bluuebunny/crossref_metadata_2025 huggingface dataset:
Geomagnetic paleointensities from radiocarbon‐dated lava flows on Hawaii and the question of the Pacific nondipole low
Authors: Coe Robert S. , Grommé Sherman , Mankinen Edward A.
Published: February 2008

Radiocarbon ages have been published for nine basaltic lava flows on the island of Hawaii; the ages range from 2600 to somewhat older than 17,900 years B.P. By using the Thelliers' method in vacuum, geomagnetic paleointensity values were obtained from eight of the lavas; the ninth proved unsuitable. The paleointensities for the four youngest flows (2600–4600 years B.P.) yield virtual dipole moments (VDM's) that are 20% greater to more than twice the worldwide values for those times obtained by V. Bucha from archeomagnetic data. The dispersion of virtual geomagnetic poles for the eight lavas is 15.5°, appreciably larger than the average for older lava flows on Hawaii. These results contrast with the historic magnetic field in the region of Hawaii, in which both secular variation and nondipole components are very low. At about 10,000 years B.P. the measured VDM is not very different from the long‐term worldwide average but differs considerably from a smooth extrapolation of Bucha's average curve. At about 18,000 years B.P. the measured VDM is very low and is associated with an unusually shallow paleomagnetic inclination for the latitude of Hawaii. These new paleointensity and paleodirectional data strongly suggest that sizable nondipole geomagnetic fields have existed in the vicinity of Hawaii at various times during the Holocene epoch and perhaps earlier.
Analysis of ablation debris from natural and artificial iron meteorites
Authors: Blanchard M. B. , Davis A. S.
Published: February 2008

Artificial ablation studies have been performed on iron and nickel‐iron samples by using an arc‐heated plasma of ionized air. Experiment conditions simulated a meteoroid traveling about 12 km/s at an altitude of 70 km. The artificially produced fusion crusts and ablation debris show features very similar to natural fusion crusts of the iron meteorites Boguslavka, Norfork, and N'Kandhla and to magnetic spherules recovered from deep‐sea Mn nodules. X ray diffraction, electron microprobe, optical, and scanning electron microscope analyses revealed that important mineralogical, elemental, and textural changes occur during ablation. Some metal is melted and ablated. The outer margin of the melted rind is oxidized and recrystallizes as a discontinuous crust of magnetite and wüstite. Adjacent to the oxidized metallic ablation zone is an unoxidized metallic zone in which structures such as Widmannstatten bands are obliterated as the metal is transformed to unequilibrated α2 nickel‐iron. Volatile elements such as S and P are vaporized in the ablation zone. Less volatile elements such as Si and Mn undergo fractionation and are concentrated in the oxide phases, while Ni is concentrated in the metal phases. Dissimilar phases form an intimate intergrowth that persists to a submicroscopic level. Identification of meteor ablation debris in particle collections cannot be based on a single parameter; elemental and mineralogical composition as well as morphological and textural features must be considered.
The control of volcanic column heights by eruption energetics and dynamics
Authors: Wilson L. , Sparks R. S. J. , Huang T. C. , Watkins N. D.
Published: February 2008

The height reached by a volcanic eruption column, together with the atmospheric wind regime, controls the dispersal of tephra. Column height is itself a function of vent radius, gas exit velocity, gas content of eruption products, and efficiency of conversion of thermal energy contained in juvenile material to potential and kinetic energy during the entrainment of atmospheric air. Different heights will be attained for the same total energy release depending on the style of the eruption: a discrete exptosion produces a transient plume, whereas a prolonged release of material forms a maintained plume. A maintained eruption plume will also be formed if discrete explosions occur within a few minutes of one another, and eruptions producing large volumes of tephra commonly lead to maintained plume formation. Observed eruption columns from eight eruptions with cloud heights in the range 2–45 km and volume rates of magma production in the range 10 to 2.3 × 105 m3/s are compared with predicted values deduced from theoretical relationships for fluid convection. Theoretical model heights were calculated in two ways: first, for a wide range of eruptive conditions by using a dynamic model of eruption column formation and second, by using a theoretical formula relating height to rate of thermal energy release. Results from the two calculations were found to agree well and furthermore showed satisfactory agreement with the eight observations. Expected cloud heights can be usefully expressed as a function of heat release rate, expressed as the equivalent volume eruption rate of magma, for three different values of the efficiency of heat use. The results imply that many eruptions involve highly efficient use of the released heat, which indicates that the particle sizes in these eruptions are sufficiently small to allow rapid heat transfer to air entrained into the column. For certain combinations of vent radius, gas exit velocity, and gas content, column collapse to form pyroclastic flows should occur. Cloud heights have been calculated for a wide range of permutations of these parameters corresponding to the onset of collapse. The maximum theoretical height expected for a stable maintained plume is about 55 km, corresponding to a volume eruption rate of 1.1 × 106 m3/s.
Tidal perturbations on the satellite 1967–92A
Authors: Felsentreger T. L. , Marsh J. G. , Williamson R. G.
Published: February 2008

