bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimulated emission of AKR in regions of a weakly unstable electron velocity distribution
Authors: Kainer S. , Krauss‐Varban D. , Gaffey J. D. , Burch J. L.
Published: February 2008

It is generally believed that auroral kilometric radiation (AKR) originates in regions in which energetic auroral electrons possess a velocity distribution with a loss‐cone feature. According to the cyclotron maser theory of AKR, the resulting instability can be substantial in some regions of the auroral zone, leading to the generation of radiation with many features that are consistent with the observations. However, in situ measurements of velocity distributions obtained with satellites only show evidence of moderate positive gradients in the perpendicular velocity component υ⊥. Such distributions prove to be only marginally unstable, and thus imply a low level of radiation. From this point of view it is of interest to study the amplification of externally generated radiation. This radiation might originate from adjacent regions with a more unstable velocity distribution or from remote sources in space. In this work we perform numerical simulation studies using alternatively an observed electron velocity distribution, obtained from DE 1 measurements, and an idealized loss‐cone distribution. The results show the expected amplification of X mode radiation as well as effects which go beyond the behavior of a simple amplifying medium. Whereas wave growth is exponential for the idealized loss‐cone distribution, it is initially linear in time in the weakly unstable case, following the increase of the external wave power in the simulation. At later stages the collective properties of the plasma lead to interesting effects, such as the continuation of wave growth after cessation of the external wave in cases which do not exhibit growth otherwise, and a threshold power level before growth occurs. We suggest that these phenomena are relevant to the understanding of the observed enhancement and triggering of AKR due to solar radio bursts.
Seismicity and tectonics of the Ninetyeast Ridge Area: Evidence for internal deformation of the Indian Plate
Authors: Stein Seth , Okal Emile A.
Published: February 2008

The Ninetyeast Ridge, far from being ‘aseismic’, has historically been a region of substantial seismicity. Since 1913, 4 magnitude 7 or greater earthquakes (including one with Ms = 7.7) and 10 magnitude 6 events have occurred in this general area. We have determined the mechanisms of several of these earthquakes, which suggest that the Ninetyeast Ridge area is presently a complex zone of deformation within the Indian plate. The northern portion (3°N–10°S) of the ridge is the active seismic zone, where both vertical and strike slip motion occur, while further south the ridge is far less seismic. This transition roughly coincides with a change in the ridge's morphology from irregular en échelon blocks to a smooth flat‐topped high. The strike slip motion is left lateral, which is consistent with the Indian (west) side encountering resistance due to the collision with Asia while the Australian (east) side is subducting smoothly at the Sumatra trench. South of about 9°S the style of deformation differs on the two sides of the ridge. To the east, normal faulting occurs, which may be related to the formation of grabenlike structures. To the west the topography can be interpreted as the result of NW‐SE compression which takes place largely aseismically but is observed for one large earthquake. This signficant intraplate deformation may explain the difficulties that occur in attempts to close the India‐Africa‐Antarctica triple junction using a rigid Indian plate.
Moments, corner frequencies, and the free surface
Authors: Langston Charles A.
Published: February 2008

Theoretical amplitude and phase spectra for P and SH body waves are computed for dislocation sources imbedded in an elastic half space to determine the effect of neglecting the free surface in earthquake spectral studies. Depending on source orientation, depth, and time function, errors in moment and corner frequency determinations may be arbitrarily large. Interference of the direct and reflected phases produces peaked amplitude spectra with pronounced spectral minima. These minima could be erroneously identified as source dimension or directivity effects. Stress drop, source dimension, and other related results obtained from body wave spectral studies of shallow earthquakes become ambiguous or misleading if the free surface effect is ignored.
Local time variation of geomagnetic induction vectors
Authors: Anderson C. W. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2008

Geomagnetic induction vectors are calculated in four frequency bands (between 0.72 and 27 mHz) at 2‐hour intervals for 12 consecutive days from data acquired at four magnetic stations spaced between ∼54° and ∼65° geomagnetic latitude near geomagnetic longitude 4°W. Significant temporal variations are found in the local time dependence of the induction vectors even when they are averaged over appreciable time periods. These variations in the induction vectors are qualitatively related to measured local time changes in the power spectral amplitudes and ellipticities during the 12‐day period. Time‐intensity data plots and wave polarization diagrams are presented that demonstrate the great variability in the magnetosphere sources in the frequency intervals studied. The present results and discussions call into question assumptions commonly made in the straightforward derivation and use of geomagnetic induction vectors for defining internal conductivity structures.
Microseisms at Palisades: 1. Source location and propagation
Authors: Rind D. H. , Donn W. L.
Published: February 2008

