bluuebunny/crossref_metadata_2025 huggingface dataset:
An experimental study of incremental surface loading of an elastic plate: Application to volcano tectonics
Authors: Williams K. K. , Zuber M. T.
Published: February 2004

Models of surface fractures due to volcanic loading of an elastic plate are commonly used to constrain the thickness of planetary lithospheres, but discrepancies exist in predictions of the style of initial failure and in the nature of subsequent fracture evolution. In this study, we perform an experiment to determine the mode of initial failure due to the incremental addition of a conical load to the surface of an elastic plate and compare the location of initial failure with that predicted by elastic theory. In all experiments, the mode of initial failure was tension cracking at the surface of the plate, with cracks oriented circumferential to the load. The cracks nucleated at a distance from load center that corresponds to the maximum radial stress predicted by analytical solutions, so a tensile failure criterion is appropriate for predictions of initial failure. With continued loading of the plate, migration of tensional cracks was observed. In the same azimuthal direction as the initial crack, subsequent cracks formed at a smaller radial distance than the initial crack. When forming in a different azimuthal direction, the subsequent cracks formed at a distance greater than the radial distance of the initial crack. The observed fracture pattern may explain the distribution of extensional structures in annular bands around many large scale, circular volcanic features.
Improved sea surface temperature measurements from space
Authors: Harris A. R. , Saunders M. A. , Foot J. S. , Smith K. F. , Mutlow C. T.
Published: February 2004

We investigate ATSR's skin SST precision by comparing all the available high resolution (1km) ATSR data coincident with global drifting buoy records for the months of February and March 1992. Comparisons are made at two spatial levels: using spatially averaged ATSR data, and at the full (1km) resolution. SSTs in each case are derived using both the standard algorithm and one using the 3.7µm and 11µm nadir channels. The mean bulk‐skin temperature bias is 0.45K (night) and 0.13K (day). The precision is greater: (i) at night than during the day, (ii) when 12µm noise contamination in the standard SST product is removed, (iii) when using a ½° subset for which a corresponding 1km match‐up exists. Our highest precision subset has an r.m.s. scatter of just 0.25K and must approach the geophysical limit set by the inherent variability of the skin effect.
Evidence for midwinter chemical ozone destruction over Antarctica
Authors: Vömel H. , Hofmann D. J. , Oltmans S. J. , Harris J. M.
Published: February 2004

Two ozone profiles on June 15 and June 19, obtained over McMurdo, Antarctica, showed a strong depletion in stratospheric ozone, and a simultaneous profile of water vapor on June 19 showed the first clear signs of dehydration. The observation of Polar Stratospheric Clouds (PSCs) beginning with the first sounding showing ozone depletion, the indication of rehydration layers, which could be a sign for recent dehydration, and trajectory calculations indicate that the observed low ozone was not the result of transport from lower latitudes. During this time the vortex was strongly distorted, transporting PSC processed air well into sunlit latitudes where photochemical ozone destruction may have occurred. The correlation of ozone depletion and dehydration indicates that water ice PSCs provided the dominant surface for chlorine activation. An analysis of the time when the observed air masses could have formed type II PSCs for the first time limits the time scale for the observed ozone destruction to about 4 days.
Temporal development of Mt. Pinatubo aerosols as observed by lidar and sun photometer at Ny‐Ålesund, Spitsbergen
Authors: Beyerle Georg , Herber Andreas , Neuber Roland , Gernandt Hartwig
Published: February 2004

Since summer 1991 multiwavelength lidar and sun photometer observations of the Pinatubo aerosol layer are performed at the Arctic NDSC station (Koldewey‐Station) in Ny‐Ålesund, Spitsbergen. The height integrated backscatter coefficient and the optical density decrease exponentially in time with time constants of 0.89±0.39 and 0.94±0.37 years, respectively. Their wavelength dependence indicates a decrease in median particle radii between 1992 and 1993. It is shown that the observations are consistent with results of parameter‐free model calculations. The calculations are based on the assumption that gravitational sedimentation is the dominant process for volcanic aerosol removal in the Arctic stratosphere.
Weak pitch angle scattering of few MV rigidity ions from measurements of anisotropies in the distribution function of interstellar pickup H+
Authors: Gloeckler G. , Schwadron N. A. , Fisk L. A. , Geiss J.
Published: February 2004

The distribution function of interstellar pickup hydrogen is measured with the SWICS instrument on Ulysses at high heliographic latitudes, over a broad range of velocities, including those below that of the solar wind. It is found that these few MV rigidity protons exhibit a pronounced inward radial streaming in the frame of the solar wind, suggesting that neither instabilities generated by the pickup ions nor ambient turbulence is effective in scattering and isotropizing these particles. These results have implications for theories for the instabilities associated with pickup ions and for the propagation of low rigidity particles in the solar wind.
Total‐electron‐content signatures of plasmaspheric motions
Authors: Jacobson Abram R. , Carlos Robert C. , Massey Robert S. , Wu Guanghui , Hoogeveen Gary
Published: February 2004

