bluuebunny/crossref_metadata_2025 huggingface dataset:
Partitioning of rare Earths and other trace elements between sanidine and coexisting volcanic glass
Authors: Leeman William P. , Phelps David W.
Published: February 2008

Major and trace element contents have been measured for sanidine‐glass pairs separated from eight high‐SiO2 rhyolite lavas from the Yellowstone volcanic field. Sanidine/glass partition coefficients for most elements based on these data are uniform to within the cumulative analytical errors. This uniformity reflects similarities in major element compositions and inferred physical conditions for the analyzed samples, which are considered representative of most Yellowstone rhyolites. Of particular note are very high partition coefficients for Ba (∼22), Sr (∼28), and Eu (∼9), relative to the sparse data for other rhyolites available in the literature. With the exception of Na, K, and Pb, which have sanidine/glass partition coefficients of approximately unity, all other elements determined are strongly fractionated into the melt phase relative to sanidine. These data may be used in modeling crystallization of sanidine from Yellowstone or other rhyolites or anatexis of crustal rocks containing sanidine (or K‐feldspar sensu lato), but only with due caution. In particular, the wide range in reported partition coefficients for those elements (Sr, Ba, Eu) strongly incorporated in sanidine suggests that compositional and physical characteristics exert significant influence on these values. Accordingly, applicable partition coefficients are poorly known for these elements in most systems. Care must be taken to select appropriate values or to measure these partition coefficients directly in specific suites of rocks. Difficulties with direct experimental studies in silicic magmas seemingly preclude systemization of element partitioning at present.
The relationship between temperature distribution and the perturbation of time‐varying electromagnetic fields for a two‐dimensional model of a subducting lithospheric slab
Authors: Jones F. W. , Pascoe L. J. , Ramaswamy V. , Sydora L. J.
Published: February 2008

A two‐dimensional numerical model is employed to investigate the perturbation of time‐varying electromagnetic fields by a subducting lithospheric slab. A simple thermal conduction model, a model with shear strain heating, and a model which takes into account the production and upward movement of partially melted material from the top of the slab are considered for two periods. Also, a continental/oceanic interface is included. The results indicate that the hot material above the slab considerably affects the spatial behavior of the electromagnetic field components at the surface. The maximum perturbation of the vertical magnetic component occurs over the point where subduction begins in the simple conduction case, but when shear strain heating is present, the maximum perturbation in this component occurs on the continental side of subduction. The nature of the vertical magnetic field perturbation changes considerably when rising melt is present. Apparent resistivity profiles across the subduction zone are substantially different for the three models. The calculated apparent resistivity is significantly reduced when partial melt is present.
An alternative explanation for intermediate‐wavelength magnetic anomalies
Authors: Shure Loren , Parker Robert L.
Published: February 2008

Harrison and Carle [this issue] and others have examined very long profiles of the magnetic field and have calculated one‐dimensional power spectra. In these they expect to see, but do not find, a minimum in power at intermediate wavelengths, between 65 and 150 km. Conventional one‐dimensional models of the field predict very little power in this band, which lies between the spectral peaks arising from sources in the crust and the core. Mantle sources or high‐intensity, long‐wavelength magnetizations have been proposed to account for the observations. An alternative, more plausible explanation is that one‐dimensional spectra of two‐dimensional fields contain contributions from wavenumbers in the perpendicular (i.e., nonsampled) direction. Unless the seafloor spreading anomalies are perfectly lineated at right angles to the profile, some low‐wavenumber energy must be attributed to this effect; we propose that such directional aliasing is a major factor in the power spectra. To support this idea, we discuss theoretical models and analyze a large‐scale marine survey.
Episodic rifting and volcanism at Krafla in north Iceland: Radon (222) emission from fumaroles near Leirhnjukur
Authors: Hauksson Egill
Published: February 2008

From June 1978 until late 1980 radon emission from the Leirhnjukur fumaroles was monitored within the Krafla caldera of north Iceland where episodic volcanism is occurring. Frequent sampling of the fumaroles shows that no easily identifiable short‐term radon precursors occur in the days prior to subsidence of the caldera, despite an observed increase in microseismicity preceding deflation. Following the onset of subsidence, however, the radon emission of the fumaroles gradually increases and reaches a maximum 3–6 days later. The radon in the fumaroles is assumed to be transported from depth by steam and noncondensing gases that slowly escape from the geothermal water table. The cause of the co‐episodic increase in radon emission appears to be a temporal rise of the water table driven by fissure closure resulting in an abbreviated transport time for radon to the surface fumaroles. Furthermore, the closing of the fissures appears to cause a transient increase in the velocity of transport, making the shape of the anomalies broader and higher than is predicted from a change in the water level alone. Changes in radon emission also coincide with fluctuations in fumarolic activity and permanent changes in the level of geothermal water that occur during periods of uplift.
Mars residual north polar cap: Earth‐based spectroscopic confirmation of water ice as a major constituent and evidence for hydrated minerals
Authors: Clark Roger N. , McCord Thomas B.
Published: February 2008

