bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of laboratory and in situ compressional‐wave velocity measurements on sediment cores from the western North Atlantic
Authors: Tucholke Brian E. , Shirley Donald J.
Published: February 2008

Laboratory and in situ velocity measurements have been made on six piston cores taken in the western North Atlantic Ocean. Sediments from the southwestern Bermuda Rise and Greater Antilles Outer Ridge are clays having velocities ranging mostly from 1500 to 1530 m/s and velocity gradients near 1 s−1. In cores from the Nares Abyssal Plain, the clayey sediments have comparable velocities, but interbedded silty turbidites exhibit much higher values (up to 1690 m/s). Velocity gradients are slightly higher in the abyssal‐plain cores. After the laboratory measurements are corrected to in situ conditions, they show reasonable agreement in average velocity and velocity gradient with in situ measurements, although the in situ velocities average 10–12 m/s higher in the clayey cores and 15–20 m/s higher in the turbidites. This difference may be caused by reduction in the dynamic frame bulk modulus and/or the dynamic shear modulus due to visually undetected coring disturbance. The profilometer used to obtain the in situ measurements does not record the fine‐scale variations in velocity that were measured in the laboratory, but it accurately determines average velocities and velocity gradient. Where cores were closely spaced (2–12 km apart), inter‐core correlations in lithology, velocity, and bulk properties are possible. Fluctuations in the latter two parameters are very similar in position and magnitude from core to core, suggesting either that effects of coring disturbance are small or that they are uniform in a given kind of sedimentary bed. Inter‐core comparison also shows that some beds are laterally discontinuous as a result of local (less than a few kilometers) patterns of seafloor erosion and deposition.
Thermal evolution of Earth and Moon growing by planetesimal impacts
Authors: Kaula William M.
Published: February 2008

Improved estimates of impact energy partitioning are combined with models of planetesimal size distribution and planetary growth to infer the early thermal evolutions of the earth and the moon. The early stage partitioning of impact energy to kinetic energy is used to limit the contribution of impacts to heat transfer by turbulent mixing. The greatest uncertainty in the models is the portion of the impact energy partitioned to internal energy which is retained after material falls back in great impacts. Models of the earth with dynamically plausible growth times of the order of 50 m.y. are found to get hot enough for vaporization to occur if more than about 12% of the heat energy is retained upon impact. There is a slight positive correlation of heating with growth time, since a longer time implies larger planetesimals. Binary accretion models of the moon allowing for enhancement of velocities by the proximity of the earth do not get hot enough for appreciable melting unless a planetesimal mass distribution starting at a rather high value, ME/20 or more, is assumed. This melting occurs deeper than is inferred from petrological and thermotectonic data. Hence these results favor formation of the moon as a consequence of a great impact (or impacts) into the earth (Kaula, 1977).
Shear stress enhancement of void compaction
Authors: Curran J. H. , Carroll M. M.
Published: February 2008

Motivated by the success of the spherical model in predicting the volumetric compaction behavior of both porous rocks and metals to a hydrostatic pressure, we consider the applicability of the spherical model to nonhydrostatic loading conditions. Specifically, the spherical model is used to examine the influence of the presence of a shear stress on the volumetric compression of a porous solid. We first obtain the linear, elastic solution for a hollow sphere subject to homogeneous tractions on the outer boundary. Then, assuming that the matrix material is governed by the Drucker‐Prager yield criterion, we use the elastic solution to derive an analytic expression for the onset of yield in the hollow sphere. The expression for the initial yield surface shows that the presence of a shear stress hastens the onset of yield in the sphere in comparison to a hydrostatic loading condition. This result agrees well with experimental data which shows that, for porous solids, permanent crush‐up begins at a lower mean stress under a nonhydrostatic loading than when the applied loading is a hydrostatic pressure. At this point, due to difficulty in obtaining an analytic solution, we turn to a numerical scheme (finite element method) to extend the analysis of the hollow sphere problem into the elasto‐plastic range. The spherical model results clearly exhibit the experimental finding that the presence of a shear stress tends to enhance the volumetric compaction of porous solids in comparison to a hydrostatic loading condition. For both a porous rock and metal sample, agreement between the spherical model and experimental results is excellent.
The mechanics of dynamic shear crack propagation
Authors: Freund L. B.
Published: February 2008

A recent trend in seismology has been to model the earthquake source as a dynamically extending shear crack, and several basic concepts which seem to be important in this modeling process are examined. First, the universal spatial dependence of the plane elastodynamic stress and velocity fields near a sharp propagating crack tip is demonstrated for both subsonic and transsonic crack speeds, and the corresponding energy release rates are considered. Next, a class of steady state shear crack propagation problems is analyzed, based on both a direct stress analysis approach and an energy integral approach which obviates the need for a complete stress analysis in some cases. Several distinct stress differences which correspond to some of the common definitions of stress drop are involved in the analysis of these simple problems. Finally, some analytical considerations are presented which are relevant to rupture velocity determination for transient shear crack growth according to various fracture criteria, such as the critical stress intensity factor criterion, the critical energy release rate criterion, and the critical stress level criterion. Possible effects of spatially nonuniform stress drop and frictional resistance on rupture propagation are also discussed.
Mechanisms for the accumulation and release of stress on the San Andreas Fault
Authors: Turcotte D. L. , Clancy R. T. , Spence D. A. , Kulhawy F. H.
Published: February 2008