Measurements of fluctuations in total electron content made with a radio interferometer reveal weak but frequent outbursts of total‐electron‐content disturbances, with trace azimuths roughly westward, with trace speeds up to ∼2 km/s, and with temporal periods in the range 100–500 seconds. Statistical tests of the dataset suggest that these disturbances are due to drift of plasmaspheric irregularities past the radio lines‐of‐sight. The phenomenon is likely to provide a new passive tracer of zonal convection in the inner magnetosphere.
Dynamics Explorer observations of the production of electron conics
Authors: Burch J. L.
Published: February 2004

Several mechanisms have been proposed as the cause of electron conics, including perpendicular, oblique, and parallel heating, as well as acceleration by both stochastic and oscillating parallel electric fields. While the conic distributions and various wave modes have been observed together, no measurements have been made of the actual acceleration mechanism while it is in progress. This study reports on data obtained by DE‐1, which measured conies at high altitudes (≥ 10,000 km) over the auroral oval, and DE‐2, which simultaneously measured electron distributions at low altitudes (∼900 km) in the same latitude/local time region. The DE‐2 electron distributions often show the low‐altitude signature of a downward field‐aligned beam, which has spread to pitch angles of about 55° as a result of the magnetic mirror force, along with an upward‐moving electron conic distribution localized in the same pitch‐angle range. Even though the electron conies observed by DE‐2 have energies of only several hundred eV, compared to the several keV conics observed by DE‐1, these observations nevertheless support models in which field‐aligned bursts and electron conics occur on the same field line, with the conic being the result of the magnetic mirroring of the part of the downward electron beam that lies just outside the loss cone.
Ulysses observations of the radial magnetic field
Authors: Smith Edward J. , Balogh A.
Published: February 2004

The radial component of the magnetic field in the southern hemisphere has been measured at Ulysses as it traveled from the equator to −80.2° latitude and returned. The radial component multiplied by the square of the radial distance, i.e., BRr², averaged over 77 day intervals (three solar rotations) is approximately constant at −3.5 nT and shows no evidence of a dependence on heliographic latitude. To discriminate against possible time variations, the measurements have been compared with simultaneous observations in the ecliptic by IMP‐8. The two sets of observations agree very well confirming the absence of a significant latitude gradient. Since the sun's dipolar magnetic field component is strong at this phase of the sunspot cycle, it is inferred that magnetic flux from the polar cap is transported to lower latitudes in the solar wind source region to produce a uniform radial field. Such a configuration would be expected if magnetic stresses are influencing the solar wind flow near the sun and are contributing to a non‐radial deflection. Estimates of the flux of open field lines (3 × 1014 webers), of the non‐radial solar wind expansion (∼3) and the polar cap magnetic field strength (∼5 Gauss) are derived.
Sources and velocities of Pc1‐2 ULF waves at high latitudes
Authors: Neudegg D. A. , Fraser B. J. , Menk F. W. , Hansen H. J. , Burns G. B. , Morris R. J. , Underwood M. J.
Published: February 2004

The polar cusp and boundary layer are important in coupling magnetospheric energy sources to the high latitude ionosphere. ULF waves are one of the processes by which this coupling is realized. To study the source regions and propagation characteristics of discrete Pc1‐2 (0.1–2 Hz) ULF wave packets, particularly unstructured emissions and Pc1 bursts at high latitudes, a triangular array of closely spaced induction magnetometers (∼150 km) was deployed beneath the average cusp projection during the 1992 Antarctic winter. From interstation time lags the wave velocity and direction of arrival were calculated with average uncertainties of ±60 kms−1 and ±8°. Wave sources were poleward of the array at low geomagnetic activity and equatorward at high activity. The sources also moved east to west with time, centred around local noon. These results are interpreted as indicative of the ionospheric signature of sources localized in the cusp, the low latitude boundary layer (LLBL) or the outer magnetosphere. Intercalibration of the results for typical events with extrapolations of PACE radar observations and DMSP satellite particle signatures support sources within these regions. Observed group velocities in the range 300–800 kms−1 with a mean of 450 kms−1 are consistent with wave propagation in the ionospheric waveguide. Signals above the waveguide lower cutoff frequency likely propagate away from the source in the ionospheric waveguide and across the magnetometer array. The results suggest a technique for monitoring the high latitude boundary regions and outer magnetosphere using local ULF wave measurements.
Height measurements of OI(557.7 nm) gravity wave structure over the Hawaiian Islands during ALOHA‐93
Authors: Taylor M. J. , Swenson G. R. , Taylor V.
Published: February 2004