Microseisms recorded at Palisades, New York are investigated as to the source location, propagation paths, and relation to ocean wave heights. The source location is estimated from analysis of the incoming wave ground motion by using an analog correlator to compute the direction of propagation and then tracing this ray to a suspected meteorological‐oceanographic source. The propagation path of Rayleigh waves approaching Palisades is determined, taking into account the geological and bathymetric structure of the western North Atlantic, and the resulting ray diagram is used to explain some of the source frequency observations. The amplitude of observed microseisms is shown to be a function of the ocean wave heights in the suspected generating region and is a result of both refraction effects as well as apparently ‘off the continental shelf’ source locations. The principal conclusions are as follows: (1) The majority of sources producing microseisms at Palisades occur either in regions on the continental shelf to the east and northeast or off the continental shelf to the southeast, although the latter produce weaker signals. (2) The relative lack of sources in the region along the southeast coast on the continental shelf appears to be due to refraction effects. (3) The closer the origin is to the east or northeast on the continental shelf, the higher are the microseisms produced by given ocean waves although the relationship has too much scatter to allow ocean wave heights to be determined with great enough certainty from microseism observations.
The effect of dilatancy on velocity anisotropy in westerly granite
Authors: Soga N. , Mizutani H. , Spetzler H. , Martin R. J.
Published: February 2008

Jacketed samples of Westerly granite were fractured at confining pressures up to 1 kbar and at strain rates of 2 × 10−5; s−1 and 3 × 10−7 s−1. Compressional and horizontally as well as vertically polarized shear velocities were measured in orthogonal directions perpendicular to the compression axis. The strains in three orthogonal directions were monitored by strain gauges and compared with the results obtained by optical holography. At approximately 60% of the ultimate axial strain the onset of dilatancy is noted by a decrease in the Vs and Vp velocities. As failure was approached, the compressional velocity across the fault decreased by nearly 40% for the low‐strain‐rate case versus about 22% for the high‐strain‐rate case. In order to characterize the stress‐induced cracks during dilatancy, an analysis of the velocity data was carried out following the approach of Anderson et al. [1974] with the assumptions that flat spheroidal cracks were oriented perpendicular to three orthogonal directions and that the effects of oblique cracks was vectorially divided between the effects of three mutually perpendicular crack types. The comparison between the observed velocities and those calculated from the three crack densities on the basis of the above assumptions showed good agreement. The degree of dilatancy was determined from the differences between the strains measured perpendicularly to the compression axis and the estimated elastic strains in those directions. The aspect ratios for the flat spheroidal cracks were calculated to be 1/400 to 1/800, which are in line with those determined by Hadley [1976].
Separation of magnetic components from af step demagnetization data by least squares computer methods
Authors: Stupavsky M. , Symons D. T. A.
Published: February 2008

A least squares mathematical modeling technique is used to analyze af step demagnetization data of an individual rock specimen. Its total magnetization is separated to give the directions, magnitudes, and decay rates of the inherent magnetization components. An estimate of the uncertainty in the directions and magnitudes of the components is also computed. The method assumes that the direction of each component remains constant as its intensity decreases approximately exponentially during demagnetization. Two tests comparing the mean directions determined by using the af demagnetization data of a single specimen per site to those determined by using traditional paleomagnetic analysis of multiple demagnetized specimens per site given identical results at the 95% confidence level.

The computer program used in this study, with explanatorycomment, is available with entire article on microfiche. Order fromAmerican Geophysical Union, 1909 K Street, N. W., Washington, D. C.20006. Document J78‐005; $1.00. Payment must accompany order.
Contemporary uplift of the diablo plateau, West Texas, from leveling measurements
Authors: Brown L. D. , Reilinger R. E. , Hagstrum J. R.
Published: February 2008

Comparison of leveling surveys carried out in 1934, 1943, and 1958 indicates that the Diablo Plateau region of Trans‐Pecos Texas has been uplifted in archlike fashion in relation to El Paso, Texas, on the west and Carlsbad, New Mexico, on the east. Assuming that all movement occurred between 1934 and 1943, the maximum amount of relative uplift measured is 13.0 ± 3.6 cm. Extending east from El Paso, the leveling route traverses the region's basin and range type structure, including the Hueco Basin, the Hueco Mountains, the Diablo Plateau, the Salt Basin, and the Guadalupe Mountains before terminating on the High Plains near Carlsbad. The maximum relative uplift occurs near the eastern edge of the Diablo Plateau, approximately 12 km west of the Salt Basin graben. Only 15 km west of this maximum, the east‐west leveling route passes near an anomalous outcrop of Precambrian rocks and is intersected by a northwest‐southeast trending zone of Tertiary intrusives. The pattern of observed uplift correlates well with a positive Bouguer gravity anomaly, and significant crustal conductivity variations have been mapped nearby by geomagnetic surveys. The observed relative uplift is consistent, at least in sign, with the broad aspects of the tectonic evolution of the Diablo Plateau, although geologic evidence of recent activity is limited to some Quaternary fault scarps and entrenched bajadas in the vicinity of the Salt Basin. The region appears to be aseismic at present. The pattern of observed movement, its geological setting, and its geological and geophysical correlations suggest that intracrustal magmatic activity may be causing the uplift, although corroborative heat flow and seismic measurements are as yet unavailable.
Q of the Earth
Authors: Anderson Don L. , Hart R. S.
Published: February 2008