The orbit of the 1967‐92A satellite has been studied to ascertain the extent to which tidal forces contribute to orbital perturbations. This study has permitted an estimation of the magnitudes of ocean tide effects on the satellite's inclination, in particular for the M2 and S2 constituents. The ocean tide estimates are based upon the use of a value of 0.3 for the solid earth tide Love number k2 and a lag angle of 0° in the orbit determination procedure. The amplitudes and phases of these tidal effects are in good agreement with those calculated from numerical models of the M2 and S2 tides derived from surface data. The results are as follows:

Supplement table is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N. W., Washington, D. C. 20006. Document J78‐002; $1.00. Payment must accompany order.
Prehistoric large earthquakes produced by slip on the San Andreas Fault at Pallett Creek, California
Authors: Sieh Kerry E.
Published: February 2008

Late Holocene marsh deposits composing a terrace about 55 km northeast of Los Angeles, California, contain geologic evidence of many large seismic events produced by slip on the San Andreas fault since the sixth century A.D. I excavated several trenches into the deposits in order to study this evidence. The principal indicators of past events are (1) sandblows and other effects of liquefaction, (2) the termination of secondary faults at distinct levels within the stratigraphic section, and (3) sedimentary deposits and faulted relationships along the main fault. The effects upon the marsh deposits of six of the eight prehistoric events are comparable to those of the great (Ms = 8¼+) 1857 event, which is the youngest of the nine events disturbing the strata and is associated with about 4½ m of right lateral slip nearby. Two large events may be smaller than this. Radiocarbon dates indicate that the events occurred in the nineteenth, eighteenth, fifteenth, thirteenth, late twelfth, tenth, ninth, seventh, and sixth centuries A.D. Recurrence intervals average 160 years but vary from ½ century to about 3 centuries. The dates may indicate a fairly systematic pattern of occurrence of large earthquakes.
Geodetic strains and large earthquakes in the axial tectonic belt of North Island, New Zealand
Authors: Walcott R. I.
Published: February 2008

The axial tectonic belt is a belt of active deformation within the boundary zone between Indian and Pacific plates that crosses both North Island and South Island between sub‐duction zones northeast and southwest of New Zealand. Shear strains determined from triangulation data within the axial tectonic belt on the east coast of North Island show variations in rate and direction that are related to the great 1931 earthquake of Hawke's Bay (province around Hawke Bay). This earth‐quake was associated with thrusting on a fault parallel to the axial tectonic belt and involved a shortening of about 2–3 m. Before the earthquake, shear strains showed a large component of compression normal to the belt and, after, a large component of extension. The Pacific plate is being subducted beneath the axial tectonic belt of North Island along a shallow dipping (about 12°) thrust some 250 km wide from the trench to where the plate abruptly bends and plunges into the asthenosphere. It is suggested that (1) the compressional stage results from locking of the subduction thrust with plate movement accumulating as an elastic strain, (2) the earthquake is caused by rupturing of the locked zone, and (3) the following extension is a relaxation and gravitational spreading within the lithosphere above the thrust. In southern North Island a phase of extension normal to the axial tectonic belt occurred up to about 1920, since which time compression has been occurring. The onset of compression is believed to mark the locking of the subduction zone. Sufficient time has elapsed for the accumulated strain to be large enough to cause a major earthquake.
Mineralogy and geochemistry of the Michigan deep hole metagabbro compared to seawater hydrothermal alteration
Authors: Stakes Debra S.
Published: February 2008

Gabbro recovered from the deep drill hole of the Michigan Basin has been altered to secondary minerals indicative of the lower greenschist facies. A metagabbro from the East Pacific Rise (EPR) and two greenstones from the Mid Atlantic Ridge (MAR) have similar secondary minerals that resulted from submarine hydrothermal alteration. The albitization of the plagioclase in the Michigan sample (MDH) provided Al and Ca for chlorite, tremolite and epidote formation. The clinopyroxene shows minimal alteration. By comparison the plagioclase in the MAR greenstones is altered to albite and epidote or calcite with the primary pyroxenes showing variable alteration. The predominant alteration phase for both the Michigan gabbro and the MAR greenstones is an aluminous(IIb) chlorite polytype with no associated expandable clays. The EPR metagabbro is similarly altered to chlorite and actinolite, but saponite, vermiculite, and talc are also present. Temperatures of alteration for MDH and MAR can be estimated at 200°–300°C. The absence of smectite indicates that both MAR and MDH were altered under low water‐rock ratios. The preservation of the primary pyroxene and the preponderance of chlorite in MDH suggest lower Pco2 and lower fo2 for this sample than for any of the oceanic samples. The mineralogical changes in MDH appear to be accompanied by a loss of Si and Ca, with an increase in Mg observed in the bulk rock composition, similar to seawater hydrothermal alteration.
Rb‐Sr geochronologic analysis of metagabbro at the bottom of the Michigan Basin deep drill hole
Authors: Van Schmus W. R.
Published: February 2008