During the ALOHA‐93 campaign simultaneous observations of gravity wave structure in the OI(557.7 nm) nightglow emission were made using two all‐sky CCD imagers; one located near the summit of Haleakala Crater, Maui and the other at Mauna Loa Observatory, Hawaii. On 19 October a set of bright, planar, monochromatic waves was imaged by both systems as it progressed rapidly over the Hawaiian Islands. Triangulation on these wave forms indicates a mean altitude of 95±2 km in good agreement with previous rocket soundings at mid‐latitudes. Two methods of triangulation were employed, both achieving similar results.
Interannual variability of the North Polar Vortex in the lower stratosphere during the UARS Mission
Authors: Zurek R. W. , Manney G. L. , Miller A. J. , Gelman M. E. , Nagatani R. M.
Published: February 2004

Northern winters since the 1991 launch of UARS are compared to earlier years (1978–1991) with respect to the potential for formation of Polar Stratospheric Clouds and for isolation of the north polar vortex. Daily NMC temperature minima at 465 K late in the 1993–94 winter and again in December 1994 were the lowest values experienced at those times of year (since 1978). Northern PV gradients were unusually strong in 1991–92 prior to late January and throughout the winter in both 1992–93 and 1994–95. Of all northern winters since 1978, 1994–95 with its early extended cold spell and persistently strong PV gradients most resembled the Antarctic winter lower stratosphere. Even so, temperatures were never as low, nor was the polar vortex as large, as during a typical southern winter. Judged by daily temperature minima and PV gradients at 465 K, meteorological conditions in the Arctic winter lower stratosphere during the UARS period were more conducive to vortex ozone loss by heterogeneous chemistry than in most previous winters since 1978–79.
A lattice BGK model for the diffusion of pore fluid pressure, including anisotropy, heterogeneity, and gravity effects
Authors: Maillot B. , Main I. G.
Published: February 2004

Fluid flow in the Earth's crust can be accurately described by a diffusion equation, with highly anisotropic, heterogeneous and time dependent diffusivities. The so‐called Bhatnagar‐Gross‐Krook (BGK) models provide a simple and efficient way to solve the scalar diffusion equation with complex boundary conditions. We show that it can be adapted to include an anisotropic, space and time dependent diffusivity, and a gravity body force, hence providing a suitable numerical model for realistic simulations of incompressible pore fluid diffusion in heterogeneous, anisotropic rocks in the presence of a gravity field. A stability condition limits in some cases the magnitude of anisotropy that can be simulated. Numerical results are in very good agreement with a known analytical solution.
The slip distribution of the 1992 Nicaragua Earthquake from tsunami run‐up data
Authors: Piatanesi Alessio , Tinti Stefano , Gavagni Ivan
Published: February 2004

The 2 September 1992 Nicaragua tsunami was induced by an MS≃ 7–7.3 earthquake and caused about 170 victims and significant damage to the coastal areas. The aim of this work is to infer the slip distribution along the causative seismic fault from the available tsunami run‐up field data. We have subdivided the fault plane into five slip‐independent subfaults and computed the corresponding numerical Green's functions by integrating shallow‐water equations via a finite‐element technique. The slip distribution on the fault has been determined by applying a least‐squares procedure to the differences between the observed run‐up values and the computed maximum water levels along the coast. The main result is that the best fit is found for a nonuniform coseismic slip, with the highest displacement taking place in the southernmost part of the fault. The measure of the improvement is given by the global rms residual of the heterogeneous‐slip case being circa 25% smaller than that of the uniform case. An ancillary outcome of the analysis is the evaluation of the mean amplification factor of the tsunami wave investing the Nicaraguan coast, that, on assuming a rigidity of 1×1010 N/m², results to be 3.45.
Raman spectroscopic study on the spatial distribution of nitrogen and oxygen in natural ice clathrates and their decomposition to air bubbles
Authors: Pauer Frank , Kipfstuhl Josef , Kuhs Werner F.
Published: February 2004

We present the results of Raman spectroscopic scans to determine the N2/O2 ratios on different planes through a clathrate in the GReenland Ice Core Project (GRIP) deep ice core, in which the two axes of the scans are perpendicular to one another. The spectra of the individual focal planes do not point to fractionation effects of the N2/O2 concentrations within a clathrate specimen in the range of the precision of Raman spectroscopy. In a second study, the frequency shift of the N2 and O2 peaks due to decomposition of a clathrate to an air bubble is shown qualitatively. From their peak integrals, there is no indication of different retransformation rates to air bubbles between the oxygen and the nitrogen contents of clathrates. The N2/O2 ratios we observe in clathrates and on clathrate decomposition at different stages are retained in our measurements of air bubbles resulting from clathrate decomposition.
Heating of the solar wind by pickup ion driven Alfvén ion cyclotron instability
Authors: Gray P. C. , Smith C. W. , Matthaeus W. H. , Otani N. F.
Published: February 2004

