then processed 10 subsets of the main data set in the same way. Almost all of the solutions show velocities of 7.9–8.1 km/s, in agreement with the velocity determined for the main data set. The station time terms of the main data set also are substantiated, and they suggest that the base of the crust dips northward by a few degrees in the region of the survey. The smallest value reported by other investigators for the inverse slope (7.6 km/s) appears to be related to the dip. The normal P wave velocity of the uppermost mantle of north‐central New Mexico places restrictions on thermal models of the rift. For instance, the results exclude the likelihood of a wide zone of asthenosphere at the base of the crust beneath the rift, but they do not exclude a narrow such zone.
A remark on the theory of measuring thermal diffusivity by the modified Ångström's method
Authors: Horai Ki‐iti
Published: February 2008

The theory of measuring thermal diffusivity by the modified Ångström's method is extended to the case in which the sample's radiative heat dissipation occurs from its end surface. The formulas deriving the sample's thermal diifusivity from the amplitude decay and phase lag of the temperature wave traveling through the sample are presented for two different sample configurations: one for a flat disk sample for which only the heat dissipation from the sample's end surface is important, and the other for a sample of finite diameter and length for which the heat dissipations from both the sample's side and end surfaces must be considered. For the flat disk sample, measurement at a single angular frequency of the temperature wave provides data necessary for the determination of the sample's thermal diffusivity. For the sample of finite diameter and length, however, measurements of the temperature wave at two discrete angular frequencies are necessary to obtain a complete solution for the thermal diffusivity and other unknown constants. In both cases, to determine the sample's thermal conductivity and the surface conductance of the sample's end surface, the heat capacity per unit volume of the sample must be known. Reanalysis of experimental data on two lunar samples under atmospheric and vacuum conditions shows that for a rectangular parallelepiped, 1 × 1 × 2 cm size, the diffusivity values obtained by the new theories are within 5% of those derived by the conventional theory for a finite bar. Probably, the effect of the sample's geometry will become more distinctive if the samples are more elongated or flatter. The analysis also shows that progressively more refined data are required as the measurement theory becomes more complicated.
Leveling arrays as multicomponent tiltmeters: Slow deformation in the New Hebrides Island Arc
Authors: Bevis Michael , Isacks Bryan L.
Published: February 2008

Two benchmark arrays located on the islands of Santo and Efate in the New Hebrides Island Arc have been releveled 2 to 4 times per year during the past five years (1975–1980). Each array is thereby used as a multicomponent tiltmeter. The arrays are located within 35–50 km of the interplate thrust zone of this convergent plate boundary. Each array has an aperture of about 1 km and in addition contains small subarrays with apertures of about 70 m. This configuration leads to a resolving power (for tilt) of about one microradian. The most important result of this study is that, at the Devils Point array on Efate Island, there has been a consistent tilting down of about five microradians in a direction subparallel to the strike of the arc. This tilting is clearly resolved by both the large array and by the small aperture sub‐arrays. The observations are accounted for by a combination of planar tilting and random error consistent with the observed misclosures. Although tilting is clearly established at the Devils Point array, it is not possible at this stage to establish a unique cause for the observed tilt. Hypotheses consistent with the tectonic setting include interseismic strain accumulation associated with the earthquake cycle of loading and interplate boundary rupture, deformations near a transition between a creeping and a locked segment of the interplate boundary, and near surface deformations associated with magmatic activity and/or block movements also reflected in the pattern of faulting of the uplifted Holocene coral terraces. At the second array at Ratard (Santo Island) no tilt trend clearly emerges from the noise level. However, two marginally significant tile excursions precede in each case large earthquakes located several hundred km north of the array. At the Ratard array the ground noise (small wavelength elevation disturbances and benchmark instability) contributes to residuals almost as much as the measurement error, thereby degrading the threshold level for detection of tilt signals. The difference between the two arrays is not clearly established but seems to reflect an inherently larger level of short wavelength ground noise associated with geological properties of the Ratard site.
A ray parameter‐intercept time spectral ratio method for seismic reflectivity analysis
Authors: Brocher Thomas M. , Phinney Robert A.
Published: February 2008

