bluuebunny/crossref_metadata_2025 huggingface dataset:
Earthquake prediction: Criterion for a tilt anomaly
Authors: Buckley C. P. , Kohlenberger C. W.
Published: February 2008

In this paper we present our current approach to the problem of defining and detecting anomalous tilt behavior. To establish what is considered to be normal tilt behavior, we isolate systematic signals such as hydrologic, thermal, tidal, cultural, and equipment‐related effects from the tilt data. The kinds of tilt signals which remain after rejection of the systematic signals are designated by ourselves as residual tilt. Residual tilt consists of asystematic random noise and anomalous tilts. To affirm or deny the contention that an anomalous tilt is present in the data requires the formulation of a statistically valid judgment criteria. Our approach adopts the hypothesis that the random walk model is not significantly different from the residual tilt and allows the application of standard statistical tests to the problem of detecting anomalous variations in random noise. In our study of the data analyzed so far, we find that the boundary for detectability is inverse frequency dependent, and this limits the way in which anomalies can be treated. The fact that the magnitude of the anomaly decreases as the tilt data span increases suggests that further criterion development is necessary and tends to imply that longer anomalies will not be detected unless there is a correspondingly larger amplitude. From our studies of three earthquake‐associated anomalies this does not appear to be the case.
Deep structure of the Rio Grande Rift: A magnetotelluric interpretation
Authors: Hermance John F. , Pedersen Jens
Published: February 2008

The interpretation of long‐period magnetotelluric data (T≤104 s) from several sites in the Rio Grande rift indicates a thin zone of anomalously high conductivity (S = 2200 S) at a depth of 10–17 km beneath the north central rift (Santa Fe) and a similar zone (S = 1600 S) at a depth of 21–28 km beneath the southern rift (El Paso). We suggest that these zones are contiguous with the tabular body previously delineated seismically over a restricted area (1700 km2) beneath Socorro, New Mexico, by Sanford and his colleagues and probably have a similar generic cause. Although the electrical conductivity itself can be affected by (1) electrolytic conduction in hydrothermal pore fluids, (2) conduction in the bulk silicate material itself, or (3) solid conduction through hydrated phases, such as layered silicates, we find the suggestion offered by Sanford and his colleagues for the Socorro structure to be the most attractive: the intracrustal zone represents a generic class of phenomena associated with the accumulation of basaltic melt derived from sources at greater depth. If this is the case, we may be seeing the effects of a thin zone (1 km) comprised totally of melt or a thicker zone containing as little as a 15–35% melt fraction. The periodic (T≤5×104 years) emplacement of such a magma accumulation and the subsequent perturbation and partial remelting of the surrounding country rock may in fact account for episodic silicic volcanism along the rift such as that which is seen, for example, in the Jemez Mountains.
The reconstruction of the Laurentide Ice Sheet of North America from sea level data: Method and preliminary results
Authors: Clark James A.
Published: February 2008

Usually, ice sheets are reconstructed using the flow law of ice and a number of assumptions regarding ice sheet temperature, accumulation rate, and basal sliding resistance. In this paper, an alternative and independent reconstruction method is given. The weight of the ice sheets deformed the earth's surface and its geoid. The amount of deformation, which can be determined from presently emerged shorelines, is related to the thickness of the overlying ice burden. Therefore elevation and age data of sea levels from North America during the past 17,000 yr can be inverted to determine the history of the most recent North American ice sheet (Laurentide ice sheet), assuming the earth behaves as a viscoelastic material. Because of errors in the data and a number of simplifying assumptions, quadratic programing methods were necessary to insure that the predicted ice thicknesses of the past were always greater than at present. The predicted ice sheet is relatively thin (2000–3000 m), which suggests that it was very dynamic and probably not in a steady state. But poor resolution indicates that other ice thickness models, perhaps of greater thickness, may fit the sea level data as well. The fit to the data is, in general, good, and it indicates that a large proportion of the North American sea level data can be explained by glacial isostatic processes. However, at regions distant from the ice sheet (e.g., Florida) the fit is poor, because the assumption that water‐loading effects can be ignored is no longer valid. For regions close to a fluctuating ice sheet margin (e.g., Connecticut) the fit to the data suffers because of the assumption that the areal extent of the ice sheet remained fixed through time. Methods are developed which obviate both of these unrealistic assumptions so that future inversion calculations will be more realistic. Furthermore, proglacial lake deleveling data can be used to extend the data set throughout the interior of North America so that the entire perimeter of the ice sheet is close to the constraining data. The error of fit and resolution may thus be improved considerably.
Seismicity changes preceding two recent central Asian earthquakes
Authors: Kristy M. J. , Simpson D. W.
Published: February 2008