Because of the highly altered nature of the metagabbro at the base of the McClure 1 Sparks deep well in the Michigan basin, no meaningful age can be determined on that unit by using Rb–Sr techniques.
Electrokinetic and magnetic anomalies associated with dilatant regions in a layered Earth
Authors: Fitterman David V.
Published: February 2008

According to the dilatancy‐diffusion earthquake model, there will be fluid motion into a dilatant zone prior to an earthquake. One possible consequence of this fluid motion is the generation of an electric potential anomaly by means of electrokinetic processes. A surface electric potential anomaly will not be produced unless there is a boundary separating regions of differing streaming potential coefficient and there is a component of pressure gradient parallel to this boundary. The magnetic anomaly produced by the current flow is calculated. It is shown for the case of an n‐layered half space that the surface magnetic field will be identically zero no matter what the pressure distribution is. There will, however, be an azimuthal component of the magnetic field inside the earth.
Paleomagnetic results from the Ordovician Moccasin, bays, and Chapman Ridge Formations of the Valley and Ridge Province, eastern Tennessee
Authors: Watts Doyle R. , Van der Voo Rob
Published: February 2008

New results from the late Early (middle) Ordovician Moccasin, Bays, and Chapman Ridge Formations of the Central Appalachian Mountains indicate that the Early Ordovician was a period of relatively rapid apparent polar wander. The available reliable Late Ordovician poles (Juniata, Beemerville, Trenton) lie in a cluster near 35°N, 120°E separated from the Late Cambrian poles (Wilberns, Lamotte, Wichita Granites) by about 50° of arc. High blocking‐temperature components with positive fold tests from the Moccasin and Bays Formations yield a pole position intermediate between the two clusters at 33°N, 147°E (K = 135, A95 = 5.8°, N=6 sites). High blocking‐temperature components, also with positive fold tests, from the underlying Chapman Ridge Formation give a pole position that agrees well with the Juniata results, and is about 30° from the Moccasin‐Bays pole (27°N, 112°E, K = 38.3, A95 = 15.0°, N = 4 sites). At least two interprettions of these data are possible: (1) the Chapman Ridge Formation acquired its characteristic magnetization during the Late Ordovician or Early Silurian sometime after deposition but before its folding and (2) the path of apparent polar wander with respect to North America described a loop during the Ordovician.
Thaumasia: A fossilized early forming tharsis uplift
Authors: Frey Herbert
Published: February 2008

The structure of the present‐day Valles Marineris rift canyons and related features is a consequence of episodic crustal uplift, dominated by two major events. The first of these occurred near the present‐day Thaumasia crustal dome and produced a structural pattern in old plains‐forming units which is principally transverse to the main trend of the Valles Marineris canyons. This pattern has provided some structural control over later tectonic and volcanic events. The major crustal deformation in this region is associated with the younger Tharsis uplift, which is responsible for the major trends observed in the Valles Marineris. These two distinct structural patterns are best observed in the resistant, least eroded parts of the canyon walls; linear trends in such resistant points provide a closer approximation to the original bounding scarps of the Martian rifts than do the modern, highly eroded canyon walls. Crustal deformation was dominated by the Tharsis uplift but began much earlier in Martian history.
Detrital magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona
Authors: Elston Donald P. , Purucker Michael E.
Published: February 2008

Specularite, apparently formed and in part altered to red hematite before deposition, carries much of the stable magnetization in sandstone, siltstone, and claystone of the Moenkopi Formation (Lower Triassic) at Gray Mountain, northern Arizona. High‐temperature exsolution textures seen in polished section indicate that some, if not much, of the specularite is detrital. Progressive thermal and chemical demagnetization analyses indicate that the most stable magnetization resides in specularite. Last, a systematic depositional inclination ‘error’ is seen in both normally and reversely polarized sandstone. Part of its cause was the deposition along fine bedding laminations of subtabular grains of specularite carrying a strong direction of magnetization in the plane of tabularity. Magnetic inclinations are significantly shallower in sandstone than in siltstone, and the amount and sense of inclination error are controlled in detail by the dip of the horizontal and cross laminations. The foregoing leads to the conclusion that the stable magnetization is a detrital magnetization acquired during the course of deposition and that the Moenkopi Formation thus is suitable for magnetostratigraphy. Neither the formation of specularite nor the acquisition of the stable magnetization appears to be the consequence of prolonged and complicated postdepositional chemical alteration, a process invoked by some workers for formation and magnetization of ancient red beds. Because specularite is common in other ancient red bed sequences, their stable magnetizations also may be partly or largely detrital.
Tsunami earthquakes and subduction processes near deep‐sea trenches
Authors: Fukao Yoshio
Published: February 2008