A method is proposed for obtaining reflectivity magnitudes from seismic record sections that are densely sampled in the horizontal dimension. The technique consists of finding the spectral ratio of subcritical (target) to postcritical (reference) reflection amplitudes. The method's novelty is that both target and reference amplitude spectra are estimated from slant (also known as ray parameter) intercept time, p‐tau or velocity) stacks of the record sections. The source level is directly determined because the reference amplitude is totally reflected; source level estimation does not require knowledge of the impedance structure of the reference reflector.The technique functions well for reflections having low S/N ratios, little or no lateralcontinuity, poor near‐surface velocity control and steep dips. Application of the method to the Consortium for Continental Reflection Profiling (COCORP) survey in the Rio Grande rift supports the interpretation that its large amplitude midcrustal reflection is a magma body.
Spectral properties (0.40 to 0.75 microns) of soils exposed at the Viking 1 landing site
Authors: Guinness Edward A.
Published: February 2008

The bidirectional reflectance and photometric function (Hapke, 1981) was determined for seven patches of soil located near the Viking Lander 1 spacecraft. The soil photometric function is strongly backscattering and has a prominent opposition effect such that the ratio of reflectances at 1° and 10° phase angles averages 1.25, 1.24, and 1.19 in blue, green, and red wavelengths, respectively. The reflectance of the soil also exhibits a wavelength dependence as a function of phase angle. For instance, red/blue ratios can vary by up to 33% as the phase angle increases from 10° to 75°. Estimates of soil reflectance at a 5° phase angle averaged over the blue and green passbands of the Viking Lander cameras are 0.11 and 0.17, respectively, while estimates of soil reflectance averaged over the red channel range from 0.30 to 0.39. There is little need to call upon mineralogical variations to explain the subtle color variations exposed at the landing sites. Rather, brightness and color variations within the soil can be correlated with particle size, with finer‐grained soil being brighter and redder than coarser‐grained soil. When compared to earth‐based reflectance data, the soil at the Viking 1 site is most like Martian bright areas. Such a result is consistent with the Lander soil being part of a globally homogenized soil unit.
Quaternary and geodetically measured crustal movements in the Tokai District, central Honshu, Japan
Authors: Thatcher Wayne , Matsuda Tokihiko
Published: February 2008

Geodetic measurements made in this century demonstrate that modern movements correlate well with deformation that has occurred during the past 2 m.y. Both show effects of subduction and of the Quaternary collision of Izu Peninsula with Central Honshu. Westward from Suruga Bay, deformation is greatest near the coast, decreases markedly towards the uplifting Akaishi Mountains, and then increases again in the conjugate block‐faulted terranes of Chubu district. Quaternary faults, historic earthquakes, and the distribution of horizontal shear straining since ∼1880 all reflect this deformation pattern. Since 1900 the west coast of Suruga Bay has undergone secular subsidence, horizontal contraction and tilting towards Suruga Trough, all typical of the strain buildup process that occurs landward of subduction zones. However, futher westward, active mountain building is occuring, and the Akaishi Ranges are rising 3 mm/a. Unlike the episodic aseismic uplift of the Transverse Ranges of southern California, movements here have been relatively uniform. It seems likely that an increase in the tilt rate of the Suruga coast recognized since ∼1973 is part of a long‐term readjustment to the great 1944 Tonankai earthquake. Data show that a monotonic increase has occurred since at least 1949, and similar effects are seen adjacent to the 1944 rupture zone over 200 km southwest along the Nankai Trough.
Near‐field surface motions excited by radiation from a slip zone of arbitrary shape
Authors: Harris John G. , Achenbach J. D.
Published: February 2008

A ray method is used to calculate the near‐field ground acceleration excited by sliding on a fault plane. The sliding action is modeled by a propagating mode II crack. The crack is planar and it has an edge of arbitrary but smooth curvature. Attention is focused upon the case in which the expanding crack abruptly changes its rate of growth to a slower one. This change in propagation speed causes a burst of wave motion to be emitted. The interaction of the high‐frequency (or wave front) part of the emitted acceleration pulse with the free surface is studied. The near field, where body waves dominate the wave motion at the surface, is considered in some detail.
The magnetic flux linkage of a moving medium: A theorem and geophysical applications
Authors: Hide Raymond
Published: February 2008