Pickup ions in a ring velocity distribution are unstable to several kinetic plasma instabilities. At large heliocentric distances where the overall plasma β (ratio of kinetic to magnetic energy) is dominated by the energy density of interstellar pickup ions and pickup is perpendicular to the interplanetary magnetic field, the dominant of these is the Alfvén ion cyclotron instability (AIC). We demonstrate by hybrid particle simulation that, for conditions where the solar wind β is low, AIC driven by the pickup ions couples to the solar wind. The result is perpendicular heating, leading to an anisotropic solar wind distribution. This process may contribute to enhanced solar wind temperatures at large heliocentric distances and may allow for indirect measurement of interstellar pickup ions.
Evidences for extended self‐similarity in hydromagnetic turbulence
Authors: Carbone Vincenzo , Bruno Roberto , Veltri Pierluigi
Published: February 2004

We investigate the occurrence of Extended Self‐Similarity in the Solar Wind Magnetohydrodynamic (MHD) turbulence by using Helios 2 bulk velocity data recorded in the inner heliosphere at five different heliocentric distances. For each analysed time interval, we compute the sets of scaling exponents for the q‐th order velocity structure functions and we show that these sets turn out to be to a good approximation the same whatever the different heliocentric distances. This result shows that a universal non gaussian probability distribution function, which depends on a unique parameter, describes the velocity differences in a MHD turbulent flow also outside the inertial range and in presence of intermittency. Moreover comparing the values of the obtained scaling exponents with those derived from the existing theoretical models of intermittency, we can infer that the most intermittent MHD structures should be two‐dimensional current sheets.
Statistical tests of VAN earthquake predictions: Comments and reflections
Authors: Kagan Y. Y. , Jackson D. D.
Published: February 2004

The papers in the debate show that precise consideration of whether various events should be considered as having been successfully predicted is one of the key issues for statistical testing of the VAN earthquake prediction efforts. Other issues involve whether the VAN prediction rules have been adjusted retroactively to fit data, how to test the prediction significance and, finally, the criteria for rejection of the null hypothesis‐that the VAN successful predictions are due to chance. The diversity of views on these subjects suggests that the seismological community needs a general strategy for evaluating earthquake prediction methods.
A comparison of MHD simulations of magnetotail dynamics
Authors: Birn J. , Iinoya F. , Brackbill J. U. , Hesse M.
Published: February 2004

The accuracy of two resistive MHD codes based on very different methods is studied by simulating the same problem of plasmoid formation and ejection in the geomagnetic tail initiated by resistive diffusion, using identical initial and boundary conditions and a 2‐D approximation. One of the codes is a modified explicit leapfrog scheme, which has been used extensively in the past for similar problems. The other scheme is an implicit particle‐in‐cell MHD code (FLIP‐MHD). The results from both schemes agree within a few percent in all major characteristics: onset and growth rate of the instability, formation and tailward motion of a plasmoid, scale size of the perturbation and details of the mode structures. FLIP, however, predicts a slightly slower tailward motion of the plasmoid. Both simulations exhibit highly variable earthward flow from the reconnection site with similar averages and similar fluctuation levels, resembling observed bursty bulk flows. Due to differences in the phase of these fluctuations, however, the instantaneous flow may differ in the two simulations.
Origin of late Cenozoic basalts at the Cima volcanic field, Mojave Desert, California
Authors: Farmer G. L. , Glazner A. F. , Wilshire H. G. , Wooden J. L. , Pickthorn W. J. , Katz M.
Published: February 2004

Major element, trace element, and isotopic data from late Cenozoic alkali basalts comprising the Cima volcanic field, southeastern California, are used to characterize basalt sources beneath this portion of the Mojave Desert over the past 8 m.y. The basalts are dominantly trachybasalts with trace element compositions similar to modern ocean‐island basalts (OIB), regardless of the presence or absence of mantle‐derived xenoliths. In detail, the basalts can be divided into three groups based on their ages and on their trace element and isotopic characteristics. Those basalts <1 m.y. in age, and the majority of those 3–5 m.y. old, belong to Group 1 defined by high εNd values (7.6 to 9.3), low 87Sr/86Sr (0.7028 to 0.7040), low whole rock δ18O (5.8‰ to to 6.4‰), and a restricted range of Pb isotopic compositions that generally plot on the mid‐ocean ridge basalt (MORB) portion of the northern hemisphere reference line. The 3 to 5‐m.y.‐old basalts have rare earth element (REE) and other incompatible element abundances that increase regularly with decreasing %MgO and apparently have undergone more extensive differentiation than the younger, <1 m.y.‐old basalts. The Group 2 and 3 basalts are minor constituents of the preserved volcanic material, but are consistently older (5–7.6 m.y.) and have lower εNd (5.1 to 7.5) values than the Group 1 basalts. These basalts have distinctive trace element signatures, with the Group 2 basalts having higher Ni, lower Hf, and slightly lower middle REE abundances than the Group 1 basalts, while the Group 3 basalts are characterized by higher and more fractionated REE abundances, as well as higher Ca, P, Ti, Th, Ta, and Sc contents. The isotopic and trace element characteristics of all the basalts are interpreted to have been largely inherited from their mantle source regions. The isotopic compositions of the Group 1 basalts overlap the values for Pacific MORB and for late Cenozoic basalts in the California Coast Ranges interpreted to have been derived from upwelling MORB asthenosphere. We suggest that the Group 1 basalts were all derived from light REE (LREE)‐enriched portions of the Pacific MORB source, which rose into the slab “gap” that developed beneath the southwestern United States during the late Cenozoic transition from a convergent to a transform plate margin. The Group 2 and 3 basalts either represent smaller degrees of melting of the MORB source, or melting of mafic portions of the subcontinental lithospheric mantle currently present beneath the region. Ancient, LREE‐enriched mantle lithosphere has not been a primary source of basaltic magmatism in this region at any time over the past 8 m.y.
A seismic reflection profile across the Cascadia Subduction Zone offshore central Oregon: New constraints on methane distribution and crustal structure
Authors: Tréhu Anne M. , Lin Guibiao , Maxwell Edward , Goldfinger Chris
Published: February 2004