A tsunami earthquake is defined as a shock which generates extensive tsunamis but relatively weak seismic waves. A comparative study is made for the two recent tsunami earthquakes, and a subduction mechanism near a deep‐sea trench is discussed. These two earthquakes occurred at extremely shallow depths far off the coasts of the Kurile Islands and of eastern Hokkaido on October 20, 1963, and on June 10, 1975, respectively. Both can be regarded as an aftershock of the preceding larger events. Their tsunami heights and seismic wave amplitudes are compared with those of the preceding events. The results show that the time constants involved in the tsunami earthquakes are relatively long but not long enough to explain the observed disproportionality between the tsunamis and the seismic waves. The process times are estimated to be less than 100 s. The spatio‐temporal characteristics of the two events suggest that they represent a seaward and upward extension of the rupture associated with a great earthquake which did not break the free surface at the coseismic stage. The amplitude and phase spectra of long‐period surface waves and the long‐period P waveforms indicate that this extension of the rupture did not take place entirely along the lithospheric interface emerging as a trench axis. It rather branched upward from the interface in a complex way through the wedge portion at the leading edge of the continental lithosphere. This wedge portion consists in large part of thick deformable sediments. A large vertical deformation and hence extensive tsunamis result from such a branching process. A shallowest source depth, steepening of rupture surfaces, and a deformable nature of the source region all enhance generation of tsunamis. The wedge portion ruptured by a tsunami earthquake is usually characterized by a very low seismic activity which is presumably due to ductility of the sediments. We suggest that this portion fractures in a brittle way to generate a tsunami earthquake when it is loaded suddenly by the occurrence of a great earthquake and that otherwise it yields slowly. Upward branching of the rupture from the lithospheric interface produces permanent deformation of the free surface which is relative uplift landward and relative subsidence trenchward of the zone of surface break. This surface break zone geomorphologically corresponds to the lower continental slope between the deep‐sea terrace and the trench. Such a mode of permanent deformation seems to be consistent with a rising feature of the outer ridge of the deep‐sea terrace and a depressional feature of the trench. This consistency implies a causal relationship between great earthquake activities and geomorphological features near the trench.
Sources of seismic events in the cooling lava lake of Kilauea Iki, Hawaii
Authors: Chouet Bernard
Published: February 2008

Seismic surveys conducted in recent years revealed a surprisingly high and sustained activity of local seismic events originating in the partially frozen lava lake of Kilauea Iki crater, Hawaii. About 8000 events per day were counted in 1976 at the center of the lake with a seismograph having a peak magnification of 280,000 at 60 Hz. The activity was found to be uniform over the whole area above the inferred magma lens and very weak in the periphery of the lake. The frequency‐amplitude relation for these shocks obeys the Ishimoto‐Iida or Gutenberg‐Richter law very well, with a b value of 1.19 (±0.06). Locations of a few selected events indicate that they occur both above and below the layer of melt, although the seismic activity appears to be much higher in the upper crust. Whenever clear, the first motion is always outward from the source, suggesting that a crack opening under tensile stress owing to cooling is the responsible source mechanism. A simple model of a circular tensile crack nucleating at a point and growing at subsonic velocity can match the far‐field P wave from these sources fairly well. Typical parameters for a large event inferred from the model are the following: radius, 2.7 m; maximum static tensile displacement between crack faces, 2.9μ; cavity volume, 4.4×10−5 m3; and a seismic moment tensor with diagonal elements only, having the values 3.8×1012, 4.5×1012, and 3.8×1012 dyn cm. The magnitude of the event is about −1, and its stress drop is of the order of 0.01 bar. A Q as low as 10 is required to satisfy the shape of the observed wave forms. Using the observed b value, the total cavity volume integrated over all cracks which is generated daily in the upper crust of Kilauea Iki is of the order of 1–20 m3, in rough agreement with the amount of volume contraction expected on the basis of our knowledge of the thermal history of the crust. As an alternate interpretation of the data, a model is proposed which views the seismic activity as reflecting the extension by up to several tens of centimeters of long existing cracks rather than the formation of new cracks. Discrimination between the two types of models does not appear to be feasible with the present available data.
Geoid anomalies and the thickness of the lithospere
Authors: Turcotte D. L. , McAdoo D. C.
Published: February 2008

Over broad regions of the earth, such as stable continental interiors and deep ocean basins, isostatic equilibrium is a good approximation. The isostatic requirement relates the density distributions in the continental and oceanic lithospheres. These density distributions are the result of differences in both composition and temperature. In regions of isostatic equilibrium the dipole moment of the near‐surface density distribution is proportional to the local geoid anomaly. By using accepted crustal density distributions and either error function or linear temperature distributions the difference in geoid height between stable continental areas and deep ocean basins has been determined as a function of the continental lithospheric thickness. If the continental lithosphere were greater than 200 km thick, the geoid anomaly over the continents would be systematically negative in relation to that over the ocean basins. By using the GEM 9 satellite geoid the mean geoid anomalies over ocean basins and stable continental areas have been obtained. No systematic difference between continental and oceanic geoids is observed. We conclude from our geoid studies that the thickness of the continental lithosphere is near 180 km. This is in good agreement with various interpretations of the surface heat flow observations.
Flat Earth upward continuation
Authors: Shebalin John V.
Published: February 2008