The time of change N (S;t) of the ‘number of intersections’ N (S;t) of the lines of force of a magnetic field
with a closed surface S satisfies a certain theorem. This leads directly to a necessary and sufficient condition for self‐exciting hydromagnetic dynamo action (involving inductive interactions between
and fluid motions) in the electrically conducting fluid ‘core’ of a ‘planet,’ in terms of the structure of
at the core surface S0. The condition shows the crucial importance of representing that structure with great accuracy in numerical or analytical models of dynamos, and it suggests that dynamo action might be associated with certain topological properties of the pattern of the normal component of
on S0. The condition also indicates that axisymmetric (but not necessarily steady) magnetic fields cannot be maintained by fluid motions; such fields must ultimately decay away, even in a compressible fluid (contrary to a recent claim in the literature to the contrary). The condition has to be modified when thermoelectric emf's are present, but when these emf's are proportional to
, they cannot prevent the decay of axisymmetric magnetic fields. This result refutes a recent claim that such fields can be maintained by Nernst‐Ettinghausen effects. The theorem for N (S;t) also shows that while fluctuations in
can take place on time scales down to that (τa ) associated with core motions (decades to centuries in the case of the earth), the quantity N (S0;t) cannot change on time scales shorter than the Ohmic decay time τd, (≥ 104 years). This result provides the basis of a method for solving the inverse problem of determining the radius of the core of the planet from observations of secular changes in
near the surface of the planet. The method has been applied to the earth and Jupiter with encouraging results.
Upper mantle viscosity derived from the difference in rebound of the Provo and Bonneville Shorelines: Lake Bonneville Basin, Utah
Authors: Passey Quinn R.
Published: February 2008

Twenty‐four new field measurements of elevation of Provo level and Bonneville level shoreline terraces provide data for reanalysis of isostatic rebound in the Lake Bonneville basin. Analysis of the differential rebound between the Provo shoreline (maximum rebound of 43 m) and the Bonneville shoreline (maximum rebound of 69 m) requires that the latter be an equilibrium shoreline. Within the possible measurement errors, the Provo shoreline formed after the elastic lithosphere had attained at least 91% equilibration to the removal of the water load between the Bonneville and Provo shorelines; within the errors the Provo may also represent an equilibrium shoreline. From the new data presented in this paper, the best estimate of the upper limit of effective viscosity of the uppermost mantle, assuming a half‐space model and a 2000‐year time interval between the Bonneville and Provo shorelines, is 2 × 1019 N s m−2 (2 × 1020 P). In addition, comparison of shoreline rebound profiles, for both shorelines, with theoretical plate flexure models indicates that the mean flexural rigidity of the Basin and Range lithosphere in this region is 1 × 1023 N m, or slightly less.
Earth rotation from lunar laser ranging
Authors: Langley R. B. , King R. W. , Shapiro I. I.
Published: February 2008

We have estimated the variation of latitude and UTO from McDonald Observatory lunar laser ranging observations for the period October 1970 to October 1979. The typical formal uncertainties of our values are about 6 milliarcseconds (marc sec) and 0.5 milliseconds (ms) of time, respectively. We compared our values of variation of latitude with those derived from pole positions determined using classical astrometric, satellite Doppler, and satellite laser ranging techniques for the period July 1976 to November 1978 when data from all of these techniques were available. This comparison shows no evidence for significant high‐frequency fluctuations and, in fact, yields the smallest root‐mean‐square difference of 5.2 marc sec between the lunar laser ranging and the satellite Doppler values, after smoothing the latter with a Gaussian‐shaped smoothing window with a full‐width‐at‐half‐maximum of about 70 days. A comparison of our UTO estimates with those calculated from the circular D smoothed pole position and UT1 values published by the Bureau International de l'Heure (1979 system) gives an rms difference of 1.5 ms for the entire 9‐year span of the laser ranging data.