The Gissar‐Kokshal fault zone south of the Tien Shan Mountains in central Asia is one of the most active areas of intraplate seismicity in the world. Since 1949 four earthquakes greater than magnitude 6.5 have occurred along the section of this fault north of the Pamir Mountains, almost completely filling a seismic gap which had existed for at least 80 years. The two most recent of these earthquakes, the 1974 Markansu (Ms = 7.4) and 1978 Zaalai (Ms = 6.7) events, were each preceded by a short period of increased activity in the future epicentral area some years prior to the main shock. The largest earthquake within this active period occurred at the same location as the future large shock. A segment adjacent to the 1978 earthquake, occupying the only unbroken portion of the former gap, has recently shown a similar burst of activity, suggesting that it may be the site of the next large earthquake of this sequence. Large premonitory earthquakes are also observed near the sites of seven other earthquakes with magnitudes 7.0 to 8.3 in the Tien Shan since 1880. In all of the cases studied, the premonitory time interval and magnitude of the premonitory earthquake increase with the size of the main shock.
Microseisms at Palisades: 3. Microseisms and microbaroms
Authors: Rind David
Published: February 2008

Microseisms (seismic surface waves) in the frequency range of 0.1–1 Hz recorded at Palisades, New York, are compared with microbaroms (infrasound) in the same frequency range recorded simultaneously. Both are thought to be generated by interfering ocean waves, and the question addressed is how often does their comparison imply that they have the same source. Compared were their approximate regions of formation, the formation conditions (both meteorological and oceanographic), amplitude variations, and frequency spectra. It is concluded that for the signals recorded at this location the two phenomena appear to have the same ocean wave sources 2/3 of the times and at least some of the remaining times one of the signals is prevented from arriving owing to propagation difficulties.
Measurement of absorption and dispersion from check shot surveys
Authors: Ganley D. C. , Kanasewich E. R.
Published: February 2008

The spectral ratio method for measuring absorption and also dispersion from seismic data has been examined. Corrections for frequency‐dependent losses due to reflections and transmissions have been shown to be an important step in the method. Synthetic examples have been used to illustrate the method, and the method has been applied to one real data case from a sedimentary basin in the Beaufort Sea. Measured Q values were 43±2 for a depth interval of 549–1193 m and 67±6 for a depth interval of 945–1311 m. Dispersion was also measured in the data and is consistent with Futterman's model.
Constraints on the absorption band model of Q
Authors: Lundquist Gary M. , Cormier Vernon C.
Published: February 2008

First order models for the combined depth and frequency dependence of Q are derived and tested using several independent constraints. (1) Using a microphysics approach, the adoption of an absorption band as a first‐order model for the frequency dependence of Q is justified, and the expected depth behavior of relaxation times in the earth is derived. The significant new parameter in this model of Q is τ2, the period at the half‐amplitude point of the high frequency end of the absorption band. (2) Using observed body‐wave spectra, the existence of a frequency dependence in Q is proved, and the average location of that frequency dependence (i.e., τ2) is estimated to be in the range 1 to 2.5 Hz. (3) Under the constraints of Q model ratios, the depth dependence of τ2 is estimated by assuming that a free‐oscillation and a body‐wave Q model both measure Q from the same absorption band. The resulting τ2 is about 0.04 s in the upper 200 km and then increases exponentially with depth in the mantle to about 1.9 at the core mantle boundary. The Q model ratios are better satisfied if a second absorption band is hypothesized to operate in the depth range of the asthenosphere. In that case, τ2 for the mantle absorption band varies from about 0.09 s in the first 200 km to 4.0 s at 2886 km, and τ2 for the asthenosphere absorption band is about 0.005 s in the depth range 35–220 km. (4) Both classes of Q models are tested in the time domain using synthetic seismograms of Russian and American nuclear explosions. Although trade‐offs between source and mantle transfer functions preclude further refinement of the models at this time, a compatibility is demonstrated between the double absorption band model and time domain constraints, including arrival time and pulse shape.
Chemomechanical weakening in the presence of surfactants
Authors: Dunning Jeremy D. , Lewis Wardell L. , Dunn David E.
Published: February 2008

Stable cracks (crack velocity of l m s−1) were propagated in 20 rectangular parallepipeds of identical orientation cut from a synthetic quartz crystal, having an initial dislocation density of 103 mm−2 or less and a water content of 5×10−2 wt %. Five tests each were conducted dry (ambient atmosphere PH2o) in distilled water ( ζ = −40 mV, where ζ is a type of electrostatic potential), and in two aqueous solutions of dodecyl trimethyl ammonium bromide (DTAB) (ζ = 0 and ζ = +70 mV). Mean values of the crack propagation stress σc were 69.7 MPa dry, 60.5 MPa in water, 48.6 MPa in DTAB at ζ = 0, and 45.3 MPa in DTAB at ζ = +70 mV. All cracks were mapped by phase contrast and transmission electron microscopy, from open crack throats to tips. Each crack consists of straight segments parallel to the basal plane and zigzag segments composed of r(
) and z(0111) cleavage steps. There is no evidence of dislocation nucleation or mobility influencing crack growth. These observations, the strength minimum at (ζ ≠ 0), and recent data from the surface chemistry literature strongly suggest that the chemomechanical models of Westwood (1974) and Rehbinder et al. (1944) cannot be applied to quartz and silicate geologic materials. The data appear to support a model presented here which relates crack propagation stress to the availability of adsorbable species.
The conversion of heat into mechanical work by mantle convection
Authors: McKenzie Dan , Jarvis Gary
Published: February 2008