Two modeling studies of the stress accumulation and release cycle on two‐dimensional strike‐slip faults have been carried out to understand better the behavior of the San Andreas fault. The first study concerns the interaction between an elastic lithosphere and an elasticoviscous asthenosphere. An analytic solution has been obtained for the stress and displacement fields due to an infinite sequence of step displacements on a two‐dimensional fault. For reasonable values of the asthenospheric viscosity the periodic fault displacement is damped out several lithospheric plate thicknesses from the fault. For relatively high viscosities the strain accumulation phase between earthquakes is nearly constant; for relatively low viscosities the strain accumulation decreases with time. In a second study a series of finite element calculations have been carried out to study a depth‐dependent rheology on the fault. A yield stress which decreases with depth is prescribed. It is found that the depth of the locked section of the fault decreases as stress on the fault increases. As a result, the rate of shear strain near the fault increases prior to a great earthquake.
Seasonal recession of Mars' south polar cap as seen by Viking
Authors: James Philip B. , Briggs Geoffrey , Barnes Jeffrey , Spruck Andrea
Published: February 2008

The spring‐summer retreat of the south polar cap of Mars is portrayed in photomosaics obtained by Viking Orbiter 2 during 1977. Comparisons of these data to Mariner 9 photos and to the record of telescopic observations attest that the polar retreat viewed by Viking was significantly slower than those previously reported. A global dust storm which occurred at an unusually early season may have effected this retarded recession by introducing dust into the atmosphere of Mars which modified the polar energy balance through scattering of incident radiation. The composition of the south residual cap cannot be unambiguiously determined at this time; however, some data suggest that CO2 or clathrate survived the entire summer viewed by Viking.
Profiler‐sonobuoy measurements in the South China Sea Basin
Authors: Ludwig W. J. , Kumar N. , Houtz R. E.
Published: February 2008

Analyses of single‐channel seismic reflection profiles, and wide‐angle reflection and refraction data from sonobuoys, indicate that the deep part of the South China Sea basin is typically oceanic in structure, except that layer 2 is about 1 km thicker than usual and layer 3 is only about half the usual thickness. The top of layer 2 has a rough upper surface and becomes deeper in the northern part of the basin, where it is overlain by sediments of velocity 1.7–3.8 km/s up to 3 km thick. A 6‐km‐thick sequence of sediments fills a basement depression just north of the outer subsurface (peripheral) ridge of the Sunda shelf that may be structurally part of the Palawan trough. Thick sediments were measured at the outer parts of the Sunda shelf, Palawan shelf, Northeast Borneo shelf, and in the Taiwan straits (Pescadores channel) south of the Penghu‐Peikang basement high. The Manila trench is divided into two sections, a sediment‐filled trench between southernmost Taiwan and Stewart bank off central northern Luzon and a topographic trench between Stewart bank southward into the Mindoro straits. Burial of part of the trench by sediments that thicken to the north and the thick sediment cover of the northern South China Sea basin indicate provenance from the north. Each of the margins of the South China Sea basin is interpreted to have a different structural style. The northern margin of the basin is predominantly a zone of tension (graben and horst block structures), the southern margin is predominantly a zone of compression (fold structures), the western margin may be a zone of shear, and the eastern margin is a subduction zone (Palawan trough and Manila trench). The geological and geophysical characteristics of the basin suggest a middle Tertiary to Recent history of development that involved spreading of the sea floor. It seems probable that the southern margin of the basin, called the Reed bank crustal block, was formerly attached to mainland Asia along the present eastern edge of Macclesfield bank. Sometime during the Paleogene the Reed bank crustal block broke away from the continent, and new sea floor was created between the southeastward migrating block and the Asian mainland. Old ocean crust in front of the advancing block was subducted along the northwestern sides of Borneo and Palawan. Subduction ceased in the early Miocene with the collision of the crustal block and the Borneo‐Palawan isthmian ridge.
Grüneisen parameter of quartz, quartzite, and fluorite at high pressures
Authors: Boehler R. , Skoropanov A. , O'Mara D. , Kennedy G. C.
Published: February 2008

Temperature changes caused by small rapid pressure changes have been measured for quartzite, single‐crystal quartz, and fluorite to 36 kbar. The Grüneisen parameter was then calculated from the adiabatic pressure derivative of temperature (∂T/∂P)s by use of the thermodynamic relationship γ = Ks/T (∂T/∂P)s where Ks is the adiabatic bulk modulus. The measurements were carried out in a piston cylinder apparatus using alcohol as a pressure transmitter. Parameter γ for quartz and quartzite slightly decreases with pressure, whereas we note a large decrease in γ for fluorite with pressure. We find values for q = −(∂ ℓn γ/∂ ln V)T of 2.0, 1.7, and 4.6 for quartz, quartzite, and fluorite, respectively.
Temporal variation of the activity of intermediate and deep focus earthquakes
Authors: Abe Katsuyuki , Kanamori Hiroo
Published: February 2008

The appendix is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J79‐003; $1.00. Payment must accompany order.