Regional body wave and surface wave studies indicate that there is a low‐Q upper mantle layer underlying a high‐Q lithosphere. Great circle surface wave attentuation is used to refine the Q structure of the upper mantle and to demonstrate that these features are consistent with the global data. Body wave results are used to constrain the average Q of various regions of the mantle and core and the Q gradient in the lower mantle. Normal mode data are used to test the hypotheses that bulk dissipation is not required in the mantle and that the inner core has low Q. Both hypotheses are consistent with the data. The data are also consistent with a smooth increase of Q with depth over most of the lower mantle and a low‐Q zone at the base of the mantle. The radial modes require bulk dissipation somewhere in the earth, probably in the inner core. A series of parametric models is presented which illustrate the sensitivity of the attentuation data to major features of the Q distribution.
Size of great earthquakes of 1837–1974 inferred from tsunami data
Authors: Abe Katsuyuki
Published: February 2008

A new magnitude scale Mt is defined by using the logarithm of the maximum amplitude of far‐field tsunami waves measured by tide gauges or their substitutes. The Mt scale is experimentally adjusted to the Mw scale introduced by Kanamori (1977), so that the Mt scale measures the seismic moment of a tsunamigenic earthquake as well as the overall size of tsunami at the source. Mt and the conventional tsunami magnitude m are distinct scales. By using many amplitude data of tsunami waves now available the values of Mt are assigned to 65 tsunamigenic earthquakes that occurred in the Pacific area during the period from 1837 to 1974. The 1960 Chilean shock has the largest Mt, 9.4. The 1946 Aleutian (Mt = 9.3), the 1837 Chilean (Mt = 9¼), and the 1964 Alaskan (Mt = 9.1) events follow. Nine great events having Mt = 9 or over occurred during this period, and their occurrence is clustered in the years around 1840, 1870, and 1960. Of all the 65 events listed, at least six unusual earthquakes having tsunamis with an amplitude disproportionately large for their surface‐wave magnitude Ms are identified from the Mt‐Ms relation.
In‐situ stress measurements near the San Andreas Fault in central California
Authors: Keys W. S. , Wolff R. G. , Bredehoeft J. D. , Shuter Eugene , Healy J. H.
Published: February 2008

A series of stress measurements using hydraulic fracturing techniques were made in five holes drilled into shale along a line normal to, and 3–18 km from, the trace of the San Andreas Fault. In all the holes, except the one farthest away from the fault, the least principal stresses were found to be approximately equal to the weight of the overburden. The drilling as well as the variation in breakdown pressures suggested a more or less visco‐elastic material. The data further suggested that the shale does not retain the regional tectonic stresses. The dip of most of the hydraulic fractures was greater than 69°. Televiewer logs showed that the natural fractures have a consistent dip of 58° principally to the west‐southwest. Although widely scattered, the orientations of the total set of natural and hydraulic fractures suggest that the direction of maximum principal stress is north‐south. The strike of the San Andreas Fault is N45°W, which falls within the limits of the relationship of stress distribution and fault trace predicted on the basis of the Mohr‐Coulomb theory for strike‐slip faults. It is suggested that more competent rocks would be better suited to a study of the relationship of tectonic stresses to the San Andreas Fault.
The role of rim slumping in the modification of lunar impact craters
Authors: Settle Mark , Head James W.
Published: February 2008

Wall failure has significantly altered the structure of virtually all large, fresh‐appearing lunar craters. Terrace blocks exposed upon a crater's interior walls are interpreted to be sections of the transient cavity rim that slumped into the cavity during the terminal stages of crater formation. Impact excavation cavities have been reconstructed by restoring the innermost terrace block exposed within a crater to its inferred original position at the cavity rim and accounting for the volume of material that slumped into the cavity. Critical model assumptions include (1) the radial variation of topography near the initial cavity rim crest, (2) the structure of the failure surface along which terrace blocks slumped into the cavity, and (3) the geometric shape of the initial cavity. This terrace restoration model has been applied to 12 fresh lunar craters with observed rim crest diameters D0 ranging from 19 to 137 km. Up to an initial cavity diameter Di of 30 km, reconstructed cavity depths are comparable to or greater than cavity depths extrapolated from nonterraced craters of less than 15 km in diameter. For a wide range of model parameters the reconstructed depths of impact cavities 15–30 km in diameter are significantly greater than depths predicted by small‐crater morphometry, implying that cavity depths inferred from depth/diameter ratios observed for small craters may substantially underestimate the depth of excavation of impact cratering events in this size class. Reconstructed cavity depths for Di > 70 km, however, are consistently less than cavity depths extrapolated from smaller craters. This indicates that the morphometric transition from small, relatively unmodified, bowl‐shaped craters (D0 < 15 km) to large, terraced, saucer‐shaped craters (D0 > 80–90 km) cannot be solely attributed to rim‐slumping modification. Ejecta fallback and basement rebound also play a role in modifying impact crater cavities; however, the manner in which the volume of fallback ejecta and basement rebound material varies with increasing crater size is unknown. The discrepancy between cavity depths extrapolated from small craters and those obtained from the terrace restoration model suggests that impact excavation cavities become relatively shallower at larger diameters. However, this cannot be conclusively demonstrated until the effects of rebound and ejecta fallback are quantitatively accounted for.
Techniques for interpretation of geoid anomalies
Authors: Chapman Michael E.
Published: February 2008