The rate at which heat is converted into mechanical work by a convective system can be estimated from the thermal expansion coefficient, the specific heat, and the depth of the layer if the convective heat flux is known. The heat flux from the mantle is reasonably well known, as is the rate of plate creation. The depth of the layer involved is more controversial, as is the thickness of the brittle layer beneath island arcs. Reasonable estimates for these two parameters yield mean stresses on plate boundaries of between 100 and 200 bars with a probable upper bound of about 500 bars. These estimates are consistent with seismically determined stress drops.
Interpretation of postseismic deformation with a viscoelastic relaxation model
Authors: Wahr J. , Wyss M.
Published: February 2008

Aleutian tide gages registered coseismic subsidence of 10–15 cm at three locations during the great earthquakes of 1957 and 1965. For approximately 5–6 years after this initial subsidence the data indicate postseismic uplift of the islands to approximately their initial level. Similar postseismic uplift in other island arcs has previously been interpreted as due to fault creep along the deep end of the rupture plane and along extensions of it. Since the lithosphere below island arcs and above deep earthquakes has anomalously low Q values, it must be modeled as a volume of anomalously low viscosity. This anomalous volume is located adjacent to the deep end of great subduction zone earthquake ruptures. By modeling the lithosphere and asthenosphere as elastic solids, with an anomalous viscoelastic inclusion below the island arc volcanoes, we find that the observed postseismic surface deformation is a corollary of the known island arc structure. A satisfactory fit to the uplift following the 1957 earthquake is found for a viscoelastic volume with 80‐km width extending from 50‐ to 200‐km depth. Similarly fitting the post‐seismic uplift following the 1964 Alaskan earthquake, we find an anomalous volume northwest of Anchorage with a width of 40 km and a depth extending from 20 to 80 km. These smaller dimensions suggest that the viscous volume pinches out near the northern end of the 1964 rupture, where the chain of volcanoes ends. The viscosity of the anomalous material is estimated to be from 1.2×1019 to 2.2×1019 P. We propose that the assumption of fault creep on deep and sometimes distant extensions of the rupture plane is a less‐plausible mechanism for postseismic surface deformation than the viscoelastic relaxation model.
Heating of the Moon by heterogeneous accretion
Authors: Ransford G. A. , Kaula W. M.
Published: February 2008

Monte Carlo models of lunar accretion are constructed by randomly sampling mass and velocity distributions of objects in the solar nebula in circumsolar and circumterrestial orbits. The thermal effect of an impact is expressed as the volume of melt produced. The melt volumes produced by each Monte Carlo case are sorted into regions to produce percentage melt estimates as a function of radius. One hundred cases are calculated to obtain a range of plausible thermal histories for lunar accretion. The principal measure of heterogeneity used here is the average percentage of surface redistributed by large impacts as a function of radius of the moon: in these studies that measure was around 15%. The average melt percentage estimates show substantial deep melting, but the spread of melt percentage values about the mean from the random sampling procedure is wide enough to include 0% melting for radii below 1000 km and 100% melting for radii below 500 km. The average melt percentages above 1000 km in the moon are below 15% with small spreads about this average. The sensitivities to reasonable variations in solar nebula and geophysical parameters are investigated. No attempt is made to model heat transfer within the moon.
Diamonds: Minor elements in silicate inclusions: Pressure‐temperature implications
Authors: Hervig R. L. , Smith J. V. , Steele I. M. , Gurney J. J. , Meyer H. O. A. , Harris J. W.
Published: February 2008

New electron and ion microprobe analyses of 36 olivines, 9 pyroxenes, and 10 garnets included in diamond were obtained. Olivine inclusions from kimberlites with Precambrian ages have higher mean Cr2O3 (850 versus 360 ppmw), Al2O3 (200 versus 90 ppmw), CaO (490 versus 210 ppmw), Na2O (110 versus 30 ppmw), and Li (686 versus 466 counts per second) than olivine inclusions found in Mesozoic kimberlites, perhaps indicating a temporal change of either temperature or chemistry or both in the mantle source regions of diamonds. In contrast to lherzolitic olivines, aluminum and sodium in olivines from diamonds are insufficient to couple with trivalent chromium; perhaps divalent chromium is inferred. Some olivines show zoning to Ca‐rich edges. Temperature and pressure estimates for inclusions in four diamonds fall in the combined pressure‐temperature range of coarse and porphyroclastic garnet lherzolites, and temperature estimates for inclusions in seven diamonds lacking a pressure estimate cover most of the temperature range for lherzolites. Taken at face value, the pressure and temperature estimates for inclusions in diamonds indicate that some diamonds might have crystallized in the presence of a liquid containing H2O, CO2, or both. Other estimates fall in the subsolidus field even for H2O‐saturated peridotite. Although the pressure‐temperature estimates overlap for lherzolites and ultramafic‐type inclusions in diamonds, differences in trace element content of olivines require that the crystallization environments were not identical. Perhaps the Cr content of the olivine inclusions indicates that the host diamonds grew in a reducing environment not found in typical lherzolites.
Thermal stability of dolomite at high temperatures and pressures
Authors: Goldsmith Julian R.
Published: February 2008