The temporal variation of the activity of intermediate and deep focus earthquakes for the period from 1904 to 1974 is investigated on the basis of a uniform body‐wave magnitude scale. The mb scale has been traditionally used for intermediate and deep focus earthquakes. However, the period of seismic waves and the Q function used for the magnitude determination have changed as a function of time, so that uniform evaluation of the activity is difficult. In this study, we used a body‐wave magnitude mB determined from the maximum amplitude of body waves recorded by broad‐band or long‐period instruments to investigate the temporal variation. For the period from 1904 to 1952, mb values were calculated from the unpublished material of Gutenberg and Richter. For 1953 to 1958, mB values were calculated from the amplitude and the period data in Gutenberg's unpublished file. For the period from 1959 to 1974, seismological bulletins from various stations were used to determine mB. Consistency between the values of mB calculated from these different sources was carefully checked. The temporal variation of the number of intermediate‐deep events with mB ≥ 7 is remarkably similar to that of shallow earthquakes with Ms ≥ 7. The temporal variation of the energy release in intermediate‐deep earthquakes shows a maximum around 1910. The sum of the energy release curve of intermediate‐deep and shallow earthquakes shows a good correlation with the amplitude of the Chandler wobble. The energy release becomes maximum at the depth of 350 and 600 km.
Rupture propagation and focusing of energy in a foam rubber model of a stick slip earthquake
Authors: Hartzell Stephen H. , Archuleta Ralph J.
Published: February 2008

Dynamic displacements and velocities measured at the surface of a block of foam rubber are analyzed for stick slip events on faults with three different geometries; elliptical, rectangular (aspect ratio of 3.33), and long thin (aspect ratio of 12.66). Over 100 separate events were recorded, each with up to eight simultaneous point measurements of particle displacement. The positions of the measurements were along the fault trace and to distances of a few fault lengths. Acceleration of the rupture front is observed as the rupture propagates out from its point of nucleation. Deceleration of the rupture is also observed when it enters a region of lower effective stress. In the direction of rupture propagation the static displacements along the fault trace increase, and the rise times decrease. The combined effect causes a rapid increase in particle velocities. For a predominantly unilateral rupture, increases in particle velocities by a factor of 2 or 3 over those recorded near the center of the fault trace are common. At higher rupture velocities, there is a larger increase in the particle velocity (or decrease in the rise time) for a given increase in the static displacement. The amplitude of the far‐field displacement is similarly strongly dependent on the nature of the rupture. Differences in far‐field displacements of an order of magnitude are seen for measurements at equal distances from a unilateral rupture. When the rupture initiates at depth, a much more uniform distribution of particle displacements and velocities is obtained. Afterslip, i.e., motion occurring significantly slower than the time required for a shear wave to travel the length of the fault, is also observed. Comparison of our model results with surface static displacements following major earthquakes suggests that observed variation in static displacements may be due in part to variations in the bedrock stress field. Such stress heterogeneity can lead to a division of a major earthquake into a series of several subruptures, each of which rupture coherently, but the event as a whole behaves incoherently.
Polar motion results from Geos 3 laser ranging
Authors: Schutz B. E. , Tapley B. D. , Ries J. , Eanes R.
Published: February 2008

The observability of polar motion from laser range data has been investigated, and the contributions from the dynamical and kinematical effects have been evaluated. Using 2‐day arcs with Geos 3 laser data, simultaneous solutions for pole position components, xp and yp, and orbit elements have been obtained for a 2‐week interval spanning August 27 to September 10, 1975, using three NASA Goddard Space Flight Center stations located at Washington, D.C. (Stalas), Bermuda, and Grand Turk. The results for yp from this limited data set differenced with the BIH linearly interpolated values yield a mean of 39 cm and a standard deviation of 1.07 m. Consideration of the variance associated with each estimate yields a mean of 20 cm and a standard deviation of 81 cm. The results for xp indicate that the mean value is in fair agreement with the BIH; however, the xp determination is weaker than the yp determination due to the distribution of laser sites (all three are between 77°W and 65°W) which results in greater sensitivity to the data distribution. In addition, the sensitivity of these results to various model parameters is discussed.
The computation of wind speed and wave heights from Geos 3 data
Authors: Mognard Nelly , Lago Bernard
Published: February 2008