A new reflectance spectrum of the Martian north polar cap is analyzed, and it shows water ice absorption features. This evidence confirms the result of the Viking IRTM and MAWD experiments, which indicate that the north residual polar cap of Mars is composed of water ice during the season observed (northern hemisphere spring, Ls = 50°). The spectra presented here show that other materials are probably present as well. These other materials may be hydrated minerals, as indicated by weak absorptions in the spectrum at 1.9, 2.3, and possibly 1.4 μm, but the materials seem to be otherwise spectrally bland.
Field dependence of magnetic blocking temperature: Analog tests using coercive force data
Authors: Dunlop David J.
Published: February 2008

The field dependence of blocking temperature TB and the temperature dependence of coercive force Hc are shown to be analogous functions. Coercive force data, obtained routinely with standard equipment, can therefore be used in analog tests of theories of thermoremanent magnetization (TRM). Analog Hc data reported in this paper for single‐domain and small multidomain magnetite dispersions and a variety of rocks are matched equally well by Néel's (1949) theory of thermally activated magnetization changes or Néel's (1955) theory of field‐induced domain wall displacements. The temperature dependence of domain wall pinning is a crucial unknown in the wall displacement model, allowing considerable latitude in curve fitting. If the thermal activation model is assumed, reasonable values are found for the volume activated in a single magnetization reversal or wall jump in the fine‐ and medium‐grained rocks tested. Additional independent evidence from hysteresis and microscopic examination is necessary, however, to choose between the competing TRM models in individual cases.
Late Cretaceous‐Early Tertiary paleomagnetism of Aruba and Bonaire (Netherlands Leeward Antilles)
Authors: Stearns Carola , Mauk Frederick J. , Van der Voo Rob
Published: February 2008

For a paleomagnetic study of the Upper Cretaceous of the Netherlands Leeward Antilles, we have analyzed 187 oriented samples from the complex norite‐tonalite batholith and the diabase‐schist‐tuff formation on Aruba and 44 oriented samples from various igneous units of the Washikemba Formation on Bonaire. Both alternating field and thermal demagnetization procedures were used to demagnetize the samples. Most of the cores yielded univectorial Zijderveld diagrams above 10–20 mT or 400° C. Virtual geomagnetic poles calculated from site means were then compared with equivalently aged poles for cratonic South America and other Caribbean sites. The Washikemba Formation of Bonaire and the diabase‐schist‐tuff formation of Aruba yield poles that are similar to those of Guajira (Colombia) and the Caribbean Mountains of northern Venezuela, whereas they are rotated approximately 90° with respect to the pole for stable South America. The batholith on Aruba yields a pole which is rotated clockwise over approximately 20° with respect to the South American pole. Thus, our data strongly support the hypothesis that Aruba and Bonaire have been rotated clockwise by as much as 90° since the Late Cretaceous. In addition, paleolatitudes calculated from the inclinations of the paleomagnetic vectors suggest that there was a northward drift with respect to South America of perhaps as much as 10° of the islands during the rotation, although this can hardly be called a statistically significant amount.
Analysis of shock wave arrival time from underground explosions
Authors: Heusinkveld Myron
Published: February 2008

The arrival times of the shocks from underground explosions in the highly inelastic region were used to determine the energy of the explosions driving the shocks. The relation between arrival time and radial distance was obtained with the use of the equation U = a + bu relating shock velocity U to particle velocity u. Also used was the similarity assumption that the peak pressure at the spherical shock front was linearly related to the average energy within the sphere. Numerical coefficients in the derived formula were determined from experimental data on peak pressure and peak particle velocity from explosions in tuff and in granite. This anlaysis was used successfully for energy determination of nuclear explosions from about 1 kiloton to several megatons energy output. The stress range covered was from about 5×108 Pa up to more than 1011 Pa. The good agreement between experimental and computed results showed that the similarity assumption was reasonable. Computed arrival times in wet tuff were compared with those from an empirical formula from the Los Alamos National Laboratory with good agreement over a limited peak stress range. Similar comparison with results from a semiempirical formula of Kurtz did not give good agreement unless the constants in the Kurtz formula were adjusted.
Amplitude and phase normalization of seismograms from multiple seismograph recording systems for the Yellowstone‐Snake River Plain Seismic Refraction Experiment
Authors: Baker M. R. , Braile L. W. , Smith R. B.
Published: February 2008

A z transform filter theory method for the normalization of the instrument responses of several seismographs is presented. In this method, an inverse filter is derived by consideration of the seismometer/recorder characteristics which may be applied to a given seismogram to convert the system frequency response to that of a reference system. Inverse filters are derived for the seismographs used on the 1978 Yellowstone‐Snake River Plain seismic profiling experiment. It is shown by application to these data that the inverse filters are effective in amplitude normalization and that their use allows improvement in the amplitude and waveform character of seismic record sections.
Seismicity of Mount Hood and structure as determined from teleseismic P wave delay studies
Authors: Weaver C. S. , Green S. M. , Iyer H. M.
Published: February 2008