The thermal decomposition (PT) curve of dolomite has been extended in piston‐cylinder apparatus from a point at 9 kbar and 1130°C to 1450°C. The piston‐cylinder runs, in which unsealed Pt sample capsules were embedded in glass pressure media, were compatible with earlier gas system determinations. At approximately 11 kbar and 1175°C the curve delineating the onset of decomposition of CaMg(CO3)2 deviates strongly from any extrapolated extension of the PT curve up to 10 kbar and 1150°C, and the dolomite composition is stabilized to significantly higher temperatures than those predicted from low‐pressure data. At 1450°C, Pco2is less than 14 kbar. The striking increase in thermal stability at pressures above 10 kbar is the result of rapidly shifting phase boundaries in TX sections at temperatures above the calcite‐dolomite solvus, and probably of an entropy increase in CaMg(CO3)2, due to a variety of disordering mechanisms.
Experimental results on the pressure dependence of the Grüneisen parameter: A review
Authors: Boehler R. , Ramakrishnan J.
Published: February 2008

The pressure dependence of the Grüneisen parameter can be determined by the measurement of temperature changes associated with adiabatic compression using the thermo‐dynamic relationship γ = Bs/T (∂T/∂P)s. Bs, the adiabatic bulk modulus, is known from ultrasonic measurements. The measurement of (∂T/∂P)s is carried out in an end‐loaded piston cylinder apparatus at pressures up to 35 kbar using a fluid cell. Results are presented for a series of fluids, metals, salts, and minerals. For fluids an increase in γ with pressure is observed. For all solids investigated, γ decreases with pressure. A drastic increase in γ is observed when a substance undergoes a phase transition. We find a systematic behavior of (∂T/∂P)s versus volume except for minerals. The average value of q = −(∂ℓ nγ/∂ℓ nV)T is 1.3. The value of γ for iron at a compression equivalent to the core mantle boundary is discussed, but the uncertainties are too large to permit us to predict with confidence γ for iron at the temperatures and pressures of the core.
A theoretical investigation of the sliding crack model of dilatancy
Authors: Stevens Jeffry L. , Holcomb David J.
Published: February 2008

Laboratory measurements of volumetric strain and velocity in Westerly granite are analyzed to determine the physical nature of microcracks in the rock. The commonly used factional ‘sliding crack’ model of dilatancy is examined in detail. The characteristics of an ensemble of sliding cracks are determined from the characteristics of the individual sliding surfaces. It is found that sliding cracks which open with a stick slip motion cannot account for the observed ‘memory’ of the peak stress in cycling experiments. A system of stably sliding cracks would have the required memory but would also produce a very long dead band at the beginning of sample unloading during which no change in inelastic volumetric strain could take place. An analysis of the elastic constants at the beginning of unloading shows that this dead band is not observed in laboratory experiments. This analysis shows that the sliding crack is an unlikely mechanism for dilatancy. An alternative is developed in the following paper.
An Earth dynamo with anisotropic resistivity
Authors: Cowan E. W.
Published: February 2008

A dynamo mechanism generating a self‐sustaining dipole field as a result of the motion of a fluid with anisotropic resistivity is a possibility for the dynamo generating the dipole field of the earth. The anisotropy appears as a large‐scale average effect resulting from small‐scale differences in the resistivity of the materials in ascending and descending convection currents within the liquid core, which are deflected in the required form by the Coriolis acceleration. A numerical analysis of time‐evolving magnetic fields in a model with anisotropic resistivity supports the basic mechanism of the dynamo effect. The differences in resistivity required to give regeneration for the core of the earth are too large to be accounted for by temperature differences but could conceivably result from differences in chemical composition.
Tidal variations of Earth rotation
Authors: Yoder Charles F. , Williams James G. , Parke Michael E.
Published: February 2008

We have rederived the periodic variations of the earth's rotation due to the tidal deformation of the earth by the sun and moon and included all terms with amplitudes ≥0.002 milliseconds (1 mm). This series applies to the mantle (plus crust) and oceans, which rotate together for characteristic tidal periods. The parameter which scales the rotational series is k/C, where k is that fraction of the Love number which causes the tidal variation in the moment of inertia of the coupled mantle and oceans while C is the dimensionless polar moment of inertia of the coupled units. If the whole earth (minus oceans) were coupled rotationally, then k = 0.30 and C = 0.33. Ocean tides increase k by 0.04. Decoupling of the fluid core from the mantle decreases k by 0.06 while C = Cmantle = 0.29, since neither pressure, viscous, nor hydromagnetic coupling locks the fluid to the mantle for periods less than 5 to 25 years. From the analysis of lunar laser ranging data we find that k/C at monthly and fortnightly frequencies equals 0.99 ±0.15 and 0.99 ± 0.20 as compared with a theoretical value of 0.94 ± 0.04. In addition, we have estimated theoretically the effects of ocean tides on earth rotation, nutation and polar motion using models based on the Laplace tidal equations.
The effect of crustal structure on the regional phases Pg and Lg at the Nevada test site
Authors: Barker B. W. , Der Z. A. , Mrazek C. P.
Published: February 2008