We have processed radar altimeter data from more than 100 Geos 3 passes. The statistical analysis of the stretching of the leading edge of the return radar pulses gives us an estimate of the significant wave heights, as reported by other scientists. Moreover, the automatic gain control level measurements of the reflected power provide an evaluation of the wind speed. The wave height and wind speed estimates are in favorable agreement with the meteorological information obtained from weather maps used as sea truth, thus resulting in interesting complementarities.
Remanent Early Cretaceous spreading center in the Central Pacific Basin
Authors: Tamaki Kensaku , Joshima Masato , Larson Roger L.
Published: February 2008

A WNW trending lineated topographic depression (Magellan Trough) and associated magnetic anomaly lineations (Magellan lineation set) were revealed in the basin northeast of the Magellan Rise. We identified symmetric magnetic anomaly lineations from M9 (121 m.y.) to M11 (126 m.y.) of Early Cretaceous age with a symmetric axis at the Magellan Trough and concluded that the Magellan Trough is a remanent spreading center which ceased its opening at the latter half of M9 time. The Magellan lineation set fans drastically to the southeast, and it represents an asymmetric spreading system. The Magellan spreading system has two separate rotation poles for the northern flank lineations and the southern flank ones, and both poles lie on the extension of the Magellan Trough. The amplitudes of the magnetic anomalies decrease toward the poles, possibly owing to the contamination of the magnetized blocks as a function of decreasing spreading rate. The Magellan spreading system shows unusually rapid rotation rates of 4.2°/m.y. for the northern flank and 3.1°/m.y. for the southern flank, which together with the proximity of the rotation poles suggest unstableness of the spreading system. This unstable spreading may have shortened the life of the Magellan spreading system. Such an unstable spreading system could have been generated in order to balance changes in movements of the Phoenix, Farallon, and Pacific plates. The spreading center may have jumped to the northeast at M9 time as part of its evolution as the Pacific‐Farallon ridge. Thus the Magellan Trough may be a fossil Pacific‐Farallon ridge and the northern flank of the Magellan set a piece of captured Farallon plate.
Analysis of star pair latitudes
Authors: Graber Michael A.
Published: February 2008

Star pair latitude observations form the basis for the pole positions reported by the International Polar Motion Service (IPMS). The IPMS processes these observations to produce a mean pole position. However, the time series of observations contains high‐frequency information which is lost in the calculation of the mean pole. In this study, 2931 star pair observations are analyzed. A possible large excitation at one cycle per solar day is observed. The average power level in the frequency band of the tesseral tides is seen to be high, although the peaks cannot be conclusively identified as tidal phenomena.
Moderate and large earthquakes in the South Sandwich Arc: Indicators of tectonic variation along a subduction zone
Authors: Frankel Arthur , McCann William
Published: February 2008

Moderate and large earthquakes of the South Sandwich region that occurred between 1929 and 1963 were relocated. Variations in the long‐term seismicity along the length of the South Sandwich arc are observed to correlate with changes in the age of the subducted lithosphere and with the position of a fracture zone inferred to be on the incoming plate. The northern portion of the arc generated several events of moderate size (6 ≤ Ms < 7) but no large events (Ms ≥ 7) in the zone of underthrusting during this century. In this region, relatively old lithosphere (∼70 m.y. old) is being subducted underneath a young plate (∼7 m.y. old). The central portion of the arc produced almost no events of magnitude 6 or larger during the period studied. This section of the arc is located across from the position of an inferred fracture zone on the plate that is being subducted. The southern portion of the South Sandwich arc is being uplifted in response to the subduction of buoyant lithosphere generated at an adjacent spreading center. This is consistent with the model of ridge‐trench convergence proposed by DeLong et al. Several events of moderate and large size occurred in both the zone of underthrusting and the area seaward of the trench in this region. The low level of seismicity of the South Sandwich arc, when compared to other island arcs of similar convergence rates (7–9 cm/yr), indicates that aseismic slip may account for a substantial portion of the convergence in this subduction zone. This paper also reports fault plane solutions for three earthquakes of the region.
Regional deformation of the Sierra Nevada, California, on conjugate microfault sets
Authors: Lockwood John P. , Moore James G.
Published: February 2008

Strike slip microfaults are pervasive throughout the granitic rocks of the eastern Sierra Nevada. Offsets typically range from less than a millimeter to several tens of centimeters but exceed 100 m in some places. The spacing between microfaults varies from a few tens of centimeters to a few tens of meters throughout much of the high Sierra Nevada. Many of these microfaults are loci of slickensided, compact fault gouge, and they are commonly mineralized by quartz veinlets with minor amounts of epidote, chlorite, and rare sulphide minerals. The microfaults are oriented in two nearly vertical conjugate sets; a north to northeast striking set showing right lateral offset and an east to northeast striking set showing left lateral offset. Microfaults with left lateral offset are more common than microfaults with right lateral offset. Most lineaments visible on aerial photographs are microfaults. The age of this microfaulting is not precisely known. It developed after consolidation of the youngest granitic plutons in the Sierra (79 m.y. B.P.) and is known to cut a late Miocene volcanic dike in one area. Slickensides along the microfaults are subhorizontal but show a slight (about 3°) westward plunging average inclination, suggesting that much of the deformation occurred prior to the westward tilting of the Sierran block in late Tertiary time. The direction of maximum horizontal extensional strain (determined as the bisector of average microfault trends) changes systematically from north to south (WNW at 38.5°N; NW at 36.5°N). A pure shear constant volume solution based on a detailed study of microfaults at 37°20′N indicates a maximum extension of 2.3% in a N61°W direction. These extension directions are remarkably parallel to late Mesozoic to present‐day tectonic extension directions in the Basin and Range province. The pattern of microfaulting demonstrates that the supposedly monolithic Sierran terrane was also affected by the late Cenozoic and possibly earlier regional extension of western North America and provides an independent criterion for determining extensional strain directions.
Reevaluation of the turn‐of‐the‐century seismicity peak
Authors: Kanamori Hiroo , Abe Katsuyuki
Published: February 2008