For a thorough understanding of flat earth upward continuation, Poisson's integral equation is transformed from a coordinate system with spherical symmetry to one with planar symmetry. The two separate cases of infinite and finite flat earths are considered. In the resulting exact forms a functional factor in the integrands is isolated, a factor which allows the translation of a spherical earth surface potential into an exact flat earth surface potential. In the infinite flat earth case the ‘upward continuation’ of Heiskanen and Moritz is discussed and is seen to result when the corresponding functional factor is set equal to 1.
Thermal infrared properties of the Martian atmosphere: 3. Local dust clouds
Authors: Peterfreund Alan R. , Kieffer Hugh H.
Published: February 2008

Prior to and during the Martian global dust storms in 1977, local dust clouds were identified from thermal observations. Local dust clouds appear up to 40 K cooler than the surrounding region in the daytime and appear warm at night. In contrast to their well‐developed and consistent thermal signatures, local dust clouds exhibited little albedo contrast. Multiple observations of the same areas show that local dust clouds form and dissipate in a few days or less, and clouds were observed to move with velocities of from 14 to at least 32 m s−1. Local dust clouds are common along the receding edge of the south polar cap (observed primarily for orbiter imaging) and in the restricted area −10° to −30°, 80° to 125°W (observed primarily by the infrared thermal mapper); several of the latter occurred over a ridge in Claritas Fossae, indicating strong topographic control. In their active growing stage, local dust clouds are probably about 8 km high and have relatively abrupt tops. The accumulation of dust from local clouds reached similar levels immediately prior to both global storms, supporting the hypothesis that such dust accumulations are responsible for triggering the global dust storms.
Reducing noise in tectonomagnetic experiments by linear regression
Authors: Steppe J. Alan
Published: February 2008

Noise in tectonomagnetic experiments caused by geomagnetic variations due to nonlocal sources can be reduced by applying multivariate linear regression to data from a network of magnetometer sites. Residuals obtained by subtracting from the observed values at a particular site the values calculated from a regression expression using the simultaneous values at a number of other sites tend to have nonlocal variations suppressed and hence to emphasize effects local to that particular site. For total field data from central California, at periods greater than a day, this method provided a moderate reduction of the noise below the level obtained by two‐site differencing. The effectiveness varied considerably from site to site, with an average improvement in the standard deviation of the corrected time series of roughly a factor of two, compared with two‐site differences. It was possible to find regression expressions that remove most of the long‐period (T > ∼100 days) variations from the available data, although they may not be statistically reliable.
Distribution of Reverberant Subbottom Layers in the Southwest Pacific Basin
Authors: Houtz Robert E. , Ludwig William J.
Published: February 2008

Vertical reflection profiles made with a large volume (7628 cm3) sound source and airgun‐sonobuoy reflection and refraction profiles in the southwest Pacific Ocean basin reveal significant accumulations of acoustically reverberant material that is difficult to penetrate with small (328–656 cm3) airguns.

An isopach map of the reverberant layers, called ‘smooth acoustic basement’ or the ‘opaque layer’ in earlier studies, is presented along with an isopach map of the overlying weakly stratified (and low velocity) sediment. The map shows that misidentification of the reverberant material as igneous basement would result in average basement depth about 500 m too shallow, and in three separate areas, 1500 to 2000 m too shallow. The new data generally agree with depth versus age curves of basement in the oldest Pacific crust as determined by DSDP boreholes.

DSDP cores from the reverberant layer in the central part of the study area show that it is volcanogenic in some places and calcareous in others. These drill sites are in areas where the reverberant layer is relatively thin, so that thick accumulations to the south have not been sampled. The calcareous nature of these thick beds is implied by the lack of consistently nearby seamount clusters, which could provide volcanogenic materials. If the thick reverberant layers are calcareous, that is, deposited at or near the equator, the net northward drift during the past 130 m.y. (or more) has been only 7–8° of latitude.
Discrete wave number representation of elastic wave fields in three‐space dimensions
Authors: Bouchon Michel
Published: February 2008

We present the generalization to three dimensions of the discrete wave number representation method of Bouchon and Aki (1977). The method is developed to study the near field of a three‐dimensional seismic source embedded in a layered medium. The elastic wave fields are represented by a superposition of plane waves propagating in discrete directions. The discretization is exact and results from a periodic two‐dimensional arrangement of sources. The accuracy of the method is checked, in the case of a rectangular dislocation source radiating in an infinite medium, by comparing the results obtained with Madariaga's (1978) exact solution. Examples of the calculation of strong ground motion produced by a thrust fault and a strike slip fault are presented.
On geoid heights derived from Geos 3 altimeter data along the Hawaiian‐Emperor Seamount Chain
Authors: Watts A. B.
Published: February 2008