For purposes of geological interpretation, techniques are developed to compute directly the geoid anomaly over models of density within the earth. Ideal bodies such as line segments, vertical sheets, and rectangles are first used to calculate the geoid anomaly. Realistic bodies are modeled with formulas for two‐dimensional polygons and three‐dimensional polyhedra. By using Fourier transform methods the two‐dimensional geoid is seen to be a filtered version of the gravity field, in which the long‐wavelength components are magnified and the short‐wavelength components diminished.
Base line estimation using single passes of laser data
Authors: Dunn P. J. , Torrence M. , Smith D. E. , Kolenkiewicz R.
Published: February 2008

During the first year's operation of the Geos 3 mission, 20 satellite passes were simultaneously observed by three laser tracking stations. Range observations of better than 7‐cm precision were taken from the sites at Greenbelt, Maryland, and Bermuda and Grand Turk islands. The three interstation base lines have been estimated simultaneously from these multiple arcs of data and were found to be quite insensitive to the dynamic model used to determine the orbit. The 1364‐km Bermuda‐Grand Turk base line values always lie within 15 cm of the best estimate when a variety of reasonable dynamic models are employed and when disjoint subsets of all available data are used. The corresponding precisions for the Greenbelt‐Grand Turk (2013 km) and Greenbelt‐Bermuda (1323 km) base lines are 30 cm and 20 cm, respectively. A clear dependence of the estimates on base line orientation relative to the orbit's ground track can be eliminated by combining data from north going passes with those from south going passes. Range observations from a fourth site at Patrick Air Force Base, in Florida, were taken during five of the 20 passes and were used to measure the distances between the three original stations and Patrick. A consistency analysis of the heights of the Bermuda and Grand Turk lasers determined in relation to Greenbelt indicates better than 35‐cm precision in relative height. When scale and orientation information is provided by conventional dynamic techniques using longer arcs of ranging data, the short‐arc method offers an important capability in the accurate triangulation of networks of laser stations.
Surface roughness slope density estimates for low sea state conditions
Authors: Brown Gary S.
Published: February 2008

In addition to the 16 wave form samplers in the Geos 3 radar altimeter there exist integrating samplers located at approximately 0.3° and 1° angles of incidence, i.e., the plateau and attitude/specular gates, respectively. For very low surface wind speed conditions where the joint probability density function for the slopes of the surface roughness is very peaked these two gates can be used to infer the form of the density. For two instances where this condition was observed, it was found that the Geos 3 altimeter data implied that the joint slope density was closer to a Laplacian form than a Gaussian form. These experimental results indicate that under conditions of very low surface wind speed the slopes of the surface roughness may depart rather markedly from the universally assumed Gaussian form. Such departures may be due to the possible trochoidal nature of small‐amplitude gravity waves.
Estimation of significant wave height and wave height density function using satellite altimeter data
Authors: Priester R. W. , Miller L. S.
Published: February 2008

A technique for estimating the ocean surface roughness probability density function from satellite altimeter data is presented. Results from the application of the technique to Geos 3 altimeter data demonstrate its ability to detect both large‐scale and small‐scale structural deviations from the Gaussian distribution. Knowledge of the surface roughness probability density enables a direct computation of significant wave height. Significant wave height computations using the estimated quantities have been found to be in general agreement with other approaches. Density function estimation is important to precision altimeter design and has possible implications in oceanographic research.
A sequential method for filtering satellite altimeter measurements
Authors: Tapley B. D. , Schutz B. E. , Abusali P. A. M.
Published: February 2008

Recent advances in the area of satellite altimetry have provided a data type which is applicable to the detection and monitoring of transient sea surface phenomena. With the extensive altimeter data set collected by the Geos 3 spacecraft there is an associated need for accurate and efficient methods for editing and filtering the altimeter data. In this investigation a sequential filter is developed assuming that the sea surface topography can be approximated by an adaptive Gauss‐Markov process. The effect of geoid model errors on the filtered results is illustrated by using both 1° × 1° and 5′ × 5′ geoid models. Finally, an application of the method to the Gulf Stream current boundary identification problem using Geos 3 altimeter data is described.
Geos 3 STDN S band Doppler tracking investigation
Authors: Rosenbaum B.
Published: February 2008

Geos 3 S band Doppler and laser ranging data were acquired in the spacecraft altimeter calibration area during an extensive tracking campaign from August 1975 to March 1976. These data have been used to support the calibration and evaluation of the Spaceflight Tracking and Data Network S band stations. The Goddard Space Flight Center laser systems are the reference for the precise positioning of the Doppler stations and for evaluating the recovery of Geos 3 arc height derived from Doppler tracking data. The solutions for station coordinates were determined from two‐way and three‐way Doppler data. A comparison of the solutions with survey in a local datum indicates a submeter precision for adjusted coordinate components. Arcs of one revolution were generated for Geos 3 from passes of two‐way and three‐way Doppler data. An evaluation for the calibration area shows arc height being recovered with an uncertainty of about 1 m. The Geos 3 beacon signal has also been tracked in the Doppler difference mode with a precision useful for geodetic purposes. In a solution for Geos 3 arc elements obtained using Doppler difference data, arc height has been recovered in the calibration area with an uncertainty of the order of 2 m.
Paleomagnetism of Lower and Middle Cambrian sedimentary rocks from the Desert Range, Nevada
Authors: Gillett Stephen L. , Van Alstine David R.
Published: February 2008

