bluuebunny/crossref_metadata_2025 huggingface dataset:
Stratospheric NOy measurements on the NASA DC‐8 during AASE II
Authors: Weinheimer A. J. , Walega J. G. , Ridley B. A. , Sachse G. W. , Anderson B. E. , Collins J. E.
Published: February 2008

We survey measurements of stratospheric NOy on the NASA DC‐8 during AASE II in early 1992. Emphasis is on correlations with other long‐lived species: O3, N2O, and CH4. No cases of extremely high NOy/O3 ratios were seen in 1992. This suggests that denitrification at higher altitudes was less pervasive than in 1989. Values of NOy/O3 in 1992 are comparable to, though perhaps lower than, those measured in 1989. The correlation of NOy with N2O changes over the course of the mission. Relative to the correlation for the whole season, low values of NOy occur in February, most notably on the 22nd. The reason for the low NOy is unknown. It is generally too warm at DC‐8 altitudes for the presence of PSC particles, but denitrification at higher altitudes, followed by descent, might account for the low values seen on 22 February. However, this may be unlikely since the NASA ER‐2, flying during the same period, saw no evidence of denitrification at higher altitudes [Loewenstein et al., 1993].
Steep subduction geometry of the Rivera Plate beneath the Jalisco Block in western Mexico
Authors: Pardo Mario , Suárez Gerardo
Published: February 2008

The morphology of the Rivera plate subducted beneath the Jalisco block in western Mexico is determined from accurately located hypocenters of locally recorded microearthquakes, and from earthquakes with magnitude mb≥4.5 recorded at teleseismic distances. The hypocenters of these latter earthquakes are relocated, and for five of them the focal depth is constrained by the inversion of long‐period body waves. The Wadati‐Benioff zone inferred from these data indicates a steep subduction of the Rivera plate that resembles the geometry of subduction of the Cocos plate beneath Central America. It is, however, very different from the shallower and almost subhorizontal subduction of the Cocos plate observed in southern Mexico, southeast of this region. The Rivera plate is comparable to the Juan de Fuca plate in terms of the small areal extent, young seafloor age, low relative velocity, and low teleseismic activity in the subduction zone. This study shows that the dip of both the Juan de Fuca and Rivera plates are similar once they are decoupled from the overriding continental crust. The downgoing Rivera plate initially starts with a dip of ∼10° down to a depth of 20 km and then increases gradually to a constant dip of ∼50° below a depth of 40 km. Intermediate‐depth seismicity is low in this zone associated with the subduction of the slow (2 cm/yr) and young (9 m.y.) Rivera plate. The maximum depth extent of earthquakes observed in the Rivera subduction zone is about 130 km. The andesitic, calc‐alkaline Colima volcano appears to be directly related to the subduction of the Rivera plate. To the NW of this volcano, the observed Quaternary volcanism in the Jalisco block, which is parallel to the trench, may also be explained by the subduction of the Rivera plate.
The 1993 Kushiro‐Oki, Japan, Earthquake: A high stress‐drop event in a subducting slab
Authors: Takeo Minoru , Ide Satoshi , Yoshida Yashuhiro
Published: February 2008

A spatial slip distribution of the 1993 Kushiro‐Oki earthquake (Mw; 7.6) is undoubtedly constrained using near‐field data and the detailed aftershock distribution. This earthquake has peculiar features in that the slip on the fault plane is restricted within an extremely small area (about 40 km × 20 km) between the upper and lower planes of the double seismic zone in the subducting slab beneath Hokkaido. The total seismic moment, averaged slip, and stress‐drop in this area are 3.3 × 1020 Nm, 5.5 m, and 4.2 × 107 Pa, respectively. This earthquake indicates that the region of the slab between the planes of the double seismic zone beneath Hokkaido is strong enough to sustain a stress of about 4 × 107 Pa. The slip motion of this event is consistent with retrograde slab migration along the Southern Kuril trench, and may represent a process by which retrograde motion of subducting slabs could occur.
Oxygen‐18 and carbon‐13 records for the last 14,000 years from Lacustrine carbonates of Siling‐Co (Lake) in the Qinghai‐Tibetan Plateau
Authors: Morinaga H. , Itota C. , Isezaki N. , Goto H. , Yaskawa K. , Kusakabe M. , Liu J. , Gu Z. , Yuan B. , Cong S.
Published: February 2008

To understand paleoenvironmental changes for the central Qinghai‐Tibetan plateau, we analyzed stable isotopes of oxygen and carbon from calcium carbonates in a bottom sediment core collected from Siling‐Co (lake). Five conventional and two TAMS 14C dates indicate that the core recovered sediments of the last 14,000 years. Calcium carbonates in the sediments seem to be primary carbonates precipitated chemically in the lake, and not clastic particles from limestones distributed around the lake, because of large variation of isotopic ratios, isotopic covariance since 6,000 yr BP and similarity between dates from total calcium carbonates and organic carbon. Their isotopic composition therefore reflects that of the lake water. We present the following paleoenvironmental history over the last 14,000 years in the central part of the plateau, from secular variations of δ18O, δ13C and CaCO3 content throughout the core: (1) Desiccation was dominant during the latter part of the Last Glacial stage (14,000 to 11,000 yr BP). (2) The Last Glacial stage abruptly terminated at 11,000 yr BP. (3) A temperate and stable climate was dominant from 11,000 to 5,000 yr BP. (4)