A 16‐station seismic network was established in the Mount Hood area of Oregon in 1977 as part of a multidisciplinary study to evaluate the geothermal potential of a typical Cascade volcano. The immediate objective was to monitor local seismicity and to study the P wave velocity structure of the crust and upper mantle. During the 13 months the network was in operation, 10 local earthquakes were recorded. All these events are spatially associated with the volcano and occurred at shallow depths roughly defining a zone striking north‐northwest. The largest earthquake, a magnitude 3.4 event, was preceded by a single foreshock and followed by three locatable aftershocks. These and the five other located earthquakes were of magnitude less than 2.0. Fifty‐five teleseisms were recorded by the network and used in a teleseismic P wave delay study. The residuals from the P delay study indicate that there is no significant velocity anomaly associated with the Mount Hood volcano and only small velocity variations in the Mount Hood region.
A method of studying variations in the clustering property of earthquakes: Application to the analysis of global seismicity
Authors: Prozorov A. G. , Dziewonski A. M.
Published: February 2008

We propose a formal algorithm for identification of aftershocks following a main event. Our approach is based on the analysis of the local statistics: the number of events following the ‘initial’ earthquake displayed as a function of distance and time. By comparing this distribution with the random catalog, we establish, for several magnitude ranges, spatiotemporal thresholds for detection of aftershocks. High‐confidence thresholds correspond to the limit at which the number of events derived from the actual catalog (corrected for the background seismicity) exceeds by a factor of 10 the expected level of background seismicity; marginal confidence thresholds correspond to the spatiotemporal boundary along which these two values are approximately equal. After several iterations, the final thresholds are derived. The swarming parameter is defined as the number of aftershocks, with a cut‐off lower magnitude range, following an initial earthquake of certain magnitude. If a correction for the recurrence law is applied, we find that the swarming parameter is magnitude independent. This allows us to apply this technique to the analysis of global seismicity. Using a catalog derived from the ISC Bulletin for the years from 1964 to 1975, we determine for shallow earthquakes the average values of the swarming parameter in 9°×9° cells. A global map produced in this manner shows a very wide range of variations in the swarming property, undoubtedly reflecting variations in the local geological conditions. Triple junctions are among the areas with the highest swarming values. At the present time there is an insufficient amount of data to study regional variations in the swarming property of deep earthquakes. Considering the clustering as a function of depth alone, we find detectable swarming for two depth ranges: from 100 to 250 km and from 450 to 650 km. Not a single aftershock has been detected among nearly 200 earthquakes withmb≥5 in a depth range from 250 to 450 km. The average swarming parameter for earthquakes deeper than 100 km is 10 times smaller than that for the shallow earthquakes.
Mean gravity anomalies from a combination of Apollo/ATS 6 and GEOS 3/ATS 6 SST Tracking Campaigns
Authors: Kahn W. D. , Klosko S. M. , Wells W. T.
Published: February 2008

The satellite to satellite tracking (SST) technique has been used to measure the earth's gravity field. Data from the Apollo/Soyuz and GEOS 3 missions, obtained at greatly differing altitudes, have been used in combination to determine 5° and 10° equal area mean gravity anomalies in the South Atlantic and Indian Oceans and over the African continent. The Apollo/ATS 6 SST data consist of 47 passes of data taken from the 230‐km Apollo altitude, while the GEOS 3/ATS 6 data set consists of 45 passes taken from 840‐km altitude. Through a combination of the two data sets with the GEOS 3 SST data providing stability for the solution at the longer wavelengths, excellent (5° at ±3 mgal, 10° ±1 mgal) results were obtained for this largely unsurveyed region. These results have been compared to an altimetric surface at comparable wavelengths, providing good agreement (±1 mgal for 10° blocks and ±5 mgal for 5° blocks) between these independent measuring techniques. The terrestrial data over the African continent is shown to differ significantly (rms difference of 8 mgals with a maximum difference of 23 mgals) from a comparison of 10° GEOS 3 SST derived blocks. A simulation has also been performed to assess the data requirements, aliasing, and noise characteristics of a low spacecraft being tracked in a high‐low SST configuration. The results of the simulation were employed in the design of the SST data reduction procedures.
On the tectonics of the Himalaya and the Tibet Plateau
Authors: Wang Chi‐yuen , Shi Yaolin , Zhou Wen‐hu
Published: February 2008

New data for the gravity anomalies and the uplift rate in the Himalaya and the Tibet Plateau have imposed strict constraints on acceptable models for present tectonic processes in this region. Through two‐dimensional finite element modeling, incorporating realistic rheology for the crust and upper mantle, a model is found which satisfactorily predicts both the observed uplift rate and the changes of the gravity anomalies. This model shows that (1) the Himalaya is dynamically supported, (2) the northern margin of the Indian plate underthrusts the Himalaya and, at the same time, the entire Tibet thickens at a rate of 2 mm/yr, (3) the partition of the total shortening across the region is about 15% in the Ganges Basin, 30% in the Himalaya and 55% in Tibet, and (4) the extent of deformation reaches beyond the northern boundary of Tibet.
Introduction to the Special Issue on Accretion Tectonics
Authors: Nur Amos
Published: February 2008