Over 500 oriented samples were collected from a homoclinal sequence of Lower and Middle Cambrian terrigenous and carbonate miogeoclinal strata in the Desert Range, Nevada. Extensive thermal demagnetization was successful in isolating similar characteristic magnetization directions in red‐purple mudstones of the Wood Canyon Formation (Lower(?) and Lower Cambrian) and in gray limestones of the Carrara and Bonanza King Formations (Lower and Middle Cambrian). Lithologic and magnetic evidence suggest that these magnetizations were acquired penecontemporaneously with deposition. The similarity of the characteristic magnetization directions in these strata implies that little apparent polar wander occurred with respect to North America from early Early through middle Middle Cambrian time. The divergence of these directions from those from the partly coeval Tapeats Sandstone of the Colorado Plateau probably resulted from a net 36° clockwise rotation of the Desert Range section about a vertical axis. This rotation is probably due to mid‐Tertiary oroflexural bending but may in part have been caused by Mesozoic thrusting. The polarity of the geomagnetic field in early Middle Cambrian time appears to have been continuously reversed. Stratigraphic intervals of mixed polarity, possibly correlative with those found in the Tapeats, may be present in the Lower Cambrian Zabriskie Quartzite.
Correlation function analysis of geomagnetic field reversals
Authors: Laj C. , Nordemann D. , Pomeau Y.
Published: February 2008

We have analyzed the time series of geomagnetic field reversals using a correlation function of the telegraphic signal obtained from the Heirtzler scale by assigning ±1 to intervals of normal/reversed polarity. Unlike the correlation function used by Naidu, the autocorrelation function we define is only very slightly sensitive to undetected short polarity intervals. Thus the results we obtain are valid irrespective of failure to detect short polarity intervals and give information about the dynamics of the time series of polarity reversals. We show that, within statistical noise limits, the successive polarity intervals are statistically independent and are distributed in time according to Poisson process, as already suggested by Cox using other mathematical means.
M2 ocean tide parameters and the deceleration of the Moon's mean longitude from satellite orbit data
Authors: Felsentreger T. L. , Marsh J. G. , Williamson R. G.
Published: February 2008

An estimation has been made of the principal long‐period spherical harmonic parameters in the representation for the M2 ocean tide from the orbital histories of three satellites: 1967–92A (Transit), Starlette, and Geos 3. The data used were primarily the evolution of the orbital inclinations of the satellites, with the addition of the longitude of the ascending node from Geos 3. The results are as follows: C22+ = 3.42 ± 0.24 cm, ε22+ = 325.5° ± 3.9°, C42+ = 0.97 ± 0.12 cm, and ε42+ = 124.0° ± 6.9°. These values agree quite well with recent numerical models and another recent determination from satellite data. Dissipational tidal friction in the oceans is known to provide the largest contribution to
, the observed deceleration in the lunar mean longitude. Further, only the second‐degree components of the ocean tide contribute significantly to this secular decay (Lambeck, 1975). The M2 parameters obtained here infer an
of −25±3 arc sec/century2, in good agreement with other investigators. The range of current determinations of
is from −24.6 to −27.2 arc sec/century2. Considerably different techniques have been used to derive the estimates: the study of ancient eclipses, transits of Mercury, lunar laser ranging, and another satellite solution.
Strain accumulation rates in the western United States between 1970 and 1978
Authors: Prescott W. H. , Savage J. C. , Kinoshita W. T.
Published: February 2008

The rate of dilatation and the rate and direction of shear have been determined from trilateration data for 23 Geodolite networks in the western United States. Sixteen nets are located along the San Andreas fault system between Point Reyes, California, and the United States‐Mexico border. Other locations are across the Garlock fault in California; across Puget Sound near Seattle, Washington; near Hanford in eastern Washington; near Hebgen Lake in Montana; across the Wasatch fault at Ogden, Utah; across the Rio Grande rift at Socorro, New Mexico; and Dixie Valley in Nevada; and at the northern end of Owens Valley on the California‐Nevada border. Implicit in the treatment are the assumptions that the strain was accumulating at a constant rate over the time period (within the interval 1970–1978) and over the local area (usually about 50‐km diameter) covered by the surveys. Of the nets located away from the San Andreas fault, only Ogden and Hebgen show significant strain accumulation. At Ogden the deformation is principally an east‐west compression of 0.23±0.05 μstrain/yr and at Hebgen Lake a northeast‐southwest extension of 0.17±0.03 μstrain/yr. Along the San Andreas fault system the rate of shear is 0.2 to 0.4μ/yr. The direction of shear agrees very well with the surface strike of nearby faults. This agreement is maintained even in regions like the ‘big bend,’ where both the fault strike and the observed shear direction are more westerly than they are elsewhere. Shear strain in northern California appears to be concentrated more closely on the faults, whereas in southern California the strain is a broader, smoother feature. In the San Francisco Bay area the strain data indicate slip at depth on both the San Andreas and the Calaveras faults. In addition to the observed shear the nets in California indicate a negative dilatation (areal decrease) of about 0.2 μstrain/yr. This dilatation is unexplained, but the following sources appear unlikely: (1) systematic survey error, (2) an association with the southern California uplift, (3) an association with the big bend in the San Andreas fault in Southern California, or (4) the result of the superposition of a uniaxial strain on the Pacific‐North American plate boundary shear.
Seasonal displacement and deformation of the Earth by the atmosphere
Authors: Stolz A. , Larden D. R.
Published: February 2008