Climatic conditions fluctuated from 5,000 to the present, including two strong desiccation periods (5,000 to 4,000 yr BP and 3,000 to 2,000 yr BP) and an intermediate period of heavy rainfall (4,000 to 3,000 yr BP). This period is also characterized by a covariant O and C isotopic trend.
A rotating, midday auroral event with northward interplanetary magnetic field
Authors: Moen J. , Burke W. J. , Sandholt P. E.
Published: February 2008

We present ground‐based observations of an isolated, daytime auroral event that was detected above Svalbard by means of an all‐sky TV camera and a multichannel, meridian scanning photometer. This 5‐min event occurred near magnetic noon, poleward of a stable cusp aurora. It emitted ∼20 kR of 557.7‐nm light, with little if any admixture of 630.0‐nm radiation. The structure rotated rapidly then underwent a sudden breakup. The spectral distribution of emitted light indicates that electrons responsible for this auroral structure were accelerated though several kilovolts between the magnetosheath and the ionosphere. Such a potential structure requires that the precipitating, arc electrons constitute a field‐aligned current out of the ionosphere. Current continuity and Ohm's law require radial electric fields within the arc. From the structure's rotational speed, we estimate that the accelerated electrons carried an upward field‐aligned current of order 100µA/m². Satellite observations of polar cap convection patterns suggest that the interplanetary magnetic field had turned northward prior to the event. The auroral structure may be explained as resulting from a transitory magnetic merging of interplanetary and lobe magnetic field lines at and/or a penetration of plasma across the high‐latitude magnetopause.
The ion cyclotron group delay for source regions near the plasmapause
Authors: Ludlow G. R. , Hughes W. J.
Published: February 2008

Electromagnetic ion cyclotron waves are generated near the magnetic equator through an instability driven by the energetic ion temperature anisotropy. Many of these waves are observed on the ground as Pc 1 magnetic pulsations, which often exhibit a distinct wave packet structure (pearl pulsations). We use the warm dispersion relation to calculate the group delay of ion cyclotron waves from the equatorial source region near the plasmapause to the foot of the field line and compare the results to both cold theory and observations. Our results differ significantly from the cold plasma result for frequencies less than 0.85 fnof;cHe. In general we find that the presence of the ring current produces longer delays and greater wave dispersion. We model three particular ion cyclotron wave events each of which was observed simultaneously by DE 1 and the Air Force Geophysics Laboratory Magnetometer Network. Plasma data from three other instruments on DE 1 provide input parameters to the model. In two out of three cases our predicted delays agree with ground/satellite correlation delays. The third event does not agree as well with theory possibly due to the presence of a significantly greater O+ concentration during this event. The enhanced O+ concentration introduces an additional stop band below fnof;cHe along the propagation path which the linear theory is unable to model.
Lunar components in Lunping scintillations
Authors: Koster John R. , Huang Yinn‐Nien , Lue H. Y. , Wu Hsi‐Shu
Published: February 2008

Fourteen consecutive years of 15‐min scintillation index values observed at Lunping, Taiwan, by Huang have been analyzed by the lunar age superposition method. Analysis covers 24 hours per day and is performed separately for each of the four seasons. The analysis method is explained briefly. Statistically significant first and second lunar harmonics have been found at all seasons, but the largest and most significant occur in the summertime data (141 days, centered on the June solstice). During the daytime the probability of scintillations at Lunping is significantly correlated with fnof;0Es (the ordinary wave top frequency of sporadic E). We conclude that our lunar harmonics probably result from the modulation, in the E region, of neutral winds of solar thermal origin by upward propagating lunar tides. The nighttime scintillations are larger, and tidal neutral air velocities seem to play a role in the production mechanism(s). There is clear evidence for both the summertime temperate latitude type of scintillations observed in Japan and the scintillations, mainly equinoctial, arising from the equatorial bubble mechanism. Four distinct, statistically significant summertime scintillation periods are found; plus one each in the autumnal and winter data. At attempt is made to identify the production mechanism associated with each. The prominent role of lunar tides in both equatorial and temperate latitude types of scintillation is noted, and the statistical significance of the results is discussed briefly. The potential value of analyzing numerous existing data bases by this method is mentioned.
Dispersed ion structures at the poleward edge of the auroral oval: Low‐altitude observations and numerical modeling
Authors: Bosqued J. M. , Ashour‐Abdalla M. , El Alaoui M. , Peroomian V. , Zelenyi L. M. , Escoubet C. P.
Published: February 2008