Abundant geological and geophysical data demonstrate tectonic complexities on land far greater than those on the ocean floor. In the oceans, geological features are well explained by plate tectonics. On land, however, plate tectonics is less successful explaining the large and complex Cordilleran chains of the world. Although some of these chains result from classical plate collisions, the origin of others remains enigmatic.
Free surface displacements in the near field of a tensile crack expanding in three dimensions
Authors: Chouet Bernard
Published: February 2008

The discrete wave number method is applied to the study of the elastic motions due to an arbitrary three‐dimensional tensile source. The source elastic field is formulated as a superposition of plane waves propagating in discrete directions. The discretization stems from an assumption of two‐dimensional periodicity in the description of the source. Examples of the free surface motion induced in the near field of circular and rectangular dislocation sources expanding in a homogeneous half space are discussed.
Using dual‐well seismic measurements to infer the mechanical properties of a Hot Dry Rock Geothermal System
Authors: Fehler Michael
Published: February 2008

High‐frequency (8–15 kHz) seismic waves that travel between two boreholes are used to probe the detailed structure of rock in the Los Alamos National Laboratory Fenton Hill hot dry rock geothermal energy reservoir. We study travel times of direct P waves and spectral characteristics of waveforms in an effort to determine whether or not large fluid‐filled fractures are present in the rock between the two boreholes. Data from two experiments, separated in time by 1 year, are studied. We find dramatic differences between the two experiments in P wave velocity and in spectral characteristics of waveforms that traversed the same region. We also find variations in P wave velocity and spectra as a function of location during one experiment.

Loss of high‐frequency content of direct‐arriving P waves during the second dual‐well seismic experiment is compared with Q, found by using the coda wave technique (Aki and Chouet, 1975), and also with theoretical results on frequency dependence of transmission through a water‐filled fracture to determine the minimum number of large fractures in the system. Two to four fractures with thickness about 4 mm and vertical extent of as much as 200 m are required to explain observations during the second experiment. Efficient transmission of high frequencies observed during the first experiment is interpreted as meaning that fracture thickness was less than 1 mm at that time. Increased fracture thickness during the second experiment is probably due to a 75‐day period of heat extraction during which in situ temperature dropped by as much as 100° below the virgin temperature. Since the coefficient of linear thermal expansion of granite is approximately 10−5/°C, a decrease in temperature of 100°C in a region on the order of 1 m around the fracture is required to explain the increased fracture thickness.
Statistical tests of ARIES data
Authors: Musman Steven
Published: February 2008

Statistical tests are performed on Project ARIES preliminary baseline measurements in the Southern California triangle formed by the Jet Propulsion Laboratory, the Owens Valley Radio Observatory, and the Goldstone tracking complex during 1976–1980. In addition to conventional one‐dimensional tests a two‐dimensional test which allows for an arbitrary correlation between errors in individual components is formulated using the Hotelling statistic. On two out of three baselines the mean rate of change in baseline vector is statistically significant. Apparent motions on all three baselines are consistent with a pure shear with north‐south compression and east‐west expansion of 1×10−7/year. The ARIES measurements are consistent with the USGS geodolite networks in Southern California and the SAFE laser satellite ranging experiment. All three experiments are consistent with a 6 cm/year motion between the Pacific and North American Plates and a band of diffuse shear 300 km wide, except that corresponding rotation of the entire triangle is not found.
Microcrack studies of basalts from the Iceland Research Drilling Project
Authors: Kowallis Bart J. , Roeloffs Evelyn A. , Wang Herbert F.
Published: February 2008

Microcavities in basaltic core from the IRDP borehole in Reydarfjördur, Iceland, can be characterized by using direct observations with the scanning electron microscope (SEM) and inversion of velocity data. In the Iceland basalts, neither method provides an accurate, complete picture of the crack aspect ratio spectrum, but by using both SEM and inversion data a complete spectrum may be obtained. SEM observations provide information on high aspect ratio pores and cracks (>0.005) that cannot be separated by velocity inversion theory, while velocity inversion theory provides information on low aspect ratio cracks (< 0.005) that are too narrow to be seen with the SEM. Alteration rims along cracks, blunt ends, and aspect ratios which imply closure pressures greater than the vertical stress all provide evidence that cracks are open in situ. Cracks coincident with grain boundaries are abundant in the basalt flows and suggest an origin early in the rock's history, probably during cooling. Crack healing and alteration are observed to increase with depth, causing a shift in crack shape distribution toward higher aspect ratios.
Deformation of an island arc: Rates of moment release and crustal shortening in intraplate Japan determined from seismicity and Quaternary fault data
Authors: Wesnousky Steven G. , Scholz Christopher H. , Shimazaki Kunihiko
Published: February 2008