According to currently available seismicity catalogues, seismicity (for example, the number of events with Ms ≥ 8) around the turn of the century, from 1897 to 1906, was significantly higher than in recent years. However, the magnitudes of the earthquakes which occurred during this period were determined by Gutenberg, who used the records obtained by the undamped Milne seismograph with the assumption that the effective magnification is 5. Because of saturation of the Milne seismogram for very large events used by Gutenberg for calibration, the gain (=5) used by Gutenberg could have been underestimated, and therefore the magnitude overestimated. Because of the lack of damping, the magnification of this instrument needs to be calibrated carefully. In order to calibrate the instrument response, a Milne seismograph was constructed and has been in operation side by side with damped seismographs at Pasadena. Eleven events have been recorded since February 1977. On the basis of (1) comparison of the amplitudes measured on the Milne seismograms with those of the standard seismograms, (2) numerical experiments simulating the response of the Milne seismographs to surface waves, and (3) examination of Gutenberg's original materials used for the calibration, we conclude that the average effective gain is as large as 20 for very large earthquakes, resulting in systematic reduction of the magnitude of up to 0.6. This reduction is large enough to suggest that the turn‐of‐the‐century seismicity peak is of marginal significance.

Tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J79‐006; $01.00. Payment must accompany order.
Focal mechanisms and seismic energy release of deep and intermediate earthquakes in the Tonga‐Kermadec Region and their bearing on the depth extent of mantle flow
Authors: Richter Frank M.
Published: February 2008

The geophysical arguments relevant to the depth extent of mantle convection are reviewed with special attention to the seismic properties of subducted lithosphere. New focal mechanism solutions of deep and intermediate earthquakes from the Tonga‐Kermadec region are presented, but it is then shown that these do not provide an unambiguous constraint regarding the ability of subducted material to penetrate below 700 km. Estimates of the seismic energy release as a function of depth in Tonga show very large energy release in the depth range 500–700 km, which is just above the depth at which all seismicity ends. This property of the seismicity proves to be the most suggestive of the inability of subducted material to penetrate below 700‐km depth.
Deep structure of the Iceland Plateau
Authors: Evans J. R. , Sacks I. S.
Published: February 2008

The topography of the sea floor between Iceland and Jan Mayen Island is flat and elevated in relation to most ocean basins. Marine geophysical observations in the area have shown that it was formed by sea floor spreading but have not revealed details of structures more than a few hundred meters beneath the sea floor. We have examined the dispersion of seismic surface waves across the Iceland Plateau and have modeled structures to depths of up to 100 km. We find that the thickness of the crustal component of the lithosphere is much greater than that of normal oceanic structures, perhaps exceeding 20 km. We suggest that the elevation of the region is due to isostatic compensation for this excess of low‐density crustal material. The total lithospheric thickness is about 50 km throughout the region, indicating that the lithosphere thickens with age at a rate similar to that found in other young oceans.
The distribution of lobate debris aprons and similar flows on Mars
Authors: Squyres Steven W.
Published: February 2008

Planet‐wide mapping of lobate debris aprons and other similar flows on Mars shows a strong concentration in two latitudinal bands roughly 25° wide and centered at 40°N and 45°S. This distribution supports the idea that these flows form when erosional debris is transported downslope and becomes mixed with ice deposited from the atmosphere, as these latitudes should receive high seasonal H2O frost deposition relative to the rest of the planet. Flows are found in the northern hemisphere band wherever old highland surfaces occur but are found in the southern hemisphere only near the two major impact basins, Argyre and Hellas. These areas are apparently characterized by mass wasting that is rapid relative to most of the southern hemisphere highlands. The rate of mass wasting may be related to the degree of consolidation of highland material.
Thermal mapping of the northern equatorial and temperate latitudes of Mars
Authors: Zimbelman James R. , Kieffer Hugh H.
Published: February 2008