In 1989, we conducted an onshore/offshore seismic experiment to image the crustal structure of the Cascadia forearc. In this paper, we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort. This profile reveals several features of the forearc that were not apparent in an earlier, coincident reflection profile. One of the most important of these features is a very strong bottom simulating reflection (BSR) beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m, where it appears to crop out on the seafloor. The pressure and temperature conditions at the BSR derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate/seawater system over a broad range of temperatures and pressures, assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during Ocean Drilling Program (ODP) leg 146. Interval velocities and reflection coefficients derived from the data indicate that the BSR represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas, consistent with results obtained during leg 146. Although the regional distribution of the anomalously strong BSR beneath the midslope is poorly known, we speculate that it may be related to apparent slope instability. The data also provide constraints on the thickness and geometry of the Siletz terrane, which is the basement beneath the shelf and acts as the subduction zone backstop. A deep reflection, which might mistakenly be interpreted to be Moho if coincident large‐aperture data were not available, is interpreted to be the base of the Siletz terrane. A “recently” active strike‐slip (?) fault zone that overlies the seaward edge of the Siletz terrane suggests that the Siletz terrane controls the location of decoupling of the subduction complex from the rest of the forearc.
A three‐dimensional gravity analysis of the East Pacific Rise from 18° to 21°30′S
Authors: Cormier Marie‐Hélène , Macdonald Ken C. , Wilson Douglas S.
Published: February 2004

Multibeam bathymetry and gravity coverage of the East Pacific Rise (EPR) between 18° and 21°30′S is used to investigate the relation between melt supply and tectonic segmentation at ultrafast spreading rates. The long‐wavelength features in the residual anomaly show a good correlation with those in the bathymetry. The highest residual anomaly values occur over the broad discordant zone of the 20°40′S overlapping spreading center (OSC), for seafloor ages of 0 Ma to at least 1.5 Ma. We interpret the deepening of the bathymetry and the increase of the residual anomaly toward that discordant zone as due to a decrease of 500 plusmn;200 m of the crustal thickness. Hence the 20°40′S OSC has been associated with a reduced magmatic budget for at least the past 1.5 m.y. and represents a persistent segmentation of the EPR. This is consistent with models in which mantle upwelling, even at the fastest spreading centers, is enhanced between large discontinuities rather than evenly distributed along axis. However, this decrease of the crustal thickness toward an axial discontinuity is several times smaller than that typically documented for slow spreading ridges, which suggests that mantle upwelling is less focused at fast spreading ridges, or that along‐axis transport of crustal material is more efficient, or both. Across the study area, the residual anomaly decreases toward the NW by 15–20 mGal. This regional gradient can be modeled with lateral temperature variations in the upper mantle of up to 60°C, increasing toward the NW. This interpretation is consistent with the numerous seamounts present to the NW and the robust magmatic budget of the ridge between 17° and 18°S, and it could also explain why the ridge segments defined by the smaller OSCs between 18° and 19°S propagate very rapidly away from the robust area. Similar patterns of ridge propagation away from the shallowest section of a ridge have been documented near the Galapagos and Easter Island hot spots. Hence these shorter ridge segments may not be associated with significant individual melt sources. Rather, they may represent a superficial segmentation due to the interaction between the EPR and a mantle heterogeneity located between 17° and 18°S.
On the penetration of an endothermic phase transition by upwellings and downwellings
Authors: Tackley Paul J.
Published: February 2004

Using a simple criterion for the deflection of a constant‐viscosity upwelling or downwelling by an endothermic phase transition, the scaling of the critical phase buoyancy parameter Pcrit with the important lengthscales is obtained. The derived trends match those previously observed in time‐dependent numerical simulations, implying that geometry is the dominant factor in determining the propensity to layering. For a sinusoidal temperature anomaly, Pcrit is found to be proportional to wavelength, so that a stronger phase change is required to stop longer wavelengths, in accord with observations from three‐dimensional numerical simulations. For more realistic Gaussian upwelling and downwelling features, the dependence of Pcrit on width of feature, spacing of features, depth of phase transition and width of phase transition are determined for idealized internally heated and basally heated systems. Narrow upwellings and downwellings are deflected more easily than broad ones, providing a first‐order explanation for the increased propensity to layering as Rayleigh number is increased. Internal heating is found to strongly favor deflection, particularly when the phase change is at shallow depth. For basally heated systems, the depth of the phase transition is found to be relatively unimportant in determining the value of Pcrit for which both upwellings and downwellings are deflected. In contrast, for internally heated systems, a shallower phase transition strongly favors layering. Only weak dependence of Pcrit on the spacing of upwellings and downwellings is found. A narrower phase transition enhances deflection.
Tomography of the crust and upper mantle in southeast Europe
Authors: Papazachos C. B. , Hatzidimitriou P. M. , Panagiotopoulos D. G. , Tsokas G. N.
Published: February 2004