The historical record of large (M > 6.9) earthquakes and geologically determined rates of slip on Quaternary faults in intraplate Japan (Honshu and Shikoku) are used to estimate the average rate of seismic moment release (
) for the last 400 years and during the late Quaternary, respectively. Values of
estimated from the two data sets are similar in regions where seismic activity is concentrated on land. We interpret this observation to suggest that
in intraplate Japan has been constant during the late Quaternary and is relatively free from secular variation when averaged over periods of several hundreds of years.
in Shikoku may be attributed almost solely to right‐lateral slip of the median tectonic line (MTL). The easterly strike of the MTL is consistent with a compressive stress field that trends northwest. Crustal shortening of the Izu Peninsula taken up on a set of strike slip faults that show north by northwest compression is ≈1 mm/yr. Northeast Honshu is characterized by a set of reverse‐type faults that trend northerly. Conversion of
in northeast Japan to strain rates suggests that about 5% of the relative plate motion between Japan and the Pacific plate (≈9.7 cm/yr) is accommodated as a permanent east‐west shortening (≈5 mm/yr) of northeast Honshu. The predominant deformation in central and western Honshu takes place as slip on a conjugate system of strike slip faults that strike northeast and southeast and show right‐lateral and left‐lateral motion, respectively. Crustal shortening, resulting from slip on faults, in central and western Honshu is 5 and 0.5 mm/yr, respectively, in an easterly direction. Central and western Honshu are in closest proximity to the Nankai trough, and hence, the stress field in these regions cannot simply be attributed to the accommodation of the relative (northwesterly) convergence of the Philippine Sea plate. The northward impingement of the Izu Peninsula into Honshu may influence stresses in central and western Japan, but a conclusive explanation of the stress field in central and western Honshu remains an enigma.
Long‐term fault creep observations in central California
Authors: Schulz Sandra S. , Mavko Gerald M. , Burford Robert O. , Stuart William D.
Published: February 2008

The U.S. Geological Survey (USGS) has been monitoring aseismic fault slip in central California for more than 10 years as part of an earthquake prediction experiment. Since 1968, the USGS creep network has grown from one creep meter at the Cienega Winery south of Hollister to a 44‐station network that stretches from Hayward, east of San Francisco Bay, to Palmdale in southern California. In general, the long‐term slip pattern is most variable on sections of the faults where several magnitude 4 and larger earthquakes occurred during the recording period (e.g., Calaveras fault near Hollister and San Andreas fault between San Juan Bautista and Bear Valley). These fault sections are the most difficult to characterize with a single long‐term slip rate. In contrast, sections of the faults that are seismically relatively quiet (e.g., San Andreas fault between Bear Valley and Parkfield) produce the steadiest creep records and are easiest to fit with a single long‐term slip rate.

Appendix is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐004; $1.00. Payment must accompany order.
Bulk attenuation in a polycrystalline Earth
Authors: Heinz Dion , Jeanloz Raymond , O'Connell Richard J.
Published: February 2008

Bulk attenuation occurs in the solid, polycrystalline regions of the earth due to the local mismatch of the elastic moduli of neighboring grains. A ratio of bulk to shear attenuation which is consistent with seismological whole earth models,QK−1/Qμ−1∼1% is predicted for typical mineral assemblages of the mantle, based on viscoelastic theory. The resultingQ derived for the normal mode 0S0, 6000, is within the observed range.
Implications of an elastic analysis of in situ stress measurements near the San Andreas Fault
Authors: McGarr A. , Zoback M. D. , Hanks T. C.
Published: February 2008

Twenty‐nine measurements of in situ stress obtained with the hydraulic fracturing technique near Palmdale, California, are the basis of an elastic analysis of the state of stress in the Mojave Desert adjacent to the San Andreas fault. The measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km. The elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7.9 MPa/km. Extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 MPa, a result that is higher than estimates of the average shear stress on the San Andreas fault based on the analysis of heat flow data. This finding is consistent, however, with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of San Andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone. If so, then the coefficient of friction of the San Andreas fault zone inferred from the stress field results is about 0.45. The state of stress does not appear to vary systematically with distance from the San Andreas fault although considerable localized variation is observed. The observations suggest an upper bound of about 0.1 MPa/km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the San Andreas fault, a result that implies a corresponding limit of about 1.4 MPa on the shear traction applied to the base of the seismogenic layer. Finally, we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip, which could be released in a large earthquake. We show that on the basis of a model involving a locked fault, extending to about 22 km, the total fault slip below the locked portion is less than 13 m. A more comprehensive set of stress data could permit the estimation of an even lower bound.
Anharmonic properties: Ionic model of the effects of compression and coordination change
Authors: Jeanloz Raymond , Roufosse Micheline
Published: February 2008