We have compared the AUREOL 3 (A3) observations of auroral ion precipitation, particularly ion beams, with the results from the global kinetic model of magnetotail plasma of Ashour‐Abdalla et al. (1993). We have identified 101 energetic 2‐20 keV H+ velocity dispersed precipitating ion structures (VDIS) with fluxes above 10−3 ergs. cm−2. s−1 in the A3 record between the end of 1981 and mid‐1984. These beams display a systematic increase in energy with increasing latitude and were observed in a narrow region within less than 1 deg in latitude of the polar cap boundary. The VDIS are the most distinctive feature in the auroral zone of the plasma sheet boundary layer. We report first on a statistical analysis of the possible relationships between magnetic activity or substorm phase and the VDIS properties. The VDIS are found on 15‐18% of the A3 orbits. In general their frequency of occurrence is not correlated with activity. However in the 2200‐0200 MLT sector, the probability of observing more energetic VDIS increases for larger values of the AE index. Our particle simulations of the precipitating ions have been extended by using a series of modified versions of the Tsyganenko (1989) magnetic field model and by varying the cross‐magnetosphere electric field. In the simulations, plasma from a mantle source is subject to strong nonlinear acceleration, forming beams which flow along the PSBL. Only 3 to 4% of these beams precipitate into the ionosphere to form the VDIS while the majority return to the equatorial plane after mirroring and form the thermalized central plasma sheet. The final energy and the dispersion of the beams in the model depend on the amplitude of the cross‐tail electric field. Two unusual observations of low‐energy (< 5 keV) O+ VDIS, shifted by 4°‐5° in invariant latitude equatorward of H+ VDIS are analyzed in detail. The sparsity of such O+ events and the absence of the changes in the flux and frequency of occurrence indicate a solar wind origin for the plasma. Finally, large‐scale kinetic modeling, even with its simplifications and assumptions (e.g., static magnetic field, solar wind source), reproduces low‐altitude auroral ion features fairly well; it may therefore be presented as an appropriate framework into which data on energization and transport of the hot plasma, obtained in the equatorial plane, could be inserted in the near future.
Equatorial ion composition, 140‐200 km, based on Atmosphere Explorer E data
Authors: Miller N. J. , Grebowsky J. M. , Hedin A. E. , Spencer N. W.
Published: February 2008

We have used in situ measurements of ion composition and horizontal winds, taken from equatorial orbiting Atmosphere Explorer E in eccentric orbit during 1975‐1976 to investigate the bottomside ionosphere at altitudes 140‐200 km. Representative daytime altitude profiles of ionization were stable against wide variations in horizontal wind patterns. On the nightside, ionization profiles displayed much structure, some of which was associated with variations in the horizontal winds. Special features that sometimes appeared in the structured nightside ionization were apparent ion composition waves, intermediate layers of enhanced ionization, and ionization depletions similar to equatorial ionization bubbles. Apparent ion composition waves displayed a horizontal wave length ∼ 650 km. A series of measurements displayed enhanced layers of ionization that appeared to be newly separated from the bottomside midnight F layer. Ions detected in the enhanced ionization layers were primarily NO+ and O2+ without significant densities of metallic ions, an indication that metallic ions are not required to produce the layers at altitudes above 140 km. Equatorial ionization depletions were observed at lower altitudes than previously reported and displayed molecular ion depletions as well as O+ depletions.
Global asymptotic equilibria and collisionless tearing stability of magnetotail plasmas
Authors: Wang Xiaogang , Bhattacharjee A.
Published: February 2008

Asymptotic tail equilibria which are slowly varying in the Earth‐Sun direction are constructed, including all three components of the magnetic field. These equilibria allow for spatial dependencies in Bn and By. Some of these equilibria are analyzed for stability with respect to collisionless electron tearing modes using a fluid model which predicts, to within a numerical factor of π1/2, the growth rates derived from kinetic theory. No ion tearing instability is found. The By field is demonstrated to have a destabilizing effect on electron tearing modes. In the asymptotic equilibria considered here, electron tearing modes can grow in the presence of By in those regions where the stabilizing effect of electron bounce is small. Implications for numerical simulations and observations are discussed.
Theoretical effects of geomagnetic activity on thermospheric tides
Authors: Fesen C. G. , Richmond A. D. , Roble R. G.
Published: February 2008