Compressional velocity structure of the crust and the upper mantle in south‐eastern Europe (broader Aegean area) is studied by inverting residuals of the first P arrivals from earthquakes in this region (16°E–31°E, 34°N–43°N). The data used are from regional events recorded by the permanent network of stations during the period 1971–1987, enriched with data from experiments with portable seismographs in four regions of this broad area. This study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern Mediterranean lithosphere under the southern Aegean and gives further detailed information on the crustal and upper mantle structure of the area. Important new information is the existence of a low‐velocity crustal layer in western Greece and Albania and that the velocity anomaly in the mantle under the southern Aegean extends much farther and deeper to the northeast than the Benioff zone of the intermediate depth earthquakes indicates. Furthermore, evidence is presented about the possible existence of older subduction in the northern Aegean and about the influence of the tectonic regime on the velocity field.
Implications for Gondwana of new Ordovician paleomagnetic data from igneous rocks in southern Victoria Land, East Antarctica
Authors: Grunow Anne M.
Published: February 2004

New paleomagnetic data presented here from the southern Victoria Land (SVL) region of East Antarctica further refine the Gondwana early Paleozoic apparent polar wander path. The results are based on paleomagnetic analyses of Early to Middle Ordovician granitoids and dike swarms from which a new SVL pole (23°E, 3.5°S, A95 = 5.9°) was calculated. The new SVL paleomagnetic pole agrees with less well‐determined Ordovician poles from other parts of East Antarctica indicating little or no translation/rotation across the East Antarctic craton since the Middle Ordovician. A new Gondwana ∼475 Ma mean pole (11°E 36°N, A95 = 7°, N = 4 studies, in African coordinates) has been calculated from African, Australian, and Antarctic Early and early Middle Ordovician paleomagnetic poles. Age reassignment of the Antarctic Sør Rondane paleopole (Zijderveld, 1968) places it into the Cambrian period and when combined with other Gondwana Cambrian poles results in a new Gondwana ∼515 Ma pole (7°E, 22°N, A95 = 9.5°, N = 7 studies, in African coordinates). The new Gondwana ∼515 Ma and ∼475 Ma poles, when compared with poles of similar age from Laurentia, allow paleogeographic reconstructions to be made that are in keeping with models predicting that Iapetus Ocean basin opening and closure may have been related first to rifting and then collision of Laurentia with Gondwana. The paleomagnetic data also suggest that most of West Gondwana moved toward lower latitudes between the Middle/Late Cambrian and the late Early Ordovician which may be reflected in the fossil record.
The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects
Authors: Hauksson Egill , Jones Lucile M. , Hutton Kate
Published: February 2004

The Mw 6.7 Northridge earthquake occurred on January 17, 1994, beneath the San Fernando Valley. Two seismicity clusters, located 25 km to the south and 35 km to the north‐northwest, preceded the mainshock by 7 days and 16 hours, respectively. The mainshock hypocenter was relatively deep, at 19 km depth in the lower crust. It had a thrust faulting focal mechanism with a rake of 100° on a fault plane dipping 35° to the south‐southwest and striking N75°W. Because the mainshock did not rupture the surface, its association with surficial geological features remains difficult to resolve. Nonetheless, its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the Transverse Ranges. The Northridge earthquake is part of the temporal increase in earthquake activity in the Los Angeles area since 1970. The mainshock was followed by an energetic aftershock sequence. Eight aftershocks of M ≥ 5.0 and 48 aftershocks of 4 ≤ M ≤ 5 occurred between January 17 and September 30, 1994. The aftershocks extend over most of the western San Fernando Valley and Santa Susana Mountains. They form a diffuse spatial distribution around the mainshock rupture plane, illuminating a previously unmapped thrust ramp, extending from 7–10 km depth into the lower crust to a depth of 23 km. No flattening of the aftershock distribution is observed near its bottom. At shallow depths, above 7–10 km, the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults. The dip of the ramp increases from east to west. The west side of the aftershock zoae is characterized by a dense, steeply dipping, and north‐northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting. These events coincided with the Gillibrand Canyon lateral ramp. Along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms. The focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks, with some strike‐slip and normal faulting in the smaller aftershocks. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge. Both earthquakes accommodated north‐south contractional deformation of the Transverse Ranges. The two earthquakes differ primarily in the dip direction of the faults and the depth of faulting. The 1971 north‐northeast trend of left‐lateral faulting (Chatsworth trend) was not activated in 1994.
Network modeling of permeability evolution during cementation and hot isostatic pressing
Authors: Zhu Wenlu , David Christian , Wong Teng‐fong
Published: February 2004