Using Viking infrared thermal mapping observations, nighttime temperatures have been mapped over the northern hemisphere of Mars. The latitude range from 10°S to 50°N was mapped near midnight local time in the northern spring and temperatures compared to those predicted by a uniform thermal model. As in earlier Viking thermal mapping, three large well‐defined regions are significantly cooler than expected. Four less well defined warm areas occur; two extend north beyond this coverage. Large variations of the temperature residual, −45 to +19 K, are related primarily to the thermal inertia of the surface. Although strong local correlations exist in some areas, there is no consistent regional‐scale correlation with elevation, albedo, geology, or geomorphology. Where studied in detail, the boundaries of the cool regions and some local thermal structures are found to be related to the occurrence of patches of dark material and streaks downwind of craters. High‐resolution imaging indicates that a mantling layer exists over at least one of the cool regions. A general hypothesis for the transport of loose material on the Martian surface invokes the stability of the smooth, fine grained surfaces to account for the bimodal thermal behavior observed. This hypothesis and thermal mapping suggest that large areas of the Martian surface are very different from those observed by the Viking landers.
Effects of dust on the heating of Mars' Surface and atmosphere
Authors: Davies Donald W.
Published: February 2008

An analysis performed to determine the effect dust particles suspended in Mars' atmosphere have on the radiation reaching the surface both directly and scattered by the dust is described. Additionally, the fraction of incident sunlight directly absorbed by the atmospheric dust is computed. These calculations are done for ranges of dust opacity τ, incidence angle i, surface albedo as, and dust albedo ad, representative of the conditions on Mars. The effect of atmospheric dust on the Bond albedo is discussed. It is shown that direct heating of the atmosphere by dust absorption of solar radiation is adequate to explain Mars' south polar spring temperature inversion. Under most circumstances the presence of dust in Mars' atmosphere produces a lowering of the average surface temperature; this is probably the cause of the anomalously slow south polar cap retreat of 1977. Explicit forms for both the surface heating and the atmospheric heating as a function (τ, i, as, ad) are given.
Photometry of Phobos and Deimos from Viking Orbiter images
Authors: Klaasen Kenneth P. , Duxbury Thomas C. , Veverka Joseph
Published: February 2008

Images of Phobos and Deimos acquired by the Viking orbiter television system have been used to determine the photometric functions of the Martian moons. Data covering wavelengths from 445 nm to 593 nm and solar phase angles between 0.5° and 122° were used. Normal reflectances of 0.066±0.006 for Phobos and 0.069±0.006 for Deimos were determined. No variations in either photometric function or average normal albedo were observed over the wavelength range studied. The photometric functions demonstrate that the surfaces of Phobos and Deimos are intricate in texture and exhibit brightness surges near opposition that are more pronounced than that of the moon.
Structure and evolution of Uranus and Neptune
Authors: Hubbard W. B. , MacFarlane J. J.
Published: February 2008

We present three‐layer interior models of Uranus and Neptune with central rocky cores, mantles of water, methane, and ammonia (the ‘ices’), and outer envelopes primarily composed of hydrogen and helium. The models incorporate a new H2O equation of state based on experimental data which is considerably ‘softer’ than previous H2O equations of state. Corrections for interior temperatures ∼5000°K are included in the models, and the thermal evolution of both planets is investigated using recent heat flow measurements. We find that evolutionary considerations are consistent with gravitational field data in supporting models with approximately solar abundances of ‘ice’ and ‘rock.’ Models with very low abundances of water, methane, and ammonia can be excluded. Evolutionary considerations indicate that initial temperatures and luminosities for Uranus and Neptune were not substantially higher than the present value. Both planets apparently have relatively small (∼1–2 earth masses) hydrogen‐helium envelopes, with Neptune's envelope smaller than Uranus'. A monotonic trend is evident among the Jovian planets: all have central rock‐ice cores of ∼15 earth masses, but with hydrogen‐helium envelopes which decrease in mass from Jupiter to Saturn to Uranus to Neptune.
Structure of the New Hebrides Arc‐Trench System
Authors: Ibrahim A. K. , Pontoise B. , Latham G. , Larue M. , Chen T. , Isacks B. , Recy J. , Louat R.
Published: February 2008

Results from refraction experiments have been combined with local earthquake data to produce a preliminary model for the structure of the New Hebrides arc‐trench system. Maximum crustal thickness of about 28 km occurs beneath the island ridge. The crust is unusually thick (about 10 km) in the zone seaward (westward) of the trench, where the velocity of the deepest layer is 7.0 km/s. The crustal thickness of the Fiji plateau, east of the New Hebrides ridge, varies between 5 and 7 km. The velocities of the materials that form the inner trench wall are similar to those found in the island ridge. A zone of relatively high velocity (4.1 km/s) is encountered at a depth of about 1 km beneath the arc‐trench slope. This zone thickens toward the trench and is overlain by material with velocity typical of surficial sediments (≈ 2 km/s). This may be an accretionary deposit, or it may be of volcanic origin.
Effect of pressure on bonding in MgO
Authors: Bukowinski M. S. T.
Published: February 2008