Geoid heights derived from the Geos 3 satellite altimeter show relatively short wavelength (λ
280 km) geoid highs of 5–12 m over the crest of the Hawaiian‐Emperor seamount chain and geoid lows of 1–3 m over flanking regions. These geoid undulations can be generally explained by a simple model in which the load of the seamount chain is supported by a strong, rigid lithospheric plate. The best fitting estimates of the effective elastic thickness of the plate based on data over the Hawaiian Ridge are in the range 25–37.5 km. The elastic thickness cannot be reliably determined from altimeter data over the Emperor Seamounts, although it is probable that it is significantly lower than that obtained for the Hawaiian Ridge. These estimates of the differences in elastic thickness along the seamount chain are in general agreement with previous studies along the chain and provide useful new constraints on the long‐term (>106 years) mechanical properties of the oceanic lithosphere.
Monitoring of thickness changes of the continental ice sheets by satellite altimetry
Authors: Brooks Ronald L.
Published: February 2008

Radar altimeter measurements from the Geos 3 satellite are being utilized to define the topography of the southern Greenland ice sheet between latitudes 60°N and 65°N. The resultant elevations have been correlated with two geoceiver sites on the ice surface; the agreement is within a few meters. The internal repeatability of the altimeter‐derived elevations has been assessed at the crossovers of the satellite ground tracks; the observed agreements varied from 0.07 to 3.10 m, intervening times being 1 day and 674 days, respectively. Improved knowledge of the temporal changes in the topographic expression of the continental ice sheets, via satellite altimetry, will significantly add to the understanding of glaciological and climatological processes.
Geos 3 wave height measurements: An assessment during high sea state conditions in the North Atlantic
Authors: Parsons C. L.
Published: February 2008

During February 1976 a concentrated effort was made to monitor the sea state in the North Atlantic Ocean using Geos 3 radar altimeter wave form data and specialized data processing techniques. The average wave form was computed and fitted in a least squares sense by an error function whose slope is directly relatable to the ocean's significant wave height. The resulting measurements were compared with various sets of truth data. Underflights of Geos 3 orbits were made by a NASA C‐54 aircraft equipped with two narrow pulse radar systems. Additionally, buoy measurements from selected National Data Buoy Office data buoys, hindcasts from experienced analysts in the National Weather Service, and shipboard observations and wave recorder data from Ocean Weather Station ships located in the North Atlantic were collected for comparison with the satellite information. Excellent agreement exists between the aircraft, buoy, and satellite data. Contour maps produced twice daily for February using Geos 3 measurements are compared with the computer‐produced operational hindcast maps from the Navy Fleet Numerical Weather Central and the National Meteorological Center and are found to compare favorably with the Navy product. Differences between the Geos 3 measured magnitudes and locations of major significant wave height features with those found on the operational maps can be explained as the result of atmospheric primitive‐equation model tendencies to underestimate systematically the intensity of oceanic cyclones and to underestimate their speed of movement across the ocean. Using the interpolated wave height values from the Geos 3 February contour maps at the location of Ocean Weather Station Lima, it is shown that the quality of the Geos 3 map is degraded mainly when the narrow swath width of the satellite ground track does not overlay the ocean region with the highest sea state. Even so, the interpolated Geos 3 measurements are in closer agreement with Ocean Weather Station Lima wave recorder data than are Lima shipboard observations of sea state. Hence the satellite altimeter measurements are of better quality than routinely used significant wave height observations, are produced globally, and should be utilized in activities such as ship routing and weather forecasting.
Comparison of the TRM of the Yellowstone Group and the DRM of some Pearlette ash beds
Authors: Reynolds Richard L.
Published: February 2008

Air fall ash beds (Pearlette) originating from rhyolitic eruptions in the Yellowstone‐Island Park region of Wyoming and Idaho are discontinuous but widespread throughout the western United States. Accumulation and deposition of ashes occurred in low‐energy fluvial and lacustrine environments. These ash beds have been correlated, according to their chemistry and remanent magnetism, with specific eruptions of tuffs of the Pleistocene Yellowstone Group. Type O Pearlette ash beds are normally magnetized and correlate with the Lava Creek Tuff (0.60 m.y.), whereas type S Pearlette ash beds are reversely magnetized and correlate with the Mesa Falls Tuff (1.22 m.y.). The mean direction of detrital remanent magnetization (DRM) of 11 type O Pearlette ash beds is identical to the mean direction of thermal remanent magnetization (TRM) of the Lava Creek Tuff after correcting for differences in site latitude and longitude. Considered separately, only 2 of the 13 Pearlette ash beds studied possess mean inclinations significantly shallower than, and only 4 have mean declinations significantly different from, those of their correlative tuffs. Identical directions of magnetization measured from evenly laminated and highly contorted type O ash suggest that a readjustment of magnetic grains in water‐saturated ash shortly after deposition may account for the close agreement of TRM and DRM.
The grabens of Canyonlands National Park, Utah: Geometry, mechanics, and kinematics
Authors: McGill George E. , Stromquist Albert W.
Published: February 2008