Thermodynamic properties resulting from anharmonicity in minerals decrease with pressure but are expected to increase across high‐pressure phase transitions that are characterized by small volume changes and an increase in coordination number. This conclusion is supported by simple ionic models (based on either a molecular approach or crystal‐independent potentials applied to the B1(NaCl structure)–B2(CsCl structure) transition), as well as by the available high‐pressure data. The enhanced anharmonicity of the high‐pressure phase is caused by the increase in first‐neighbor interatomic distance with a coordination change. The Grüneisen parameter and coefficient of thermal expansion are expected to increase by factors of order 20 to 50% across transitions involving changes from fourfold to sixfold and from sixfold to eightfold coordination in halides and oxides. Thus, lower mantle phases are expected to exhibit relatively large coefficients of thermal expansion (e.g., perovskite: α ∼ 3 to 4×10−5 K−1 at 300–1000 K and zero pressure), and the driving force for convective heat transfer may be augmented by the occurrence of phase transformations deep within the Earth's mantle.
Technique for measuring ultrasonic velocity and attenuation spectra in rocks under pressure
Authors: Winkler K. W. , Plona T. J.
Published: February 2008

An ultrasonic pulse technique has been developed for measuring phase velocity and attenuation spectra in rocks inside a pressure vessel. This technique has been adapted from those used in the nondestructive evaluation field. In essence, a broadband acoustic pulse is directed into a lucite buffer, which is followed by the rock sample and another lucite buffer. The pulse partially reflects off both the front and back faces of the sample, and the reflected pulses are received by the transmitting transducer. Digital recording and signal processing techniques are used to analyze the signal. After Fourier transforming each pulse, correcting for diffraction effects, and allowing for partial reflections at the interfaces, the phase shift between pulses at each frequency yields the phase velocity and the amplitude reduction gives the attenuation. Tests on synthetic materials show that as currently implemented, the technique produces reliable measurements at frequencies above 400 kHz. Velocity and attenuation spectra are shown for brine saturated Massilon and Boise sandstones at different effective pressures.
Fractionation of mineral species by electrophoresis
Authors: Dunning J. D. , Herren B. J. , Tipps R. W. , Snyder R. S.
Published: February 2008

The fractionation of fine‐grained aggregates into their major components is a problem in many scientific areas including earth and planetary science. Electrophoresis, the transport of electrically charged particles, immersed in a suspension medium, by a direct current field (Bier, 1959), was employed in this study as a means of separating simulated lunar soil into its constituent minerals. In these tests, conducted in a static analytical cylindrical microelectrophoresis apparatus, samples of simulated lunar soil and samples of pure mineral constituents were placed in the chamber; the electrophoretic mobilities of the lunar soil and the individual mineral constituents were measured. In most of the suspension buffers employed separability was indicated, on the basis of differences in mobility, for all the constituent mineral species except ilmenite and pyroxene, which were not efficiently separable in any of the buffers. Although only a few suspension media were employed, the success of this initial study suggests that electrophoresis may be an important mineral fractionation option in fine‐grained aggregate processing.
Subcrater lithification of polymict regolith breccias
Authors: Nagle J. Stewart
Published: February 2008

Crater bottom structures show a consistent orientation in lunar cores. They have dust‐free vesicular schlieren glass at the top, which grades downward to mafic arenite—a concentration of densely packed mafic mineral grains in a dark glass matrix. Next in the succession is a coarse‐grained, polymict, compact and lineated breccia that has a nonporous brown glass matrix; it grades downward into a fine‐grained, polymict lineated breccia that has a microporous glass matrix that is laden with fine fragments. All polymict regolith breccias formed by subcrater lithification show evidence of compaction (>2 contacts/grain in thin section), directional movement (linear fabric in at least some places) and rapid cooling (glassy matrix but no reaction rims or obvious annealed grain margins). In contrast, suevitic polymict regolith breccias, which are lithified by heating, are not compacted (<2 contacts/grain in thin section, highly porous matrix), show evidence of rolling (radial‐arcuate lapillar structures instead of linear fabric) and show sintered grain margins and/or reaction rims on mineral grains. The presence of relatively large fragments of shock‐resistant minerals in the upper parts of the crater bottom structure, with other minerals being not clearly visible, suggests that fines and minerals easily damaged by shock are selectively fused to form the glass matrix.
Numerical simulation of crystal fractionation in shergottite meteorites
Authors: Grimm Robert E. , McSween Harry Y.
Published: February 2008

Cumulus clinopyroxenes in the Shergotty and Zagami meteorites suggest crystal fractionation occurred, possibly by gravitative settling. Numerical models of this process in a nonconvecting environment argue that the small phenocrysts can segregate only under extreme conditions of cooling time or gravitational field strength. Since textures indicate that cooling time was not excessive, a large (planetary) g is required by these models, in agreement with other suggestions that the shergottite parent body may be Mars. Other calculations indicate that it is extremely difficult to produce the observed textures in a convecting environment, unless crystal setting occurred in a quiescent zone at the bottom of the magma chamber.
Porosity and hydraulic properties of sediments from the Galapagos Spreading Center and their relation to hydrothermal circulation in the oceanic crust
Authors: Karato Shun‐ichiro , Becker Keir
Published: February 2008