Permeability of rocks may evolve as a complicated function of porosity during hydrothermal compaction. Recent laboratory studies on hot‐pressed calcite and naturally lithified Fontainebleau sandstone reveal that there exist three regimes with distinctly different permeability‐porosity relationships. At relatively high porosities, permeability changes with porosity reduction following a power law (k∝ϕα) with an exponent α≈3 (regime 1). At low porosities, the power law no longer applies and an accelerated reduction in permeability is found (regime 2). Finally, the permeability becomes too small to be measured, which implies that the pore space is disconnected and there is no percolation (regime 3). Similar behavior and three separate regimes have also been observed in the evolution of electrical conductivity with porosity in hot‐pressed quartz. In this study, we developed a unified model based on percolation theory and the simulation of random networks to analyze the evolution of permeability and electrical conductivity in regimes 1 and 2. We incorporated a random shrinkage model and a connectivity loss model in a three‐dimensional cubic network to account for the two distinct regimes. In regime 1, we kept the network connectivity at 100% and reduced the diameter of an arbitrary bond by a shrinkage factor randomly distributed between 0 and 1. In regime 2, we reduced the network connectivity from 100% to the percolation threshold while maintaining the same pore size distribution. For Fontainebleau sandstone, the pore size distribution is constrained by microstructural observations from automated image analysis and stereological measurements. For hot‐pressed calcite and quartz, since microstructural data were not available, we made preliminary measurements on one available micrograph of a calcite sample. Our simulations predict changes in permeability and pore statistics as a function of porosity which show good agreement with the laboratory data. In accordance with percolation theory, the ratio between interconnected and total porosities is given by the ratio between the order parameter and the bond occupation probability of the network. Detailed observations of the interconnected porosity and total porosity of calcite during hot isostatic pressing are in good agreement with the theoretical prediction. Implications of our modeling results on the kinetics of healing and diagenetic processes are also discussed.
Determination of thermal conductivity for deep boreholes
Authors: Pribnow Daniel F. C. , Sass John H.
Published: February 2004

Two methods for thermal conductivity determinations on rock cores and fragments were tested on a suite of samples from the Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) superdeep drill hole in Germany. They were also compared with estimates of thermal conductivity using the mineral composition of the rock and physical well logs and with in situ thermal conductivity measurements. Laboratory methods provide reasonably precise determinations of the thermal conductivities of both solid core (±5%) and drill cuttings (±10%) at room temperature and pressure. The most common methods presently used for crystalline rocks are the steady state “divided‐bar” (DB) technique and the transient “half‐space” line source (LS). Sample preparation and measurement times are comparable for the DB and LS, with sample preparation being more time consuming on average. For isotropic rocks there is little to choose from between the two methods, which both give reliable values of conductivity in the vertical direction. The LS is easier to set up and use in field laboratory situations, which renders it the preferred method for field reconnaissance. The gneissic crystalline rocks penetrated by the KTB boreholes typically have anisotropy of the order of 10–20%. The DB provides unambiguous values of conductivity in a given direction, so its use is preferable for obtaining both principal conductivities and the vertical component. Anisotropy can be estimated using LS measurements in many different directions, but the potential for large random errors is much greater than with the more straightforward DB approach. For deep research wells the difficulties of extrapolating laboratory results to in situ conditions (particularly temperature) present additional obstacles to determining heat flow. Laboratory measurements of water‐saturated samples under in situ conditions, combined with in situ measurements and judicious use of calculations based on mineralogy and well log derived physical properties, can aid in the accurate characterization of thermal conductivity in deep wells. The application of different methods helped to link variations of heat flow with depth in the KTB hole to the anisotropy of thermal conductivity or thermal refraction and thus allowed the calculation of background heat flux in this geologically complex area.
The kinematics of northern South Island, New Zealand, determined from geologic strain rates
Authors: Holt William E. , Haines A. J.
Published: February 2004