The effect of local structure on the characteristics of the regional phases Pg and Lg was investigated by analyzing events at epicentral distances less than 10° from the Nevada Test Site. The stations used are on markedly different geological structures; three on granitic intrusions, two on Pahute Mesa, a feature overlaying a buried volcanic caldera, and, finally, four on Yucca Flats, a deep alluvial valley. The crustal effect on amplitudes is quite significant for both Lg and Pg; Yucca Flats gives amplitudes as much as a full order of magnitude higher than the stations on granite, while Pahute Mesa stations are 0.7–0.8 magnitude units higher in amplitude than the granite sites. Besides gross amplitude effects, the local crust also induces prolonged ringing at Yucca Flats, resulting in the lengthening of the signal wave trains. The observed site amplification effects show no clear dependence on the azimuths of arrivals, indicating that the phenomenon is due to local structures and is not caused by lateral refraction of surface wave modes. The observed amplification is similar in magnitude for the vertical component of both Pg and Lg, thus not affecting the Pg/Lg amplitude ratio, a possible discriminant between earthquakes and explosions at regional distances. Finite difference theoretical simulations using two‐dimensional models of the Yucca basin reproduce, in a qualitative sense, the ringing effect observed at the Yucca Flats. Any attempt at matching characteristics of Pg and Lg with synthetic seismograms computed for laterally homogeneous layered structures is apt to lead to false conclusions about the source and path properties unless the recording sites used are carefully chosen for small crustal effects.
Interpretation of self‐potential survey results from the East Mesa Geothermal Field, California
Authors: Corwin Robert F. , DeMoully Gregory T. , Harding Richard S. , Morrison H. F.
Published: February 2008

A dipolar self‐potential anomaly of about 90 mV peak‐to‐trough amplitude and 5km peak‐to‐trough length has been measured over the East Mesa geothermal field in the Imperial Valley of Southern California. The anomaly does not appear to be related to surface features. A surface field similar in form to the measured self‐potential anomaly is generated by a source configuration consisting of dipolar current distributions along a series of three steeply dipping planes that are roughly coincident in location and depth extent with known faults. The source currents could be generated by the interaction of heat or fluid flow with changes in thermoelectric or electrokinetic coupling coefficients across faults. However, source potentials associated with these currents are considerably greater than those generated by estimated in situ coupling coefficients and heat and fluid flows. This implies that in situ coupling coefficients are much larger than those derived from room temperature measurements; that heat or fluid flows are much greater than those assumed for the East Mesa field; or that some other mechanism is responsible for the generation of the anomaly.
Estimation of the seabed acoustic impedance structure from normal‐incidence reflections: Somali Basin
Authors: Knott S. T. , Hoskins Hartley , LaCasce E. O.
Published: February 2008

Geological interpretation of marine seismic reflection profiles is usefully supplemented by systematically measuring the energy (and its variations) of the reflection events typically observed in low‐frequency, normal‐incidence profiles. Analyses of the low frequency signals received via a passband of at least an octave (e.g., 30–60 Hz) indicate that much of the variations in reflection energy can for the purposes of the experimentalist be ascribed to vertical and lateral changes in the acoustic impedance of the seabed. Using a simple multilayer reflection model, impedances can be estimated from the energetics measurements. Techniques and rationale are given for making these observations of seabed reflectance and deriving impedance estimates. Reflectance and estimated impedance characteristics of the seabed are compared at three stations in the Somali Basin northwest of Chain Ridge and three southeast of the southwesterly extension of the ridge, which bounds the basin to the southeast. One station to the southeast is near Deep Sea Drilling Project site 235, where the impedance structure down through the sediments into basement was estimated from core measurements. The impedance structure inferred from the reflectance at the station is in good agreement with that measured at the site. Having established this agreement, the impedance structures estimated from the reflectance measurements at the stations within the basin northwest of the ridge are compared with the structures at the stations to the southeast. Although recent sedimentation is similar throughout the region, as evidenced by surficial cores, seafloor reflectivity measurements, and a water depth of about 5100 m at each station, reflectance measurements and impedance estimates show the sedimentation within the basin evolved differently from that southeast of the ridge. Two clearly different impedance structures are evident. Northwest of Chain Ridge estimated impedance increases nearly linearly from about 2 × 106 rayls (ρv, kg/m2‐sec) down through the sediment to about 16–18 × 106 rayls in basement. South of the basin proper a nearly uniform low impedance of about 4 × 106 rayls is evident throughout the sediment column with sharp increases at, or near basement. Reflectance profiles show that the lateral continuity of material properties within the seabed in the southern area is more uniform than in the northern area. Northwest of the ridge small scale variations over distances of about 2 km are observed in seafloor reflectivity and in seabed reflectance.
Size distribution of siderophile element concentrations in black magnetic spherules from deep‐sea sediments
Authors: Yamakoshi Kazuo , Nogami Ken‐ichi , Shimamura Tadashi
Published: February 2008