The electron band structure and charge density of MgO are investigated theoretically at V0/V = 1.0, 1.25, 1.50. It is found that compression increases the band gap to a value of the order of 10 eV at lower mantle conditions. The prediction of increasing band gap is consistent with the experimentally observed decrease of the index of refraction of MgO. The valence charge density shows a significant amount of delocalization which further increases with compression. Ionic radii and ionicities cannot be uniquely defined, and in particular, the distribution of the valence charge, and therefore the properties of the oxygen ‘ion,’ are strongly influenced by the nature of the cation. Some geophysically important consequences are a likely increase in the transparency of MgO and its silicates to thermal radiation and a very large electrical resistivity at lower mantle conditions.
Preliminary heat flow and radioactivity studies in Wyoming
Authors: Decker Edward R. , Baker Keith R. , Bucher Gerald J. , Heasler Henry P.
Published: February 2008

Twelve new heat flow values in Wyoming are in the range 0.6–2.1 μcal/cm2 s (25–88 mW/m2). Radioactive heat productions at eight localities range from ∼0 to ∼1.3 μμcal/cm3 s (∼0−5.4 μW/m3). These data are consistent with the following interpretations: (1) The Laramie Mountains‐eastern Wyoming Basin area is a zone of normal heat flow (0.6–1.6 μcal/cm2 s (25–67 mW/m2)) that is characterized by low flux (∼0.6 μcal/cm2 s (∼25 mW/m2)) from the lower crust and upper mantle. (2) The eastern boundary of the Yellowstone caldera heat flow high (≥2.5 μcal/cm2 s (≥105 mW/m2)) is narrow. (3) The heat flow is high (1.9–2.1 μcal/cm2 s (79–88 mW/m2)) in parts of the Black Hills in northeastern Wyoming and western South Dakota. From the data presented, a major heat flow transition occurs between the Medicine Bow and Laramie mountains in Wyoming (0.6–1.3 μcal/cm2 s (25–54 mW/m2)) and the Rocky Mountains in northern Colorado (2.2–3.0 μcal/cm2 s (92–125 mW/m2)). The high flux in this part of the Southern Rockies may mean that the zone of high heat flow associated with the Rio Grande rift extends to the Colorado‐Wyoming Border. The normal heat flow in the Laramie Mountains‐eastern Wyoming Basin area implies submelting temperatures in the upper mantle. In contrast, the very high flux in northern Colorado may be related to high‐temperature, nonradiogenic heat sources in the lower crust and upper mantle because the width of the transition to normal flux in the Laramie Mountains in southern Wyoming is narrow (≤70 km).
The structure of the Kurile Trench–Hokkaido Rise System computed by an elastic time‐dependent plastic plate model incorporating rock deformation data
Authors: Liu Hsi‐Ping
Published: February 2008

The flexure profile of the Kurile trench‐Hokkaido rise system has been fitted numerically to observational accuracy by an elastic, time‐dependent plastic plate model. The elastic part of the constitutive relation is derived from seismology (relaxed moduli = 56% of the seismic values), while the strain‐rate‐dependent plastic part is derived from dunite deformation data extrapolated to the appropriate loading rates. The numerical fit of the flexure profile depends on a number of parameters such as the rock flow law parameters, the temperature distribution within the lithosphere, and the state of prestress of the plate before it enters the subduction zone. Two examples of fitting, one with Kirby‐Raleigh flow law parameters and one with Carter‐Avé Lallement flow law parameters, are given. The oceanic crust is considered to be made of strong and competent rocks in the former example, and it is considered to be made of fractured rocks with negligible strength in the latter. Because the solution is nonunique, conclusions regarding the values of the individual parameters cannot be drawn from the flexure profile alone. The present calculation, however, demonstrates the consistency between the rock deformation data and the observed plate flexure profile.
Periodicity and chaos in great earthquake occurrrence
Authors: Ito Keisuke
Published: February 2008

This paper is an application of the theory of ‘chaos’ to geophysics. It is pointed out that interactions between seismic events may play an important role in causing the apparently random behavior of earthquake occurrence. The great earthquake belt is modeled by a number of large‐scale fault blocks having the same strength and the same interaction parameters. Simulated seismic events show spatiotemporal random behavior if the blocks interact with each other in such a way that when an earthquake occurs in one block, the energy of blocks in the nearest region increases while the energy in the distant blocks decreases. A simple model composed of only two blocks also can exhibit nonperiodic behavior, if the blocks mutually interact in such a way that when an earthquake occurs in one block, the rate of energy increase decreases in the other block. An electrical analog model of the coupled fault blocks showing nonperiodic oscillation is presented.
The Galapagos Rift at 86° W: 4. Structure and morphology of hydrothermal fields and their relationship to the volcanic and tectonic processes of the Rift Valley
Authors: Crane Kathleen , Ballard Robert D.
Published: February 2008