Mass transports on the surface impart a space motion to the earth. The shift induced by seasonal atmospheric pressure variations is calculated. The magnitude of the displacement is approximately 1.6 | cos [(sun ‐ 9°)] | mm where (sun) is the sun's longitude measured from the beginning of the year. The motion is thus too small to be detected at present. Deformations of a model earth caused by atmospheric loading are estimated. The seasonal distortions over parts of Asia and Greenland approach the lcm measuring goal of the new laser ranging techniques and exceed it over the Antarctic continent. They are not generally significant in other parts of the globe. The results are depicted in contour form on world maps for each of the four seasons.
The accuracy of geoid undulations by degree implied by mean gravity anomalies on a sphere
Authors: Sjöberg Lars
Published: February 2008

Formulae are derived for the resolution of the geoid undulations by degree implied by mean gravity anomalies. The two error sources considered are the propagation of the mean anomaly errors and the error due to lack of more detailed data than the mean anomalies. The errors are computed for the degree variances of Tscherning and Rapp and the β function of Meissl. Finally, the corresponding errors of point gravity anomalies are derived.
Sample fields of the Viking Landers, physical properties, and aeolian processes
Authors: Moore H. J. , Spitzer C. R. , Bradford K. Z. , Cates P. M. , Hutton R. E. , Shorthill R. W.
Published: February 2008

After the Viking primary mission the surface samplers and cameras continued to operate during the extended mission until early May 1978. Major extended mission accomplishments include (1) excavation of deep trenches, (2) acquisition of more samples (chiefly for the X ray fluorescence experiment(, (3) construction of conical piles of materials in the sample fields, (4) backhoe touchdown experiments, (5) acquisition of contiguous pictures of the surface beneath #2 terminal descent engines using mirrors, (6) pushing and pulling rocks, and (7) other experiments for the Physical Properties Investigation. The landing sites have continued to be monitored with the cameras during the lander continuation automatic mission, and the Lander 1 site will be monitored for a long period of time during the Viking survey mission (perhaps to December 1990 and beyond). Activities of and experiments performed by the surface samplers have disturbed the equilibrium of the surface so that wind and other processes have produced changes. Both pictures and surface sampler data acquired chiefly during the extended mission indicate that the surface materials in the sample fields of the Viking landers may be grouped into four categories (in order of increasing strength(: (1) drift material, (2) crusty to cloddy material, (3) blocky material, and (4) rocks. The response of the surface materials to engine exhaust erosion combined with data from other experiments, rock populations at the sites, and theory indicates that the surface should be relatively stable and resistant to wind erosion. Although relatively stable, the erosion of the surface may occur when wind speeds are sufficiently high and when local conditions permit erosion.
Changes in the crust and upper mantle near the Japan‐Bonin Trench
Authors: Houtz R. , Windisch C. , Murauchi S.
Published: February 2008

Depths and reflection times to mantle have been computed in the west Pacific from 60 sonobuoy refraction solutions, many of which could be compared with observed mantle reflection depths from multi‐channel data obtained at the same time. After repicking some of these sonobuoy records, all were eventually adjusted to agree within 0.05 s with the observed mantle reflection times. This added constraint produces solutions that are clearly more reliable. Crustal velocities (exclusive of water and sediment) from the study area are rather tightly distributed about a mean value of 6.53 km/s with a standard deviation of only 0.23 km/s (n = 47). Results show that the crust thickens in a westerly direction from the west Pacific basin, where mantle depths are 11–11.5 km to a belt 200 km east of the Japan trench, coinciding with the outer gravity high, where mantle is at an average depth of 14 km. Several sonobuoys in the zone of maximum crustal thickness just east of the outer slope of the Japan trench record two deep reflectors about 0.6 s apart in the vicinity of the upper mantle. Two values of interval velocity obtained from a reduced T2/X2 analysis of the layer bounded by these reflectors are 7.5 and 7.2 km/s. These sonobuoys and a few others with weaker double reflections are all located within the outer gravity high. To the south a well‐observed mantle reflection and its strong 8.2‐km/s refraction disappear from our records just as the crust begins its descent into the Bonin trench. Within the outer trench slope a 7.3‐km/s refractor, which is a weak arrival elsewhere, becomes the dominant refractor. The peculiar double reflector near mantle and the marked change in velocity structure and upper mantle reflectivity at the edge of the outer slopes seem to confirm the changes in upper mantle refraction velocity reported by Talwani et al. (1977) in the Curacao trench, Caribbean Sea, but our evidence is not conclusive. In spite of the ambiguity it is clear that velocity structures near active trenches cannot be confidently defined by correlating sections with similar velocities.
The motion of the ground during an earthquake: 1. The case of a strike slip fault
Authors: Bouchon Michel
Published: February 2008