The theoretical effects of auroral activity on thermospheric tides during equinox solar cycle minimum are investigated using simulations from the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model. One set of model runs examined the effects of increasing levels of geomagnetic activity on the neutral horizontal winds and temperatures. A second set of model runs examined the generation of diurnal and semidiurnal waves in the neutral horizontal winds and temperatures by solar forcing, auroral forcing, and waves propagating vertically from the lower atmosphere. The model simulations were made for four levels of geomagnetic activity, parameterized principally by the total hemispheric power index and the potential drop across the polar cap. The resulting neutral horizontal wind and temperature fields were examined at geographic latitudes of 17.5°N, 42.5°N, and 67.5°N at 70°W longitude. The modeled response to the level of geomagnetic activity varies with altitude and latitude: the effects tend to maximize at high altitudes and high latitudes and penetrate lower in altitude as geomagnetic activity increases. The simulated mean temperatures increase and the mean winds become more southward and westward at all latitudes with increasing auroral activity. In the upper thermosphere, the model diurnal temperature amplitudes decrease with increasing activity, while the diurnal meridional wind amplitudes increase. The modeled semidiurnal winds are strongly affected by the level of geomagnetic activity, while the semidiurnal temperatures are not. Small but discernible effects are predicted at low latitudes. Analysis of the second set of model simulations focusing on the generation of the tidal waves indicates that the tidal response to auroral activity is largely determined by the interference between the waves due to upward propagating tides and in situ solar forcing and those generated by the auroral momentum and energy sources. Depending on location, the waves forced by these sources may have comparable amplitudes but different phases or may be dominated by one source. Thus at high latitudes the wave response is largely driven by the auroral sources; at low latitudes the upward propagating tides dominate. At midlatitudes, complex tidal structures are predicted, since all sources contribute.
Effects of magnetospheric electric fields and neutral winds on the low‐middle latitude ionosphere during the March 20‐21, 1990, storm
Authors: Buonsanto M. J. , Foster J. C.
Published: February 2008

During the geomagnetic storm of March 20‐21, 1990, substorm activity is clearly evident in magnetometer data collected during the night at the middle‐ and low‐latitude stations Fredericksburg (38.2°N, 282.6°E) and San Juan (18.1°N, 293.8°E). At the same time, incoherent scatter radars at Millstone Hill (42.6°N, 288.5°E) and Arecibo (18.3°N, 293.25°E) observed ionospheric storm effects, which included the penetration of magnetospheric electric fields and disturbance neutral winds to the latitude of Arecibo. The eastward electric fields associated with the substorm disturbances result in increases in the F2 peak height (hmF2) at Arecibo. Decreases in hmF2 follow as a result of increased downward diffusion and/or the effects of an ion drag induced poleward wind. During the intervals between the electric field penetration events, equatorward surges in the neutral wind result in westward electric fields by the disturbance dynamo mechanism. At these times the horizontal ionization drifts are not as strong as the neutral winds, apparently because of a partial shorting out of the dynamo electric fields as a result of some E region conductivity. The anticorrelation between the components of the ion drift parallel (V∥) and perpendicular to the magnetic field in the northward direction (V⊥N) results in approximately horizontal (constant altitude) ion drift motion throughout the interval. Calculations of spatial gradients in the electron density and in the components of the ion velocity are carried out using the multi‐directional incoherent scatter observations at Arecibo. The results show that the variations in electron density during this disturbed interval follow closely the motion term in the F2 region continuity equation, with both advection of spatial gradients and divergence of the ion flow important at times.
Birkeland currents in the plasma sheet
Authors: Tsyganenko Nikolai A. , Stern David P. , Kaymaz Zerefşan
Published: February 2008

A search was conducted for the signatures of Birkeland currents in the Earth's magnetic tail, using observed values of Bx and By from large sets of spacecraft data. The data were binned by x and y for −10 > xGSM > −35 and |yGSM| ≤ 20RE (≤ 30 RE for xGSM ≤ −25 RE) and in each bin their distribution in the (Bx, By) plane was fitted by least squares to a piecewise linear function. That gave average x‐y distributions of the flaring angle between Bxy and the x direction, as well as that angle's variation across the thickness of the plasma sheet. Angles obtained in the central plasma sheet differed from those derived near the lobe boundary. That is the expected signature if earthward or tailward Birkeland current sheets are embedded in the plasma sheet, and from this difference we derived the dawn‐dusk profiles of the tail Birkeland currents for several xGSM intervals. It was found that (1) the Birkeland currents have the sense of region 1 currents, when mapped to the ionosphere; (2) both the linear current density (kiloamperes/RE) and the net magnitude of the field‐aligned currents decrease rapidly down the tail; (3) the total Birkeland current at x ≈ −10 RE equals ≈500‐700 kA, which is ∼30% of the net region 1 current observed at ionospheric altitudes, in agreement with model mapping results; and (4) the Bz and By components of the interplanetary magnetic field influence the distribution of Birkeland currents in the tail.
Role of grain size and particle velocity distribution in secondary electron emission in space plasmas
Authors: Chow V. W. , Mendis D. A. , Rosenberg M.
Published: February 2008