The Angus camera system is used to investigate the detailed structure and morphology of the active hydrothermal vent fields of the Galapagos Rift. Precision navigational data are combined with microtopographic information and detailed geological and biological observations obtained from an analysis of the color bottom pictures to create a series of three‐dimensional models for each vent field. The five isolated vents are distributed along the southern boundary of a fissure wedge that cuts along the active axial high. All but one vent occur in pillow terrain. The fifth vent is at the contact between a pillow flow and sheet flow. Vents range in size from 400 m2 to 1600 m2 and are spread out along 2.8 km of a lineated fissure system that trends 095°. They are spaced at intervals ranging from 250 m to 1600 m with an average separation of 700 m. Vents are bound by tectonic features such as fissures or small throw faults. The egress of water is localized in small pockets within the fields. These zones of egress are distinctly unfissured. Instead, the water emanates out of rock to rock brecciated contacts. These thermal pockets range from 15 m to 40 m in diameter and are characterized by large concentrations of benthic fauna. On the basis of animal communities the fields may be divided into two different types, the western and the eastern groups. The two western fields are predominantly populated by clams (Vesicomyidae) and mussels (Mytilidae). The eastern three fields are dominated by tube worms (Vestimentifera), mussels, serpulid worms, and the spherical, colonial animal dandelion. The positioning of the vents on the fissure wedge implies a tectonic propagation and age gradation up and down the rift. At any one time, adjacent segments of the rift valley may be in different stages of a volcanic and tectonic cycle. As cooling of a central magma intrusion occurs, fissures propagate toward the remaining heated core, opening up pathways for the egress of heated bottom water. Cooling continues and the outermost vents die out. Precipitates block passages, capping the terrain. As extension continues, the very oldest sector of the valley will fracture. The incipient stress drop generates the eruption of a new magma center down strike from the former center, thus ensuring a volcanic and tectonic periodicity in space and time up and down the rift valley.
Leveling circuits and crustal movements
Authors: Chi S. C. , Reilinger R. E. , Brown L. D. , Oliver J. E.
Published: February 2008

Circuit misclosure analysis is used to investigate further possible vertical crustal movements in three areas in the western United States: along the San Andreas fault north of San Francisco, California; across the Nevada seismic zone in west central Nevada; and in the Sierra Nevada of northeastern California. These areas were chosen for this study because they are characterized by long‐wavelength apparent movements and because existing survey data are sufficient to investigate the reliability of at least some of the leveling observations defining these apparent movements. The analysis used here is based on the supposition that in areas undergoing crustal movement the misclosure for a particular circuit should have the smallest value when the circuit is formed from the most temporally homogeneous survey data and larger, predictable values when the circuit is closed with surveys conducted at other times. For example, leveling surveys conducted in 1935 and 1942 along the San Andreas fault show a pronounced zone of relative uplift north of San Francisco. However, analysis of the leveling circuit containing these surveys reveals large misclosures, suggesting that the apparent uplift is due to errors in the 1935 survey and not to real crustal movement. Leveling surveys conducted across the Nevada seismic zone in 1934, 1955, and 1967 were used to study crustal movements associated with the 1954 series earthquakes in this area (Savage and Church, 1974). Although comparison of the 1955 and 1967 surveys suggests an overall tilt of 2 mm/km down to the west over a 90‐km distance crossing the fault zone, Savage and Church (1974) showed that the 1967 survey was probably contaminated by systematic error. Comparison of the 1934 and 1955 surveys also suggests a regional tilting, 0.8 mm/km down to the east over a distance of about 200 km. The loop data in this area are sufficient to investigate the reliability of the 1955 survey. Our study suggests that the 1955 survey was not affected by unusual errors, thus implying but not confirming the possibility of regional tilting in the Great Basin between 1934 and 1955. Bennett et al. (1977) presented and interpreted elevation changes in the northern Sierra Nevada Range using results from leveling surveys conducted in 1912, 1938, 1947, and 1969 between Roseville, California, and Reno, Nevada. Their study suggests temporally complex regional movements with uplift of the range occurring between 1947 and 1969. Our analysis of leveling circuits in this area suggests that the 1947 survey was contaminated by systematic error. In addition, comparison of the 1947–1969 movement profile with topography indicates a strong correlation, possibly due to elevation‐correlated systematic errors. These results reemphasize the need for caution in applying tectonic interpretations to apparent movements indicated by releveling measurements.
Cyclic loading of simulated fault gouge to large strains
Authors: Jones Lucile M.
Published: February 2008

As part of a study of the mechanics of simulated fault gouge, deformation of Kayenta Sandstone (24% initial porosity) was observed in triaxial stress tests through several stress cycles. Between 50‐ and 300‐MPa effective pressure the specimens deformed stably without stress drops and with deformation occurring throughout the sample. At 400‐MPa effective pressure the specimens underwent strain softening with the deformation occurring along one plane. However, the difference in behavior seems to be due to the density variation at different pressures rather than to the difference in pressure. After peak stress was reached in each cycle, the samples dilated such that the volumetric strain and the linear strain maintained a constant ratio (approximately 0.1) at all pressures. The behavior was independent of the number of stress cycles to linear strains up to 90% and was in general agreement with laws of soil behavior derived from experiments conducted at low pressure (below 5 MPa).