The grabens of the Needles District, Canyonlands National Park, Utah occur in a 460 m‐thick plate of unfolded, brittle rocks overlying ductile evaporites. These rocks dip gently northwest towards Cataract Canyon of the Colorado River. The river has eroded entirely through the brittle plate, thus permitting downdip flow in the evaporites with consequent extension and faulting within the brittle plate producing systematically spaced grabens which in plan are curved, concave in the direction of dip. Stresses in the brittle plate are similar to those in the brittle surface ice of a valley glacier in extending flow, and stress analyses of crevasse geometry are applicable to the Canyonlands grabens; graben curvature is due to shear stresses acting on vertical planes parallel to the lateral boundaries of the zone of evaporite flow. Flow in the evaporites relaxes basal shear stress on the brittle plate, hence the spacing of grabens is a function only of the thickness of the brittle plate and the strength of the contained rocks. Because both are essentially uniform throughout the area, the grabens tend to be uniformly spaced. Evidence from the field and from experiments suggests that the graben faults initiated at or close to the contact between brittle and ductile rocks and propagated both upward and laterally within the brittle plate. At the surface, the resulting incipient grabens thus had initial widths that were functions only of thickness of the brittle plate and dips of the faults. Because plate thickness and mechanical properties of the rocks parallel to bedding do not vary significantly within the area, these initial widths were similar for all major grabens formed. Grabens terminate in ramps which retain this initial width. An equation based on this model of graben geometry and kinematics relates thickness of faulted plate to graben parameters measurable on photographs. Near the surface, graben geometry and trend indicate a compromise between a curved intermediate principal stress trajectory and two sets of pre‐existing, nearly rectilinear, vertical joints, resulting in grabens characterized by en‐echelon offsets and saw‐toothed walls and which have bounding faults that are vertical near the surface but dip inward at depth.
Attenuated white noise statistical gravity model
Authors: Heller Warren G. , Jordan Stanley K.
Published: February 2008

A new statistical gravimetric model is derived which applies to the spherical earth. The model characterizes the gravity disturbing potential by a superposition of white noise spherical shells at selected depths beneath the earth's surface. Asymptotic forms of the model are presented which are equivalent to previously developed models but are obtained in a manner which allows errors of the ‘flat earth’ approximation to be assessed. Detailed formulas are given for covariance functions involving the disturbing potential and its derivatives. Power spectra expressions are developed for the Legendre, Hankel and one‐dimensional Fourier frequency domains. Numerical comparisons are made between the spherical earth and asymptotic forms of the model. A set of model parameters is obtained by fitting to worldwide gravity anomaly data and GEM 10B degree variances.
Seismic refraction and reflection measurements in the Middle America Trench offshore Guatemala
Authors: Ibrahim Abou‐Bakr K. , Latham Gary V. , Ladd John
Published: February 2008

Seismic refraction, 24‐fold, common‐depth‐point reflection and magnetic field profiles were obtained during the summer of 1977 on the inner slopes of the Middle America Trench off Guatemala. In addition, piston cores and dredge samples were collected along the midslope region. Five unreversed refraction profiles between 20‐ and 80‐km length were recorded by Texas ocean bottom seismographs using air guns, Maxipulse, and conventional explosives as the sound sources. Travel time‐distance curves based on first and later arrivals show velocities ranging between 1.6 km/s and 8.1 km/s. Some of the refracting horizons are observed as reflectors in the multichannel reflection sections. Relatively high velocities (4+ km/s) occur at shallow depth in the midslope region and may indicate imbricate slices of indurated sediment within the slope. From the combination of the reflection and refraction data and previous refraction work we infer the presence of at least two separate landward dipping slabs of oceanic crust within the upper slope. The presence of serpentine, basalt, chert, and mudstone in dredge and core samples collected near midslope is consistent with this interpretation. The top of the oceanic crust, with a velocity of 6.5 km/s, can be traced as a refractor approximately 30 km landward from the trench axis. The mantle with a velocity of 8.1 km/s is found as a refractor at a depth ranging from 13 to 17 km near the trench axis and can be traced landward to increasing depth, for about 15 km.
The Galapagos Rift at 86°W: Morphological wave forms; Evidence for a propagating rift
Authors: Crane Kathleen
Published: February 2008

A combination of near‐bottom and multibeam surveys of the N‐S spreading Galapagos Rift reveals periodic morphological transitions from highs to lows along fault scarps with amplitudes of 90–140 m and wavelengths of 24–30 km. All fault scarps reflect similar wave forms, yet the highs and lows are offset by a consistent phase shift with each successive increment of time. Fault scarp morphology realigns approximately every 0.33 m.y. This progression along strike, distance, and time infers either postemplacement tectonics resulting from antisymmetric distribution of stresses in young crust, oscillating volcanism, or propagation of a magma chamber(s) along the axis of the spreading center.
Gravity anomalies and origin of the Walvis Ridge
Authors: Kogan M. G.
Published: February 2008