A theoretical study of the ground motion produced by strike slip faults is presented. The source is modeled as a propagating dislocation embedded in a layered medium. The resulting surface displacement is computed at more than a thousand locations covering the whole near‐source region (0–100 km from the source). The seismograms obtained are then combined together to yield the space and time dependence of the ground motion. Various source‐medium configurations are studied. The results show the strong directivity effect of the propagating rupture. Large surface displacements and high‐frequency motions are confined to a narrow zone around the fault and to the region which extends beyond the source along the trend of the fault. SH waves and Love waves are the dominant contribution to the ground shaking. The vertical displacement is small but exhibits high‐frequency oscillatory motions. The presence of low‐velocity surface layers has a very severe effect on the amplitude and duration of the ground shaking.
Mechanical behavior of New Mexico rock salt in triaxial compression up to 200°C
Authors: Wawersik W. R. , Hannum D. W.
Published: February 2008

Experiments were conducted to determine the ‘quasi‐static’ mechanical behavior of rock salt in support of the structural design of a waste isolation pilot plant. The salt studied was from the Salado formation in the Delaware Basin, southeastern New Mexico. Three groups of tests are discussed to identify the relative and site‐specific importance of deviator stress, confining pressure (mean stress), temperature, time (loading rate), and stress path. The three groups of experiments consist of (1) hydrostatic loading, (2) triaxial compression tests (σ1 > σ2 = σ3 = const), and (3) variable stress path tests including experiments at approximately constant σ1 and at constant mean stress. The range of parameters considered was 0 ≤p (hydrostatic pressure) ≤ 34.5 MPa, 0 ≤ σ 2 = σ3 ≤ 20.7 MPa, 0 ≤ (σ1 − σ3) ≤ 62 MPa, 23 ≤ T (temperature) ≤ 200°C, and 1.7 ≤ d(σ1 − σ3)/dt ≤ 3.5 kPa/s. All data were generated on 100‐mm‐diameter specimens. The rock salt exhibited nonlinear response under all loading coditions, a low initial elastic limit (σ1 − σ3) ≤ 0.7 MPa, and an apparent inseparability of permanent deformations into time‐independent and time‐dependent components. Pressure and temperature did not alter the elastic constants over the range 29.6 ≤ E ≤ 36.5 GPa and 0.17 ≤ v ≤ 0.26, depending on core lithology. Pressure and temperature affected the principal strain ratio, the ratio of volumetric to shear strain, rock salt ductility, and the ultimate stress. In particular, low pressure and temperature permitted pronounced dilatancy and loss in load bearing ability. Under such conditions the volumetric strains reached sizeable fractions of the shear strains, ∣∈/(∈1 − ∈3)∣ ≥ 0.2. Pressure remained important even at 200°C temperature because it influenced the rate of shearing at constant loading rates. Load path and stress history may be significant under deviatoric loading conditions and for large variations in pressure.
Mechanical properties of clays at high pressure
Authors: Wang Chi‐Yuen , Mao Nai‐Hsien , Wu Francis T.
Published: February 2008

The mechanical properties of preconsolidated clays are studied at high pressures under undrained, triaxial conditions. At confining pressures equivalent to those at midcrust, different clays have the following common characteristics which differ significantly from those at low pressures: (1) the clays possess significant strength of several hundred bars (10 bars = 1 MPa); (2) the constitutive relations are marked by ductile yielding and strain hardening, followed by a broad peak strength and a gradual decrease in strength at greater deformation; and (3) the change in volume during shearing is small. On the other hand, the behavior of montmorillonite differs from that of the other clays (illite, chlorite, and kaolinite) in the following ways: (l) its peak strength at a given confining pressure is about half of the strengths of the other clays; (2) its peak strength occurs at a shortening of about 10%, while for the illite and chlorite the peak strength occurs at a shortening of 20–25% (for kaolinite, strain hardening continues even at 30% shortening); and (3) the fracture surfaces of some montmorillonite samples deformed at relatively low confining pressures show features resembling those in natural clayey fault gouge, whereas at higher confining pressures, montmorillonite and the other clays remain unfractured at shortening up to 30–40%.
The determination of surface harmonic expansion coefficients from mean gravity anomalies by orthogonal collocation
Authors: Shebalin John V.
Published: February 2008

Orthogonal collocation methods are utilized in determining the information content of global mean gravity anomalies. In particular, it is shown that the maximum degree of a spherical harmonic expansion derived from 5°×5° (unequal area) mean anomalies is fifty. Also, a practical algorithm is given for determining the coefficients of the expansion which exactly reproduces a set of mean gravity anomaly values.
Reconnaissance electrical surveys in the Coso Range, California
Authors: Jackson Dallas B. , O'Donnell James E.
Published: February 2008