Density and porosity of sediments were measured on DSDP legs 69 and 70 samples from the Galapagos spreading center. Permeability and the hydraulic impedance of each sediment layer were estimated from measured values of porosity. The gradients of porosity and density with depth where sediment layers are thin ( ≲50 m thick ) are anomalously high compared with those of other areas and with the upper part of thicker sediment layers in this area. A good correlation was found between the anomalous porosity and density gradients and the present‐day heat flow. We interpret these observations to suggest that these high gradients may be due to active hydrothermal circulation through a thin sediment cover, which is inhibited by a thicker sediment layer, and that the pattern of hydrothermal circulation may be essentially fixed with the moving plate. Hydraulic impedance of the sediment layer was estimated from the observed depth variation of porosity and was shown to increase rapidly with its thickness. Our interpretation that a threshold thickness of about 50 m would inhibit direct diffuse discharge or recharge of hydrothermal flow through the undisturbed sediment layer yields after an average permeability of the underlying basement layer of about 3 −6×10−14 m2 ( 30–60 mdarcies ).
Thermal parameters of the oceanic lithosphere estimated from geoid height data
Authors: Cazenave Anny , Lago Bernard , Dominh Kien
Published: February 2008

Geoid height anomalies derived from SEASAT altimeter data have been analyzed across fracture zones and over ocean ridges in two limited regions of the South Pacific (Eltanin fracture zone system; East Pacific Rise) and southeast Indian Ocean. Observed geoid height‐age and geoid height derivative (with respect to age)‐age relationships have been established. Comparison with theoretical relationships computed for the plate model of lithospheric cooling permits an estimation of the thermal parameters entering into the model. Two quantities can be derived: the product ακTm (α, thermal diffusivity; κ, volume coefficient of thermal expansion; Tm, bottom boundary temperature) and the thickness H of the plate. The best fitting values for the South Pacific and the southeast Indian Ocean are ακTm = 0.52×10−3 cm2 s−1 and H in the range 50–70 km for ages less than 30 m.y., whereas for larger ages (≥30 m.y.), geoid observations are better explained by a larger H value, in the range 70–90 km. The depth‐age relationship observed in the South Pacific region is consistent with these parameters. On the other hand, data for the northeast Pacific argue rather for a lithospheric thickness larger than 100 km. This suggests that local and regional variations exist in the thermal properties of the oceanic lithosphere.
Assigning probability gain for precursors of four large Chinese earthquakes
Authors: Cao Tianqing , Aki Keiiti
Published: February 2008

We extend the concept of probability gain associated with a precursor (Aki, 1981) to a set of precursors which may be mutually dependent. Making use of a new formula, we derive a criterion for selecting precursors from a given data set in order to calculate the probability gain. The probabilities per unit time immediately before four large Chinese earthquakes are calculated. They are approximately 0.09, 0.09, 0.07 and 0.08 per day for 1975 Haicheng (M = 7.3), 1976 Tangshan (M = 7.8), 1976 Longling (M = 7.6), and Songpan (M ‐ 7.2) earthquakes, respectively. These results are encouraging because they suggest that the investigated precursory phenomena may have included the complete information for earthquake prediction, at least for the above earthquakes. With this method, the step‐by‐step approach to prediction used in China may be quantified in terms of the probability of earthquake occurrence. The ln P versus t curve (where P is the probability of earthquake occurrence at time t) shows that ln P does not increase with t linearly but more rapidly as the time of earthquake approaches.
A teleseismic analysis of the New Brunswick Earthquake of January 9, 1982
Authors: Choy George L. , Boatwright John , Dewey James W. , Sipkin Stuart A.
Published: February 2008

The analysis of the New Brunswick earthquake of January 9, 1982, has important implications for the evaluation of seismic hazards in eastern North America. Although moderate in size (mb 5.7), it was well‐recorded teleseismically. Source characteristics of this earthquake have been determined from analysis of data that were digitally recorded by the Global Digital Seismograph Network. From broadband displacement and velocity records of P waves, we have obtained a dynamic description of the rupture process as well as conventional static properties of the source. The depth of the hypocenter is estimated to be 9 km from depth phases. The focal mechanism determined from the broadband data corresponds to predominantly thrust faulting. From the variation in the waveforms the direction of slip is inferred to be updip on a west dipping NNE striking fault plane. The steep dip of the inferred fault plane suggests that the earthquake occurred on a preexisting fault that was at one time a normal fault. From an inversion of bodywave pulse durations, the estimated rupture length is 5.5 km. Average properties of the rupture process were examined by a moment tensor analysis of long‐period P and SH body waves. The long‐period moment of this earthquake was 5.3 × 1024 dyne cm. The static and dynamic stress drops are 41 and 65 bars, respectively, similar to those of many earthquakes with similar moment in regions that are more seismically active. The joint epicenter determination algorithm was used to locate, relative to the mainshock, the three teleseismically recorded aftershocks that occurred through March 31, 1982. The relocated hypocenters of the aftershocks are significantly different from each other and from that of the mainshock; they provide additional support for the source dimensions inferred from the waveform analysis.
Comment on ‘A schematic model of crater modification by gravity’ by H. J. Melosh
Authors: Pike Richard J.
Published: February 2008