Gravity anomalies over the western Walvis Ridge are well explained by a model of the regional depression of the rigid lithosphere under the load of the ridge. Computation of the best‐fitting flexural rigidity yielded small scatter about a single value for all profiles across the ridge. Depending on the value adopted for the density of the ridge, an estimate of the rigidity ranges from 2×1028 to 8×1028 dyne‐cm. The values computed are an order of magnitude lower than those published for the Hawaiian‐Emperor seamount chain in the central Pacific Ocean. This fact cannot be attributed to the change in the effective flexural rigidity with time of loading, since we compare the segments of the Walvis Ridge and of the Emperor chain, which are of similar age. The difference in rigidity found favors the formation of the Walvis Ridge on relatively thin lithosphere close to the axis of the Mid‐Atlantic Ridge, while the Hawaiian‐Emperor chain was produced on old lithosphere far from the East Pacific Rise crest.
Properties of iron at high pressures and the state of the core
Authors: Jeanloz Raymond
Published: February 2008

Shock‐wave (Hugoniot) data for initially porous and nonporous samples of iron are inverted to yield values of the Grüneisen parameter (γ), adiabatic bulk modulus (Ks), and coefficient of thermal expansion (α) along the Hugoniot to pressures of about 150 GPa (1.5 Mbar). This represents the first reliable estimate of thermal properties (e.g., α) to such high pressures based directly on experimental data. The values of γ fit the conventional function γ = γ0(V/V0)n but with n significantly larger than 1, while extrapolation formulas suggested to date for α appear not to provide the best fit to the data. The present analysis yields values of γ between about 1.4 and 1.0 (essentially temperature independent) and α between about 1.0 and 0.4×10−5 K−1 throughout the earth's core, therefore implying that simple dynamical models of the core are quite viable. These results provide experimental confirmation of the Vaschenko‐Zubarev/Irvine‐Stacey (or ‘free‐volume’) formulation for γ of iron at high pressures, as well as support for Stacey's recent model of the thermal state of the core. The data on density, sound speed, and bulk modulus of iron are extrapolated and corrected to pressures and temperatures existing throughout the earth's core, and compared with current seismological information. This leads to the following conclusions: (1) both densities and bulk moduli in the outer core are less than those of Fe under equivalent conditions (by about 10 and 12%, respectively) but (2) their gradients through the outer core are consistent with gross chemical homogeneity (i.e., uniform intermixing of Fe and a lighter, more compressible element or compound); (3) both densities and bulk moduli for the inner core are compatible with those of iron, suggesting that (4) the inner core‐outer core boundary is likely to be a compositional as well as a phase boundary. Assuming that the outer core consists of Fe and a lighter element or compound, X, the constraints on density, bulk modulus and mass fration of X which must be simultaneously satisfied are given.
Convective instabilities in vertical fractures and faults
Authors: Murphy Hugh D.
Published: February 2008

Natural convection of water contained in a vertical fracture or fault in which the temperature increases with depth is strongly influenced by the heat transport processes not only within the water itself but also by the heat transferred to and from the surrounding rock mass. The results of a linear stability analysis indicate that the critical Rayleigh number R* is time dependent. For spontaneous neutral stability, R*(t = 0) ≅ 10(h/a)2, where h and a are the fault height and aperture, respectively. Since h ≅ a, R*(0) is several orders of magnitude greater than the value 4π2 that would pertain to the same situation without the influence of the surrounding rock masses, e.g., a porous bed with large horizontal dimensions. The resultant cell motion consists of rolls about axes parallel to the aperture. These rolls are of height h and closely spaced in the strike direction. Cases of spontaneous instabilities in fractures or faults are expected to be infrequent, but initially subcritical convection could be fostered by other means such as tectonic displacements at the fault. Because R* diminishes as time−½, eventually, this subcritical convection becomes unstable, and exponential growth ensues. As the heat of the surrounding rock is depleted and an isothermal state is approached, the convection eventually dampens until a period of thermal recovery allows its resumption.
Low‐velocity impact craters in ice and ice‐saturated sand with implications for Martian crater count ages
Authors: Croft S. K. , Kieffer S. W. , Ahrens T. J.
Published: February 2008

We produced a series of decimeter‐sized impact craters in blocks of ice near 0°C and −70°C and in ice‐saturated sand near −70°C as a preliminary investigation of cratering in materials analogous to those found on Mars and the outer solar system satellites. The projectiles used were standard 0.22 and 0.30 caliber bullets fired at velocities between 0.3 and 1.5 km/s, with kinetic energies at impact between 109 and 4×1010 ergs. Crater diameters in the ice‐saturated sand were ∼2 times larger than craters in the same energy and velocity range in competent blocks of granite, basalt and cement. Craters in ice were ∼3 times larger. If this dependence of crater size on strength persists to large hypervelocity impact craters, then surfaces of geologic units composed of ice or ice‐saturated soil would have greater crater count ages than rocky surfaces with identical influx histories. The magnitude of the correction to crater counts required by this strength effect is comparable to the magnitudes of corrections required by variations in impact velocity and surface gravity used in determining relative interplanetary chronologies. The relative sizes of craters in ice and ice‐saturated sand imply that the tensile strength of ice‐saturated sand is a strong inverse function of temperature. If this is true, then Martian impact crater energy versus diameter scaling may also be a function of latitude.