By virtue of being generally immersed in a plasma environment, cosmic dust is necessarily electrically charged. The fact that secondary emission plays an important role in determining the equilibrium grain potential has long been recognized, but the fact that the grain size plays a crucial role in this equilibrium potential, when secondary emission is important, has not been widely appreciated. Using both conducting and insulating spherical grains of various sizes and also both Maxwellian and generalized Lorentzian plasmas (which are believed to represent certain space plasmas), we have made a detailed study of this problem. In general, we find that the secondary emission yield δ increases with decreasing size and becomes very large for grains whose dimensions are comparable to the primary electron penetration depth, such as in the case of the very small grains observed at comet Halley and inferred in the interstellar medium. Moreover, we observe that δ is larger for insulators and equilibrium potentials are generally more positive when the plasma has a broad non‐Maxwellian tail. Interestingly, we find that for thermal energies that are expected in several cosmic regions, grains of different sizes can have opposite charge, the smaller ones being positive while the larger ones are negative. This may have important consequences for grain accretion in polydisperse dusty space plasmas.
Low‐frequency electromagnetic plasma waves at comet P/Grigg‐Skjellerup: Overview and spectral characteristics
Authors: Glassmeier Karl‐Heinz , Neubauer Fritz M.
Published: February 2008

Large‐amplitude electromagnetic plasma waves are one of the dominant features of the solar wind‐comet interaction. Wave characteristics strongly depend on parameters such as the solar wind flow and Alfvén velocities and the angle between flow and interplanetary magnetic field as well as the production rate. With respect to the latter the flyby of the spacecraft Giotto at comet P/Grigg‐Skjellerup provides a unique possibility to study such waves in further detail. Pickup ion‐related wave signatures have been observed up to a distance of 600,000 km from the nucleus. Peak spectral power in the spacecraft frame of reference occurs at frequencies mainly somewhat below the water group ion gyrofrequency. From this the waves are determined to be mainly left‐hand polarized waves, causing one‐sided pitch angle diffusion outbound. The wave activity strongly increases close to the comet; upstream it exhibits a quadratic dependence on the water group pickup ion free energy. Furthermore, a phenomenological study of the wave characteristics provides a unique description of the fine‐structure of the interaction region. Indications of steepened magnetosonic waves have been found in the outbound magnetosheath region.
Upper limits to Jovian hard X radiation from the Ulysses Gamma Ray Burst Experiment
Authors: Hurley K. , Sommer M. , Waite J. H.
Published: February 2008

We describe the first in situ search for Jovian X radiation with a dedicated X ray instrument. Upper limits to Jupiter's X ray power in the 27‐48 keV range are presented using data taken by the gamma ray burst experiment on the inbound Ulysses trajectory, at distances between 163 and 110 RJ. The data were analyzed using three methods for detection of (1) a steady flux, (2) a flux linked to System III rotation, and (3) a sporadic flux. The 3σ upper limits lie between 1.1 × 108 and 5.6 × 108 W. The results are consistent with the idea that the bulk of the Jovian UV auroral emissions are due to energetic electrons, while the previously observed low‐energy X radiation is due to ion precipitation.
In situ velocities in pelagic carbonates: New insights from Ocean Drilling Program Leg 130, Ontong Java Plateau
Authors: Urmos Jozsef , Wilkens Roy H.
Published: February 2008

Ocean Drilling Program Leg 130 returned an extensive suite of in situ borehole logging and closely spaced shipboard physical properties measurements from four sites drilled in carbonate‐rich pelagic oozes and chalks on the Ontong Java Plateau. Using these data, we evaluated factors responsible for observed differences between laboratory data and in situ values. Laboratory and log differences for bulk density/porosity are small and nearly constant and can be accounted for by hydraulic rebound of pore fluid following sediment recovery. Evidence for mechanical rebound of sediments was not observed. Velocity‐porosity relationships cannot be used to indirectly correct laboratory velocities to in situ because fundamental assumptions required for rebound corrections are not satisfied. Velocity‐porosity systematics are also complicated by foraminiferal intraparticle porosity variations. We derived empirical laboratory velocity corrections using log and laboratory velocity differences expressed as functions of depth and effective pressure. We tested the velocity corrections using data for carbonates recovered from Ocean Drilling Program Sites 704, 722, and 762; comparisons with logs show that corrected velocities accurately estimate in situ values for oozes and chalks (CaCO3 >60%). For applications to unlogged Deep Sea Drilling Project Leg 85 sites, igneous basement travel times calculated using corrected velocities agree with acoustic basement travel times from seismic records, implying that corrected velocities are realistic in situ estimates. At Site 574, we used our corrected velocities to reevaluate previous lithostratigraphic and seismostratigraphic correlations based on mechanical rebound‐corrected velocities. The new velocities correlate reflectors to progressively greater core depths. Although changes are minor above 200 m below seafloor, the lowest assigned reflector is shifted 36 m deeper to more closely coincide with a diagenetic boundary and acoustic basement correlates with igneous basement instead of with postulated high‐impedance reflectors within the sediments.
Viscosity determinations of some frictionally generated silicate melts: Implications for fault zone rheology at high strain rates
Authors: Spray John G.
Published: February 2008

Analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths. The data show that the matrices of the friction melts (which cool to form pseudotachylytes) are relatively basic and hydrous, even when their protoliths are intermediate to acid. This is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points, not the formation of minimum melts through equilibrium mineral interaction. This means that hydrous ferromagnesian minerals (e.g., micas and amphiboles) melt preferentially to form the liquid matrix, while feldspars and especially quartz more readily survive as clasts. Pseudotachylytes generated by frictional melting are therefore not bulk melts, and as clast‐melt suspensions, they cannot be considered as simple Newtonian fluids. The calculated viscosities of the friction melts are low. For example, at 1200°C, most friction melts possess zero‐shear suspension viscosities of 102–104 dPa s (1 dPa s = 1 P). This is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature. These viscosities are maximum determinations because, as clast‐melt suspensions, friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates (i.e., during slip on a fault surface). Contrary to earlier suggestions, where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding, this work shows that most pseudotachylytes are partial melts possessing low viscosities. The formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault. Interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation. This would be manifest as an increase in high‐frequency seismic wave radiation and vibrational.
Seismicity of the Gorda Plate, structure of the continental margin, and an eastward jump of the Mendocino Triple Junction
Authors: Smith Stewart W. , Knapp John S. , McPherson Robert C.
Published: February 2008

Analysis of 9 years of data from the Humboldt Bay seismic network sheds new light on the structure and evolution of the Gorda plate and Mendocino triple junction. Significant findings include the buttressing effect of the Pacific plate which demonstrates that there is no underthrusting of the Gorda plate along the Mendocino fault, and the pattern of left‐lateral northeast‐trending faults, which demonstrates how the Gorda plate accommodates N‐S shortening without such underthrusting. Focal mechanisms are consistent with northward compression of the Gorda plate by the Pacific plate until the Gorda plate passes the triple junction, beyond which the N‐S compressive stress is effectively removed, and the focal mechanisms show a change from strike slip to normal faulting with downslab tension. Another significant feature is the shallow Benioff zone (10–40 km) which shows a double seismogenic layer. Unlike most other double seismic layers in deep subduction zones, this one appears to be due to underplating, with a new subduction zone being developed east of the former one. A corner of continental margin material has been partially subducted. We attribute this double seismic zone to reactivation of both the old and new subduction boundaries under N‐S compression produced by Pacific‐Gorda plate interaction at the triple junction. Because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the San Andreas fault has migrated eastward, we have constructed a model that accounts for both the eastward migration of the San Andreas fault and the doubling of the seismic zone in the Gorda plate. This can be done by assuming that at the time of the last eastward jump of the fault, the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the San Andreas fault. Other scenarios for the evolution of the Mendocino triple junction do not require that the subduction zone migrate eastward. However, the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place. This is consistent with other evidence for an eastward jump of the San Andreas fault.
High‐resolution global upper mantle structure and plate tectonics
Authors: Zhang Yu‐Shen , Tanimoto Toshiro
Published: February 2008

A global high‐resolution S wave velocity model RG5.5 is obtained for the upper 500 km of Earth's mantle using a 5° × 5° equal‐area block parameterization. The data set consists of some 18,000 seismograms associated with 971 events with magnitudes larger than 5.5. Fundamental modes (Love and Rayleigh waves) are used with periods from 75 to 250 s. The horizontal resolution length is around 1000 km, and the vertical resolution varies with depth from 60 to 250 km. Model RG5.5 has many features consistent with previous three‐dimensional global and local seismic studies, but many new features are found. The S waves under mid‐ocean ridges have broad slow velocity and have very slow velocity in the upper 100 km below the surface. The minimum velocity is at depths near 50 km or shallower. The lateral extent of the slow velocity region across ridges increases with spreading rate. The S wave velocities under ridges are strongly correlated with spreading rates at shallow depth, but the correlation decreases with depth and almost disappears at 100 km. The slow velocities shift off the current spreading positions below 100 km depth under the Mid‐Atlantic Ridge and may record past positions of the ridge and/or be related to hotspots near the ridge. Some major hotspots are associated with slow‐velocity anomalies with magnitudes of about 1–2% slower than the global average and with lateral dimension larger than 1000 km at depths between 100 and 200 km. Differences in the upwelling structure between ridges and hotspots are indicated. The S wave velocity structures may suggest an active mechanism for the East African Rift Valley and a plate extension mechanism for the Baikal Rift Valley.
Subcrustal microearthquake seismicity and fault plane solutions beneath the Hellenic Arc
Authors: Hatzfeld D. , Besnard M. , Makropoulos K. , Voulgaris N. , Kouskouna V. , Hatzidimitriou P. , Panagiotopoulos D. , Karakaisis G. , Deschamps A. , Lyon‐Caen H.
Published: February 2008