A size spectrum of black magnetic spherules which were collected from deep‐sea sediments was obtained. Siderophile element (Fe, Co, Ir, and Au) concentrations in 100 spherules, each of them larger than 100 μm, were determined with instrumental neutron activation analysis. About 20% of the spherules showed (Ir/Fe) and (Co/Fe) ratios which cannot be explained clearly by an iron meteorite parent for the spherules. Further, there was no strong correlation between the size and the chemical concentration ratios ((Ir/Fe), (Co/Fe), and (Au/Fe)) of the spherules.
Evidence for the subducting lithosphere under southern Vancouver Island and western Oregon from teleseismic P wave conversions
Authors: Langston Charles A.
Published: February 2008

Long‐period teleseismic P waves recorded at VIC (Victoria, British Columbia) and COR (Corvallis, Oregon) show anomalously large Ps conversions and later arriving P‐to‐S reverberations not observed from typical continental crustal sections or from previously proposed structures for these stations determined from refraction surveys. The timing and large amplitude of the Ps phase, relative to direct P, suggests a high velocity‐contrast interface at 45 to 50 km depth under VIC and COR forming the base of a distinct low velocity zone. This interface is proposed to be the oceanic Moho which is being subducted under North America. Off azimuth Ps recorded at COR is consistent with a 20° eastward dip for the interface. Horizontal particle motion at both sites show evidence for lateral heterogeneity in local crustal structure. The distinct low velocity zone and its negative velocity gradient with depth has important consequences for refraction interpretation in the region since the usual assumption of increasing velocity with depth is violated. Crustal thicknesses derived from such misinterpretation may be overestimated. In principle, this type of structure suggests a solution for the Vancouver Island crustal thickness problem in which the observed positive Bouguer gravity anomaly is inconsistent with the 50 km thick crustal thickness derived from previous refraction work.
Origin of Martian outflow channels: The Eolian hypothesis
Authors: Cutts James A. , Blasius Karl R.
Published: February 2008

The ancient outflow channels on Mars exhibit fossil flow patterns similar to current flow patterns in terrestrial desert regions eroded by wind. We have modeled outflow channel formation by a cascade process in which sand‐sized particles produced by rock weathering in the chaotic terrain ‘source areas’ of the outflow channels were transported by the wind across the adjacent plateau surfaces, and sand blasting and deflating the surface. Comparisons of model predictions with photogeologic data suggest that rates of rock breakdown by physical and chemical action along the channels during the channel‐forming era must have been much higher than they have been on similar terrains formed after the channel era ended. A tenuous atmosphere, comparable to the present one, could have transported materials sufficiently rapidly to excavate outflow channels in about 106 years. However, there may be difficulties in explaining how abrading particles could survive high‐velocity impacts in a thin atmosphere over the many hundreds or even thousands of kilometers that they were transported in outflow channels. It is difficult to avoid the conclusion that a climate change affecting rock breakdown rates, atmospheric density, or both occurred early in Martian history to end the era of outflow channel formation. Particulate materials generated during this era may be important components of the north circumpolar dune field and the polar layered deposits.
Time‐dependent friction of granite: Implications for precursory slip on faults
Authors: Johnson Tracy
Published: February 2008

Frictional forces were measured during sliding between saw cut cylinders of Barre granite deformed in a servo‐controlled triaxial loading machine. Two different effects occur, depending on the type of loading. When steady shortening of a stably sliding sample is halted suddenly, slip continues at a diminishing rate, which depends on the logarithm of the initial sliding velocity. Thus, the frictional strength of the surface is decreasing with time. When shortening at a constant rate resumes after the holding period, however, the frictional resistance temporarily rises to a peak proportional to the logarithm of time of no driving, showing a well‐known increase in friction with time. The subsequent decrease in resistance from this peak is continuous. The observed frictional resistance of a surface thus results from the interaction of several processes and is not a constant, in general, but depends on the immediate and preceding deformation. When shortening is resumed after a period of holding, slip rate across the friction surface temporarily accelerates above the driving rate. The slip rate can then either decrease to the driving rate and remain stable or accelerate further and lead to stick slip motion, at stress levels below the peak value. There is no apparent difference in the frictional resistance, until the unloading rate of the loading machine is exceeded, between slip episodes which settle stably to the constant driving rate and those which become unstable and result in stick slip. The stick slip failure criterion is not a peak stress but rather loss of strength more rapidly that the unloading rate of the loading machine. The evidence that accelerating motion before stick slip is due to the time variable strength of rock suggests that accelerating slip will occur before earthquakes. Unfortunately for easy observation, experiments indicate that slip need not be uniform over a fault surface. Also to be established is whether slip will be sufficiently large in the earth to be detectable. Nevertheless, a plausible mechanism for generating observable phenomena prior to earthquakes is suggested by the laboratory observations.
A refraction study of deep crustal structure in the Basin and Range:Colorado Plateau of eastern Arizona
Authors: Gish Dan M. , Keller G. R. , Sbar Marc L.
Published: February 2008