Telluric current, audiomagnetotelluric (AMT), and direct current (dc) methods were used to study the electrical structure of the Coso Range and Coso geothermal area. Telluric current mapping outlined major resistivity lows associated with conductive valley fill of the Rose Valley basin, the Coso Basin, and the northern extension of the Coso Basin east of Coso Hot Springs. A secondary resistivity low with a north‐south trend runs through the Coso Hot Springs‐Devil's Kitchen geothermal area. The secondary low in the geothermal area, best defined by the 7.5‐Hz AMT map and dc soundings, is caused by a shallow conductive zone (5–30 ohm m) interpreted to be hydrothermally altered Sierra Nevada basement rocks containing saline water of a hot water geothermal system. This zone of lowest apparent resistivities over the basement rocks lies within a closed contour of a heat flow anomaly where all values are greater than 10 heat flow units.
Seismic waves at the epicenter's antipode
Authors: Rial J. A. , Cormier V. F.
Published: February 2008

The antipodal region (178° ≤ Δ ≤ 180°) of a seismic wave source is investigated in detail and shown to provide a new set of remarkable data to use in the exploration of the earth's interior. Body and surface waves converge individually at antipodal distances after having sampled laterally the totality of the planet. The waves are focused and strongly amplified up to 1 order of magnitude with respect to the normal phase recorded 2° or more away. The delicate interference patterns thus formed yield information on departures from lateral homogeneity and sphericity of the core and mantle, the structure of the inner core, global dissipation characteristics of the upper mantle, and provide strong constraints on earth models. Seismograms have been synthesized that closely reproduce the phases Pdiff, PKIKP, PKIIKP, PKP(BC), PKP, and PP observed at World‐Wide Standard Seismographic Network long‐period instruments located within 5° from the antipode of the New Zealand Inangahua earthquake of May 23, 1968. Preliminary results indicate that the lower mantle and upper core are laterally homogeneous as seen by 15‐s waves, but the core‐mantle boundary region is probably laterally inhomogeneous. The inner core‐outer core boundary appears to be a sharp transition with a P wave velocity jump of the order of 0.8 km/s. The resolution of the long‐period data is poor, but the potential richness of the method when better data sets are available strongly motivated the investigation. Suggested future lines of research using antipodal observations include monitoring of inner core phases, study of focal processes of large earthquakes, and the exploration of planetary interiors.
Observations from a seismic network in the Virgin Islands Region: Tectonic structures and earthquake swarms
Authors: Frankel Arthur , McCann William R. , Murphy Andrew J.
Published: February 2008

Hypocenters of earthquakes located by a telemetered seismic network in the Virgin Islands provide a detailed picture of the seismicity along the northeast boundary of the Caribbean plate. The Virgin Islands‐Puerto Rico group represents an island arc characterized by plate motion highly oblique to the arc. The network data indicate a complex distribution of epicenters that is not homogeneous along the strike of the plate boundary. Earthquake foci between 25‐ and 120‐km depth align along a downgoing seismic zone that dips to the south beneath the Virgin Islands, indicating the presence of a subducted portion of the North American plate. Shallow events above 20‐km depth are concentrated between 64.5° and 65.1°W and appear to be associated with the boundary between the North American plate and the Caribbean plate. Twenty swarms of earthquakes (localized surges of shocks of nearly equal size) were detected by the network during the 2½ year period of study. Three of the swarms consisted of over a hundred events between magnitude 2 to 4½. Certain areas produced more than one swarm within this time interval. Some of the environments that generated the swarms include the region beneath the inner wall of the Puerto Rico trench near 19°N, 65°W, one portion of the outer wall of the trench, and a zone 40–50 km deep within the North American plate at about 19°N. During the period of observation, each swarm that occurred near 19°N between 64° and 65°W was followed by a second swarm in that area within 28 days. Results from the network indicate that the north wall of the Virgin Islands basin, which forms a portion of the Anegada passage, is a significant locus of seismicity within the Caribbean plate. This activity was largely manifested as swarms to the south of the islands of Vieques and St. Thomas during the time of network observation.
Soil radon concentration changes preceding and following four magnitude 4.2–4.7 Earthquakes on the San Jacinto Fault in southern California
Authors: Birchard G. F. , Libby W. F.
Published: February 2008

Radon concentrations in soil gas measured in shallow holes by plastic Track Etch® detectors show variations which may be associated with four magnitude 4.2 to 4.7 earthquakes on the San Jacinto Fault in southern California. A simple sinusoidal correction for the annual soil temperature cycle removes most of the variability in radon concentration that cannot be correlated with earthquakes. The two earthquakes located approximately 4 and 5.5 km away from the array of detectors showed spatially coherent responses with the largest radon concentration changes occurring closest to the earthquake. The two earthquakes 10–15 km distant from the closest site showed changes apparent when all sites were averaged together. The two nearby earthquakes were right lateral strike‐slip events which had quadrants of seismic compression and dilation which correlate with radon concentration increases and decreases, respectively. Radon increases occur when the upward velocity of soil gas increases, since a sharp radon concentration gradient exists in the top few meters of soil. Gas outflow may occur in regions of compression and inflow in areas of dilation, thus producing the observed radon concentration changes.