Subcrustal seismicity recorded in the southern Aegean sea during a 7‐week microearthquake study was low compared with shallow seismicity. Most intermediate depth seismicity occurred beneath the western and eastern ends of the Hellenic arc. This distribution confirms that a slab of lithosphere is being subducted at a very shallow (<15°) angle for 200 km beneath the western end (Peloponnese) but more steeply beneath the eastern end (Dodecanese). We could locate only one intermediate depth event beneath the central pan of the arc, where teleseimically located intermediate depth earthquakes also are infrequent. T axes for most of the 22 focal mechanisms of subcrustal earthquakes are roughly parallel to the local dip direction of the seismic zone. Between depths of 40 and 80 km, the mechanisms are more confused than at greater depth, perhaps because some of these earthquakes did not occur within the downgoing slab. Earthquakes deeper than 80 km, and within the subducted slab, have nearly horizontal P axes that trend NNE‐SSW in the eastern part and NNW‐SSE in the western part of the arc. These deeper mechanisms show horizontal P axes along strike, perhaps in response to the contortion of the slab or to the westward motion of Turkey, as well as lengthening downdip, probably in response to gravity acting on excess mass in the slab. Thus the short slab, both downdip and along strike, subducting beneath the Aegean is subjected to a more complex set of forces than the long slabs of the Pacific.
Magnetic grain‐size variations through an ash flow sheet: Influence on magnetic properties and implications for cooling history
Authors: Rosenbaum J. G.
Published: February 2008

Rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks, provide a basis for interpretation of aeromagnetic data over volcanic terranes, and yield insights into the depositional and cooling histories of ash flow sheets. A rhyolitic ash flow sheet, the Miocene‐aged Tiva Canyon Member of the Paintbrush Tuff, contains both titanomagnetite phenocrysts, present in the magma prior to eruption, and cubic Fe‐oxide microcrystals that grew after emplacement. Systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the Nevada Test Site. Natural remanent magnetization varies from less than 1 × 10−4 to more than 8 × 10−4 A m3 kg−1, and in‐phase magnetic susceptibility varies from less than 1 × 10−6 to more than 10 × 10−6 m3 kg−1. The microcrystals, which include both magnetite and maghemite, have Curie points and maximum unblocking temperatures between 580°C and 640°C. Rock magnetic data, including in‐phase and quadrature magnetic susceptibilities as well as hysteresis parameters, demonstrate that these microcrystals are of superparamagnetic and single‐domain sizes. Titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section, whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15‐m‐thick zones at the top and bottom. Within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet. The decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break; possibly represented by a concentration of pumice. Laboratory heating produces changes in magnetic properties which vary complexly from sample to sample. Such changes include (1) decrease in saturation magnetization, (2) both increase and decrease in in‐phase and quadrature magnetic susceptibilities, and (3) shifts in coercivity spectra. The complex sample‐to‐sample variation in response to heating is attributed to differences in initial grain‐size distributions.
Magma‐water interaction in closed systems and application to lava tunnels and volcanic conduits
Authors: Dobran Flavio , Papale Paolo
Published: February 2008

The magma‐water interaction in closed systems was studied by a lumped‐parameter model which employs the conservation of mass and balance of energy equations for magma, water, and gas (water vapor). The resulting system of ordinary differential equations models the distributions of system pressure, gas volume, water droplet size, and magma, water, and gas temperatures as a function of time and system parameters. The system parameters include the characteristics of the surrounding rocks and degrees of magma and water fragmentation as specified by the initial water and magma drop sizes. The resulting model was applied to the rupturing of lava tunnels at Etna and magma‐water interaction in the magmatic conduit of Vesuvius during the gray eruption phase in A.D. 79. The results demonstrate that very rapid system pressurization times of the order of a tenth of a second are possible when the water and magma are highly fragmented. The magma‐water interaction within the conduit of Vesuvius was associated with the water inflow into the conduit due to the decrease of the magmatic pressure below the hydrostatic pressure of the surrounding aquifer and fracturing of the conduit wall. The predicted times of pressure increase within the conduit and the high pressure achieved are consistent with observations of phreatomagmatic eruptions and characteristics of the deposits which suggest that these eruptions are characterized by a series of eruptive pulses associated with the transition between magmatic and hydromagmatic phases.
Holocene slip rate of the Central Garlock Fault in southeastern Searles Valley, California
Authors: McGill Sally , Sieh Kerry
Published: February 2008

A Late Pleistocene shoreline at the overflow level of Searles Lake has been displaced 82 to 106 m (preferred value is 90 m) in a left‐lateral sense and 2.5 m (net) north side up along the Garlock fault, at the southeastern corner of Searles Valley. Previously published radiocarbon dates from both surface and subsurface strata indicate that the most recent highstand of Searles Lake ended sometime between 10,000 and 13,800 14C years ago. The maximum slip rate of the Garlock fault in southeastern Searles Valley is thus 11 mm/14C yr. If part of the offset of the shoreline occurred during older lakestands, then the slip rate may be somewhat less. A channel incised after the most recent highstand, however, is offset about 68 m, indicating that the minimum slip rate is 5 mm/14C yr. Subjective evaluation of the constraints on the offset and on the age of the shoreline yields a preferred rate of 6–8 mm/14C yr at this site. Assuming Bard et al.'s (1990) recent calibration of the radiocarbon time scale, the calibrated slip rate of the Garlock fault is between 4 and 9 mm/yr with a preferred value of 5–7 mm/yr. This estimate is similar to a previous estimate of the Holocene slip rate and is slightly less than an estimate derived from modelling of geodetic data. Extension north of the Garlock fault in Indian Wells and Searles valleys contributes no more than 3 mm/yr left slip to the Garlock fault.
On the role of the upper plate in great subduction zone earthquakes
Authors: Mccaffrey Robert
Published: February 2008