Relative motions within the distributed plate boundary zone of northern South Island, New Zealand, are determined through an inversion of geologic strain rate estimates. The Quaternary fault slip rate estimates define the shear strain rates, and rock uplift rates provide information on the horizontal divergence rates. An erosion rate to rock uplift rate ratio along with a crustal compensation factor is estimated in order to convert rock uplift rates to horizontal divergence rates. Because of the uncertainty in erosion rates, horizontal divergence rates
are given a large standard error of
. The three horizontal strain rate components obtained from these data completely define the horizontal velocity gradient tensor. Strain rate distributions are matched with spline polynomial functions, which can be constrained to behave rigidly within specified regions, such as the Pacific or Australian plates. Inversion of the strain rate distribution, assuming uniform erosion rates across the northern South Island, yields a velocity field that has small differences in both magnitude (10% larger) and direction with the NUVEL‐1A plate motion model between Pacific and Australian plates. A revised strain rate data set, obtained from a variable erosion model in which erosion rates are a linear function of the log of the average annual rainfall magnitudes, yields a velocity field with expected directions that are indistinguishable from the NUVEL‐1A plate motion model between Pacific and Australian plates, but velocity magnitudes are still 10–15% higher than the plate motion model. Therefore the average values of slip rate on strike‐slip faults in Marlborough, required by the NUVEL‐1A plate motion model, are typically close to the low end of the published range of slip values for those structures. The major strike‐slip structures within the Marlborough region are accommodating 80–100% of the total plate motion between Australia and Pacific plates on northern South Island; as much as 20% of the relative motion could be accommodated by shear generated by folding or thrust faulting with a single orientation between the major structures. The magnitude of plate motion contributed by the major faults indicates that the distributed geodetic shear strains measured across northern South Island, and discussed by Bibby [1981] and Walcott [1984], are not a form of irrecoverable strain but rather a feature of the strain field that will eventually be released primarily as concentrated slip on the strike‐slip structures within the region. Strain modeling indicates that the earthquake moment release over the last 150 years within the northwest Nelson province, which is west of the Marlborough region, is at least half an order of magnitude higher than the long‐term rate of moment release. Overall, the northwest Nelson region is playing only a minor role in taking up the long‐term plate motion.
Paleomagnetism of external southern and central Dinarides and northern Albanides: Implications for the Cenozoic activity of the Scutari‐Pec Transverse Zone
Authors: Kissel Catherine , Speranza Fabio , Milicevic Vlado
Published: February 2004

Paleomagnetic investigations have been conducted on 43 Cretaceous to Miocene sites (500 oriented cores) from the southern and central external Dinarides and from the northern external Albanides. The 24 reliable sites (out of 33) from the Dinarides do not document any significant rotation with respect to Africa since at least early Eocene. This result contrasts with the very coherent post‐Eocene 45° clockwise rotation previously observed in the external Albano‐Hellenides from Peloponesus to the Tirana basin. These new results suggest that the Scutari‐Pec line, located between the two orogens and considered as a Mesozoic transform fault on the basis of geological observations, is the place where the torsion between the Dinarides and the Albanides recently occurred. On a local scale, four sites (out of 10) from two ridges located in northern Albania, south of the Scutari‐Pec line but with a Dinaric trend, show that these two structures have undergone a ∼30° post‐Eocene clockwise rotation. This intermediate value of rotation between the north and the south suggests that the Scutari‐Pec line is, in fact, a rather wide transversal zone in which the two ridges are located. It acted as a decoupling zone between the Dinarides and the Albano‐Hellenides during the Cenozoic and probably until the Pleistocene time. The strong seismic activity recorded in this region and the anomalously high positive heat flow (documented by other authors) are also evidence for the recent activity of the Scutari‐Pec transverse zone.
Introduction to Special Section: Mechanisms and Consequences of Melt Segregation From Crustal Protoliths
Authors: Brown Michael , Rushmer Tracy , Sawyer Edward W.
Published: February 2004

… much of petrological effort has been concentrated on attempts to discern, define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis… I do not wish to deny the existence of discernable order in petrological phenomena… but… every geological event… is unique. If we concentrate too heavily on discernment of order and pattern, may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode? Possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology.

Francis J. Turner

Presidential Address to the Mineralogical Society of America

November 11, 1969
Seismic anisotropy beneath the Shumagin Islands segment of the Aleutian‐Alaska subduction zone
Authors: Yang Xiaoping , Fischer Karen M. , Abers Geoffrey A.
Published: February 2004

Lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction. In this study we inferred possible strain geometries in the mantle beneath the eastern Aleutians by mapping seismic anisotropy with shear wave splitting. We analyzed S phases of local events recorded by the Shumagin Seismic Network and obtained splitting parameters (fast direction ϕ and splitting time δt) and their 95% confidence regions. To avoid complexity from converted phases, data were restricted to paths at incidence angles less than 32° and to events deeper than 50 km. The S phases sample the forearc and back arc of the Shumagin segment from 159.7°W to 163.8°W at depths up to 216 km. In our data set of 84 local events, 44 of 127 S phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc (roughly 60°). The remaining 83 S phases produced null measurements, and the fast directions inferred from these phase polarizations agree with the fast‐axis alignment in the constrained measurements. Splitting times vary from 0.1 to 0.35 s and show an overall increase with hypocentral depth. Observed splitting parameters are well matched by predicted values for olivine‐rich mantle wedge models with ∼1% SV anisotropy where the olivine b axis is arc‐orthogonal and the a axis is vertical (no arc‐parallel strain) or arcparallel (no vertical strain). These results are consistent with mantle wedge strain models in which arc‐normal compression is accompanied by arc‐parallel or vertical shearing or extension.