Size‐dependent variations in the morphology of impact craters on planets and satellites pose controversial problems. H. J. Melosh has continued to emphasize theoretical analysis in modeling the morphologic transition from simple to complex craters. His latest results [Melosh, 1982] outline a comprehensive hypothesis wherein deep‐seated collapse of the transient cavity as a Bingham plastic material to produce central peaks and, seemingly, slump terraces is brought about by acoustic fluidization. However, Melosh's paper also contains inconsistent or ambiguously reported observations and some statements and interpretations that need to be clarified.
Seismic rupture patterns in Oaxaca, Mexico
Authors: Tajima Fumiko , McNally Karen C.
Published: February 2008

The spatio‐temporal patterns of seismic activity for events with body wave magnitude mb ≥ 4.0 are investigated for the three most recent large earthquakes in the Oaxaca region of southern Mexico (August 23, 1965, MS = 7.6, MW = 7.5; August 2, 1968, MS = 7.1, MW = 7.3; November 29, 1978, MS = 7.8, MW = 7.6). A master catalogue of earthquakes is compiled for the analyses, using the International Seismological Center (ISC) catalogue supplemented by the National Earthquake Information Service (NEIS) catalogue; the events are then relocated with the Joint Hypocenter Determination (JHD) method. At this magnitude threshold, we find that the aftershock zone of the 1965 earthquake was seismically quiescent for at least 20 months prior to the main event; the 1968 earthquake was preceded by 1 year of foreshock type activity clustered in the aftershock zone; the 1978 earthquake was preceded by 43 months of quiescence and one event (mb = 4.7) within the aftershock zone 4 months prior to the mainshock. We also find that a segment of the subduction zone in Oaxaca remains unbroken by these earthquakes. In addition, some catalogue problems are pointed out which are important to interpretations of seismicity patterns.
Geological significance of surface gravity measurements in the vicinity of the Illinois Deep Drill Holes
Authors: Aiken O. W. , Keller G. R. , Hinze W. J.
Published: February 2008

A gravity study of northeastern Illinois and southwestern Wisconsin was undertaken to determine the three‐dimensional geometry of the Precambrian granitic body encountered in the Illinois deep drill holes. Gravity data acquired during this study and existing observations show a marked negative gravity anomaly in the vicinity of these drill holes which is interpreted as originating from a negative density contrast between the granitic body and the intruded crystalline rocks. A fifteenth‐order polynomial surface fitted to the Bouguer anomaly data (which includes only wave‐lengths greater than ∼35 km) was chosen as best displaying the effects of the granitic body, and profiles of this surface were modeled employing a variety of techniques and assumptions. The preferred model of the causitive body has a depth to the top of 2 km, a density contrast of −100 kg m−3, and a maximum vertical extent of 5 km. An isopach map of the body constructed from the modeling places the Illinois deep drill holes on the edge of an oval‐shaped mass.
Results from an absolute gravity survey in the United States
Authors: Zumberge M. A. , Faller J. E. , Gschwind J.
Published: February 2008

Using the recently completed JILA absolute gravity meter, we made an absolute gravity survey which covered 12 sites in the United States. Over a period of 8 weeks, the instrument was driven a total distance of nearly 20,000 km to sites in California, New Mexico, Colorado, Wyoming, Maryland, and Massachusetts. The time spent in carrying out a measurement at a single location was typically 1 day. A measurement accuracy of around 1×10−7 m/s2 (10 μGal) is believed to have been obtained at each of the sites.
Earthquakes, plate subduction, and stress reversals in the Eastern Aleutian Arc
Authors: House L. S. , Jacob K. H.
Published: February 2008

Plate subduction beneath the 1500‐km‐long segment of the eastern Aleutian arc between Kodiak and Atka islands (154°W and 176°W longitude) is studied with observations from teleseismic data. The primary data base consists of hypocenters of earthquakes (for the period 1965–1975), carefully selected from the bulletins of the International Seismological Centre, and of 44 new focal mechanism solutions. The principal results of this study are that hypocenters of intermediate‐depth earthquakes in the eastern Aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km. Additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip‐directed P axes for earthquakes in the upper zone and downdip‐directed T axes in the lower zone. Major features of the double zone can be explained by thermoelastic stresses in the downgoing plate. The observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc. This stress reversal coincides in map view with a change from a continental to an oceanic arc. The coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding (oceanic versus continental) mantle. Alternatively, the stress reversal may be related to the time since the last great earthquake. If the observed stress polarity relates to the elapsed time since the last great earthquake, it may be a significant observation for long‐ to intermediate‐term prediction of great thrust earthquakes in subduction zones. Portions of the eastern Aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades. A moderate‐sized bathymetric feature in the Pacific plate, the Amlia Fracture Zone, modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate. Subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100‐km‐long segment of the main thrust zone.