At subduction zones where convergence is not perpendicular to the trench (i.e., oblique), slip vectors of interplate thrust earthquakes are often systematically deflected away from the expected plate vector, probably by deformation of the leading edge of the upper plate. Near the epicenters of great thrust earthquakes of Mw≥8.0 in this century that occurred at trenches, systematic deflections of slip vectors for recent (1977–1992.5) interplate thrust earthquakes are significantly less than global averages. Statistical tests show that great earthquakes nucleate where deviations of slip vectors (residuals) are smaller and plate convergence is faster but not preferentially where the angle of obliquity is smaller than global averages. The small slip vector residuals suggest that great earthquakes nucleate where forces in the forearc due to plate convergence are sustained elastically, rather than anelastically, and that shear stress on subduction thrust faults with great earthquakes is not necessarily higher than on faults without them. Shear stress magnitude is less important than rheology in determining the seismic behavior of subduction zones. An observed decrease in the slip vector residuals with increasing rates of trench‐normal convergence is interpreted as evidence that the forearc is increasingly cooled and strengthened by faster subduction. For the forearc to cool with increasing convergence rate, the shear stress on the subduction thrust fault must be less than about 40 MPa. Lower temperatures on the fault surface and the possibility that thrust faults below undeforming forearcs are smoother than those below deforming forearcs both act to increase the frictional instability of the fault zone. Great thrust earthquakes at subduction zones seem to occur where convergence is fast and the forearc is cool and predominantly elastic. The ability of the forearc of the upper plate to store elastic strain energy appears to be a major factor controlling where great subduction thrust earthquakes can and cannot occur.
Impact and volcanism: A momentum scaling law for erosion
Authors: Simonds Charles H. , Kieffer Susan Werner
Published: February 2008

The controversy that raged through the 1950s–1970s over similarities and differences between meteorite impact and volcanic processes is revisited in this paper. We propose that there are quantitative similarities in erosion caused by high‐speed ejecta produced by either impacts or volcanic processes. Field and petrographic data from the Manicouagan impact crater, Canada, are used to demonstrate that during the emplacement of the impact melt sheet, erosion occurred at a rate of ∼2562 kg m−2 s−1. Field data for the Mount St. Helens lateral blast of May 18, 1980, suggest an erosion rate of 21 kg m−2 s−1, and field data for a small pyroclastic flow on August 7, 1980, suggest an erosion rate of ∼14 kg m−2 s−1. It is proposed that these three rates were determined dominantly by the momentum of the ejecta, and a quantitative formulation based on lofting theory is given. A new application of the Monte Carlo approach to analysis provides minimum, most likely, and maximum estimates for both the field and the theoretical analyses. The substantial erosion that occurs by fast moving flows results in mixing of stratigraphic components over large distances and to very fine scales.
Critical review of electrical conductivity measurements and charge distribution analysis of magnesium oxide
Authors: Freund Friedemann , Freund Minoru M. , Batllo François
Published: February 2008

The electrical conductivity σ of MgO single crystals shows a sharp increase at 500–800°C, in particular of σsurface, generally attributed to surface contamination. Charge Distribution Analysis (CDA), a new technique providing information on fundamental properties that was previously unavailable, allows for the determination of surface charges, their sign and associated internal electric field. Data on 99.99% purity, arc‐fusion grown MgO crystals show that mobile charge carriers start to appear in the bulk of the MgO crystals between 200 and 400°C when σ (measured by conventional techniques) is in the 10−14 to 10−16 Ω−1cm−1 range. Above 500°C, as σ increases to 10−6 to 10−7 Ω−1cm−1, more charges appear giving rise to a strong positive surface charge supported by a strong internal field. This indicates that charges are generated in the bulk and diffuse to the surface by an internally controlled process. On the basis of their positive sign they are identified as holes (defect electrons). Because of the low cation content of these very pure MgO crystals, these holes cannot be associated with transition metal impurities. Instead, they are associated with the O2‐ sublattice, e.g. consist of O− states or positive holes. This conclusion is supported by magnetic susceptibility data showing the appearance of 1000 ± 500 ppm paramagnetic species between 200–500°C. The magnetic data are consistent with strongly coupled, diamagnetic O− pairs below 200–500°C, chemically equivalent to peroxy anions, O2−2 and probably associated with cation vacancies in the MgO matrix. The formation of O2−2 in arc‐fusion grown MgO crystals is very unexpected because of the highly reducing growth conditions. Their presence implies an internal redox reaction involving dissolved “water” by which OH− pairs convert to O2−2 plus H2 molecules. This redox conversion is supported by mass spectroscopic measurements of the H2 release from highly OH−‐ doped, finely divided MgO and by wet‐chemical analysis of its oxidant concentration.