A reversed seismic refraction profile extending 260 km between Globe, Arizona and Tyrone, New Mexico has been recorded using quarry blasts from open pit copper mines as energy sources. Interpretation of these data suggest a 28‐km‐thick crust for the Basin and Range in east‐central Arizona and a 32‐km‐thick crust for the Transition Zone in eastern Arizona and western New Mexico. Delays in Pn arrivals have been interpreted as evidence for approximately 4 km of abrupt crustal thickening near Morenci, Arizona. The area of abrupt Moho offset corresponds to rapid changes in tectonic style, late Quaternary faulting, Quaternary and late Tertiary volcanism, high heat flow and evidence for partial melting of the lower crust. Uniformity of layer thicknesses along the profile can be interpreted as upward displacement of the Basin and Range relative to the Transition Zone. The above evidence suggests that the Transition Zone of eastern Arizona and western New Mexico may be experiencing active tectonic readjustment.
The inverse problem of the shear modulus and density profiles of a layered Earth
Authors: Coen Shimon
Published: February 2008

It is shown that the shear modulus and density profiles of a layered, isotropic elastic earth can be uniquely inferred from the displacement of the surface due to an impulsive SH line source. An inversion algorithm is presented which constructs these profiles from the surface data. Two illustrative examples are given in which the shear modulus and density profiles are reconstructed analytically from an analytic surface data.
Reflectivity applied to peak pressure estimates in silicates of shocked rocks
Authors: Lambert Philippe
Published: February 2008

Shock pressures in framework silicates and glasses of shocked rocks are deduced from the values of their refractive index (n); n is directly determined in a polished thin section by measuring reflectivity using a micro‐reflectometry technique adapted to transparent minerals. The technique is tested on experimentally and naturally shocked crystalline rocks and on unshocked material. The technique is fast, nondestructive, directly adapted to the study of naturally shocked materials and can be used for both crystalline and glassy phases whatever the structure and the composition. The major limitation actually comes from the scarcity of experimental data relating shock pressure and refractive index for calibration. Accuracy is excellent for tectosilicates in the range 15–45 GPa. The small size of the measured field (10 μm or less) allows the recognition and quantitative estimate of pressure gradients within shocked minerals. All intermediate values for n are found ranging from n of the unshocked minerals to n of the corresponding diaplectic glass. Some diaplectic glasses of quartz and plagioclase An27 have considerably higher refractive index than previously known, 1.525 and 1.538 respectively, suggesting that diaplectic glasses of quartz and plagioclase can form at relatively lower pressures than previously thought, about 23.5 and 27.5 GPa, respectively. The ranges of n of quartz and diaplectic glass of quartz are distinct and do not overlap in the experimentally shocked grains. The same feature characterizes An27, while the ranges widely overlap in microcline of a naturally shocked granite. Diaplectic crystals and diaplectic glasses are possibly a mixture of crystalline and amorphous submicroscopic domains. Diaplectic glass of microcline and oligoclase gives pressure estimates 15–20% higher than does diaplectic glass of quartz in the same naturally and artificially shocked rocks. Average pressure measured in quartz and feldspars of a gneiss sample shocked at 35.3 GPa are significantly lower than the shock pressure of the whole rock. Further work is required to understand these discrepancies. Possible applications of the technique are introduced.
Double seismic zone beneath the Mariana Island Arc
Authors: Samowitz Ilene R. , Forsyth Donald W.
Published: February 2008

A double zone of seismicity has been found beneath the northern Marianas in the depth range 80–120 km. The Benioff zone undergoes a pronounced bend, with radius of curvature about 200 km, before straightening and descending nearly vertically to a depth of more than 600 km. Thus this is an ideal area in which to search for evidence of stresses associated with unbending of the subducted oceanic plate. We relocated all teleseismic events from 1963 through 1975 by using a regional, composite master event technique, which significantly decreased the scatter of the hypocenters. The three events of the lower seismic zone are separated from the upper zone by about 30–35 km. The true thickness of the upper seismic zone is probably 20 km or less. The limited focal mechanism data indicate that there is downdip tension in the lower zone and downdip compression in the upper zone. The seismicity patterns could be generated either by thermal stresses or by unbending a plate which was originally deformed anelastically. We show that an elastic‐perfectly plastic model of the rheology of the plate can account for the separation of the two seismic zones, the focal mechanisms, and the greater activity of the upper seismic zone.
Venezuela Basin crustal structure
Authors: Diebold J. B. , Stoffa P. L. , Buhl P. , Truchan M.
Published: February 2008

Velocity‐depth profiles derived from six two‐ship expanding spread experiments, in combination with other geophysical data, define the characteristics of two distinct types of Venezuela Basin crust and the boundary between them. Each two‐ship common midpoint reflection/refraction profile is automatically transformed into the τ‐p plane, ‘picked’ and interpreted to provide V(Z) functions with appropriate confidence bounds. The results, together with gravity, magnetic, and near‐vertical incidence reflection data, reveal a 50,000 km2 triangle of Venezuela Basin crust which resembles normal oceanic crust in a magnetic quiet zone. North and west of this triangle lies the previously defined, thick ‘Caribbean’ crust, having two distinct layers above the M discontinuity. Acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at Deep Sea Drilling Project sites 146/149(sills), and 150 (flows); also, depths to mantle are greater than normal. Interpretations of near‐vertical and wide‐angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal. The boundary between the two crustal areas has a NE‐SW trend which parallels the dominant structural and magnetic lineations.This boundary coincides in position, though not in trend, with the previously defined ‘central Venezuela Basin fault zone’. Further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production.
Flushing geothermal heat from moderately permeable sediments
Authors: Andrews J. G. , Richardson S. W. , White A. A. L.
Published: February 2008

Geothermal exploitation systems consisting of parallel vertical hydraulic fractures might be used to flush heat from porous rock formations with permeabilities in the tens of millidarcy range. Such systems are analyzed using the conservation equations to find the temperature history of the fluid produced, the thermal power output, and the fraction of the heat recovered, in terms of the applied pressure drop, the permeability, the anisotropy of the medium, and the scale over which the permeability is effectively homogeneous. In effectively homogeneous media, a thermal front is driven outward from the injection fracture. Rock outside this boundary is hot; within it is cool; for flows with Péclet numbers over 100 the front itself is sharp. The shape of the cooled region depends strongly upon the anisotropy of the medium. The variation of permeability is suitably scaled and related to the Péclet number to elucidate those cases which are favorable for geothermal exploitation. For a range of reasonable parameters the power output is more than three times that obtained using boreholes not having hydrofractures to extend the collection and reinjection surfaces.
Pressure dependence of the velocity of lunar soil: The velocity/depth variation in the shallow lunar crust
Authors: Gangi Anthony F.
Published: February 2008

Some recent measurements (Johnson et al., 1981) of the velocity variation with pressure (up to 2.0 bars) for lunar soil are compared with results from the Hertzian‐contact theory. Contrary to the original contention of Johnson et al. (1981), the data are shown to be consistent with the results of the Hertz theory when the effects of nonrecoverable compaction are taken into consideration. A simple analysis is given which shows that the velocity of loosely packed soil (which has only a fraction, ƒ, of the grain contacts of a well packed soil) will be smaller than that of the well packed soil by the factor ƒ1/3 (0 < ƒ < 1). A reanalysis of some earlier experimental data by Talwani et al. (1973) for the velocity variation with pressure (up to 2.5 kbar) of a volcanic ash also shows that the modified Hertzian‐contact theory can be used to fit those data. It is found that the velocity function υ(P) = 2.94P0.232 gives a better fit (rms error: ±65 m/s) to the experimental data than a best fit equation of the form proposed by Talwani et al. (1973): υ(P) = 1.147(1 + (P/0.711)1/2 − (P/2.45) which has an rms error of ±84 m/s for the same data. The curves fitted to the data of Johnson et al. (1981) would give a velocity variation for the shallow lunar crust equal to υ(z) = 225 (±25)z1/6 m/s for z in meters (down to 64 m depth). This compares quite well with the velocity variation for the first 10 m of the Moon as interpreted by Gangi and Yen (1979), υ(z) = 220z1/6, using the data from the Apollo 14 and Apollo 16 Active Seismic Experiments (up to 32 m separation) on the Moon (Kovach et al., 1971, 1972). The results reinforce the proposal by Gold and Soter (1970) and Gangi (1971, 1972) that the velocity variation in the very shallow lunar crust (the top 10 to 20 m) is due to self compaction of the lunar soil.
A closer examination of the reduction of satellite magnetometer data for geological studies
Authors: Regan Robert D. , Handschumacher David W. , Sugiura Masahisa
Published: February 2008

Global and regional studies of the data abstracted from the POGO satellites have demonstrated the utility of satellite magnetometer measurements in geological studies. However, detailed examination of these data reveals that there exists a considerable amount of noise resulting from external fields and data reduction techniques. Such noise inhibits more detailed studies with the POGO data and could similarly affect the utility of data from the Magsat mission. Correlation of POGO satellite data with ground observatory records and geomagnetic indices was employed to identify the existence of external field effects and document the data reduction problems. Significant effects of the Sq current system, including the equatorial electrojet, are shown to be still in the data despite the local time screening. Such effects are totally in agreement with theoretical models of this current system and should be considered in Magsat data reduction, as the theoretical model predicts that the Sq current may at times have significant effects even at the dawn/dusk local times of the Magsat orbit. The state of inflation and deflation of the magnetosphere is vividly portrayed in the POGO measurements and is well correlated with the Dst index. The ring current correction applied to the POGO data was seen to be of varying quality depending upon the state of the magnetosphere and the length of satellite track data reduced. These effects combined with the irregular distribution of data from the individual POGO satellites can result in pseudo anomalies as evidenced by a comparison of satellite data reduced in a fashion to minimize them with those previously published over the same area.